38 - NPTEL

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

38


Module 6:

Lecture -3: 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:


As the load is transmitted down, the effective areaincreases and the pressure decreases.

Boussinesq equations can be used to calculate thepressure of such loads on embedded pipes.

Details of live load calculations are presented in thesection on minimum cover.

Live load


Seismic Load

As a general rule, the stresses induced in pipe wallsdue to seismic strains are quite small and do notadversely affect the design.

The design codes usually allow for an increase in theallowable stress, or conversely, a decrease in loadfactors, when seismic loads are included in a loadcombination.

Thus, buried pipes that are sized to sustain other designloads usually have sufficient strength to resist seismic-imposed stresses also.


Internal Pressure and Vacuum

Underground pipe systems have to operate under varyinglevels of internal pressure.

Gravity sewer lines normally operate under fairly lowinternal pressure, whereas water supply mains and industrialprocess pipes operate under high internal pressures.

The high-pressure pipelines have to be designed for acontinuous operating pressure, as well as, a short-termtransient pressure.

Internal Pressure


Internal Pressure and Vacuum

Certain operational events may cause a temporaryvacuum in buried conduits.

In most cases the duration of application of vacuumloading is extremely short and its effects can bedelineated from other live loads.

The magnitude and time variation of the transientsneed to be considered for both the positive andnegative internal pressures.

Vacuum


Pipe and associated contents

The effects of the dead weight of the pipe wall and thefluid carried must be resisted by the structural capacity ofthe pipe.

In practice, loads from these two sources are often neglected inthe design of steel or plastic pipes, but accounted in the designof prestressed and reinforced concrete pressure pipes, andconcrete non-pressure pipes.

For simplicity, these loads are added to the vertical soil loads,keeping in mind the small magnitude of these loads.


In pipeline design, analyses of minimum soil cover requiredare essential to protect the integrity of the buried pipeunder different loading and environmental conditions.

Soil is the major component of a flexible buried pipesystem. Soil protects the pipe by holding the pipe in shapeand in alignment.

The following are analyses of minimum soil covers requiredfor protection against wheel loads, flotation, uplift, andfrost.

Minimum soil cover


Minimum soil cover

Case I: Wheel Load, W, Directly over the Pipe Whidden(2009)


Wheel load directlyover the pipe

Burried PipeD/2

hP=W/2h2

Ground surface

Wheel load directlyover the pipe

Burried PipeD/2

H

P=W/2H2

Ground surface

Magnitude of live loads get reduced as the depth of backfill isincreased.

(After NYSDOT Geotechnical design manual)


If an empty circular cross-section is deflected into an ellipse, then pxrx = pyry.

Ratio of radii rr = ry/rx= (b/a)3 = (1+d)3/(1-d)3

Minimum soil cover

d = ring deflection


The horizontal pressure of pipe on soil at spring line is Px = Pyrr

Minimum soil cover

Where,

d = 0% 2% 5% 10%rr = 1 1.13 1.35 1.83


The horizontal stress applied by pipe on soil at mid height of the pipe

=

The resisting force applied by soil atmid height of the pipe

=

( )2/ 2x rP r H W Hg= +

( )/ 2x pk H Ds g= +

( ) ( )[ ] 3 = 1 / 1 = Deflection of pipe ring as a

fraction of pipe diameter Unit weight of soil

H = Minimum coverWheel load

= Passive strength coefficient

1 sin =1 sin

= Diameter

r

p

r d dd

Wk

D

g

ff

+ -

=

=

+-

of pipe = Friction angle of soilf

Wheel Loads Case 1: Wheel load directly over the pipe


Minimum soil cover

Schematic variation of Hwith W for different ringdeflections


As soil cover decreases, live load pressure on aburied pipe increases. There exists a minimumheight of soil cover.

If the soil cover is less than the minimum, the surfacelive load may damage the pipe.

Less obvious is a minimum height of soil cover fordead load (weight of soil only).

Minimum soil cover


Minimum cover

Load distribution diagram with wheel load on one side of the pipe (Whidden, 2009)

Case 2: Wheel load is on one side of the pipe

Deformation is caused bypunch-through of atruncated pyramid of soilunder the wheel.

If ring stiffness is notadequate, top of the pipeinverts as the wheel rollsacross the pipe.


Minimum soil cover Inversion is triggered by the maximum moment, M =

0.022Pr2, about 10° to the right of center line.

The moment, M, causes bending stress σ = Mc/I.

The ring compression stress is σ = (P/2)(D/t) for flexiblepipe, where P = γH = negligible.By simplifying, we get

---(1)


Wheel load, W, on compacted granular soil punches outa pyramid with slopes of about 1H:2V (slope angle =37°). Pressure on the pipe is:

Minimum soil cover

---(2)

By solving (1) and (2), minimum cover H can be determined.


For typical granular backfill, based on analysis confirmedby tests, minimum cover is about H = D/10.

An often specified minimum allowable is H = D/6, but this applies to a perfectly flexible ring.

In fact, pipes have ring stiffness and so provide resistance todead load collapse.

Minimum soil cover


Pipe floatation The possibility of pipe flotation exists when the pipeline is

constructed in areas which will be inundated, such as streamcrossings, flood plains and high ground water areas. When suchconditions exist, evaluate the possibility of pipe flotation.

The buoyancy of a pipeline depends upon the weight of the pipe,the weight of the volume of water displaced by the pipe, theweight of the liquid load carried by the pipe and the weight ofthe backfill.

As a conservative analytical practice, consider the pipeline emptyfor two reasons; so the weight of the liquid will be considered asan additional safety factor and the possibility of the pipeline notbeing in use during a period of time.


Pipe floatation

Minimum Cover of Buoyant Soil Under Water to Prevent Flotation of Empty Pipe

For granular soil with shallow covers(H < 5D), soil slips on parabolic surfaces


Pipe floatationBuoyancy, Q, of the empty pipe is the weight of water displaced:

Weight of soil is the weight of soil wedges bounded by the soilslip planes at an angle equal to the soil friction angle. Forcohesion-less soil, a reasonable soil friction angle is 37°, forwhich soil slips at roughly 1H : 2V (based on model tests)The resisting force, W, is the buoyant weight of soil cover (i.e. cross-hatched) is given by:

W = γsubA ↓A = area shown cross-hatched = (DZ + Z2/3 − πD2/8)

A safety factor is recommended. For design purposes,increase the calculated H by a factor of 2.0.


Minimum Cover of Granular Soil Under WaterWhat is minimum cover, H, of granular soil under water to prevent flotation when a pipe is empty?

The empty pipe floats if Q exceeds W.

Equating Q = W,


Pipe uplift

Analysis for determination of the force required to upliftthe pipe in the soil is also useful.

Q is the force that lifts the pipe. The pipe uplift equation is:

σp = kpγ (H +D/2)By substituting and simplifying, 3Q/γD2 = (H/D + 2)2 − 3.428


Q=20ɣD2

Generalshear

θf

Bow wedgeof soil

Soil flow(Long term)

For H/D >5

Flotation and Uplift forcesIf the height of soil cover, H, isgreater than 5D InvertedTerzaghi Model for Uplift Forceon a Pipe Buried Under High SoilCover needs to be adopted.

A “bow wave” of soil is formed asthe pipe plows up through the soil.Shear planes (soil slip planes) do notbreak out at the ground surface.

From analysis and at H/D > 5,Q = 20 γD2


Remediation measures for reducing Pipe floatationBuoyancy effects are probably of greatest concern in areas such as flood plainsand estuaries where massive liquefaction could take place in a majorearthquake.The following recommendations may be followed to minimize the buoyancy effects onpipeline.

i) Pipelines may be encased with concrete pipes to reduce the buoyancy effects, butthe increased diameter will also increase lateral drag force on pipeline duringlateral spreading due to liquefaction.

ii) Concrete weights or gravel filled blankets can be utilized to provideadditional resistance to buoyancy.

iii) Buoyancy effect can also be minimized by shallow burial of pipeline above the ground water table.

iv) Where uplift is the main concern, anchors may be provided with a close spacing (~150 m) to prevent uplift.


If there is any possibility of soil liquefaction, flotationwill be a major concern and additionalconsiderations are required. Soil can liquefy if it issaturated and shaken, and if the density is less thanabout 80 percent modified Proctor density.

The shaking can be a result of seismic activity. If thesoil is completely saturated to ground level and thepipe is empty, there will be little resistance to flotationand the empty pipe will rise through the liquid soil.

Liquefaction


Pressure distribution on a pipe in a liquefiedsoil. Buckling at bottom is possible.

LiquefactionIf the embedmentliquefies when acircular pipe is empty,the ring may besubjected to thehydrostatic pressuresshown in the figure.


If somehow flotation is prevented, catastrophic collapse mayoccur from the bottom according to the classical bucklingequation:

Liquefaction

i.e. this gives, What is the height h of the water tableabove the bottom of a steel pipe in embedment so loosethat it can liquefy and cause catastrophic ring collapse?


Forces acting on a pipe in liquefied soil

Liquefaction The resistance forces inhibiting the uplift force due to buoyancy (FB,P) are

provided by the weight of the pipe (FS,P), the weight of the overlying soil(FWS,P) and the shear developed in the soil (FSP,P). However, in the event ofsoil liquefaction during an earthquake, the shear contribution could bereduced significantly.

In addition, the excess porepressure at the invert of the pipecan also contribute to the upliftforce acting on the structure (FEPP,P)shown in the figure.

When there is a positive net uplift force (FNET,P), the pipe may float as a result.


Mitigation technique using gravelsand geogrid:

Geogrid: confining the gravels

Gravels: counter-weight toflotation

Liquefaction


Corrosion of buried pipes

Three environmental agents usually exert stronginfluence on corrosion of the pipe wall material inburied installations.

These are :

(a)The water or other fluid carried by the pipe(b)The soil in contact with the buried pipe(c)The groundwater


Moreover, in the case of certain process watersystems, such as power plant condenser coolingsystems, the water is circulated continuously within aclosed loop.

In such cases, the concentrations of certaincompounds like chlorides, sulfates in the water mayincrease and cause elevated corrosion rates in thesteel and concrete pipes.

Corrosion of buried pipes


Corrosion Protection

(a) Application of coating

(b) Cathodic protection : Proven to be verysuccessful in providing leak-free high-pressure oiland natural gas pipelines throughout the U.S.


Coatings can be used to inhibit corrosion or otherforms of deterioration in both concrete and steel pipes.

Type and extent of coatings depends on the service environment.

Coal tar enamel and wrapping has been usedsuccessfully in the U.S. for decades. Epoxies andurethanes, among others, have become popular inmore recent times.

Corrosion Protection : (a) Coating


Applicable in power plant sites, that have widelydispersed grounding systems, which can causeunpredictable stray currents promoting steelcorrosion.

Cathodic protection must be designed by competentengineers based on information regarding the extentof buried facilities, the soil resistivity measurements,and the plant grounding system

Corrosion Protection : (b) Cathodic protection


Load Considerations

Stresses due to pressure generated by the flow(internal pressure)

External pressure by the fluid if the pipe issubmerged under water

External pressure generated by the weight of theearth and live loads on buried pipes

Loads due to thermal expansion, earthquakes


Installation of a flexible pipe at the site


Installed flexible pipe at the site


Basic mesh adopted in GEOSTUDIO (2012)


Typical failure of a flexible pipe

Classic example of buckling failure of flexible pipe due to insufficient lateral support

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