37 · 2017. 8. 4. · prof. b v s viswanadham, department of civil engineering, iit bombay...

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

37


Module 6:

Lecture -2: 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:


Materials transported through buried pipes

Crude and refined petroleum,

Fuels - such as oil, natural gas and biofuels

Waste products in a fluid sate including sewage,slurry and industrial wastes.

Water used for drinking or irrigation

In some cases, hydrogen gas, and highly toxicammonia have been transported.


Buried pipelines

Pipes that are buried underground are required tosustain other loads besides the internal fluidpressure.

That is, they must support the soil overburden,groundwater, loads applied at the ground surface(like vehicular traffic) and forces induced by seismicmotion in seismic prone zones.


Buried pipes are, therefore, structures as well asconduits for conveying fluid.

Special design procedures are required to beadopted while designing buried pipelines, toensure that both the functions aresimultaneously fulfilled.

Buried pipelines


Significance of Buried pipelines

As the pipelines are buried, more land space canbe utilized for construction above the ground.

Buried pipelines ensure minimum number ofbends/crossing along the length of the pipe, asthey can pass in a relatively straight manner belowthe ground.

This, in turn ensures minimum losses due tobending and joints.


Significance of Buried pipelines

As the pipes are placed below the ground, theyare relatively safe from sabotage point of view.

Moreover, buried pipelines are a necessity in caseof pipelines carrying sewage, oil or hazardousend-products of complex industrial processes inorder to prevent contamination to the surroundingenvironment.


Functions of buried pipe lines

Hydraulic:

Designed to carry fluids produced by public watersystems, sewers, drainage facilities, and manyindustrial processes.

Structural:

Designed to carry the weight of the ground andany load acting on it.


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

Various classes of conduits installation (Spangler & Handy, 1973) Special case, similar to negative

embankment condition, but morefavorable from standpoint of loadreduction on pipe, used in verydeep installations.

Difficult to achieve for large-diameter pipes. This type ofconstruction is called imperfect-ditch conduit or induced-trenchconduit.

Not recommended for wet areas


Active arching (Flexible pipes)

Stress distribution across plane above and below pipe

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

If the structure deforms uniformly on plane above andbelow pipe, the stresses on it tend to be lower towardthe edges due to mobilized shear stresses in the soil.


Passive arching (rigid pipes) 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.

Stress distribution across plane above and below pipe


Effect of soil settlement on flexible and rigid pipes

Flexible pipe Rigid pipe

Actual load on the pipe is less thanthe load of the central prism due tothe direction in which the shearingstresses

Actual load on the pipe is more thanthe load of the central prism due tothe direction in which the shearingstresses


Arching effect in underground conduits

Rigid pipes

Flexible pipes


What is the maximum load on a very rigid pipe in aditch excavated in sand? The pipe outside diameter(OD) is 0.45 m, the trench width is 1 m , the depth ofburial is 2.5 m, and the soil unit weight is 18.4 kN/m3.

Example problem:

Solution:

Determine Cd


Variation of Cdwith z/B for different values of Kµ

For wet sand, K = 0.33 and µ =0.5 Kµ = 0.165

z/B = H/B = 2.5/1 = 2.5

For H/B = 2.5 & Kµ = 0.165

Cd = 1.4

W = CdγB2 = 1.4 x 18.4 x (1)2

= 25.76 kN/m


In conjunction with positive projecting conduits, Marstondetermined the existence of a horizontal plane above the pipewhere the shearing forces are zero. This plane is called theplane of equal settlement.

Above this plane, the interior and exterior prisms of soil settleequally. The condition where the plane of equal settlement isreal (it is located within the embankment) is called anincomplete projection or an incomplete ditch condition.

If the plane of equal settlement is imaginary (the shear forcesextend all the way to the top of the embankment), it is called acomplete ditch or complete projection condition.

Marston load theory (Embankment conditions)



In Case I, the ground at the sides of the pipe settlesmore than the top of the pipe.

In Case II, the top of the pipe settles more than the soilat the sides of the pipe.

Case I was called the projection condition by Marston and ischaracterized by a positive settlement ratio rsd

The shear forces are downward and cause a greater load on theburied pipe for Case I.

Case II is called the ditch condition and is characterized by anegative settlement ratio rsd . The shear forces are directedupward in this case and result in a reduced load on the pipe.


Incomplete Projection condition

Embankment conditions Not all pipes are installed in

ditches (trenches); therefore, itis necessary to treat theproblem of pipes buried inembankments.

An embankment is where thetop of the pipe above thenatural ground.

This type of installation isdefined as positive projectingconduit.

Case I

Sf +dc < Sm +Sg

H > He


Case II is called the ditchcondition and is characterizedby a negative settlement ratiorsd . The shear forces aredirected upward in this caseand result in a reduced load onthe pipe.

Ditch conditionCase II

After Spangler and Handy(1982)

Incomplete ditch condition

H > He

Sf +dc > Sm +Sg


sm = compression of soil at sides of pipe;sg = settlement of natural ground surface at sides of pipe;sf = settlement of foundation underneath pipe;dc = deflection of the top of pipe.


In conjunction with positive projecting conduits, Marstondetermined the existence of a horizontal plane above the pipewhere the shearing forces are zero. This plane is called the planeof equal settlement. Above this plane, the interior and exteriorprisms of soil settle equally.

Where:

Critical plane settlement = Sm (strainin side soil) +Sg (ground settlement).

Settlement of the top of the pipe =Sf (conduit settlement) + dc (vertical

pipe deflection).



The condition where the plane of equal settlement is real (it islocated within the embankment) is called an incompleteprojection or an incomplete ditch condition.

If the plane of equal settlement is imaginary (the shear forcesextend all the way to the top of the embankment), it is called acomplete ditch or complete projection condition.

Marston’s load equation for positive projecting(embankment) conduits is given by:

For complete condition



The minus signs are for the complete ditch, and the plussigns are for the complete projection condition.For incomplete condition:

where the minus signs are for the incomplete ditch andthe plus signs are for the incomplete projection condition.And He is the height of the plane of equal settlement.

At H = He, the incomplete case becomes complete case.


Cc is a function of the ratio of height of cover to pipediameter (H/Bc), the product of the settlement ratio (rsd) andprojection ratio (p), Rankine’s constant (K), and thecoefficient of friction (µ).


The value of the product Kµ is generally taken as 0.19 for the projection condition and 0.13 for the ditch condition.


Complete projection condition: The top of the conduit settles lessthan the critical plane and the height of the embankment is lessthan the theoretical height of equal settlement.

Incomplete projection condition: The top of the conduit settlesless than the critical plane and the height of the embankment isgreater than the height of equal settlement.

Complete ditch condition: The top of the conduit settles morethan the critical plane and the height of the embankment is lessthan the height of equal settlement.

Incomplete ditch condition: The top of the conduit settles morethan the critical plane and the height of the embankment is morethan the height of equal settlement.

Four conditions classified according to Spangler


Complete projection condition

Incompleteprojectioncondition

H < He

Sf +dc < Sm +Sg Sf +dc <

Sm +Sg

H > He


Diagram for coefficient Cc for positive projecting conduits


rsd p = 0, Cc = H/Bc


The settlement ratio rsd is difficult, if not impossible, todetermine even empirically from direct observations.

Note that when rsd p = 0, Cc = H/Bc and Wc =γ HBc . This isthe prism load (i.e., the weight of the prism of soil over thetop of the pipe).

When rsd = 0, the plane at the top of the pipe called thecritical plane settles the same amount as the top of theconduit


Design values of settlement ratio After Spangler and Handy(1982)

When a pipe is installed in a narrow,shallow trench with the top of the pipelevel with the adjacent naturalground, the projection ratio p is zero.

The distance from the top of the structure to thenatural ground surface is represented by pBc


Load transfer pattern in flexible and rigid pipesAs the loading increases, the vertical diameterdecreases and the horizontal diameter increases.


Typical failure modes for flexible pipes



For flexible pipes, Moser proposed the following formula(American Water Works Association manual) to calculateload on pipes:

h

W

PipeD

Trench with a flexible pipe

W = γ Bd h


Importance of proper bedding on buried pipe

P1

Width of trench

Undisturbed soil

Load on the pipe

P1

Width of trench

Undisturbed soil

Load on the pipe

Proper beddingImproper bedding

The area of contact between thepipe and soil is small so stress ismore and may damage the pipe.

The area of contact between thepipe and soil is large so stress isless.

(After NYSDOT Geotechnical design manual)


Soil bed

Cracks

Soil bed

Load is concentrated and creating cracks

Load is uniformly distributed

Importance of proper bedding on buried pipe

(After NYSDOT Geotechnical design manual)


Types of load on pipe lines

1. External Loads

A. Dead load or overburden pressureB. Live LoadsC. Seismic Loads

2. Internal Loads

A. Internal Pressure and VacuumB. Pipe and associated contents


1.A. Dead load or overburden pressure

This is the pressure due to weight of the soil and water above the pipe. The pressure increases with depth of the pipe, i.e. dead load for (P1


1.B. Live load

This is the static or dynamic load acting on theground above the pipe and transmitted to thepipe through the soil.

Vehicles, trains, aircrafts etc are the source of suchloads.

Magnitude of such loads get reduced as thedepth of embedment is increased.

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37 · 2017. 8. 4. · prof. b v s viswanadham, department of civil engineering, iit bombay...

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