1-3-1 Deep Fundation on Bored Piles and on Wells v Imp

8/6/2019 1-3-1 Deep Fundation on Bored Piles and on Wells v Imp

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GUIDELINES FOR ROAD DESIGN,

CONSTRUCTION, MAINTENANCE ANDSUPERVISION

Volume I: DESIGNING

Section 3: DESIGNING STRUCTURES

DESIGN GUIDELINES (DG 1.3.1)

Part 1: DEEP FOUNDATION ON BORED PILES AND ONWELLS


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Guidelines for Road Design, Construction, Maintenance and Supervision Deep foundation on bored piles

RS-FB&H/3CS DDC 433/94 Volume 1 - Section 3 - Part 1 Strana 3 od 44

INTRODUCTION

In difficult geological morphological conditions where a load bearing ground, i.e. a rock geologicalbase, is located at a depth greater than 6.0 m, it is appropriate to foresee deep foundation. In theup-to-date practice of bridge and civil engineering structure foundation bored piles, and wells aremostly used, as these types of foundations can reach a rock base at a depth of up to 40 m.

Foundation on bored piles is one of the most frequent methods of the deep foundation. In particularthe feasibility of being effectively incorporated into the bridge load bearing structures as well as agood adaptability to site conditions and geomorphologic ground properties are attributes, whichopen the door to this foundation method in numerous bridges and civil engineering structures.

A rapid, reliable, and relatively inexpensive execution, made possible by means of the up-to-datemechanization, places the foundation on bored piles among those technologies, which are, in mostcases, able to fulfil all the economical requirements of construction.

Last but not least such a type of foundation attains high standards in protecting health of theconstruction manpower, as well as affects the environment insignificantly, for that reason it can beconsidered as ecologically adequate.

The foundation on wells is such a foundation method where the vertical shaft is excavated in asimilar way, as it is usual for wells in the narrower sense of the word. As the wells are essentialmembers of the bridge load bearing structure, they influence the structural design, the constructioncosts and progress, the structural stability and durability, as well as the acceptability of the plannedinterventions in the space from the point of view of the ecology and the environment protection.

The Design Guideline 1.3.1 provides fundamental lines for the design and execution of deepfoundation on piles and wells in bridges and civil engineering structures. The contents of theDesign Guideline are divided in several chapters: in the introductory part conditions of and basesfor the use of deep foundation are indicated, followed by the chapters dealing with the design,where conceptions and constructive principles are discussed, then by a chapter presentinggeostatic analysis, and, finally, by a chapter, discussing the deep foundation construction.

The Design Guideline 1.3.1 is intended for design and construction of new structures, as well as forreconstruction of existing ones. It provides requirements and recommendations for designers andcontractors, and shall be considered as a base for designing up-to-date civil engineering structures.The engineers are obliged, on the basis of Code of Engineers, and of the rules of their profession,to supplement these bases by technically and economically well considered, safe, as well as

aesthetically conceived and executed structures.


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Deep foundation on bored piles Guidelines for Road Design, Construction, Maintenance and Supervision

Strana 4 od 44 Volume 1 - Section 3 - Part 1 RS-FB&H/3CS DDC 433/94

C O N T E N T S

1 SUBJECT OF DESIGN GUIDELINES.......................................................................................... 52 REFERENCE REGULATIONS.....................................................................................................53 EXPLANATION OF TERMS .........................................................................................................64 INTRODUCTORY CHAPTER.......................................................................................................7

4.1 Bases for deep foundation design....................................................................................... 74.2 Circumstances in which foundation on bored piles is recommended .................................84.3 Circumstances under which foundation on wells is recommended ..................................10

5 DESIGN OF FOUNDATION ON BORED PILES........................................................................115.1 Selection of pile diameter, length, number, and arrangement ..........................................115.2 Design of bored pile steel reinforcement ..........................................................................145.3 Bored piles in water and soft soil ......................................................................................175.4 Design of connection of piles with bridge substructures ...................................................19

6 DESIGN OF FOUNDATION ON WELLS....................................................................................216.1 General conception principles...........................................................................................216.2 Structural elements of excavation protection ....................................................................266.3 Foundation block and design of contact between well toe and foundation bottom ..........266.4 Methods of connecting a pier with a well ..........................................................................286.5 Anchoring a well in unstable ground .................................................................................306.6 Particularities of a well executed by lowering ...................................................................33

7 GEOSTATIC ANALYSIS OF BORED PILES .............................................................................357.1 Input data ..........................................................................................................................357.2 Bearing capacity of piles loaded with axial force ..............................................................357.3 Bearing capacity of piles loaded with horizontal force ......................................................377.4 Bearing capacity of piles arranged in a group...................................................................38

8 GEOSTATIC ANALYSIS OF WELLS .........................................................................................388.1 Design models...................................................................................................................398.2 Assessment of actions on wells ........................................................................................408.3 Load due to earth pressure ...............................................................................................408.4 Ultimate limit states and serviceability limit states ............................................................41

9 EXECUTION OF FOUNDATION ON BORED PILES.................................................................4110 EXECUTION OF FOUNDATION ON WELLS ............................................................................42

10.1 Execution of wells by gradual unearthing .........................................................................4210.2 Execution of wells by lowering ..........................................................................................4410.3 Particularities of execution of wells in landslide active slope............................................ 4510.4 Supervision, monitoring, and maintenance.......................................................................45


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1 SUBJECT OF DESIGNGUIDELINES

The present Design Guideline is intended forall the participants in planning, designing,constructing, and maintaining bridges andcivil engineering structures.

The goal of this Design Guideline is topresent, discuss, and analyse common soilmechanical, constructive, technological, andorganizational cognitions related to deepfoundation of bridges. The selection offoundation method affects the investmentprocess, conception, design, construction,and maintenance of bridges.

The Design Guideline ensures a linkage ofprofound theoretical and professionalknowledge as well as information from the

literature with practical experiences in thisfield of activity as well as with technicalregulations and standards.

The Design Guideline is mainly intended forconstruction of new bridges; however, it issufficiently common to be applicable toreconstruction of existing bridges andconstruction of civil engineering structures(supporting/retaining walls, galleries, cut-and-covers, tunnels).

Bored piles are piles executed in-place by

placing steel reinforcement and castingconcrete into a previously bored or excavatedcircular hole in the foundation soil.

The present Design Guideline deals withbored piles of diameters of 80 cm to 150 cm,executed vertically. The minimum pile lengthin the bearing layer of the foundation groundamounts to 6.0 m.

The Design Guideline only deals with suchbored piles, which are foreseen forfoundation purposes, i.e. for the transfer ofsupporting forces from the structure into thefoundation soil. Partially, such bored piles arediscussed, where primary loads act in adirection perpendicular to the pile axis, andwhich are used for other purposes such asretaining of earth masses, protection ofexcavations, etc. (e.g. pile walls).

A well is a bearing element, transferringsupporting forces from the structure into thefoundation soil through strata of lower, or nilbearing capacity. A well is constructed bygradual excavation of a vertical shaftincluding all the necessary protectivemeasures.

Such wells are discusses, which areexecuted by gradual excavation in stagesand by simultaneous protection, and wellscarried out by gradual lowering of segments,which have been previously constructedabove the ground.

Usually, wells are of either circular or ellipticalcross-section of a minimum diameter of up toD = 2.5 m. The maximum diameter dependsof the pier conception, load magnitude, andfoundation depth. Wells, constructed bygradual lowering of segments, are generallyof a square or rectangular cross-section.

In the up-to-date civil engineering, thefoundation on caissons is no more used, thusit is not discussed in the present DesignGuideline.

2 REFERENCE REGULATIONS

Rulebook of technical norms for foundation ofstructures, Official Gazette of SFRYugoslavia No.15-295/90;

Rulebook of technical norms for concrete andreinforced concrete, Official Gazette of SFRYugoslavia No. 11/1987, Official Gazette ofRepublic of Slovenia No. 52/200;

- EN 1990:2002 Eurocode 0 Basis of design

- prEN 1991 Eurocode 1 Actions onstructures

- prEN 1992 Eurocode 2 Design of concretestructures

- prEN 1997 Eurocode 7 Geotechnicaldesign

- prEN 1998 Eurocode 8 Design ofstructures for earthquake resistance

EN 1537:1999 Execution of specialgeotechnical work Ground anchors

EN 1536:2002 Execution of special

geotechnical work Bored piles

DIN 4014 Bored piles - execution,dimensioning, bearing capacity

SIA 192 Pile foundation

EN 206-1:2003 Concrete Part 1 Specification, properties, production, andconformity


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3 EXPLANATION OF TERMS

Deep foundation signifies foundation onbored piles and foundation on wells at depthsgreater than 6.0 m.

Shallow foundation means foundation on

blocks, slabs or strip foundations; it is used,where strata of sufficient bearing capacityand low deformability are located at relativelysmall depths (up to 6.0 m).

Pile is a bearing element, which function is totransfer supporting forces from the structureinto the bearing soil through strata of lower,or nil bearing capacity.

Bored pile is an element, executed in-placeby placing steel reinforcement and castingconcrete into a previously bored or excavated

circular hole in the foundation soil.

Pile cap is the upper part of a pile, usuallyconnected with substructure members; ittakes the load from the substructure.

Pile foot is the lower base of the cylindricalbody of a pile, transferring the load into thefoundation soil by activating normal contactstresses.

Pile cylinder circumference is thecylindrical body of a pile transferring the load

by activating shear contact stresses.

Upright pile is a pile, which transfers all thesupporting force or a major part of it into thefoundation soil by activating compressivestresses below the pile foot.

Friction pile is a pile, transferring a majorityof the supporting force into the ground byactivating shear stresses at the pile cylinder.

Compression pile is a pile, which takescompressive forces, i.e. the compressive

axial force.

Tension pile is a pile, which takes tensileforces.

Pile bearing capacity is a physical quantityexpressed with symbols of engineeringmechanics for axial force (N), bendingmoment (M), and shearing force (V); itrepresents a limiting value at which safety isstill ensured in compliance with the criteria offailure and serviceability.

Pile axial bearing capacity is a pile bearingcapacity limited to the consideration of theaxial force ensured by the internal bearingcapacity of the pile (i.e. of materials within apile), and by the external bearing capacity ofthe foundation soil, achieved by the bearingcapacity of the soil below the pile foot, as well

as of the soil at the pile cylinder.

Pile bending bearing capacity is a pilebearing capacity limited to the considerationof the bending moment ensured by theinternal bearing capacity of the pile (i.e. ofmaterials within a pile), and by the externalbearing capacity of the foundation soil,achieved by the lateral resistance of the soilat the pile cylinder.

Protective pipe is usually a steel pipeserving as a supporting shuttering in the

excavated hole for a pile, preventingcrumbling of the wall into the excavated hole.

Flushing liquid is dispersion in a liquidaggregate state, usually a mixture of colloidalclayey grains and water (or only water),acting by its hydrostatic pressure on theexcavation hole wall thus having a function ofa retaining medium, which preventscrumbling of the wall into the excavated hole.

Pile beam is a load bearing beam element ofthe substructure, usually of reinforced

concrete, interconnecting pile caps intoplanar bearing units; by means of a pilebeam, supporting forces are transferred intoseveral piles at the same time.

Pile block is a reinforced concrete loadbearing element of the substructure,connecting pile caps in spatial bearing units;by means of a pile block, supporting forcesare transferred and distributed into severalpiles.

Well is a load bearing element transferring

supporting forces from the structure into thebearing foundation soil. It is carried out byexcavating through non-bearing strata of thefoundation soil, or strata of a low bearingcapacity.

Filled well is a well, which vertical shaft isfilled up with partially reinforced fillingconcrete, or with gravel. A pier is fixed to thewell at the top of the latter.

Hollow well is a well, where the spacebetween the pier and the well cylinder is not

filled up. The pier is fixed to the well at thewell foundation toe.


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Excavation protection is all the protectivemeasures, performed during excavationworks for a well.

Ring is a reinforced concrete wall element,which takes earth pressures duringexcavating vertical shaft for a well.

Shot cement concrete is a mix ofaggregate, cement, water, and admixtures,applied by spraying into or onto a structure. Itcan either form a structural concrete or it isonly a faade cover.

Well cylinder circumference is thecylindrical bodyof a well, transferring forcesinto the foundation soil by activating shearcontact stresses.

Well cylinder wall is a reinforced concrete

wall of a hollow well, or of a well filled up withgravel.

Well toe is the lower part of a well,transferring loads into the foundation soil byactivating normal contact stresses.

Foundation body is the soil below theground level, composed of strata of differentproperties, decisive to determine thefoundation soil bearing capacity.

Drainage signifies evacuation of water from

behind a supporting/retaining wall.

Working field is an area, or a cut into slopeto execute a well.

4 INTRODUCTORY CHAPTER

4.1 Bases for deep foundation design

Bored piles and wells are constituent parts ofa bridge or a civil engineering structure,where the following bases for design apply:

surveying, road-traffic, spatial-town planning,hydrological-hydrotechnical, meteorological-climatic, seismic, and geological-soilmechanical data in the influence area of astructure. Input design data shall beacquired, documented, and interpreted bytaking account of current regulations and DG1.2.1 General Guidelines for Designing RoadBridges.

A fundamental document indicatinggeotechnical data for designing deepfoundations is a geological soil mechanical

report on soil composition and foundationconditions. The extent of such report

depends on the level of the bridge foundationdesign. Generally, it shall comprise thefollowing geotechnical information:

a geographic-geomorphologic descriptionof the road alignment area;

geological and soil mechanical conditionsin the road alignment area;

a description of field and laboratoryinvestigations;

an information on seismic conditions in theinvestigated area;

a definition of geotechnical conditions ofbridge foundation and construction;

a master layout of the motorway alignmentin the bridge area;

a geological soil mechanical chart of thebridge area, with boreholes indicated;

a hydro-geological chart of the bridge area; a geological soil mechanical chart of the

bridge area;

a longitudinal geological - geotechnicalprofile;

transverse geotechnical profiles at locationof individual piers and abutments, withstrata of individual rock types, locations ofshallow and deep landslips, and groundwater levels indicated.

Geological boreholes shall be carried out ateach pier/abutment location, and shall extendby at least 7.0 m below the foreseen bottomof a pile or a well. By the geotechnical

foundation conditions the followinginformation shall be provided:

division of rock in deformational strata withindividual characteristics indicated: volume

weight , shear angle , cohesion c,modulus of elasticity and deformability, andPoissons ratio (for a finite elementanalysis), as well as modulus ofcompression Mv, and both vertical andhorizontal coefficients of ground reaction:Kv, Kn;

allowable bearing pressure and settlements

of the foundation soil; stability analyses including calculations of

earth pressures to the well circumference(active, passive, at rest, at landslip);

general stability of the slope for pier deepfoundation.

The types of data, that structural designersmay require, depend on the design modeland interaction foundation foundation soilrespectively.


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4.2 Circumstances in which foundation onbored piles is recommended

4.2.1 Introduction

Nowadays, piles of large diameters are usedto the greatest possible extent. Therefore,

this Design Guideline deals with the designand construction of such piles.

Pile foundation belongs to widespread deepfoundation types. It is particularly introducedin cases where shallow foundation is notfeasible due to foundation soils of lowbearing capacities, or where structuresfounded on shallow foundations wouldexperience excessive settlements,inclinations, or even sinking into the ground.

Considering all the advantages, the use of

piles is in full swing. This particularly appliesin cases where significant concentratedforces need to be transferred into the loadbearing soil by means of bored piles of largediameters. Such a solution is technical justified and economical; the loads can alsobe taken by several smaller piles, takingaccount of both magnitude and direction ofsupporting forces.

Pile foundation is feasible in a soil of lowbearing capacity, a solid soil, ground water,and surface waters. Such a foundation is

economical, safe, and acceptable from theecological point of view.

The up-to-date construction mechanizationenables a quick, effective, and economicalpile execution. However, suitable accessroads and working fields are required.

4.2.2 Geological soil mechanicalconditions suitable for pilefoundation

Basically, a pile foundation shall be foreseen

in case of a soft soil in the upper part of thefoundation ground at depths greater than 6m, from the level where supporting forcesfrom the structure are taken, up to the level ofa load bearing stratum suitable to foundation.

Supporting forces are transferred to a greaterdepth via supporting elements, i.e. piles.

As a rule, a pile foundation is a correctdecision in cases where the soil bearingcapacity at the foundation level is sufficient totake supporting forces, but it does not ensure

a safe foundation due to insufficient stability.Such circumstances usually occur on slopes.

Areas that seem to be favourable to ashallow foundation, might be unsafe for otherinfluences such as river erosion, possiblesubsequent changes in the ground profile,etc.

A pile foundation is frequently required due to

the ground water level and regime resultingfrom the pit excavation (excessive inflow ofwater into the construction pit, problem ofhydraulic fracture of the foundation soil,impacts on adjacent structures, etc.).

Piles are also designed in cases where aconstruction pit might represent a risk oflosing equilibrium of soil strata at the pit, orcomplex measures to protect the constructionpit would be necessary.

4.2.3 Static conception of the structure as

a condition to foresee a pilefoundation

Foundation on bored piles is especiallysuitable to structures, which conceptions aresensitive to major settlements of piers.

Particularly such structures are in question,which are located next below the roadcarriageway, where settlements can cause arisky deformation of the carriageway.

In statically indeterminate continuous and

frame structures, differential settlements canbecome the most important load case. Tosuch structures, foundation on piles isparticularly suitable, as a direct transfer ofsupporting forces into strata of high bearingcapacity is generally ensured, which meansthat the settlements are relatively small aswell, and they are not related to consolidationprocesses of the foundation soil belowfoundations, as well as to the settlements ofconnecting fills.

Tending to conceive structures without any

expansion joints and bearings (integralstructures), or to reduce those elements to aminimum possible extent, static conceptionsforeseeing piles are generally favourable, asbase parts of piers and abutments are moreflexible, thus allowing greater displacementsat relatively insignificant internal forces andmoments.


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4.2.4 Location of the structure as acondition to foresee a pilefoundation

As a rule, pile foundation is much lessdependent on all the conditions at theconstruction location and in the ground below

the structure, as construction impacts arepracticably negligible in comparison withshallow foundation.

A pile foundation does not represent anymajor problem, when a structure is to befounded in water (river, sea), as theconstruction carried out from a platform onpontoons are proven. The technology of anunderwater extending of piles to piers isabsolutely realizable to all well qualifiedcontractors.

In riverbeds where changes in the bed,particularly deepening of the bottom, due toerosion are quite probable, a pile foundationis a reliable solution.

When construction works are carried throughin confined conditions (in towns, narrowgorges), where impacts on adjoiningstructures and plots of land shall be reducedto minimum, the foundation on piles is themost appropriate solution.

Foundation on bored piles is not an adequate

solution on steep, and often unstable slopes,where construction of access roads andworking fields is problematic and mayprovoke an unstable behaviour of the slopes.

4.2.5 Conditions under which bored pilesare feasible

Structural conception shall take account ofthe conditions under which piles are feasible,as well as of warnings by the performer of thefoundation soil investigations:- execution of piles in cohesive soil of low

plasticity, where vibrations duringconstruction may bring the soil to acondition of a viscous consistency;

- possible obstacles during boring (hiddenexisting structures, foundations, etc.);

- excavation in soft cohesive soil, which ispasting on the protective pipe; duringcasting, the concrete can escape laterallydue to insufficient supporting effect of thesurrounding soil on the fresh concrete;

- execution in gravel consisting ofpredominantly large grains, where, due to asignificant permeability, retaining of fresh

concrete in the excavated profile is

questionable, and the fresh concrete tendsto flow through gravel grains;

- encountering large stones or rocks incohesive or non-cohesive soil; when hit bya chopper, such stones or rocks behave asspring-elements, thus the chopper isineffective;

- excavation in stratified rocks, where achopper is ineffective;

- inclined strata where the protective pipetends to incline;

- in fill slopes and different thicknesses ofstrata at the pile, the protective pipe tendsto incline;

- execution in strata containing ground waterunder hydrostatic pressure (artesian water)is particularly hazardous; there is apossibility of soil fracture within theprotective pipe, and a risky execution of anartesian well in the pile borehole, including

all the consequences resulting from thewater outflow from the artesian stratum;

- execution in aggressive ground or a groundcontaining aggressive water where harmfulactions on the completed pile shall betaken into consideration;

- other possible particularities of the soil.

The feasibility of piles shall also be verifieddue to a particularity of the constructionlocation. The following restrictions occurfrequently:- accessibility for boring devices (machine

dimensions), and required size of theworking field;

- insufficient available (confined) workingspace to execute the piles;

- insufficient clear height to execute the piles(e.g. below high-tension transmissionlines);

- height position of the working field, whichmight limit possible constructionalternatives;

- load bearing capacity of the working field toallow machines to enter the site (e.g.ground of very low bearing capacity);

- public utilities in the ground and in the air,particularly gas mains and high-tensiontransmission lines);

- required safety distances;- noise restrictions due to vicinity of

population;- working time restrictions due to prohibited

noise emissions in settlements.

From the conditions/restrictions mentionedabove it is evident, that the number of inputparameters is quite large, and that thoseparameters are specific for each location,

therefore the design shall be carried out verydeliberately and carefully.


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4.2.6 Construction technology as acondition for pile foundation

Pile foundation can be carried out quitequickly and it generally does not cause anyunexpected situations, which could extendthe construction time (soil crumbling into

construction pits, unforeseen braking in ofground water or surface water).

As a rule, pile foundation is independent onclimatic conditions such as low or hightemperatures, long lasting rainfall causingsoaked ground, increased water levels, etc.,on condition that suitable measures havebeen taken in due time.

A structure founded on piles needs not beburied deeply, as it can be practicallyfounded on the ground surface, thus the

quantities of excavated and placed materialare reduced, and the costs related toconstruction work are diminished.

In deep foundation of viaducts and bridges oflarge spans, where significant supportingforces are transferred into the foundationsconsisting of a large number of piles, boredpiles are often not the most adequatesolution, as they require construction of pileblocks of extremely great dimensions,including all the accompanying problems.

It is advantageous to construct abutments onhigh fills, where piles are bored through acompleted fill, thus a time-consuming fillcompaction between supporting columnsdoes not take place; at a correct execution,abutment settlement is independent on the fillsettlement.

The probability of polluting the ground wateris essentially smaller in case of pileconstruction in comparison with aconstruction in open pits, however oncondition that the machinery employed for

the pile execution is maintainedappropriately.

Specialized companies for execution ofgeotechnical works and other civilengineering enterprises have sufficientnumber of up-to-date boring sets on theirdisposal. As such mechanization is notapplicable to other construction works, it shallbe used as expediently as possible.

4.3 Circumstances under whichfoundation on wells is recommended

Foundation on wells as a type of deepfoundation is recommended particularly in thefollowing cases:

in founding bridge piers on a slope, i.e. in

slope viaducts running along a slope, or inviaducts crossing valleys, where this isrequired by the geological composition ofsoil, slope inclination, and where theaccess for heavy mechanization (boringsets for pile execution) is rendered difficultor even impossible;

in founding bridges consisting of longspans, where a large number of piles foreach individual pier or abutment wouldrequire uneconomically large dimensions ofpile blocks;

in cases, where a well foundation is more

favourable in view of the constructioncosts;

the transfer of forces from the pier to thefoundation soil is more direct in wellfoundation than in foundation on boredpiles.

A deep foundation on wells is selectedparticularly in the following cases:

to preserve the slope natural stability(loosened and soaked ground);

to ensure stability of foundations andpiers/abutments also in cases where a

weathered ground part becomes unstablein the bridge area;

at transferring significant load into thefoundation soil at greater depth, where theupper strata are of a low bearing capacity,or where the conditions for a shallowfoundation are not fulfilled due to theunstable ground; the deformations shall beas small as possible;

when the ground at the excavation stagelose their strength or become unstable in ashort time;

where a greater height of piers is requiredand their stiffness should be reducedrespectively (hollow wells shall beintroduced);

where an execution of access roads andworking fields for the mechanization wouldgive rise to additional slope instabilities.

Advantages of well foundation are thefollowing:

a direct load transfer from a pier into thefoundation soil is ensured;

the foundation soil is visible and kept under

control at the entire excavation depth;


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the actual correct depth of the foundationsoil can be determined during excavationworks in view of the actual groundproperties;

a well is such a protection of theconstruction pit, which does not provokeany ground movements;

the intervention in the environment isminimum.

Well foundation is suitable in a relativelycohesive soil, and in cases where the groundwater level is lower than the foundation level.However, the shaft circumference can alsobe protected in non-cohesive soil (groutedapron, shot cement concrete). Where theground permeability is relatively low, theground water level can be lowered below thefoundation level by pumping the water.

In landslide areas, in addition tomorphological and geological conditions, thefollowing requirements shall be fulfilled whena well foundation is foreseen:

the well circumference shall primarilyprotect a pier from the earth pressureaction;

when executing a well circumference it isnecessary to ensure surface waterdrainage to prevent an additionaldestabilization of a slope prone to land-sliding;

the well circumference shall ensureprotection at well excavation at bothconstruction and service stage of astructure;

the well circumference shall transfer theearth pressure load, and the pressure dueto slope movements of reasonableprobability, into the foundation soil withoutany damage.

Both technical and economical restrictions ofuse apply to the wells. These aspects arefully interconnected. From the technical pointof view those restrictions apply, when theground becomes unstable in a short time andat a small excavation depth. This particularlyapplies to fine sand and silt exposed towater, which might break in, and toweathered rock below the ground water level,or in the presence of the pore water.

Where piers are founded in water (e.g. rivers,lakes, etc.), such wells shall be introduced,which are carried out by segments onprovisionally made peninsulas or islands,lowered gradually by a simultaneousundermining. A suitable foundation depth inthe water amounts to 6-8 m, depending onthe working level of the water.

On a dry and flat ground a well foundation isreasonable at a depth greater than 6.0 m; onthe contrary, the foundation shall be carriedthrough as a shallow one with a spreadexcavation.

5 DESIGN OF FOUNDATION ONBORED PILES

5.1 Selection of pile diameter, length,number, and arrangement

5.1.1 General

Piles shall be selected taken account of theparameters indicated in 4.2 above.- Pile diameter shall be determined

particularly on the basis of the requiredload bearing capacity (axial, bending) and

feasibility of a pile, as well as availabletechnology.

- For smaller bridges and therefore minor

actions piles of smaller diameters ( 80and 100 cm), whereas for larger bridgesand major actions piles of larger diameters

( 125 and 150 cm) are selected.- The pile length, particularly the toe depth,

generally depends on geotechnicalconditions, whilst the pile cap is determinedon the basis of the selected conception,taking account of the structural geometryand ground profile, as well as otherspecificities at the construction site.

- The pile arrangement shall be adjusted tothe conception of piers/abutments. Thegoal shall be to foresee a smaller numberof piles of larger diameters, where theunfavourable interaction of the piles is lessimportant, whilst the model of taking theloads is clearer, as the flow of forces canbe followed easier.

- If feasible, piles shall be so arranged as toavoid construction of large pile beams andblocks of significant loading and greatamount of reinforcing steel.

5.1.2 Selection of pile diameter

First, the pile diameter is determined on thebasis of the external bearing capacitycalculation to be performed by one of thedesign methods taking account of:- results of static load testing,- results of empirical or analytical calculation

methods,- results of dynamical testing- observations of behaviour of comparable

pile foundation.


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Irrespective of the method adopted, theresults shall comply with relevantexperiences in similar foundations.

When the pile bearing capacity, on the basisof which both structure and its foundation areconceived, is determined approximately,

informative values can be introduced, beingindicated for common soil and rock types indifferent literature. In chapter 7 informativecharacteristics in compliance with the DIN V1054-100, and simplified equations todetermine the pile bearing capacity areindicated.

After the pile external load bearing capacity iscalculated, the internal one shall be verifiedas well. Design methods for a circular cross-section with or without steel reinforcementshall be applied.

As a rule, the deflection needs not be takeninto consideration, with exception of longpiles located in foundation soil consisting ofvery soft or viscous strata along the pile, or ofpiles extended by piers in water or air. Forcesresulting from water streaming or from vesselor ice impacts on the piers increase the initialgeometrical imperfections, and consequentlythe hazard of pile deflection in connectionwith the pier.

The pile diameter selection is also influenced

by the execution method (protective pipe orflushing liquid), and by the piling depth.

The ratio of the diameter to the length of abored pile is indicated in table 1:

Table 1: Pile length in dependence on theexecution method

Pile in a borehole with protective pipe

diameter 0.8 m 1.2 m 1.5 mlength max 20 m up to 25 m 35 m

Pile in a borehole with flushing liquid

diameter 0.8 m 1.2 m 1.5 mlength max 20 m up to 30 m 40 m

Piles of larger diameters are generally moreeconomical, as the load bearing capacity isincreasing approximately by the square of thediameter; in addition, geometrical perfectionand more favourable conditions for castingconcrete, and therefore more reliableprotective cover to the reinforcement can beensured easier; the hazard of a the pile

discontinuity is diminished, etc.

5.1.3 Selection of pile length

The pile length depends particularly on thegeotechnical conditions of the foundation soil,or the depth of the soil stratum suitable tofoundation, and the depth of the rockrespectively. It is reasonable to consider a

depth proposed by the soil mechanicsspecialist on the basis of geotechnicalinvestigations.

To specify the definitive length (depth) of apile, data on the soil composition acquiredduring pile excavation works, are often used.As circumstances require, the length of thefist piles as well of the next ones can beincreased. Hazards indicated in the passagebelow shall be taken into account.

Definition of pile length (depth) requires

special attention in view of thickness of thestratum where the pile foot will be located, aspunching may occur in case of an insufficientstratum thickness.

For the selection of greater pile lengths, theforeseen construction method is alsoimportant, as some restrictions due to frictionat pressing-in the protective pipe, anddifficulties at placing longer and heavierreinforcement cages may arise.

By means of the up-to-date equipment, bored

piles can be reliably executed up to a depthof 35 m.

5.1.4 Arrangement of bored piles

Two basic arrangements of piles below abridge pier or abutment are possible:- arrangement of individual piles below the

substructure where geotechnical conditionsand pile spacing ensure, that each pile isfunctioning individually,

- arrangement of piles below thesubstructure, taking account of

geotechnical conditions, in such a numberand at such a spacing, that one canconsider them as a group of piles.

In practice bridge piers/abutments arefounded on several piles. For smallerbridges, piers can be founded on a single pile

of larger diameter ( 150 cm), which iscontinued in the pier. The effect of adjoiningpiles need not be taken into account, i.e. theload bearing capacity needs not be reduced,when the axial spacing amounts to at least 3times pile diameter. Of course, this is only a

rough estimation, as geotechnical conditionsas well as conditions of transferring the load


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from the pile into the foundation soil (normalforce below the pile foot / friction force at pilecircumference) have an essential influenceon the bearing capacity. On the basis ofanalysis of the mechanism of transferringforces from piles into the foundation soil, atsimultaneous consideration of geotechnical

conditions, it is possible to analyse the pileinteractions quite accurately.

It is particularly important to take account ofinteraction of long friction piles, whilstinteractions of piles standing on a hard rockbase are essentially reduced. The procedureof determining bearing capacity of a group ofpiles to take both vertical and horizontalforces is indicated in chapter 7.

Pile arrangement is important not only to theload bearing capacity, but also to the transferof supporting forces from the structure intothe piles. If possible, piles shall be soarranged as to ensure an optimum model oftransferring forces into the foundation soil,and enable an economical design ofsubstructure members.

The Rulebook of technical norms forfoundation of structures specifies theminimum admissible pile spacing (refer totable 2); the absolute minimum pile spacingis determined by the construction feasibilityand the foundation soil properties.

Some fundamental conceptions of thesystem piles foundation beam are indicatedbelow, taking account of the pile arrangement

resulting from the direction of supportingforces from the structure, and the pile bearingcapacity (Fig. 5.1).

Table 2: Minimum pile spacing

Piles transferring the load into

the foundation soil mainly viapile foot

2.5 d

Piles in non-cohesive soil ofhigher density, transferring theload into the foundation soilmainly by friction

3 d

Piles in non-cohesive soil of lowdensity, and in cohesive soil,transferring the load into thefoundation ground mainly byfriction

5 d

In major bridges where supporting forces aresignificant, piers/abutments are founded ongroups of piles with large and massive pileblocks, or on wells.

Fig. 5.1: Possible arrangements of bored piles below bridge piers



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5.2 Design of bored pile steelreinforcement

5.2.1 General instructions

The steel reinforcement amount along a pileis determined on the basis of calculated

internal forces and moments in a pile. Thefollowing shall be considered when designingpile reinforcement:- amount of longitudinal (main) and stirrup

reinforcement assessed by calculation,- technical regulations dealing with

reinforced concrete structures,- reinforcement design principles, which

apply to circular cross-section,- physical-technological characteristics of

reinforcing steel, and- specific requirements resulting from

construction technology.

The first three of the above provisions can befulfilled by an usual knowledge of structuralanalysis and dimensioning, which apply tocircular cross-sections under compression ortension, and of low axial force eccentricity. Inpiles, bending and shear is specific and oftenrelatively significant, which also applies toexposition conditions.

Physical-technological properties ofreinforcing steel to be used in pileconstruction are essential, as anchor and

overlapping lengths shall be determinedcorrectly, and the reinforcement shall besuitable to bending and welding.The reinforcing steel producer is obliged tosubmit adequate certificates indicating all theproperties required.

By far the most specific requirements in thepile reinforcement design result from theconditions in which the reinforcement isplaced, from the pile construction,geotechnical conditions, hydrology, and aseries of other specific requirements; if these

conditions are not taken into account, a poorperformance or even unfeasibility may result.In view of that fact, cooperation between thedesigner and the qualified piling contractor orauthorized method engineer is indispensable.

The pile reinforcement can be placed eitherin a single piece for the entire pile length, orby extending the reinforcement cage duringconstruction. Self-supporting reinforcementcages shall be sufficiently solid and stiff toprevent their deformation due to the deadweight during transportation, lifting, and

placing into the pile shaft. Further loads arekinematical forces of liquid concrete at pile

casting by means of a funnel (contractor).Reinforcement cages are also supportingstructures for steel shuttering pipes, whenpiles are cast in water.Taking account of all the indicatedconstruction conditions two fundamentaltypes of reinforcement cages are possible:

- tied reinforcement cages on weldedsupporting framework, and

- welded self-supporting cages.

5.2.2 Tied reinforcement cages

Tied reinforcement cages consist ofsupporting framework made of welded steelof sufficient impact strength (toughness), towhich the load bearing reinforcement is tiedby means of annealed wire. The framework iscomposed of longitudinal bearing bars andbearing rings, of a frame at the bottom, and

of suspension elements on the top of thecage. Longitudinal bearing bars of theframework can be welded with bearing ringsinside or outside the ring.The framework can be built of non-certifiedstructural steel, reinforcing steel, and by non-attested welding.

Longitudinal bars of the bearing framework(Fig. 5.2) are usually of the same diameter asthe longitudinal rebars refer to table 6.When bars of a larger diameter are requiredto ensure the necessary bearing capacity of

the cage, they shall be welded on the internalside of the rings. Therefore, longitudinal barsshall be of steel that is capable of beingwelded.

Framework rings are usually made ofstructural flat or round steel, or reinforcingsteel, which shall be suitable to welding. It isessential to take account of technologicalrequirements, in particular of the ring externaldiameter, as the ring determines the externaldiameter of the entire cage and, therefore,the adequateness of the latter to installation

(Fig. 5.2).

Fig. 5.2: Structure of pile reinforcement cage



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In table 3 spacing of rings made of flat orround steel is indicated in dependence on thelongitudinal bar diameter. In case that therings made of round steel or reinforcing steelare doubled at spacing of 10 20 cm, axialspacing for rings made of flat steel apply.

Table 3: Spacing of load bearing rings

Diameter offrameworklongitudinal

bars

Ringsmade offlat steel

Ringsmade of

round steel

20 mm 2,5 m 1,75 m> 20 mm 3,0 m 2,0 m


In table 4 recommendable dimensions of flatsteel are indicated in dependence on the pilediameter.

Table 4: Cross-section of flat steel of loadbearing rings

Pile diameter Cross-section of flat steelof a ring

80 cm 60 x 8 mm 120 cm 80 x 8 mm 150 cm 100 x 10 mm

Upon placing the cage into the pile boreholeand casting the concrete into the pile body,the cage receives significant loads.

Therefore, the stiffness of rings is ofteninsufficient for piles of larger diameter, thusreinforcement shall be inserted to ensureadditional safety from deforming.

Commonly, crosses of reinforcing steel arebuilt-in, which, in case of piles of largerdiameters, do not represent any essentialobstacle to concrete casting pipe (Fig. 5.3).

In case of heavy cages, diagonal bars shouldbe installed to prevent transversaldeformation of the cage.

Fig. 5.3: Cross bracing

Spacers are extremely important elements ofthe reinforcement cage, ensuring necessarydistance between the cage and the boreholeprotective pipe, as well as the final distancebetween the cage and the borehole wall,which guarantees the required thickness ofprotective concrete cover to the

reinforcement. In table 5, minimumthicknesses of concrete cover in dependenceon the pile construction technology areindicated.

Table 5: Minimum thickness of protectiveconcrete cover to the reinforcement

Constructiontechnology

Concretecover

thickness

For piles of 80 cmexecuted in borehole protected

with pipe

c = 6 cm

For piles executed in boreholewithout protective pipeFor piles made of underwaterconcrete and concrete ofmaximum grain size up to 32mmFor piles where majorunevenness in the boreholewall exist

c = 7.5cm

Where piles are carried out using boreholeprotective pipes, spacers made of reinforcing

steel welded onto the bearing framework ofthe cage, or spacers made of fibre concreteare used. Where no protective pipes areintroduced, it is more recommendable toforesee spacers made of flat steel to achievea more reliable fix contact between thespacers and the borehole wall. In Fig. 5.4,proven methods of spacer execution arepresented.

Fig. 5.4: Methods of executing spacers

Cage foot shall be carried out is such a waythat the concrete casting pipecan reach thebottom of the pile borehole, that lifting of thecage upon pulling out the borehole protectivepipe and/or concrete casting pipe is

prevented, and that plunging of the cage intothe borehole bottom is rendered impossible.


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Bent longitudinal bars or welded-on crossspacers in combination with load bearingrings welded entirely at the bottom oflongitudinal bars are used. In Fig. 5.5, themost typical executions are presented.


Fig. 5.5: Execution of cage foot

Longitudinal load bearing reinforcement isplaced symmetrically or non-symmetrically,

depending on static loading. To avoideventual errors in placing non-symmetricalreinforcement, it is recommen-ded to foreseethe symmetrical one. Usually, deformed(ribbed) reinforcing steel is used.The longitudinal reinforcement of the cageshall be determined by circular cross-sectiondesign, taking account of forces andmoments assessed by the structural analysis.As the calculated reinforcement (or minimumreinforcement) is often too flexible to achievesufficient stiffness of the cage,recommendations for minimum diameters

and spacing of longitudinal reinforcing bars,indicated in table 6, shall be taken intoconsideration.In case of long cages, which are installed bysegments, the longitudinal reinforcementshall be extended by means of overlapping orany other method, which takes account ofprovisions of current regulations andstandards.

Table 6: Diameter and spacing of loadbearing longitudinal bars of areinforcement cage

Pilediameter

Longitudinalbar

diameter

Longitudinalbar spacing

100 cm 16 mm 120 cm 18 mm 150 cm 20 mm

20 cm

Cross load bearing reinforcement of a pileare stirrups, which shall be carried out incompliance with the relevant rules. Thestirrup reinforcement diameter must not be

less than a quarter of the minimum diameterof the longitudinal reinforcement. In table 7,recommendable diameters of stirrups in

dependence on the diameter of thelongitudinal reinforcement are indicated.

Table 7: Diameter of pile stirrup reinforcement

Longitudinal bar

diameter

Stirrup

diameter16 mm 8 10 mm20 mm 12 14 mm25 mm 12 16 mm28 mm 16 mm

Up to a diameter of 12 mm, stirrupreinforcement can be formed as spiral one,which is a common solution. For spiralreinforcement, smooth steel can also beused.Stirrup spacing or the spiral step must not begreater than 12-times the minimum diameter

of the longitudinal reinforcement.For spiral reinforcement it is recommendedthat the spiral step does not exceed 1/5 ofthe pile diameter; the maximumrecommendable spacing shall not be greaterthan 25 cm.In the area where forces are transferred intoa bored pile, the stirrup spacing or the spiralstep should be halved in compliance with theabovementioned recommendations.The length of overlapping of spiralreinforcement, which can be carried out withor without hooks, shall be sufficient. Thestirrup reinforcement shall be extended to thesubstructure members as well (e.g. to the bilebeam), when this is required by the loadtransfer conditions.The entire cage shall be shown in thedrawing, including all the items and details. InFig. 5.6, an example of reinforcementdrawing of a bored pile cage is shown.


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Fig. 5.6: Pile reinforcement drawingThe welded cage design shall fully takeaccount of the technological peculiarities ofmechanical equipment to producereinforcement cages. Prior to purchasing theproduction machines, the cage maker isobliged to check, if cages produced by thosemachines, comply with the relevant technical

regulations and standards.

5.2.3 Welded reinforcement cages

Welded reinforcement cages shall be built ofboth longitudinal and stirrup reinforcement(spiral), having a certificate proving that theyare suitable to welding, as all the joints are of

a welded type. Both longitudinal and crossreinforcement create a meshed structure,which is, thanks to the welded knots,sufficiently rigid, thus no bearing frameworkof the reinforcement cage is necessary. The joints between the longitudinal and crossreinforcement can be carried through eithermanually or by means of welding machines.

In addition, the contractor shall ensure such amaterial, which technological properties aresuitable to an automatic cage production, anda permanent control of the production by anauthorized third party institution.

5.3 Bored piles in water and soft soilWelded reinforcement cages may bemanufactured manually using portablewelding automatic machines, which ensure astandardized execution of a spot weld using

verified current and voltage, and by anexactly defined pressing force and weldingtime. At known structure of the steel for bothlongitudinal and cross reinforcement theprocedure shall be so programmed as toavoid structural changes in the steel, whichcould lead to changes of physical technological properties of reinforcing steel.The geometrical adequacy of thereinforcement cage shall be ensured bymeans of suitable templates.

5.3.1 Circumstances in which piles areexecuted in a shuttering pipe

When bored piles are carried out usingprotective steel pipe, which is subsequentlypulled-out (common method), piles shall beexecuted in a shuttering pipe in the followingcases:- when bored piles are extended through the

water up to the piers/abutments (Fig. 5.7);- when a river water flows through the

foundation soil with such a speed, whichmight result in washing-out of concreteafter the protective steel pipe has beenpulled out (Fig. 5.8);

when piles are constructed in very soft or

viscous soil (cu 0.015 MN/m2), or in a soilof low volume density, where the supportingeffect of the borehole wall does not ensurean equilibrium between the hydrostaticpressure of the fresh concrete and thesurrounding soil at the pile. In such cases it ispossible to press out the surroundingmaterial laterally, at higher concreteoverpressures also towards the surface (Fig.5.9).

Execution of welded reinforcement cages in

welding machines is more often, as theprocedure is fully automatic, and theproduction is permanently controlled andcertified. The human factor is completelyexcluded, thus the cages are produced incompliance with the highest qualitystandards, and with negligible deviationsfrom the design geometry and otherrequirements provided by the design.



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Fig. 5.7 Fig. 5.8 Fig. 5.9

5.3.2 Structure of steel shuttering pipes

Owing to the pile execution method steelshuttering pipes shall always be fixed to thereinforcement cage.

Steel shuttering pipes shall be erected aseither provisional (auxiliary) or permanentstructural members. When those pipes play arole of a formwork protecting the concreteduring casting from washing-out or pressing-out (Figs. 5.8 and 5.9), their function can betemporary. In such cases the steel pipesdisappear in the course of time due tocorrosion. A protective concrete cover to thereinforcement cage shall be ensured. To theexternal diameter of the steel pipe the samerequirements apply as to the externaldiameter of the reinforcement cage.

Therefore, in case of a provisional steelshuttering pipe, the cage diameter shall bereduced by the thickness of the shutteringpipe wall, and by the concrete cover.Corrosion bridges represented by theelements to fix the shuttering pipe onto thereinforcement cage shall be taken intoconsideration as well. In such a case steelpipes need not be permanently protectedfrom corrosion, but only temporary, i.e. forthe installation period), by a priming coat. Ittherefore recommendable to select shutteringpipes made of thinner steel plates. In table 8

recommendable steel plate thicknesses fortemporary shuttering pipes in dependence onthe pile diameter are given.

Steel shuttering pipes shall be carried out aspermanent structural members in theaforementioned cases, and particularly wherea pile is completely executed in water, orwhen it is directly extended to a river pierbelow the riverbed bottom or below the lowwater level. In such a case, the design shallprovide for stronger shuttering pipes (refer totable 9), being adequately protected from

corrosion.

Table 8: Minimum wall thickness ofprovisional shuttering pipe made ofstructural steel

Pile diameter Minimum wall thicknessof provisional shuttering

pipe

80 cm 4 mm 100 cm 5 mm 150 cm 6 mm

Table 9: Minimum wall thickness ofpermanent shuttering pipe made ofstructural steel

Pile diameter Minimum wall thickness

of provisional shutteringpipe

80 cm 6 mm 100 cm 8 mm 150 cm 8 mm

When piles or piers are located in a river ofconsiderable abrasion characteristics, thecorrosion protection shall also ensure arequired resistance to abrasion. To thecorrosion protection, epoxy paint coats aresuitable. A topcoat can be applied foraesthetic appearance, as necessary. A

minimum total dry film thickness of the epoxypaint system shall amount to 200 m. It isalso recommendable to apply galvanized zincas corrosion protection, however this is lessappropriate in case of intensive abrasion.

To specify the length and height of placing ashuttering pipe, the designer shall takeaccount of all the technological andexploitation aspects. As circumstancesrequire, the designer shall consult thecontractor, the soil mechanics specialist, andalso the hydro-meteorological service toprovide all the required information on thewater level to be expected during theconstruction works. The upper edge of theprovisional shuttering pipe is usually placedat the level of the working field carriedthrough to enable the pile construction. Thedepth of the pipe lower edge depends on theconditions of equilibrium between the freshconcrete hydrostatic pressure within theshuttering pipe and the passive resistance ofthe earth surrounding the pile, as well as onthe pile casting progress.

The shuttering pipe shall be firmly fixed to thereinforcement cage. It shall be welded to thebearing framework of the cage by means ofappropriate spacers. The contractor shallspecify the maximum admissible externaldiameter of the shuttering pipe, takingaccount of the borehole protective pipedimensions. On account of the requiredcentring of the reinforcement cage, spacersshall be welded onto the shuttering pipe. Incase of a permanent execution, thosespacers can be cut off or ground off

afterwards. However, corrosion protectioncoating shall be touched-up in this instance,therefore it could be reasonable to foresee


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patented stuck-on spacers made of asynthetic material resistant to corrosion.

5.4 Design of connection of piles withbridge substructures

5.4.1 Connection of piles with pile beamor pile block

To achieve a regular transfer of axial forces,bending moments, and shear forces from thebridge substructure members into the piles,pile beams shall be designed on the top ofthe piles, where the piles are arranged in asingle plane, or pile blocks, where the pilesare arranged in two or more planes.

Pile beams and blocks are usually stiffelements of larger and reasonablydetermined dimensions, and of substantialload bearing capacities, ensuring a

continuous flow of supporting forces from thesubstructure into the piles. Pile beams andblocks shall be so conceived as to enablelaying of properly shaped reinforcing steel totake all the forces and moments occurring inthe conceived model of piers/abutments, andto take all the local actions as well (e.g.splitting force). Pile beams shall be sodesigned as to be, on each side, at least 15cm wider than the pile outer circumference.In other words, they shall be sufficiently wide,so that the reinforced core of the beamextends over the pile diameter. Where piles

are constructed in difficult conditions notensuring a correct pile position (e.g. fromworking fields of low bearing capacity, frompontoon systems, etc.), the pile beam shallbe widened in proportion to expecteddeviations from the designed position.

The minimum height of a pile beam is usuallyselected in such a way that the requiredanchoring or overlapping lengths or thereinforcement from piles or attachedsubstructure members can be ensured.Where the piles are connected to a pile beam

located in ground containing aggressivesubstances, it is recommendable to executethe pile cap 20 cm above the pile beambottom (Fig. 5.10).

The anchoring length of the reinforcementcoming out from a pile and anchored into thepile beam shall be assessed on the basis ofreference technical regulations andstandards indicated in chapter 2. In Fig. 5.11fundamental principles of pile beamconception are presented.

Fig. 5.10: Pile cap plunged into the pilebeam

The pile beam reinforcement shall be sodesigned as to encompass the pilereinforcement completely. At least onecorner-bar encompassed by the stirrupreinforcement shall reach out of the line ofthe force transfer from the pile into the beam,taking into account an angle of force transferof 45. The same also applies to the loadtransfer from a substructure member into thepile beam.

Fig. 5.11: Principles of pile beam reinforcing

In case of transferring substantial forces intothe beam, a splitting force occurs in the areawhere the load is introduced. Such a splittingforce shall be taken by adding adequatereinforcement in a form of closed stirrups; it isalso recommendable to extend the spiralreinforcement from the pile into the beam. Todetermine the splitting force, eventual

eccentricity of the pile axial force and/or the



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axial force in the attached substructuremember shall be considered.

The splitting force depends on the ratio of thecontact area pier beam to the contact areabeam pile. For calculation of the splittingforce, empirical formulae as well as software

based on finite element method are available.

Fig. 5.12: Splitting force in the pile beam

Special attention shall be paid to thedetermination of the required reinforcementin pile beams and blocks, when piers (pointsupports) or walls (line supports) aresupporting them out of the pile axes. In suchcases it is always reasonable to verify thecalculated reinforcement by applyingsimplified models, where an analogy of truss

shall be considered (Fig. 5.13). It is alsoalways necessary to provide an appropriateanchoring of the tensile reinforcement, asboth beams and blocks are often reinforcedwith significant reinforcement cross-sections,which are quite difficult to be anchoredproperly. In such cases the reinforcement islooped, or anchor nuts or welded-on anchorplates are placed to the ends of reinforcingbars (refer to Fig. 5.13).

Fig. 5.13: Compressive and tensile bars in

pile beam, reinforcementanchorages

To ensure a reliable anchoring it is necessaryto provide sufficient cross reinforcement(stirrups, loops, etc.), (Fig. 5.13).Instructions for the pile beam design shall be

logically applied to the pile block design aswell, where spatial orientation vectors ofinternal forces and moments shall beconsidered, including the presented maintensile reinforcement above the piles (Fig.5.14).

Fig. 5.14: Primary tensile reinforcement inusual pile blocks

The reinforcement design shall also takeaccount of the force transfer from the pierinto the pile block, and the zones below thepier shall be adequately reinforced as well.

In Fig. 5.14a an extension of a single pile intoa pier is shown. Such solution isrecommendable, when a support to place thepier formwork and/or the superstructurefalsework shall be ensured.

When laying reinforcement to large pilebeams, a correct position of thereinforcement shall be assured. Structures tosupport the reinforcement shall be foreseen.They remain embedded in concrete togetherwith the reinforcement.

In case of necessity, pile beams and blocks

can also be executed in ground water orsurface water. Sheet piling, wells, or caissonsshall be foreseen to enable the construction.

5.4.2 Direct connection between piles andpiers

When bridge supports are designed as piers,standing independently, direct connectionsbetween piles and piers are frequentlyforeseen. Where such an extension is carriedout in dry conditions, a pile is extended to apier in the same way as it applies to shallow

foundation. Some hours after the boreholeprotective pipe has been pulled out, theupper layer of weak concrete is chiselled off,



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the construction joint surface is arranged,and the pile is extended to a pier of the samediameter by casting the concrete and taking

account of the necessary overlapping lengthof the reinforcement (Fig. 5.15a). When thepier diameter or cross-section is other thanthat of the pile, a pile extension is

preliminarily carried out for laying the pierconnecting reinforcement, and the transitionelement of the pier is executed (Fig. 5.15b),or the connecting reinforcement is alreadypreviously fixed to the pile reinforcementcage.

Fig. 5.15: Extending a pile in dry conditions

Under water, piles are usually extended topiers of a smaller diameter. The underwaterpart of the pier is cast in a shuttering pipe,reaching with its upper edge above the waterlevel, and with lower edge into the pile tosuch a depth that, after the boreholeprotective pipe has been pilled out, thematerial cannot be pressed out laterally atthe borehole, and/or the fresh concretecannot flow out from the borehole.

Fig. 5.16: Execution of underwater joint bymeans of a shuttering pipe

Where the viscosity of the concrete located inthe space between the borehole wall andouter circumference of the shuttering pipe

does not ensure the required equilibriumbetween the fresh concrete within the piershuttering pipe, and concrete viscid

resistance to flowing out, the pile upper partshall also be carried out in a shuttering pipe,welded-on to the pier shuttering pipe bymeans of a ring made of steel plate. Such a

steel ring can be substituted with differenttypes of seals.

Fig. 5.17: Execution of underwater joint usingshuttering pipe of both pier and pile

6 DESIGN OF FOUNDATION ONWELLS

6.1 General conception principles

When conceiving and designing foundationon wells, the following basic criteria for a safeuse of a structure shall be ensured:

structural strength, stability, serviceability,and durability;

current European (Eurocodes) and nationalregulations and standards dealing withfoundation, materials, assessing actions onstructures, reinforced concrete, andearthquake resistance shall be considered;

to analyse actions, approved internationalcalculation methods, design models, and

software shall be used to the greatestpossible extent.

In addition to selecting the foundation depth,dimensions of a well, and the method toexecute a well, the following factors shall betaken into account in the bridge design:

type and size of a bridge; conditions at the bridge location referring to

the overall stability and ground movements;

conditions of surroundings (impact onadjoining structures, traffic, public utilitystructures and services);

foundation soil conditions;



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conditions dictated by ground wateractions;

seismic conditions at the bridge location;

environmental factors such as hydrology,surface water, settlements, seasonalhumidity alterations;

construction economy.

The following shall be considered wherefoundation in a solid rock is foreseen:

admissible settlements of the supportedstructure;

deformability and solidness of rockmasses;

presence of strata of low bearing capacity,phenomena of rock solution, faults belowwells;

presence of contact bearing surfaces orother discontinuities, and theircharacteristics (e.g. filling up, continuity,width, spacing);

weathering, decomposition, and fracturesof rocks;

damage to rock in the natural conditionnext to the wells.

Wells in rock are usually designed by themethod of assumed contact bearingpressures. For solid intact eruptive rock,gneiss, limestone, and sandstone, theassumed pressure is limited by the

compressive strength of the foundationconcrete.

The following design situations shall beconsidered in the well design:

design situation of the initial state of theslope, existing structures, andinfrastructure in the influence area prior tocommencement of construction works;

technological design situations comprisingconstruction of access roads, workingfields, excavation of well shafts, and other

working stages such as prestressing ofground anchors, maintenance and eventualrepairs, interventions in slopes to maintaindrainage systems;

design situations of a permanentexploitation of the structures in its designservice life;

accidental and seismic design situations.

Well foundation is a method of deepfoundation where the vertical shaft isexcavated in a similar way, as it is usual forwells in the narrower sense of the word. The

essence of the method is a gradualexcavation and a simultaneous protection ofthe shaft circumference.

There are no essential differences betweenwells and piles in view of their bearingcapacity and deformability behaviour. When

a deep foundation is compared to a shallowone, one can establish that the interactionbetween the soil and the foundation is muchmore substantial in case of the first. Thedifference between both deep foundationmethods is particularly in the constructionmethod.

Deep foundation is any foundation carried outto a depth greater than 6.0 m measured fromthe flat ground level, or from the lower side ofthe slope.

Two excavation methods are used:

gradual excavation and simultaneousprotection of the shaft circumference (Fig.6.1);

gradual lowering of a well that has beenpreviously cast above the ground level (Fig.6.2).

By the first method, the excavation is carriedout gradually in vertical segments of 0.8 to1.5 m, and the excavated shaftcircumference is protected simultaneously

either with reinforced concrete rings or withshot cement concrete; in dependence on thesoil quality and earth pressure magnitudes,steel rings can be foreseen in addition to theshot cement concrete.

By the second method, wells are constructedat the location of excavation above theground in a height of 2.0 m to 4.0 m. Eithercast-in-situ or pre-cast solution is feasible.Both mechanized excavation and loweringthe well are carried out simultaneously. Afterthe first well segment has been lowered, the

next segment is cast on the upper side, andthe lowering procedure at simultaneousundermining is repeated.

In the spirit of the geotechnical design incompliance with the Eurocode 7, wells areclassified in the geotechnical categories 2and 3, where wells are actually notmentioned, but their individual constituentelements indeed are.

The geotechnical category 2 includes thefollowing well elements or structural

members: foundation slabs,


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walls and other structures retaining orsupporting soil or water,

very deep wells of substantial dimensions, excavations, wells where high risks or uncommon and

extremely difficult ground conditions exist; bridge abutments and piers, structures in areas of a significant seismic

hazard; ground anchors and other anchoring

systems. wells in areas of possible instabilities at the

construction location, or of permanentground movements requiring separateinvestigations or special measures.

The geotechnical category 3 includesstructures or structural parts not covered bycategories 1 and 2. Within the context ofwells, the following is classified in thecategory 3:

1 initial stage of excavation (working field)2 well excavation by stages, and by performing protection with reinforced concrete partial of

complete rings (in a soil of low bearing capacity)3 protection of excavated shaft with shot cement concrete lining (in a weathered rock)4 excavating with a dredger, and removal of excavated material with a mobile crane5 executed well and pier (example of a hollow well)

Fig. 6.1: Well execution by gradual excavation and simultaneous protection of the shaft circumference

1 working field (provisional fill)2 initial well segment with a steel shoe3 lowering of well by undermining and construction of new well segments

4 concrete underlay blinding (underwater concrete)5 execution of foundation and pier6 removal of well lining above the foundation



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Fig. 6.2: Execution by gradual lowering of a well that has been previously cast above the ground level

With regard to the way of introducing the loadfrom a pier into the foundation soil, wells canbe divided in standing (Fig. 6.3), and floatingwells (Fig. 6.4). In standing wells, thecomplete load is transferred into the

foundation soil via foundation slab or well toe.

The function of the well cylindercircumference is particularly to protect theexcavation, eventually to secure the pieragainst rock slip, to form the space aroundthe pier, and to reduce the load indirectly. Infloating wells a portion of the load istransferred into the foundation ground byfriction on the cylinder circumference. In thiscase, a massive foundation slab is carriedout on the upper side of the well shaft, or thepier is rigidly connected with the well cylinder

over the entire shaft height.

Foundation on wells can be carried througheither of individual wells of circular or ellipticalcross-section, or of a group of wells, i.e. twoto four wells being rigidly interconnected by aslab or a crossbar.

Wells executed by gradual excavation andsimultaneous protection, are usually of acircular or elliptical cross-section. Both theshape and dimension of a well particularlydepends on the dimension and shape of the

pier, order of magnitude of static actions,ground stability conditions, and height ordepth of the well. For wells carried out bygradual lowering, a rectangular or squarecross-section can also be designed. In viewof dimensions of the well cross-section nofixed restrictions exist. Where the wellexcavation is protected with shot cementconcrete, the well diameter shall be generallylimited to 2.0 or 2.5 m. Namely, the requiredworking area for both excavation andapplication of shot cement concrete dependson the well diameter. In the constructional

practice, executed wells of D = 2.0 m areknown. There are practically no limitationsregarding the maximum cross-sectionaldimensions of a well. Wells of elliptical cross-section of dimensions 21.0 x 15.0 m areknown.

Fig. 6.3: Standing well

Fig. 6.4: Floating well

To conceive a well, two principles can beadopted: an ideally stiff or an ideally flexible(deformable) structure (Fig. 6.5). A stiff wellstructure is a monolithic continuousreinforced concrete cylinder, resistant tobending, whilst a flexible well structure iscreated by well cylinder elements (rings)interconnected by means of slidingexpansion joints.



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Fig. 6.5: General principles of well design for viaduct piers

A stiff structure is advantageous for theexplicit stability of its shape, and a relativelylow sensitivity to local discontinuities andnon-homogeneities in the foundation soil,whereas a flexible structure is favourable for

the fact that earth pressures acting on thewell are lower, thus the well wall thicknesscan be reduced.

At relatively substantial movements due toslope sliding, and at periodical intensivediscontinuous slips, such a well iseconomical to a depth of 15 20 m. Atgreater depths it is more favourable toforesee a deformable well cylinder. A mixedprinciple is often designed taking account ofeconomy as well as of the static andkinematical behaviour of the well cylinder

circumference.

The well depth (height) particularly dependson the depth where a load bearing soil islocated. It is important to fix the well into arelatively sound load bearing rock. Floatingwells are only exceptionally used in caseswhere the rock is practically unattainable. Ingeneral, at depths of 15 18 m the wellconstruction costs increase substantially, inparticular for the aggravated vertical transportof the material excavated. Up to thosedepths, and at adequate cross-sectional

dimension of the well, the excavated soil istransported upwards by means of a hydraulicdredger equipped with an extended arm. Atgreater depths, the excavated material istransported with a mechanical dredger.Maximum well depths amount to 30.0 35.0m.

6.2 Structural elements of excavationprotection

The thickness of the shot cement concretelining (Fig. 6.6) executed to protect the well

shaft at gradual excavation depends on thefoundation soil conditions and the selectedwell cross-section. For habitual welldimensions it amounts to 10 15 cm. Thelining can be partly non-reinforced, however itis usually reinforced with mesh reinforcementplaced in two layers. As circumstancesrequire, steel braces can be foreseen atgreater depths. On the top of the well, areinforced concrete ring is carried out,increasing stability against the earthpressures, which act from one side only. Inthe lowest excavation layers, where a solid

rock is present, the shaft circumferenceneeds not urgently be protected with shot

cement concrete (Fig. 6.6), on condition thatthe well toe is immediately cast after thefoundation surface has been manuallycleaned.

Excavated well shafts shall be protected withcast-in-situ reinforced concrete rings (Fig.6.7), where well of larger diameters arelocated in a soft soil, and in particular, whenthe well is later on not filled up with concrete.The excavation depths for individualsegments depend on actual soil propertiesand well cross-section, and generallyamount, with regard to the working-technicalconditions, 1.0 to 1.5 m. In a soil of lowcohesion, the excavation depth has often tobe reduced to 0.2 0.3 m to preventcrumbling of the soil.

In hollow wells, after the foundation toe iscompleted, the wall of the well shall beconstructed in a thickness of 30 60 cm,which depends on the earth pressuremagnitude.

Fig. 6.6: Protection of excavated well shaft withshot cement concrete lining

6.3 Foundation block and design ofcontact between well toe andfoundation bottom

The well shall be fixed into the bearingground in a thickness of 1.50 2.50minimum. A widened well toe is reasonableparticularly in cases where the well issurrounded by non-cohesive material andweaker rock; where a well is fixed in a solidrock, such a measure is not required.Consequently, one shall commence to

execute the cross-sectional widening in thearea of non-cohesive soil, where it is



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indispensable to know the final well depth inadvance. When a well is fixed into the rock ata greater depth, a spread load transfer intothe foundation soil shall be achieved by

friction between the jagged cylindercircumference and the rock. In case of agreater inclination of the rock base, the

foundation toe can be so executed as to have

steps at the contact between the well and thesolid rock. A better connection between thewell toe and the foundation soil can beachieved by means of vertical anchors (Fig.6.8).

Fig. 6.7: Excavation protection with reinforced concrete rings



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Fig. 6.8: Examples of well toe design

6.4 Methods of connecting a pier with awell

Where a pier is fixed into the well top, a solidwell (Figs. 6.5a, 6.5b, and 6.9) shall beconstructed. Such a solution is appropriate in

the following cases:

for wells of smaller diameters (< 4.0 5.0m);

for wells of larger diameters, and heightsup to 6.0 10.0 m;

where there is not enough space betweenthe pier and the well, thus the well diametershould be increased;

where this is admitted by the pier height inview of taking horizontal loads;

at a presence of greater amounts of water.

The well shall be filled up with a filling,partially reinforced concrete, or the wellcylinder is executed of reinforced concreteand filled up with gravel. Filling-up with gravelshall be foreseen in case of greater depthsand larger cross-sections of wells, where

filling-up with concrete would not beeconomical. Drainage shall be arranged inthe pier area on the top of the well.

In hollow wells (Figs. 6.5c, 6.5.d, and 6.10), apier is fixed into the foundation block of thewell toe particularly in the following cases:

where it is necessary to reduce the pierstiffness by increasing its height;

in an unstable slope where the well cylinderis carried out as a protective structure.



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Fig. 6.9: Constructive particularities of a solid well



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Fig. 6.10: Constructive particularities of a hollow well

In bridges, an individual pier can be foundedeither on a single well or on two or morewells interconnected by a stiff beam or slab.A foundation on more wells is advantageou

Documents

1-3-1 Deep Fundation on Bored Piles and on Wells v Imp