by - trl · 0-200 kn/m 2. initial calibration was against a precision bourdon type gauge; using...

TRANSPORT and ROAD

RESEARCH LABORATORY

Department of the Environment

TRRL REPORT LR 472

EXPERIMENTAL WORK ON LARGE STEEL

PIPELINE AT KIRTLING

by

J.J. Trott and J. Gaunt

(Appendix 2 by P. Nath)

Pavement Design Division

Structures Department

Transport and Road Research Laboratory

Crowthorne, Berkshire

1972

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on l S t April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. Details of pipeline

3. Details of experiment

3.1 Experimental layout

3.2 Instrumentation

3.3 Tests during installation of pipe

3.4 Static surcharge loading tests

3.5 Vehicle loading tests

3.6 Tests at Moulton Crossing

3.7 Measurements of the road pavement deflection

4. Results

4.1 Pipe deflections

4.1.1 During backfill and surcharge loading

4.1.2 During vehicle loading (first series)

4. 1.3 During vehicle loading (second series)

4.1.4 Measurements at the Moulton Crossing

4.2 Strains in pipe wall

4.3 Pressures around and above pipe

4.3.1 During backfilling and surcharge loading

4.3.2 During vehicle loading (first series)

4.3.3 During vehicle loading (second series)

4.4 Results of soil tests to BS 1377 : 1967 on sand backfill and natural ground

4.5 Deflection studies on road pavement

5. Discussion of results

5.1 Loadings applied to the pipe

5.2 Horizontal soil pressure, and modulus of soil reaction

5.3 Comparison of horizontal and vertical pressures

5.4 Comparison of calculated horizontal deflection with observed value

5.5 Impact factors from test vehicle

5.6 Comparison of measured stresses and deflections with values calculated using the finite element method

6. Conclusions

7. Acknowledgements

8. References

9. Appendix 1 - Comparison of the stiffness of the original road structure, and the reinstatement over the pipeline

9.1 Description of test

9.2 Tests at Kirtling

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CONTENTS (continued)

9.3 Tests at Moulton

9.4 Discussion of results

Appendix 2 - Application of finite element analysis to the calculation of stresses and deflections in pipelines

10.1

10.2

10.3

10.4

10.5

Introduction

The finite element method

Application to pipelines

Application to the pipe-line at Kirtling

Conclusions from a comparison of experimental and theoretical results

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Q CROWN COPYRIGHT 1972

Extracts from the text may be reproduced

provided the source is acknowledged

EXPERIMENTAL WORK ON LARGE STEEL

PIPELINE AT KIRTLING

ABSTRACT

The Underground Pipes Group at the Laboratory is carrying out work to determine the loads on buried pipelines, and their effects on the pipes concerned. As part of the programme a steel pipeline was instrumented where it passes under a minor road, and observations were made during the backfilling and subsequent loading tests in the period between October 1970 and June 1971. The pipes are 1.83 m in diameter, with a wall thickness of 12.7 mm. Over the test length sand was used for bedding the pipe and for backfilling the trench, the crown of the pipe being 1.60 mm below road level.

This report gives the results of soil pressure measurements around and above the pipe, changes in the internal diameter, and strain in the pipe wall around the circumference.

The measured deflections were small (no more than 4.0 millimetres at any stage), as were the strains in the pipe wall. Comparisons are made with calculated deflections by the Spangler formula, and by a finite element analysis.

1. INTRODUCTION

There is at present a lack of detailed information on the design and performance of large diameter thin-walled

'flexible' steel pipelines. The American Spangler formula 1, although used to predict the deflection of flexible

pipes under load, is thought to give a conservative value for design purposes.

In the Autumn of 1969, through the cooperation of the Consultants, Binnie & Partners, the Laboratory

was given the opportunity to study the installation of a large diameter steel pipe-line near Newmarket. It was

thought that a detailed programme of tests would provide useful information on the structuraI performance

of the pipe-line and provide data for a verification of the Spangler formula and of relatively new structural

design methods. The project provided an opportunity to check the electronic measuring systems to be used

in later experimental work. The work was carried out as part of a general programme of research on under-

ground pipes.

It is hoped to continue some of these measurements over a period of one to two years, during which

time the pipeline should have been emptied and filled several times.

2. DETAILS OF PIPELINE

The pipeline section chosen for the experiment forms part of the Ely Ouse-Essex water scheme, where

untreated water is carried in a steel pipe of 1.83 m diameter buried at an average cover depth of 1.2 m.

The individual sections of pipeline are 9 m long and comprise two welded semi-circular sections of

12.7 mm thick rolled steel. A bituminous sheathing 6.4 mm thick is applied to both the interior and exterior

of the pipe, with a fibre glass layer incorporated in the outer sheath for added strength during handling.

The pipeline sections are connected by socket joints, which are arc-welded at both sides following installation

in the trench.

The general specification called for the pipes to be laid in a trench 2.75 m wide, on a shaped sand bed

forming a bedding angle of 30 ° from the vertical diameter as shown in Fig. 1. The sand was continued as

side-fill for 150 mm above the horizontal diameter, and backfill was then continued with excavated material.

At road crossings, however, concrete bedding and surround was specified with lean concrete backfill above

this, extending to the underside of the road pavement. A small vibrating tamper was used to compact the

sand around the sides of the pipe, and a pedestrian operated vibrating roller compacted the back-fill above

the level of the crown. Material was compacted in nominal 250 mm layers.

3. DETAILS OF EXPERIMENT

3.1 Experimental layout

It was decided to attempt both deflection and strain measurements in the pipe, and to measure soil

pressures around the pipe, for both static backfill loading conditions and for dynamic loading due to vehicles.

To obtain vehicle loading, it was necessary to locate the instrumented section at a road crossing and, although

a trunk or major road would have been preferable, at the time the experiment was proposed the only available

site was across a minor road at Kirtling (near Newmarket). Here the pipeline crosses obliquely under the road

at a depth of 1.6 m below the finished surface, at an angle of 52 ° with the road centre line. Due to the prob-

ability of obtaining very small deflections with the lean concrete bedding and backfill specified for a road

crossing, it was decided to replace the lean concrete with sand for the purpose of the experiment. Thus the

experimental section of pipe was bedded in sand and this material was continued as backfill up to the level

of the road base. The strain and deflection measurements were confined to two circumferential elements of

the pipe 1.98 m apart, corresponding to the average wheel track of a commercial vehicle. The elements were

spaced equally about the centre of a 9 m length of pipe and, when installed, one ring was under the centre

line Of the road and the other under the near side. With this configuration the maximum effect was likely

to be 'obtained from dynamic vehicle loads.

Due to the oblique angle of the pipe crossing, the road was widened on both sides for short lengths in

order that a vehicle could be driven across the trench normal to the line of pipe. The layout is shown in

Fig. 1. The soil pressure measurements were made in the plane of ring A on Fig. 1.

An air-valve chamber was sited in the pipeline near to the road crossing, and this allowed access to the

pipe for the purpose of carrying out deflection measurements, and for leading out strain gauge cables.

2

3.2 I nstru mentation

The measurements, listed below, all demanded instrumentation work prior to the installation of the

pipe:-

(a)

(b)

(c)

Strain in the pipe wall

Deflection of the pipe

Soil pressure around the pipe.

(a) After removal of the bituminous sheathing in 300 mm wide strips, resistance foil strain gauges

were cemented circumferentially at 45 ° spacing around the inside and outside of the pipe. Heat

cured epoxy resin was used as an adhesive, and gauges were connected through a junction box

to 30 m lengths of multi-core cable for subsequent connection to data-logging equipment.

'Dummy' gauges were attached to small steel sections, cut from a spare length of pipe, individually

paired with the 'active' gauges to form one half of the Wheatstone bridge circuit for measuring

strain. Following attachment of the gauges, the protective bituminous sheath was replaced

externally. The interior protection was not replaced until after the dynamic tests described in

Section 3.5.

(b) Pipe deflections were studied by means of internal diameter measurements. Small stainless steel

studs were cemented to the pipe wall at 45 ° spacings adjacent to the strain gauge positions, and

diameters were checked with the gauge as shown in Fig. 2a. Dynamic deflections were monitored

with a similar device shown in Fig. 2b but fitted with an inductive displacement transducer, the

output of which was fed to a high speed galvanometer recorder.

(c) Soil pressures were measured with a small cell recently developed at the Laboratory, and shown

in Fig. 3. This cell is capable of resolving both static and dynamic pressures in the range e

0 -200 kN/m 2. Initial calibration was against a precision Bourdon type gauge; using applied

hydrostatic pressure. '~"

3.3 Tests during installation of pipe

After the pipe was installed in the trench and before backfilling, the strain gauges were connected to

I00-channel data logging equipment, all strains being then recorded automatically. Pipe diameter measure-

ments were taken manually with the pipe gauge. One pressure cell had been previously installed in the sand

below the pipe, and connected to the data logger. The remaining cells were installed and connected as back-

filling progressed. Readings of strain, deflection and pressure were all repeated at intervals during the back-

filling process and 3 days after completion of backfilling to road formation level.

- L

Concurrently with the above tests, soil density tests to BS 1377 : 1967 were carried out on the sand

backfill using the core-cutter method.

3.4 Static surcharge loading tests

Before reinstatement of the road, concrete blocks of total mass 20 Mg were placed on the backfill over

an area of 10.5m 2 to apply an average stress over the width of the trench of about 19 kN/m 2 at the surface.

The effects on strain in the pipe wall, deflection of the pipe and soil pressure were all observed. In an attempt

to increase pipe deflection, the same number of concrete blocks were stacked in one-half the area, to give an

average stress of 38 kN/m 2 over the reduced width, and the measurements repeated. 3

3.5 Vehicle loading tests

After a period of about 4 weeks and following reinstatement of the road surface, loading tests were

carried out with a stationary and moving vehicle. The reinstatement comprised a nominal 200 mm thick

layer of hard core, surfaced with 60 mm of single course tarmacadam. Further tests were carried out after

a period of 7 months, during which time the pipe had been filled with water and emptied. These tests are

referred to subsequently as first and second series respectively.

A two-axle lorry with a mass distribution of 10 Mg at the rear axle and 4 Mg at the front axle was used.

The tests involved standing the lorry at various distances from the centre line of the pipe, and obtaining

deflections and soil pressure readings for static conditions, followed by dynamic tests at vehicle speeds of

5 - 3 2 kin/h, both normal and oblique to the pipe. During each run the lorry was positioned so that the

back wheels passed directly over the rings of instrumentation around the pipe. To obtain a value for the

impact factor, a 50 mm thick plank 300 mm wide was placed over the pipe, normal to the direction of the

vehicle. Pipe deflections were recorded with two of the inductive displacement diameter gauges, installed

at right angles to each other across the horizontal and vertical diameter of the pipe. The output was fed to

a high speed galvanometer recorder along with that from the pressure cells, and the traces obtained .were

related to displacement and pressure.

3.6 Tests at Moulton Crossing

During the second series of loading tests (June, 1971), the opportunity was taken to observe pipe

deflections when the same vehicle and impact conditions were used at another road crossing at Moulton,

where the normally specified concrete surround and lean concrete backfill had been used (see Section 2).

At this site the pipe crossed the road at right angles. Two inductive transducers were installed inside the

pipe to measure vertical and horizontal deflections.

3.7 Measurements of the road pavement deflection

To compare the stiffness of the reinstated road above the pipe with the adjacent undisturbed construct-

ion, deflection beam measurements were made both at Kirtling and at Moulton. These are discussed in

Appendix 1.

4. RESULTS

4.1 Pipe deflections

4.1 .1 During backfill and surcharge loading. Figs. 4 and 5 show the pattern of static pipe deflection

throughout backfill and surcharge loading. After compaction of the sand sidefill, the vertical diameter of

the pipe increased by 4.0 mm, and the horizontal diameter decreased 3.3 mm. Following completion of

backfilling and surcharge loading, the pipe returned more closely to its circular shape, as illustrated in Fig.5.

The maximum vertical deflection was only about 0.2 per cent of the pipe diameter.

4.1.2 During vehicle loading (first series). The static pipe deflections measured with the rear wheels

of the test vehicle at various positions with respect to the pipe and after completion of the pavement were

very small, reaching maxima of only 0.14 mm and 0.10 mm respectively in the vertical and horizontal

direction, as shown in Fig. 6. Dynamic deflections with the vehicle running at both normal and oblique

angles to the pipe, and at an average speed of 9 kin/h, were of the same order as the static ones. The

4

lorry formed an oblique angle to the pipe, when travelling along the centre line of the road, due to the 52 °

angle subtended by the centre lines of pipe and road. Tests with the vehicle travelling oblique to the pipe

at an average speed of 30 km/h and passing over the 50 mm thick plank, gave a maximum vertical deflection

of 0.27 mm, corresponding to ari impact factor approximately 2.1 for the rear axle. Slower impact runs at

an average speed of 6 km/h gave a maximum vertical deflection of 0.22 mm, corresponding to an impact

factor of approximately 1.7. Tests with the vehicle normal to the pipe, and travelling at similar speeds, gave

a vertical deflection of only 0.14 mm corresponding to an impact factor of 1.3. Fig. 7 shows the results of

the dynamic pipe deflections tests. The impact tests are further discussed in Section 5.5.

4 .1 .3 During vehicle loading (second series). These tests were carried out after a period of 7 months,

and were a repeat of the first series. Results were very similar (Fig. 7) with static deflections ofO.16 mm

and 0.10 mm in the vertical and horizontal directions (0.14 and 0.10 respectively in December 1970).

Impact factors were 1.5 and 2.2 at the rear axle for the fast and slow runs oblique to the pipe, and 1.8 for

the slow run normal to the pipe (1.7, 2.1 and 1.3 respectively in December 1970).

4.1.4 Measurements at the Moul ton Crossing. Attempts to measure the horizontal and vertical

deflections in the pipe at Moulton, when the vehicle was driven over it showed that the pipe deflections were

too small to measure (less than 0.0001 mm). This was also the case when impact tests were made, as at Kirtling.

4.2 Strains in pipe wall -~

Strains measured during backfilling and following surcharge loading were very low for a steel pipe, and •

at no time exceeded 100 microstrain. The distribution and magnitude of the compressive and tensile strains

are shown in Fig. 8. The drift towards a tensile component at the conclusion of the surcharge tests and on

removal of load could be explained by a rise in temperature at the inside of the pipe of from 8 ° C in the

morning to 25 ° C at the conclusion of tests• This was mainly due to welding operations and to the application,~

of a hot bituminous coating to the joints. Strains recorded during the tests with the heavy vehicle were small

(generally less than 10 micro strain) and have therefore not been reported in detail.

4.3 Pressures around and above pipe

4.3.1 During backf i l l ing and surcharge loading. The distributions of radial and vertical pressures

during backfilling and surcharge loading are shown in Figs. 9 and 10. The radial pressures were highest around

the top half of the pipe initially, but tended to increase over the lower half with increasing load, and to

approach a state of nearly uniform compression under sustained surcharge. The negative pressure recorded

at the invert following compaction around the sides of the pipe is probably due to the tendency of the pipe

to be lifted off the bedding by compaction of the lower part of the side filling. The vertical pressures show

a markedly asymmetric distribution about the centre over one edge both initially and when repeated in

June 197 I. Th6 reason for this effect, which is repeated throughout the subsequent dynamic loading tests,

is not known and will be studied in more detail in later experiments.

Also shown on Fig. 10 are calculated vertical pressures acting on the horizontal plane containing the

pressure gauges above the pipe. For the conditions without surcharge it is assumed that the pressure exerted

by the overburden acts uniformly and the pressures are calculated directly from the wet weight of the over-

burden. For the surcharge condition the component added to the overburden pressure was calculated from

the Boussinesq equations for strip loading. The comparatively small increase in pressure for the larger

surcharge is explained by the method used to increase the surcharge; the width of surcharge was halved,

whilst the intensity was doubled. The calculated values neglect the presence of the pipe, which, as would be

expected, reduces the soil pressures over the crown and increases it towards the trench boundaries. A more

rigorous finite-element analysis taking the presence of the pipe into account is discussed in Appendix 2.

4.3.2 During vehicle loading (first series). Pressure changes recorded during the vehicle tests are shown

in Fig. 11 ; they are of a low order, being generally below 10 kN/m 2. The pressure changes recorded by

gauges 1 and 2 are greater than those for the other gauges leading to a similar asymmetry to that observed in

the static loading tests.

Fig. 11 also shows the calculated maximum vertical stress generated by the passage of the rear wheel of

the test vehicle. The assumptions made were (a) that the load was transmitted over a circular area calculated

from the nominal wheel load and tyre pressure; (b) that the pavement, soil and backfill constituted a material

uniform with depth and (c) no pipe was present. If the pavement had an elastic modulus 50 times that of

the soil (as was probably the case) the calculated vertical stress would be reduced to about one half the value

shown and consequently more in line with the values measured. As with the results discussed in relation to

Fig. 10, the presence of the pipe would materially affect pressures measured above it. Again a more rigorous

theoretical treatment is required.

The soil pressures measured with the vehicle moving obliquely to the pipe are consistently greater than

those observed when it passed normal to the pipe. The reason for this is not understood and it is a matter

which will need further investigation at other sites.

4.3.3 During vehicle loading (second series). Pressures recorded are also shown in Fig. 11, and it will

"be seen that vertical and radial pressures are all slightly lower than in the first series.

4.4 Results of soil tests to BS 1377 : 1967 on sand backfill and natural ground

A sieve analysis of the sand backfill is given in Table 1, and shows the material to be a uniformly graded

fine sand. A standard 'Proctor ' compaction test gave a maximum dry density of 1.71 Mg/m 3 at an optimum

moisture content of 7 per cent. The results of in-situ density tests carried out on the sand during backfill

operations are shown in Table 2. The variation was small, and ranged from 1.59 to 1.61 Mg/m 3 (93 - 95 per

cent of Proctor compaction). CBR tests carried out on remoulded samples of the sand backfill, gave an

approximate value of 6 corresponding to the average field density of 1.60 Mg/m3; the results are given in

Table 3.

In-situ density tests carried out on the natural ground, gave an average dry density of 1.63 Mg/m 3 at

a moisture content of 20.9 per cent.

4.5 Deflection studies on road pavement

The elastic deflection of the road pavement at Kirtling was rather smaller over the pipe than on the

adjacent undisturbed pavement which is Weak, and exhibits some crazing. This would be due to the relatively

high stiffness of the reinstatement and confirms that pipe deflection is small in comparison with the elastic

deflection of the pavement materials and the supporting subgrade. At Moulton the use of the lean concrete

surround and backfill reduced the measured deflections by about 80 per cent.

6

5. DISCUSSION OF RESULTS

5.1 Loadings applied to the pipe

The static vertical soil pressures across the pipe crown produced by the backfill and by the application

of surcharge shown in Fig. 10, and the recorded dynamic soil pressures across the pipe crown produced by

the test vehicle shown in Fig. 11, indicate a minimum immediately above the pipe crown.

In Fig. 10, the calculated pressures corresponding to the applied loads have been indicafed, and are

shown to be in fairly good agreement with the average pressures measured across the pipe crown, except in

the case of the 38 kN/m 2 surcharge load. This can probably be explained by the fact that when the concrete

blocks were piled up over the pipe to double the applied pressure compared with that used in the previous

test, (19 kN/m 2) the width of loading was reduced.

The recorded dynamic pressures across the pipe crown for the rear axle-load of the test vehicle (Fig. 11)

are much lower than the value predicted by a Boussinesq analysis. This is to be expected, as the calculated

pressure at the pipe crown does not take account of the load-spreading effect of the road construction.

5.2 Horizontal soil pressure, and modulus of soil reaction

In Fig. 12, the measured horizontal pressure is shown plotted against horizontal deflection of the pipe

when the trench was loaded by the rear axle of the test Vehicle (10 Mg), both for the vehicle stationary, and

when driven over a 50 mm 'bump'. From the results, a mean value of the modulus of the soil reaction E' of

55,000 kN/m 2 has been calculated.

It has not been possible to produce a similar calculation for the modulus of soil reaction from the static

loading tests, since the pipe deflection produced by back-filling and by the surcharge loading were measured • :

only to + 0.25 mm, which was of the same order as the deflections produced by the loading. (A more accurate

deflection gauge had been used for the dynamic work).

5.3 Comparison of horizontal and vertical pressures

Although it is realised that the Spangler equation 1 strictly applies to a wide-trench condition, it is

interesting to compare the horizontal and vertical pressures measured in the experiment with those predicted

by the equation.

The Spangler equation i s : -

DKWcR3

A X = 'El + 0.061 E'R 3 (i)

where AX =

D =

K =

W c =

- R =

E =

change in horizontal diameter

deflection lag factor

Bedding constant, depending on bedding angle

vertical load per unit length of pipe

mean radius of pipe

elastic modulus of pipe material

7

I

E' = modulus of soil reaction

I f it is assumed that

W c = 2RP v

and E' - 2RPh AX

where, Pv = vertical pressure at pipe crown

Ph = horizontal pressure at side of pipe

= Second moment of area per unit length of cross-section of pipe wall

(2)

(3)

Then ignoring the term El, which is small compared with 0.061E'R 3, and substituting equations (2) and (3)

in equation (1)

AX = DKP v A X

0.061P h

.'. 0.061P h = DKP v (4)

Since the tests were all short-term D = 1, and putting K = 0.1

Equation 4 reduces to

0.61P h = Pv (5)

Table 4 gives the values of horizontal and vertical soil pressures as measured for various static and dynamic

loading conditions. These are shown plottin in Fig. 13. The regression line calculated from these results

gives the relat ionship:-

Pv = 0"79Ph + 1.22 (6)

The difference between this relationship and that predicted using the Spangler equation may be explained

by the fact that the experimental pipe was buried at a shallow depth in relation to its diameter, and that

Spangler's assumption of a uniform verticalpressure acting across the diameter of the pipe was not in accord-

ance with the observed pressure distribution.

5.4 Comparison of calculated horizontal deflection with observed value

I f the value of E' = 4820 kN/m 2 as recommended by Spangler (1) for general use with compacted

material is substituted in his formula, the change in horizontal deflection can be predicted for the loading

produced by backfilling from stage 5 to stage 6 (see Fig. 4).

From Fig. 9 vertical pressure above the pipe is 26 kN/m 2 due to the backfill.

Taking equation (2) in Section 5.3 above

We= 2RP v

= 26 x 1.83 kN/m

8

Assuming D = 1, and K = 0.1 as before

Then n X = 0 . 1 x 2 6 x 1 .83xR 3

0.061 x 4820 x R 3

.'. A X = 16.5 mm

By comparison, the measured change in horizontal deflection for this backfilling as seen in Fig. 4 was : -

A X = 0.5 mm

The dynamic measurements Of the horizontal pipe deflection, and of soil pressure given in Fig. 12

Correspond to a value of E' of 55,000 kN/m 2. If this value is used in the equation, the pipe deflection

obtained i s : -

/ ~ = 1.4 mm

This is of the same order as the measured value, but the measurements of the static deflections were too coarse,

at these very low values, for a reliable comparison to be possible.

It should also be borne in mind that Spangler's recommended value for E' was based upon field

measurements made mainly upon culverts installed in embankment conditions. In trench conditions, as

at Kirtling, it is to be expected that the effective value "of E' would be higher.

5.5 Impact factors from test vehicle

The Impact Factors shown in Fig. 14 are derived from the horizontal and vertical deflections of the

pipe caused by the front and rear wheels of the test vehicle. The ratio between the recorded peak deflections

when the vehicle was driven over the 50 mm 'bump' and the deflections caused when the vehicle was driven

slowly over the corresponding position without the bump gave the value of the impact factor.

In Fig. 14 values obtained at 6 km/h and at 30 km/h, which represented the maximum speed obtainable

at the site, are shown compared with the predicted values. These latter values z're derived from earlier un-

published work at RRL, in which the same vehicle with nearly the same axle loads was driven over a dynamic

weighbridge on the surface of which were attached artificial 'bumps' up to 17.5 mm in height. These values

quoted in Fig. 14 represent an extrapolation of these results to a 'bump' of 50 mm height, and are therefore

only approximate. The higher impact factors for the front axle are as expected since the factor tends to be

greater for a lower load on a given wheel, at a given speed. The results, however, indicate that the trench and

pipe behaved elastically under dynamic loading, and that impact factors relating to 'flexible' pipes are unlikely

to be very different from those relating to rigid pipes 2.

5.6 Comparison of measured stresses and deflections with values calculated usinq the finite element method

A preliminary attempt is made in Appendix 2 to apply the finite element method to the calculation of

backfill stresses and pipe deflections. The results appear very promising.

9

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

6. CONCLUSIONS

The largest deflection measured in the pipe during installation, backfilling and subsequent loading

(4 mm) was produced by the compaction of the sidefdl material around the pipe in the trench. This

produced a measurable ovality of the pipe in the vertical direction, which was subsequently only partly

reduced by the backfilling of the trench.

The observed deflections of the pipe were consistent with a value of E' for the combinations of the sand

and the trench walls of 55,000 kN/m 2. This was derived from the relation between the horizontal soil

pressure and the horizontal pipe deflection. It should be noted that this value only applies to the con-

ditions of this experiment.

The deflection and strains measured in the pipe were less than one-tenth of the values calculated by the

Spangler equation, using the recommended value for E' of 4820 kN/m 2. When calculated on the basis

of the value of E' obtained experimentally, the pipe deflection obtained was of the same order as the

measured value.

Comparison of the horizontal and vertical soil pressures around the pipe gave a relationship somewhat

different from that predicted by the Spangler equation. This may, however, be due to the large diameter

of the pipe in relation to the depth of fill, and to the difference in pressure distribution across the pipe

from that assumed by Spangler.

Using a structural approach based on assumed elastic behaviour the finite element method indicated

soil stresses and pipe deflections reasonably consistent with the measured values (Appendix 2).

The strains and deflections measured when the test vehicle (10 Mg rear axle load) was driven over the

pipe, both with and without the 50 mm 'bump' on the road surface, were all extremely small, the

strains being in general too small for reliable measurement.

The measured impact factors obtained for a 50 mm 'bump' on the road surface and derived from com-

parison of the pipe deflections with and without the 'bump' were, in general, of the magnitude which

would be expected for the vehicle used. There was no evidence of any difference of behaviour between

the flexible steel pipe and that observed in the earlier work on rigid pipes 2.

Comparison tests carried out at a second road crossing, where the pipe had been encased in concrete

• and lean concrete backfill had been used, gave dynamic pipe deflections which were too small to record

(0.0001 mm) from the passage of the same test vehicle as was used at Kirtling.

Measurements of the road stiffness at Kirtling, using a deflection beam with a standard wheel load of

3.2 Mg, indicated that the stiffness immediately above the pipe was consistent with a road of this form

of construction. It was however appreciably higher than the original roadway, which visual observation

indicated was in rather a weak condition with some crazing.

The results in general indicate that the use of well-compacted sand as a bedding and backfill material

was a satisfactory substitute for concrete encasement at this road crossing.

10

7. ACKNOWLEDGEMENTS

The Laboratory gratefully acknowledges the cooperation in this experiment of Messrs. Binnie and Partners

and their Resident Engineer Mr. Lee and his Staff; also that of Messrs. Biggs Wall the main contractors, and

the Essex River Authority who agreed to the use of their pipeline for the experiment. The County Surveyor

for Cambridgeshire, Mr. R. Lacey, O.B.E., F,I.C.I., M.I.Mun.E., is thanked for his cooperation in allowing

the use of sand backfill, and for the construction of lay-bys.

The following members of the research team assisted with the exper iment : -

H.G. Longlands, J.B. Stevens Who carried out the road stiffness tests, and P.D. Hopes and MJ . Corney who

were responsible for the electronic instrumentation. Mr. P. Nath was responsible for the finite element

analysis.

8. REFERENCES

1. SPANGLER, M.G. 'Soil Engineering'. International Textbook Company, Scranton, Penn. 1960.

. PAGE, J. ' Impact Tests on Pipes Buried under Roads'. Ministry of Transport, RRL Report No.35,

Harmondsworth 1966 (Road Research Laboratory).

. LISTER, N.W. Deflection criteria for flexible pavements. Ministry of Transport, RRL Report No.375.

Crowthorne, 1971 (Road Research Laboratory).

9. APPENDIX 1

COMPARISON OF THE sTIFFNESS OF THE ORIGINAL ROAD STRUCTURE, AND THE REINSTATEMENT OVER THE PIPELINE

9.1 Description of test

The measurement of the elastic deflections of a road pavement when a moderate load is applied at the

surface provides a useful means of comparing the load-spreading properties of various pavements. A simple

instrument for the measurement of the transient deformation of road surfaces when subjected to a dual

wheel load at creep speeds is the 'deflection beam '3.

The beam is designed to pass between the dual rear wheels of a loaded lorry so that the lorry can be

driven slowly past the tip of the beam, which rests on the road surface. The beam is pivoted at a point one

third of the way along its 3.6 m length, and the movement of the tip is recorded on a dial gauge at the oppo-

site end of the beam.

9.2 Tests at Kirtling

A plan of the measuring points chosen for the deflection beam survey carried out at Kirtling is shown

in Fig. 15A. A standard 3.2 Mg wheel load was used. One transverse line of points was located over the

trench and the other some 10 m away on the undisturbed construction. Results are shown in Fig. 16.

11

9.3 Tests at Moulton

Measurements were made only over the trench at this site. The average deflection was very low, being

less than 110 x 10"3mm.

9.4 Discussion of results

At Kirtling significantly lower deflections were obtained on the pipe reinstatement than on the original

road structure indicating an increase in stiffness over the pipe. The value of 500-600 x l f f3mm measured

over the pipe is normal for this type of construction; the higher values observed on the undisturbed road

indicated that this is in a relatively weak condition. This conclusion is supported by the crazing observed.

The tests were carried out at a road temperature of 30* C (measured at 100 mm depth below the road

surface). No temperature correction has been made to the results because of lack of knowledge of the sus-

ceptibility to temperature changes of the original road structure, and the lack of a large thickness of temper-

ature-susceptible material in the reinstatement. The very low deflections at Moulton are consistent with a

road pavement utilising a lean concrete base and are consistent with the negligible deflections observed in

the pipe at the site (see section 9.3).

10. A P P E N D I X 2

APPLICATION OF FINITE ELEMENT ANALYSIS TO THE CALCULATION OF STRESSES AND DEFLECTIONS IN PIPELINES

10.1 I ntrodu ction

Buried pipelines embrace a wide range of variables such as pipe diameter and material, depth of instal-

lation, width of trench, nature of soil and backfdl, bedding and imposed loads. The effects of these variables

can be studied in broad terms by full-scale experiments carried out under controlled conditions. A theoretical

approach, once validated by experiment, would however be invaluable in generalising the conclusions from

field studies such as those described in this report.

Various modem methods of analysis are available, of which the most applicable to this problem is the

finite element method, now being used in the solution of a wide range of structural problems. A brief outline

of the method is given below.

1,0.2 The finite element method

The complete structure is divided into a finite number of sub-structures (elements) and the structural

characteristics of each of these elements are determined. The behaviour of the whole structure is then estab-

lished by combining the structural effects of all the elements. For plane structures, whe whole structure can

be divided into a network or mesh of triangular elements, each apex being referred to as a node. For the

sake o f simplicity the following assumptions are m a d e : -

(a) each triangular element is connected to its neighbours only at the nodes. Hence any element

experiences forces from other elements only through its nodes, and no forces are transmitted

through the sides of the elements. It follows that external forces are also applied at the nodes

only.

(b) Inside each element the stress (or strain) remains constant. 12

From these assumptions it follows that the sides of the triangles remain straight before and after defor-

mation, thus assuring a condition of comparability throughout the structure. The element of approximation introduced by the assumptions decreases with decreasing size of element.

Using these assumptions a relationship can be derived between the nodal forces IF]and the nodal displacements [:6 ] in the form:-

[F i] = [ki][6 i ] (1)

where

[k i] = / f f [ B i ] T [ D i ] [B i ] d× dy dt

For triangular element of unit thickness and plane area a i

[k i ] = [B i } T [ D i][Bi]/xi

The stiffness [k i ] for any element t will depend on the geometrical properties contained in the matrix [B i ]

and the elastic properties contained in the matrix [D i ].

• Having established [k i ] for all the elements one can build up the stiffness matrix for the whole structure

[K ] of which each [k i ] is a sub-matrix.

If [R ] is the applied load to the whole structure, then one can write

[R] = [K] [6] (2)

where [ 6 ] is the combination of all the sub-matrices [ 6 i ]

All that remains now is to apply the restraints in the form [6j ] where j = 1, 2, . . ~. . (total number of

restraint boundaries of the problem) and solve the system of equation (2) for [ 6 ] .

Having found [ 6 ] , the stresses and strains for any element 'i' follows from equation (1). :~

The advantage of the finite element method lies in the fact that the calculation of k i (equation (1) )

involves the same set of mathematical manipulations for all the elements, thus making the solution suitable

for computer programming. Moreover, the resolution of the stress pattern can be increased by simply using

smaller elements for those parts of the structure where stress concentrations occur.

10.3 Application to pipelines

Fig. 17 shows the cross-section of the most complicated case likely to arise with a buried pipeline, i.e.

the case where the pipe passes Under a road. The finite element method is capable of producing solutions for

the stresses and displacements in such a case, but the results will depend on how representative the values

chosen for the properties of the various materials are of the conditions in practice.

10.4 Application to the pipe-line at Kirtling

The method has been applied to the pipeline at Kirtling mainly to illustrate its potential value. Sufficient

data relating to the soil and the friction between the pipe and its backfill were not available to justify a complete analysis in this case.

13

The assumption has been made that the trench walls were rigid, i.e. stress conditions in the sand backfill

only have been examined. To test the implications of this assumption the effects of widening and deepening

the trench beyond the dimensions actually used have been explored.

The mesh used is shown in Fig. 18. The boundary ABC represents the half-width of the trench with

the pipe to scale. The other boundaries FDE and FGH represent the cases where the trench is deepened by

300 mm and widened by 300 or 900 mm. The sand backfill is represented by traingular elements and the

pipe is represented by 'line' elements, each having two nodes. They are assumed to be able to sustain only

axial forces (the bending strength of the pipe being ignored). These elements are assembled with the triangular

elements in the stiffness matrix [K ] in equation (2).

The force due to the self weight of the backfill is taken into account as follows. The total force on each

element is represented by equivalent forces at the nodes. When this has been done for all the elements, the

total resultant forces at each node are calculated. These nodal forces are treated as external loads and included

in the matrix [R] in equation (2).

Since each element has constant strain, the calculated stresses of two adjacent elements will in general

be different. These discontinuous jumps in stress values from one element to the next can be made less

obvious by using more, and smaller, elements. In the present application such smaller elements have been

used adjacent tO the pipe. It will in the future be probably desirable to use even smaller elements in some

areas, although care is necessary to keep the zomputations within the scope of the computer available. In

addition, stresses have been calculated for each node by taking the average of the stresses of all the elements

connected to that node. These nodal average stresses show more clearly the pattern of change in stress.

Of the various loading conditions investigated experimentally at Kirtling that involving a uniform sur-

charge of 19 kN/m 2 was chosen for this preliminary analysis. For the first case examined values of Young's

Modulus of 130 MN/m 2 and 21 x l04 MN/m 2 were taken for the sand backfill and the steel pipe respectively,

the value for the sand being based on measurements made on a material of similar grading. Fig. 19 shows the

displacement vectors for the nodes calculated for this case, and Figs. 20 and 21 give the vertical and horizontal

stress contours and the stresses in the pipe wall. The average stresses at the nodes are included in the diagrams.

The assumptions made relative to the bar elements and their associated triangular elements imply com-

plete friction between the pipe and the backfill. In practice a degree of slip must occur and this would

materially affect the stress pattern in the backf'dl. The degree of friction can be established by experiment

and included in the analysis. However the effect of reducing the friction in terms of backfill stress can be

examined with reasonable accuracy by reducing the elastic modulus value adopted for the pipe. Such a

reduction to a value less than that of the sand would correspond to a no-friction condition. Figs. 21 and 22

show respectively the nodal deflections and vertical stress contours when the elastic modulus of the pipe is

reduced to a value of 13 MN/m 2, i.e. one-tenth that of the backf'dl. The experimental results both for stresses

and pipe deflections are compared with the theoretical values in Figs. 23 and 24. The effect of increasing the

trench dimensions to the limits represented by the boundaries FDE and FGH shown in Fig. 18 on stresses and

deflections, is also shown in Figs. 23 and 24. Rigid boundaries are again assumed.

10.5 Conclusions from a comparison of experimental and theoretical results

Considering the coarseness of the finite element mesh and the assumptions made, the correlation

between computed and experimental stress and deflection values is sufficiently good to merit refinement of

the theoretical curves for full and zero friction between the pipe and the backfill. (By selecting an inter-

mediate friction condition closer agreement could be established). There is some evidence to suggest that

the friction may vary round the pipe circumference.

The distance between the pipe and the trench boundaries has a significant effort on the stress regime

in the backfdl. Replacing the rigid boundaries by an elastic soil is likely to have a bigger effect. This is being

investigated.

15

TABLE 1

Sieve analysis of sand backfill

B.S. sieve Percentage passing

3/8 in

3/16 in

No. 7

14

25

52

100

100

100

96

92

87

65

13.5

TABLE 2

In-situ density tests on sand backfill

Depth below Dry density Moisture content surface (mm) :(Mg/m 3) (per cent)

0

500

900

1200

1800

2300

2800

1.61

1.59

1.60

1.60

1.59

1.62

1.61

11.9

11.0

12.5

12.7

7.3

11.2

10.1

Average wet density (Unit weight) = 1.78 Mg/m 3

TABLE 3

CBR tests on remoulded sand backfdl

Average CBR

(per cent)

3.0"

5.0

10.0

6.5

Average dry

densit~ (Mg/m j)

1.36

1.62

1.72

1.67

Average moisture content

(per cent)

2.9

5.6

7.3

15.4

1 6

TABLE 4

Measured vertical andhorizontal soil pressures upon the pipe

Loading

Backfill to road base

Surcharge 19 kN/m 2

Surcharge, after 18h

Surcharge 38 kN/m 2

Surcharge removed

Stationary vehicle

Vertical pressure

(Pv) (kN/m2)

Vehicle moving normal to pipe at 9 km/h

Vehicle over bump normal to pipe at 6 km/h

Vehicle moving oblique to pipe at 9 km[h

Vehicle over bump oblique to pipe at 32 km/h

Vehicle over bump oblique to pipe at 6 km/h

Dec. 1970

25.0

25.5

22.I

14.5

-0.5

Jun. 1971

0.5

1.5

2.0

2.5

3.5

2.0

Horizontal pressure

(Pit) (kN/m2)

Dec. 1970

1.2

2.1

9.2

6.6

4.8

22.7

29.6

31.0

30.3

20.2

2.3

2.5

2 . 8

2.6 5.2

5.5

Jun. 1971

2.0

2.0

2.0

2.0

4.9

4.1

17

(a) Splays for vehicle manoeuvres during dynamic testing

(b) Hardstanding for mobile laboratory

//'I ¢ I

/ /

I !

(~)(~)Position of Jl (a) instrumented J I rings I

I (~ Strain gauge and ~

pressure ce,l,eads ~ ,~ are stored in \\ cable pit ~ t / S "

%% %

5 2 *

(a~ I ! I ! i ! i !

I- ~_ road

N

% ,% \

(b) ., ~ pipe

j j ~ ' . I .'<,/~/

~Y~.~vs~'~Air valve chamber ~ / ; " " (used for entry to pipe)

J s

~ lns t rumented pipe

Bedding angle of pipe

Fig. 1. LAYOUT OF EXPERIMENTAL PIPELINE SECTION

Stainless steel studs f i xed to crown and invert of pipe (adjacent t o st ra in gaug, s) ~

ac i n p u t / o u t p u t rec t i f ied to dc for record ing on data logger or ga lvanometer recorder

Calibrated scale w i t h 0 .5mm resolut ion

Plastic clamp holding LVDT to plunqer.

Spring loaded plunger

Linear var iable d i f f e r e n t i a l transformer. (LVDT) capable of 0-02 mm resolution

1"83m

30 ram O.D. aluminium tube

\ /

Fig. 20. G,~UGE FOR STATIC MEASUREMENT OF PiPE OEFLECTION

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Test veh ic le : - Loaded 'Mandator ' l o r ry Front axle load 4 mg Rear 10 mg

,p

f J ,r

f f

Front

f f

/ /

/ f /

4-

I IRear

_1 _o Dec 1970

0 Using ver t ica l deformat ion ~Obl ique z~ Using horizontal de fo rmat ion . / to pipe n Using ver t i ca l de fo rma t i on \ N o r m a l 0 Using horizontal deformat ion. / to pipe

June 1971 as above but points shaded

+ Pred ic ted values f rom ear l ie r tes ts w i t h the same vehicle and ax le load, using a dynamic we ighb r idge

0 10 20 30 Vehicle speed ( k i n / h )

Fig.IL,.IMPACT FACTORS OBTAINED FROM PIPE DEFLECTIONS WITH TEST VEHICLE PASSING OVER A 50tam'BUMP'

o. K I R T L I N G

l O m ,"-

\ \

Pipe C..

K i r t l i n g

~.Road

Q

b. M O U L T O N

M o u I t o n

~E

D C

B

IA

Pipe

Newmarket

Road

Fig.15. LOCATION OF ROAD OEFLECTION MEASURMENT POINTS

2 000

'0 X

E E

v

c- 0

u

0

1 750

1 500

1 250

1 000

750

500

250

O ~ 0

10m from pipe

0.~0 Over pipe

I Crazed

P° i i t

J ,

1 2 3 4 Distance from east side of road ( m )

5

Fig.16. OEFLECTIONS OF ROAD AND PIPELINE REINSTATEMENT, KIRTLING

~~ Wheel load

Pavement

• , . . . . . . . . . . . . ~ . . . . . , . . . . !

o o ~ ~%~o%o g. Oo~ ~,oO o gE 20°0°o oo°o~,o~%o'#c'~8°oo°c

' ~Base Sub-base

Backfi l l

Soil

~ P i p e

- ~ B e d d i n g

Fig.17. COMPONENTS OF" PIPELINE STRUCTURE

C £ H

Y

' ~ X

S c a l e :- I I I

o qso 300 m m

A

Fig.18. FINITE ELEMENT MESH

D G

L

I

Y

I

i I

I I I

I I

LOAD Body force

wy = -17-3 k N / m 3 Surface f o r ce

py = - 1 9 k N l m 2 MATERIALS Sand

y = 0"4 E = 0"13 x 106 k N I m 2 G = 4 6 - 4 x 10 3 k N / m 2

STEEL E = 2 0 6 ' 8 x 106 k N / m 2 As = 1 2 7 m m 2

- - - X

I I

/ J / / /

Sca le •

M e s h I I I I

0 150 300 4 5 0 m m

D i s p l a c e m e n t s I i t t 0 0"2 0-4 0 6 mm

Fig.19. DISPLACEMENT VECIORS (FULL FRICION CASE )

22 209 21 ~4 2 ~

. . . . . . _ _ . . . . . 2 . 5

127 • • • 2"9 2'7 2"66 2"8

" - - 3 0

3.15 e3"39 • 3.34 • 3.33 3-48

/ / 3-5

• . ~ ' - - - , . , . . ~ . ~ , , . _= , . . . . , ~ ' % . .

~ 4-0 " ~

~ 6 ' 6 . 3 ~ , , , , ~ 4 . 3 7 •4.37

Y 4"36

4o4 4"3

.-X

Units: kNI m2(x 10) Compr. + ve

i • 44 /

4.1 ~ . / 4 - ~ 1

/ / 4.71 4-56

4.52 . , . . . . ._

I 0 5-49, 5-26

~ . 5 "5 "-" ---. ~ 1 1 1

I %_5"0 / ~ I ~ 6 . 0 6 0 Z

~,-sl ~.51 -6.08 ~ .e9 ° 6 1 3

Fig.20. VERTICAL STRESSESANO STRESSES IN THE PIPE-WALL (FULL FRICTION CASE )

'1'11 0"96 0 .96 0 " 8 9 0 .14

1.5

1-70 1-66

1-65

%

• 1.52 1"81

2"0

~ ' 07 2 "18

2.23

2 . 5 2"42 ~ - - - - - - 2 • ' 5 3 ~

• - - ' ~ • 2 -54

2-4

2 . 0 5

2 "74

2-68

2.5.6

Y

=X

Unit" k N / m 2 ( x 10)

1"48

2"95 3"0 . . - --- -

2 ' 9 7 ~ ~ e /

f 3"41

/ 3-38 4.0

~ 4.27 f o 4"32

/ 4"5

', 5. 'q 5 . ~ \\g62 -, ~ 4'98'

3-97 4 . 5 ~ \

1-99 4.3, / / e4'O I

~ 3 - 5 4.o~ \ 3"30 ~ ~ 6 8 3.79 \ 4.11_ 4.15

Fig.21. HORIZONTAL STRESSES (FULL FRICTION CASE)

i i_ i i

.! i

1 X : X : X~

X

×

t,

I • Original node x Displaced node

- - -D isp laced outline

Scale.

1 I I

0 2 3 mm

Fig. 22, NODAL DISPLACEMENTS (NO FRICTION CASE)

2.11 2-~29 2.53 2-52 2-59 ~

\ \ \ \ \

- 1 . 6 4 ~ " 2. 2

\ \ \ \

1.26 1-78 /

( ) ' 6 3 / •2.28 I I

• 1-83

2.82

/

J

2.83 3.13 3.1

/

f O

/ 4-17 4.73 /

/ / z / /

4 ' 4 5 5 ' 9 2 /

/~/..0 735 J

f f e6.26

8.0 • 8.51

Uni t " kN I m 2 ( x l O )

. ~ 11"28 10. -0~ - . . .~

• 1055 . 10"97 \

\

/

e~.~12'17"

11"98 " " 12"0

1"14

0 '6

4-07 I

~ 11.58

~ % • 11.56- . ~,0~.0~ 11-35

I / \~ 8.17 I

\ \ / 3-75 \ ~ 110"6 11.83

Fig.23. VERTICAl STRESSES (NO FRICTION CASE)

h

- 80

-60 I

-40

_.ol. ",-%..x.,,,,-" J \ ~ ~#'t Vert icai stress

I ~, ~ " L 0

B

Scale :-

G e o m e t r y I i I mm O 150 3O0

Pressure I I kNIm 2 O 10 20

Displacement I I I mm 0 1 .2

z ~ ~ . !nitial posit ion

I 1!i No friction, trench FDE -----

• No fr ict ion, trench FGH -----

N O friction,-trenchABC --on

Full friction, trench .A B C

Exper iment, trench AISC

A

Pipe def lect ion

Fig.2/..VERIICAL SI"RESS ABOVE PIPE AgO PIPE OEFLEC/IONS

(962) Dd891996 3,.250 7/72 H PL t d . , So'ton G1915 PRINTED IN ENGLAND

- 4 0

- 3 0

- 2 0

-10

0 -

k N / m 2

o

Pipe wall

Y

R = 9 1 4 m m

TrenCh FoE No friction

Trench FOH No friction

Trench ABC Full friction No •friction Experiment

0 O

- 1 0

- 2 0

- 3 0

- 4 0

Fig. 25. RAOIAL PRESSURE OISTRIBUTION (OR A RAOIUS = d.,.150mm WHERE 2d=OlA OF PIPE)

ABSTRACT

Experimental work on large steel pipeline at Kirtling: J J TROTT and J GAUNT (Appendix 2 by P NATH): Department of the Environment, TRRL Report LR 472: Crowthorne, 1972 (Transport and Road Research Laboratory). The Underground Pipes Group at the Laboratory is carrying out work to determine the loads on buried pipelines, and their effects on the pipes concerned. As part of the programme a steel pipeline was instrumented where it passes under a minor road, and observations were made during the backfilling and subsequent loading tests in the period between October 1970 and June 1971. The pipes are 1.83 m in diameter, with a wall thickness of 12.7 mm. Over the test length sand was used for bedding the pipe and for backfilling the trench, the crown of the pipe being 1.60 mm below road level.



ABSTRACT

Experimental work on large steel pipeline at Kirtling: J J TROTT and J GAUNT (Appendix 2 by P NATH): Department of the Environment, TRRL Report LR 472: Crowthorne, 1972 (Transport and Road Research Laboratory). The Underground Pipes Group at the Laboratory is carrying out work to determine the loads on buried pipelines, and their effects on the pipes concerned. As part of the programme a steel pipeline was instrumented where it passes under a minor road, and observations were made during the backfilling and subsequent loading tests in the period between October 1970 and June 1971. The pipes are 1.83 m in diameter, with a wall thickness of 12.7 mm. Over the test length sand was used for bedding the pipe and for backfilling the trench, the crown of the pipe being 1.60 m~a below road level.



by - trl · 0-200 kn/m 2. initial calibration was against a precision bourdon type gauge; using...

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