36 · 2017. 8. 4. · Soil arching Arching can be best described as a transfer of forces between a yielding mass of geomaterial and adjoining stationary members. A redistribution

Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

36


Module 6:

Lecture -1: Buried Structures


Load on Pipes,

Marston’s load theory for rigid and flexible pipes,

Trench and Projection conditions,

minimum cover,

Pipe floatation and Liquefaction.

Content in this module:


Ditch conduit

Various classes of conduits installation (Spangler & Handy, 1973)

Pipe is installed in a narrowtrench (generally, trench width≤ 2 d) in undisturbed soil, thenbackfilled to natural groundsurface level.

Examples of this type ofconduit are sewers, drains,water mains, gas mains, andburied oil pipelines.

Buried pipes are divided into twomain categories: ditch conduits(trench conduits) and projectingconduits (embakment conduits)

d

BackfillH



Positive projecting conduit

Projecting conduits are further divided into two groups: Positive and Negative Projecting conduits.

A positive projecting conduitis a conduit or pipe installedin shallow bedding with thetop of the pipe cross-sectionprojecting above the naturalground surface.

Highway and railroad culvertsare often installed in this way.

H



Negative projective conduit

A negative projecting conduit is aconduit installed in a relativelynarrow and shallow ditch with thetop of the conduit below thenatural ground surface; the ditch isthen backfilled with loose soil andan embankment is constructed.

Effective in reducing the load onthe conduit, especially if thebackfill above the conduit is loosesoil.

Compacted soil


Imperfect ditch conduit


Special case, similar to negativeembankment condition, but morefavorable from standpoint of loadreduction on pipe, used in verydeep installations. Difficult toachieve for large-diameter pipes.This type of construction is calledimperfect-ditch conduit or induced-trench conduit.

Although effective in reducingloads on conduits, this type ofconstruction with loose backfillencourages channeling of seepageflow through the embankment. Notrecommended for wet areas.


Types of buried pipes or conduits

Ditch conduit Positive projecting conduit

Top of the pipeprojects above theground surface


Types of buried pipes or conduits

NegativeProjecting Conduit

Imperfect Ditch Conduit

Pipe is placed in a shallowtrench and the top liesbelow the ground surface


Because of the shear resistance provided by thewalls of the trench, known as soil arching action, asignificant fraction of the weight of the soil abovethe conduit is transferred to the walls of the ditch,thus reducing the load on the conduit.

Marston load theory

In 1913, Anson Marston developed at theory toexplain the characteristics of a soil column abovea buried conduit.


Soil arching

Arching can be best described as a transfer of forcesbetween a yielding mass of geomaterial andadjoining stationary members.

A redistribution of stresses in the soil body takes place.The shearing resistance tends to keep the yieldingmass in its original portion resulting in a change ofpressure on both the yielding parts support and theadjoining part of soil.


Soil arching

If the yielding part moves downward, the shearresistance will act upward and reduce the stress atthe base of yielding mass.

If the yielding part moves upward, the shearresistance will act downward to impede thismovement and cause of increase of stress at thesupport of the yielding part.


Displacements under pressure Ps when structure is more compressible than surrounding soil

Active arching


Active arching

Stress distribution across plane AA or BB

Active arching occurs when the structure is morecompressible than the surrounding soil.

If the structure deforms uniformly on plane AA andBB, the stresses on it tend to be lower toward theedges due to mobilized shear stresses in the soil.


Passive arching

Displacements under pressure Ps when structure is less compressible than surrounding soil.


Stress distribution across plane AA or BB

Passive arching In passive arching, the soil is more compressible than the

structure. As a result, the soil undergoes large displacements, mobilizing

shear stresses which increase the total pressure on the structurewhile decreasing the pressure on the adjacent ground.

Assuming the structural deformations are uniform, the stresses are highest at the edges and lowest at the centerline.


Rigid pipe When the side columns of soil or the external

prisms are more compressible than the pipe due toits inherent rigidity, this causes the pipe to assumeload generated across the width of the trench.

The shearing stresses or friction forces that developdue to the differential settlement of the externalprisms and the central prism are additive to theload of the central prism alone. Rigid pipes

Rigid pipes show signs of distress before beingvertically deflected 2%


Effect of soil settlement on rigid pipe

Rigidpipes include reinforcedconcrete cylinder,prestressed concretecylinder,vitrifiedclay,polymer concrete, castiron,asbestos cementand cast-in-situ pipes.


Effect of soil settlement on flexible pipe

A flexible pipe has beendefined as one that willdeflect at least 2% withoutstructural distress.

If a pipe is more compressible thanthe external soil columns as a result ofits vertical deflection allowing thecentral prism to settle more in relationto the external prisms, the actual loadon the pipe is less than the load of thecentral prism due to the direction inwhich the shearing stresses

Flexible pipes.

Deflection < 2% (with rigid liningand coating)<3% (with rigid lining

and flexible coating)


Effect of soil settlement on flexible pipeFlexible pipes include:

steel,

ductile iron,

thermoplastics such as Polyvinyl Chloride (PVC)and High Density Polyethylene (HDPE),

thermosetting plastics such as fiberglass-reinforcedpolymer (FRP),

bar-wrapped concrete cylinder pipe.

d t d t l i


Arching effect in underground conduits

Rigid pipes

Flexible pipes


Semi-rigid pipeSome pipe materials exhibit characteristics of both rigidand flexible pipes, primarily controlled by theirdiameters, are referred as Semi-rigid pipes.

Semi-rigid pipes deflect between 0.1% to 3% withoutcausing harmful or potentially harmful cracks.

For example, bar-wrapped concrete cylinder


The cohesion, if any, between the trench fill and the soilin the trench sides is ignored because of its variable anduncertain value, depending on the moisture condition.[as 1) considerable time would have to elapse before

cohesion could develop and 2) the assumption of no cohesionwould yield the maximum load on the pipe]

The soil density and the frictional properties areassumed to remain constant over the depth, and the soilfriction is assumed to vary in direct proportion to theactive horizontal pressure of the fill against the trenchfaces

Marston load theory (Narrow trench)Assumptions:


After Spangler and Handy (2007)

FBD of ditch conduit

For ditch conduits, the weightof the overburden soiltransferred to the underlyingsoil, with due consideration tosoil arching action,Equating upward anddownward forces onhorizontal slice, we get:

z

B = Bd

The Marston load theory is based on theconcept of a prism of soil in the trenchthat imposes a load on the pipe

Per unit length


FBD of ditch conduit


FBD of ditch conduitwhere γ = unit weight of granular backfill; B = Bdtrench width; and Cd = load coefficient expressedas:

where K = coefficient of earth pressure; and µ =coefficient of friction for the granular soil backfill-ditchwall interface, which can vary between 0 for a smoothwall and tanφ for a very rough wall, where φ is the angleof shearing resistance of the granular soil backfill.


Variation of Cdwith z/B for different values of Kµ

Handy and Spangler (2007)suggested that Kµ can betaken conservatively as 0.11for saturated clays and 0.19for harsh granular soils, withall other soils lying withinthis range.

When Kµ increases, there issignificant reduction in Cdand hence the loadtransferred to the conduit.

Marston proposed Ka for K originally. Now KKo

tanµ = δ


Approximate values of ratio of lateral to vertical earth pressure (K) and Coefficient of friction against

sides of trench


Substituting h = H (i.e. z =H), we get the totalvertical pressure at the elevation of the top of theconduit.

How much of this vertical load V is imposed on theconduit is dependent upon the relativecompressibility (stiffness) of the pipe and soil.

For very rigid pipe (clay, concrete, heavy-walledcast iron, and so forth), the side fills may be verycompressible in relation to the pipe, and the pipemay carry practically all the load V.

For flexible pipe, the imposed load will besubstantially less than V since the pipe will be lessrigid than the side fill soil.


Schematic representation of soil-pipe contribution inload carrying capacity


Effect of ditch width on the loading on pipe

P1 P2

Width of trench Width of trench

Undisturbed soil

Less load on the pipe

More load on the pipe

The load carried by pipe increases with the width of the trench.So, the width of the trench should just be enough for compactionof the soil on the sides of the pipe.

(After NYSDOT Geotechnical design manual)


P1P2

Well compacted soil provides good lateral support to pipes

Poorly compacted soil provides reduced lateral support to pipes

Mode of failure of Flexible pipes


P1 P2

Structural strength and well compacted soil provides good lateral support to pipes

Poor structural strength provides reduced lateral support to pipes

Cracking

Mode of failure of Rigid pipes

Documents

36 · 2017. 8. 4. · Soil arching Arching can be best described as a transfer of forces between a yielding mass of geomaterial and adjoining stationary members. A redistribution