Numerical Methods in Soil Mechanics D.PDF

Numerical Methods in Soil Mechanics D.PDF Numerical Methods in Geotechnical Engineering contains the proceedings of the 8th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE 2014, Delft, The Netherlands, 18-20 June 2014). It is the eighth in a series of conferences organised by the European Regional Technical Committee ERTC7 under the auspices of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The first conference was held in 1986 in Stuttgart, Germany and the series has continued every four years (Santander, Spain 1990; Manchester, United Kingdom 1994; Udine, Italy 1998; Paris, France 2002; Graz, Austria 2006; Trondheim, Norway 2010). Numerical Methods in Geotechnical Engineering presents the latest developments relating to the use of numerical methods in geotechnical engineering, including scientific achievements, innovations and engineering applications related to, or employing, numerical methods. Topics include: constitutive modelling, parameter determination in field and laboratory tests, finite element related numerical methods, other numerical methods, probabilistic methods and neural networks, ground improvement and reinforcement, dams, embankments and slopes, shallow and deep foundations, excavations and retaining walls, tunnels, infrastructure, groundwater flow, thermal and coupled analysis, dynamic applications, offshore applications and cyclic loading models. The book is aimed at academics, researchers and practitioners in geotechnical engineering and geomechanics.

Anderson, Loren Runar et al "HISTORICAL SKETCH"

Structural Mechanics of Buried Pipes

Boca Raton: CRC Press LLC,2000

APPENDIX D HISTORICAL SKETCH

Pipeline engineering dates from prehistory The

ganats of ancient Persia were underground tunnels

bored back under the mountains to collect fresh

water for the cities on the plains The catacombs of

Egypt were remarkable underground conduits

Medieval Paris and London had brick-lined sewers

The subway tunnels of Saint Louis, long since

abandoned, are rediscovered as engineers study light

rail systems The technology of buried pipes of the

past arose from experience — including failures

The modern approach to buried pipeline engineering

began in the early 1920s by Anson Marston, Dean

of Engineering at Iowa State College Each spring

he saw the plight of Iowa farmers as they bogged

down in quagmires of mud on the rural roads His

concern became a rallying cry, "Let's get Iowa out

of the mud." Because of this effort, Marston was

named the first Chairman of the Highway Research

Board As such, he reasoned, correctly, that the

first step toward adequate roads was drainage

That meant buried drain pipes, and a procedure for

designing buried drain pipes He proposed a theory

for predicting soil loads on buried rigid pipes The

strengths of the pipes were determined by crushing

samples of the pipe between parallel plates For

design, the soil loads on the buried pipes had to be

less than the parallel plate loads that caused failure

But how much less? Tests were needed Marston

assigned the testing to a student, M.G Spangler,

who was instructed to bury samples of rigid pipe and

measure the soil loads on them The objective was

to relate parallel plate loads to soil loads at pipe

failure, and thus provide a design procedure for

highway pipes and culverts

During the time Spangler was testing rigid pipes,

flexible corrugated steel pipes appeared on the

market Spangler realized that for flexible pipes, a

parallel plate test was not representative of field

conditions In the field, soil at the sides of the buried

pipe supports the pipe and resists deflection So

Spangler derived the Iowa Formula for predicting

the ring deflection of buried flexible pipes The formula was based on: 1 the Marston soil load on the pipe; 2 ring stiffness and 3 soil stiffness which Spangler called the modulus of passive resistance of soil The Iowa Formula required a number of adjustments such as deflection time lag factor, bedding angle, and load factors The load was soon changed from the Marston load to prismatic soil load plus the influence of live load Both soil and pipe were assumed to be elastic The boundaries included a plane of equal settlement which was affected by trench or embankment condition, and positive or negative projection The Iowa Formula

was published in 1941 in the Iowa Engineering Experiment Station Bulletin 153

Spangler was convinced that buried corrugated steel culverts invert at the top when ring deflection is about 20% So he applied a safety factor of four and proposed that buried flexible pipes be limited to 5% ring deflection Kelly of Armco Corporation attempted to apply the Iowa Formula to corrugated steel pipes But the formula broke down With 5% ring deflection and all else constant in the formula, Kelly plotted height of soil cover as a function of pipe diameter The result was that in diameters over

5 ft, the allowable height of soil cover increased as

the diameter increased This seemed irrational The Iowa Formula was abandoned

In 1957, Spangler's student R.K Watkins, discovered that Spangler's modulus of passive resistance of soil had to be redefined in order to be

a correct property of material A modified Iowa

Formula overcame the irrationality demonstrated by

Kelly It was published in 1958 in the Proceedings

of the Highway Research Board.

Pipeline agencies commenced to publish values for the corrected soil modulus, now called the modulus

of soil reaction Published values were excessively conservative They became a catch-all for the many assumptions in the Iowa formula, and for

STRUCTURAL MECHANICS OF BURIED PIPES

430

incautions in installation Published values of the soil

modulus, at best, yielded only a rough, conservative

estimate of ring deflection

From field and laboratory testing, Watkins found the

modulus of soil reaction to be elusive and

undependable — a sidewise modulus based on

vertical soil loading It was not constant E-prime is

a function of depth of soil cover (confinement) and

ring stiffness Similar findings were reported by

Duncan, Molin and others

The many factors and assumptions required to solve

the Iowa Formula made the prediction of ring

deflection less precise than direct prediction of ring

deflection based on vertical compression of the

sidefill soil Ring deflection is related to vertical soil

compression and to the ratio of soil stiffness to ring

stiffness Soil stiffness is found by standard

laboratory compression tests Ring stiffness is a

form of the spring constant for a diametral line load

on the ring For flexible pipes buried in good

granular soil, the stiffness ratio is so large that the

influence of ring stiffness is negligible and the soil

alone determines ring deflection;

Research at USU showed that: 1 Ring deflection

of buried flexible pipes is equal to (or less than)

vertical compression of the embedment (the sidefill)

2 For high soil cover, pipe "failure" is not necessarily

ring deflection (Spangler's 20%), nor is it necessarily

Marston's parallel plate load The pipe wall can

buckle or crush by ring compression at deflection

less than 20% In fact, the wall can buckle or crush

when deflection is zero A pipe with high ratio of

wall strength to stiffness, such as a thin-wall steel

pipe, may buckle at less than 20% ring deflection A

pipe with low ratio of wall strength to stiffness, such

as plastic pipe, may crush at less than 20% ring

deflection

These observations have since been confirmed by

finite element analyses, and by tests — especially on

large diameter buried flexible steel pipes

For flexible pipes in select soil envelopes, engineers

can predict ring deflection as a function of vertical soil strain Whoever uses the Iowa Formula must reduce the number of variables by substituting average or assumed values for those variables that have the least effect on the result The Iowa

Formula is not a procedure for design It is an approximate procedure for predicting ring deflection.

Ring deflection has proven to be an important limit to

be specified Other important performance limits include soil slip, ring collapse, and ring compression stress as reported by White and Layer Ring compression is described in Chapter 6

Other models for analysis have been proposed by Hoeg, Luscher, Meyerhof, and others Most are based on elastic theory

An elegant analysis of elastic soil embedment was presented by Burns, J.Q., and Richards, R.M., Attenuation of stresses for buried cylinder s ,

Proceedings of the Symposium on Soil Structure Interaction, University of Arizona, Sept 1964.

Both pipe and soil were assumed to be elastic One analysis was for full bond between soil and pipe, and the other was for zero friction between soil and pipe They provide a "feel" for pipe-soil interaction Performance limits require special analysis for a variety of embedment conditions, such as backpacking and encasements, and for new pipe materials and configurations — especially plastics Stresses in plastic pipes relax under constant strain, and creep (even to a long-term regressed strength) under constant stress Clearly, pipe-soil interaction becomes complex Basic principles of engineering mechanics of materials are proving to be the most dependable tools for analysis Worst-case condi-tions are assumed Greater precision is not justified because of imprecisions in soils and in installations

Because of their versatility and corrosion resistance, plastic pipes have increased and dominated some buried pipe markets since World War II Bombing

of German cities destroyed not only the industries

APPENDIX D HISTORICAL SKETCH 431 that provided steel for guns (and pipes), but also the

water supply pipelines that served the cities In

desperation, one quick remedy seemed to be PVC

pipes The Germans had led in processing and

fabricating PVC (polyvinyl-chloride) PVC pipes

were successful Other plastic pipes soon came on

the market

With computers available for complex pipe analyses, with new pipe configurations and materials on the market, and with an urgent and sustained need for buried pipes, present-day technology is only a primer for future design of buried pipes