Lecture 1 Ground Investigation v3

8/11/2019 Lecture 1 Ground Investigation v3

1/44

BES203 CONSTRUCTION IIILECTURE 1: GROUND

INVESTIGATIONJuly 2014

Uhudhu Ahmed

B. Eng. (Civil) (Hons)

1


2/44

Contents

1. Need for a Ground Investigation

2. Primary Investigation

Surface evaluation

Subsurface investigation

3. Nature of Ground Material

Material classification and allowable bearing pressure Cohesion less - Sand and gravel

Cohesive - Silt and clay

Rock

4. Subsurface Sampling

5. Sounding rod, auger, test pits, wash boring, drilling and geophysical

testing.6. Typical Geotechnical Test Bore Information

7. Ground water level

8. Surface Testing

9. Tutorial Questions

2


3/44

Need for a Ground

Investigation

Design requirements of the proposed structurerequire a detailed site investigation

Ensure proper planning and design of theproposed structure

Many projects exceed construction designersbudgets and completion dates because ofunforeseen problems during the excavation andconstruction of their foundations.

Geotechnical experts retained by the projectmanagers to establish the parameters that will beused in the design of the building foundations

3


4/44

Need for a Ground Investigation

The amount of testing done on the site depends on a numberof conditions; Size and complexity of the structure,

Type of soil encountered,

Proximity of the proposed structure to existing buildings,

The level of the groundwater table are the more

The information is then passed on to the structural designers,who must decide on the type and size of foundations that willbe used.

The site evaluation can be considered in two phases Primary investigation which deals with the evaluation of the

physical state of the building site during the design stages of theproject

Secondary investigation which is done by the general contractorjust before the actual construction process begins

4


5/44

Primary Investigation5

Stage 1: Surface evaluation of the building site

Normally consists of a topographic survey to establishgrades for drainage, landscaping requirements, and theplacement of services.

Stage 2: Subsurface investigation Consists of the evaluation of the soil below the surface to

establish criteria for the foundation requirements of theproposed structure

Standard procedures are followed in all stages ofthe soil investigation to ensure that valid results areobtained from the samples when analyzed in thelaboratory


6/44

Surface evaluation6

The topography of the site is of prime concern

to both the designer and the builder in a

surface investigation

The structure must be designed to complementthe site

Builder must be aware of the conditions under

which construction must proceed

The survey information gathered at this time can

be used by the contractor to estimate fill and

excavation quantities


7/44

Surface evaluation7

Common Questions Is the site relatively level?

Will it be necessary to remove large quantities ofearth or, alternatively, will extensive fill material be

required?Are there any rock outcrops that will require blasting?

Is there surface water to be drained and will the sitehave drainage problems in the future?

Has the site been used as a landfill site in the past?

Each condition must be evaluated and dealt within the surface evaluation, for each will ultimatelyhave a bearing on the cost of excavation and thetype of foundation required.


8/44

Subsurface investigation8

Large buildings impose substantial loads on their foundations and dependon soil of good bearing capacitybedrock in some casesto provide thenecessary support.

The depth at which this bearing is available will dictate the type and cost ofthe foundation.

Standard laboratory tests done on subsurface samples provide the

necessary data from which the load-bearing properties of the soil can beestablished

If the project is feasible from this standpoint, this investigation will help toestimate the cost and will influence the design of the foundations

The methods used in obtaining subsurface soil samples vary significantlydepending on the complexity and size of the proposed structure

For shallow foundations, subsurface samples may be obtained from a simple testpit only several feet deep

For deep foundations, samples are usually obtained at various depths up to 100ft (30 m) or more below the surface. To obtain samples at these depths, a drill rigis used to provide the test borehole, and special methods are used for extractingthe required samples.


9/44

Nature of Ground Materials9

To make use of the samples taken duringsubsurface exploration, it is necessary tounderstand something of the nature of soil, typesof soil, and how they react under various

circumstances.

Soil denotes all the fragmented material found inthe earths crust

Includes material ranging from individual rocks of

various sizes, through sand and gravel, to fine-grained clays

particles of sand and gravel are visible to the nakedeye

particles of some fine-grained clays cannot bedistinguished even when viewed through low-powered


10/44

Nature of Ground Materials10

Soil Formations

Eol ian soi l which were deposited by wind

Glacial Til l a mixture of sand, gravel, silt, and

clay, moved and deposited by glaciersAlluvial so i lwhich were deposited by actions of

water

Residu al so i ls consist of rock particles that have

not been moved from their original location but

are products of the deterioration of solid rock


11/44

Soil Classification11

Boulders: 12 in. (300 mm) or larger in diameter.

Cobbles: larger than 3 in. (75 mm), smaller than12in. (300 mm).

Gravel: smaller than 3 in. (75 mm) and larger than

#4 (5 mm) sieve (approximately in.). Sand: particles smaller than #4 (5 mm) sieve and

larger than #200 (630 m) sieve (40,000 openingsper square inch).

Silts: particles smaller than 0.02 mm andlargerthan 0.002 mm in diameter.

Clays: particles smaller than 0.002 mm indiameter


12/44

Soil Classification12

For engineering purposes, soils are loosely grouped into categories that reflect theirability to support loads

Cohesion less soils include sand and gravelsoils in which the particles have little orno tendency to stick together under pressure.

Cohesive soils mayinclude dense silt, medium dense silt, hard clay, stiffclay, firmclay, and soft clay. The particles of these soils tend to stick together, particularly withthe addition of water.

Miscellaneous soils include glacial till and conglomerate. The latter is a mixture ofsand, gravel, and clay, with the clay acting as a cement to hold the particles together.

Rock is subdivided into massive, foliated, sedimentary, and soft or shattered.

Massive rocks are very hard; have no visible bedding planes or laminations; and have widelyspaced, nearly vertical or horizontal joints. They are comparable to the best concrete.

Foliated rocks are also hard but have sloping joints, which preclude equal compressivestrength in all directions. They are comparable to sound structural concrete.

Sedimentary rocks include hard shales, sandstones, limestones, and siltstones, with softercomponents. Rocks in this category may be likened to good brick masonry.

Soft or shattered rocks include those that are soft or broken but not displaced from theirnatural beds. They do not become plastic when wet and are comparable to poor brickmasonry


13/44

Relative Bearing Strength of

Soil13

Table 1:AllowableBearingCapacity forDifferentTypes of

Soils (Andesand Smith,2001).

The givenvalues arevery general

in nature andshould not beused forspecificapplications.


14/44

Settlement of Soil 14

Subsurface soil, in its natural state, is compacted by the weight of overlyingsoil and, in many instances, has been consolidated by geologic forces suchas the pressures due to glaciers

When soil is disturbed, as is the case during excavation, the release inpressure and the rearrangement of the soil particles result in an increase involume or bulk.

When this soil is returned to the excavation in this expanded or bulkedstate, and unless some special measures are taken during backfilling, timeand weather will bring about a return to the original volume.

This shrinkage of the soil back to its near natural condition results insettlement of the overlying soils and any structures that may have beenplaced on the unconsolidated material.

the amount of settlement can be minimized by the use of mechanicalcompaction equipment during placement.

Tests have shown that mixing a certain amount of water with the soilenables the particles to realign, reducing the voids that existed between theparticles during the bulked state


15/44

Settlement of Soil15

If soil can be placed and compacted with an optimum amountof water, excessive shrinkage and settlement due to undercompaction are less likely to occur.

When structures are placed on fill or remediated material, it isimportant that the material has undergone sufficient uniform

compaction to a specified density to minimize non uniform ordifferential settlements

To control the amount of settlement, it is recommended thatthe fill material be placed in layers, not more than 6 in. (150mm) thick at or near its optimum moisture content, to achieveits maximum density

Each layer must be uniformly compacted by some type ofcompaction machinery to ensure that the soil is sufficientlydensified


16/44

Subsurface Sampling16

The purpose of subsurface exploration is

primarily to establish three things:

to obtain samples at various depths below the

surface for the purpose of laboratory evaluation toestablish the structural characteristics of the soil,

to determine the variation of the soil (soil profile)

that exists on the site, and

to determine the depth at which free water isencountered; that is, the location of the

groundwater table


17/44


Usually, a truck-mounted drilling rig is employed to drill the testholes from which samples of soil are obtained using specialsampling tools.

The two most common are the split spoon sampler for soilcharacterization tests and the Shelby tube for relatively undisturbedsamples

The Shelby tube is a thin-walled cylinder that is pressed down intothe soil at the bottom of the test hole and then raised with a core ofsoil inside

The split spoon sampler is used to test the condition of the soil inthe bottom of the test hole and to extract a soil sample for testing.By using a standard weight to drive the spoon into the soil andcounting the number of blows required to drive the spoon into thesoil a predetermined distance (usually a total of 18 in. [450 mm] inthree 6 in. [150 mm] increments), an indication of the soil-bearingcapacity can be established


18/44

18

Split Spoon Sampler Truck Mounted Drill

Shelby Tubes


19/44


Other complimentary methods of sampling

include use of

Sounding Rods,

Augers, Test pits,

Wash borings,

Rock drillings,Geophysical instruments,

Crosshole logging


20/44

SoundingRods

20

Used for exploring layers of soil with an erratic

structures

To ensure that subsoil does not contain

exceptionally soft spots between drill holesand to get information on the relative density of

soils


21/44

Hand Auger21

A hand auger is useful for bringing up samples fromrelatively shallow depths in soils that are cohesive enough tobe retained in the tool while it is being raised to the surface. Itconsists of a cylinder, usually 2 in. (50 mm) in diameter, withcutting lips on the lower end.

It is connected by ordinary couplings to a series of pipesections. As the auger is turned, layers of earth are peeled offand forced up into the auger cylinder. When the cylinder isfull, the auger is brought to the surface, emptied, cleaned,and returned.

For deeper depths, power augers may be used to expedite

the process. Because of the cutting action of auger bits, soilsamples obtained by auguring are known as disturbedsamples and certain soil characteristics are lost.


22/44

22

Hand Augers are

manually driven into the

ground to extract

disturbed soil samples


23/44

Test Pits23

To examine the layers of earth exactly as they

exist

soil moisture conditions are evident, and load

tests can be made at any desired depth. This method is relatively expensive, and the

depth to which examination can be carried out

is limited. The excavation is normally done with an

excavator or a tractor mounted backhoe.


24/44

24

Test pits are relatively expensive but

gives a direct visual representation

of the nearby subsurface soilparameters


25/44

Wash borings25

The wash boring method requires the use of water; asa result, the borings are in the form of mud.

Any given sample may be a mixture of two or morelayers of soil. Thus, a particular stratum may not bedetected at all.

The equipment for wash boring consists of an outercasing, inside of which is a hollow drill rod with across-chopping bit.

Boring is done by raising, lowering, and turning the

rod while water is forced down the rod and outthrough ports in the sides of the bit.

Loose material is forced up between the drill rod andthe outer casing by water pressure and washesacross a sieve so that general soil types may be

assessed


26/44

Rock Drilling26

A number of systems are employed for drilling rock; among them arediamond drilling, shot drilling, and churn drilling.

A diamond drill consists of a diamond-studded bit attached to a core barrel.The barrel is in turn attached to a drill rod mounted in a rig. The drill rod isrotated, and water is forced down through the hollow rod to cool the bit andcarry the drill cuttings to the surface. The bit cuts a circular groove, and the

core is forced up into the core barrel. When it is of the desired length, thecore is broken off by a special device and brought to the surface forinspection. Diamond drilling may be done either vertically or at an angle.

Shot drilling is similar to diamond drilling except for the bit. It consists of acircular, hollow, hard steel bit with a slot around the bottom edge to allowcirculation of shot. A flow of chilled steel shot is fed through the drill rod tothe bit; as the bit turns, the shot cuts the rock and forms a core, which is

forced up into the core barrel. Cores as large as 72 in. (1800 mm) indiameter can be taken with a shot bit.

Churn drilling consists of operating a hard steel chopping bit attached to adrill stem inside a casing. A cable, to which is fastened a set of weights,raises and drops the bit so that it strikes the bottom, chipping the rock.Water is forced down the side of the casing, and the resulting slurry isremoved from the hole with a bucket.


27/44

Rock Drilling27

Diamond Bit

Rotary Bit

Cross Chopping Bit


28/44

Geophysical Instruments28

Two basic types of geophysical instruments are used forshallow 100 ft (30 m or less) subsurface investigation andexploration: Refraction seismographs

Earth electrical resistivity units.

Other methods such as radiographic and ultrasonictechniques may also be used.

The seismic refraction theory is based on the fact that shockwaves travel at particular and well-defined velocities throughmaterials of various densities. The denser the material is, the

greater the speed. The velocities may range from as low as600 ft/sec (180 m/sec) in light, dry top soil to 20,000 ft/sec(6000 m/sec) in unseasoned granite. If the speed of theshock wave is known, the type, hardness, and depth of thestratum responsible for the refracted wave may be accuratelydetermined


29/44

Geophysical Instruments29

The electrical resistivity measurement method ofanalysis depends on the ability of earth materialsand formations to conduct electrical current, whichfollows relatively good conductors and avoids

poor ones. Conductivity of the material depends on its

electrolytic properties.

Conductivity and resistivity vary according to thepresence and quantity of fluids,

mineral salt content of the fluids,

volume of pore spaces,

pore size and distribution,

degree of saturation, and a number of other factors


30/44

30

Top: Schematic Diagram of Seismic-Wave

Refraction Principle.

Right: Automatic Engineering Seismograph


31/44

Cross-Hole Logging31

Cross-hole logging is a geophysical method thatprovides soil logs between boreholes

Seismic wave velocities are used to determineengineering properties of soils and rock for use in

engineering design applications. This method can also locate rock fractures,

cavities and soil continuity.

The real advantage of this method is that itprovides an almost continuous soil profile for thedepth of the borehole without losing anyresolution.

It can also be used in a single borehole situation.

T i l G t h i l T t B


32/44

Typical Geotechnical Test Bore

Information32

A typical borehole log provides

A description of the soil types encountered

Methods that were used in obtaining the test

samples, The depths at which these samples were taken,

The condition of the samples,

The results of any tests performed on the

samples at the time of drilling,

The location where free water was encountered.


33/44

33


34/44

Groundwater Level34

Knowing the location of the groundwater level is

important for two main reasons.

From the contractors point of view, a high water

table means that extra costs will be encounteredduring the excavation and construction of

foundations.

For the designer, accurate information is of utmost

importance because the location of the watertable affects the soil-bearing capacity, which in

turn determines the size and type of foundation

required for the building.


35/44

Surface Testing35

Surface testing is necessary when (1)excavations are made in native or fill soils, (2)deleterious soils are removed and backfilledwith select materials, and (3) fill is placed on asite

Various tests are available and use specifictest equipment for checking the surface

conditions of backfill material: thepenetrometer for measuring penetration

resistance,

density equipment to check the in-place density,

andmoisture testers for testin the moisture content


36/44

Penetration Tests36

Penetration tests provide a fast and easy way ofgaining an indication of the bearing capacity ofsoils.

Two approaches have been developed: the staticpenetration test and the dynamic penetration test.

Both methods measure the resistance of the soilto a device, commonly known as a penetrometer,which is pushed or driven into the soil in a

predetermined manner.

In general, the static test is used in the evaluationof cohesive soils (clays) and the dynamicapproach is used in the evaluation of

cohesionless soils (gravel deposits)


37/44

Penetration Tests37

The instrument developed for the static approach istheproving ring penetrometer.

This instrument has a cone like probe equipped with aproving ring and a dial gauge that indicates soilpenetration resistance readings.

To determine the penetration resistance at a givenlocation in the soil, the penetrometer is pushed firmlyinto the soil, at a uniform rate, until the top of thepenetration cone is level with the soil surface. The dialis then read and the corresponding penetration load isdetermined from a calibration chart supplied with theinstrument.

Correlation between penetration load, bearingcapacity, and density of soil is also provided.


38/44

Shear Tests38

The ability of a cohesive soil to support load is relatedto its shear strength.

an indication of the strength of these soils can bedetermined by a quick test known as the shear test.

handheld vane tester consists of four-bladed vane,connected by a rod to a torsion head containing ahelical spring

To make a test, the vane is pushed into the soil andthe torsion head is rotated at the rate of 1 rpm.

When the clay shears, the load on the torsion spring isreleased and a pointer registers the maximumdeflection of the spring. The shear strength of the soilis then read from a calibration curve


39/44

39

Handheld Vane Shear Test Kit


40/44

Compaction Testing40

Compaction testing provides the site contractor with a specificcompaction value that must be achieved.

Compaction testing is based on the relationship between soildry density and the moisture content of the soil.

A practical application of this relationship is spraying water

over fill material to help it settle in. Soil samples undergo a standard lab test known as aproctor

test

A plot of the dry density of the soil versus the percentagemoisture content is made producing a bell-shaped curve. The

peak of the curve represents the maximum dry density andthe optimum moisture content of the soil.

Compaction values for the site are usually specified as somepercentage of the laboratory results


41/44

41


42/44

Moisture Tests42

Several Methods of Moisture tests areavailable

One method involves the use of a tester that

operates on the principle of a calcium carbidereagent being introduced into the free moisturein the sample.

This forms a gas, the amount of which

depends on the amount of free moisture incontact with the reagent. The gas is confinedin a sealed chamber, and by measuring thegas pressure, the amount of free moisture in

the sample is determined.


43/44

Secondary Testing43

The secondary investigation can be as

extensive as the primary and is usually done in

response to adverse findings in the primary

investigation.Areas of consideration include

Access to the Site

Availability of Services

Site Safety and Local Building Bylaws

Local Labor Supply

Local Weather Conditions


44/44

Tutorial Questions44

What two items might be found on the soil profile that couldaffect the type of foundation used in a structure?

What four pieces of information may be found in a field logreport concerning the soil test samples?

Give a brief explanation of each of the following terms: (a)

complementary usage of soil testing methods, (b) bulking ofsoil, (c) residual soil

Explain (a) what is meant by the shear strength of clay, (b)what is meant by degree of compaction, (c) why it isimportant to know the degree of compaction in certainbackfilling operations, and (d) why moisture control isimportant in compacting soil

State the difference between cohesive and cohesionlesssoils.

Give three reasons for obtaining subsurface soil samples ona building site

Documents

Lecture 1 Ground Investigation v3