ROAD RESEARCH LABORATORY

Ministry of Transport

RRL REPORT LR 309

SETTLEMENT BEHIND BRIDGE ABUTMENTS

THE PERFORMANCE OF A SILTY CLAY FILL IN: AN

APPROACH EMBANKMENT ON THE M4 MOTORWAY

by

D. McLaren

Earthworks and Foundations Section

Road; Research Laboratory

Crowthorne, Berkshire

1970

CONTENTS

Abstract

I. Introduction

2, Details of the site

2.1 Bridge

2 2 Subsoil conditions

.

.

S.

o

7.

8.

2.3 Fill materials

2.4 Construction procedure

Experimental procedure

3.1 Compaction observations

3.2 Settlement of subsoil

3,3 Settlement of road surface

Results

4.1 Compaction of fill materials

4.2 Settlement of measurements

4.2.1 Distributionof settlement on road surface

4.2.2 Rate of settlement

Discussion of results

5,1 Performance of the silty clay fill

5.2 Performance of the sandy gravel fill

Conclusions

Acknowledgements

References

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Q CROWN COPYRIGHT 1970

Extracts from the text may be reproduced provided the source is acknowledged

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.

SETTLEMENT BEHIND BRIDGE ABUTMENTS

THE PERFORMANCE OF A SILTY CLAY FILL IN AN APPROACH EMBANKMENT ON THE M4 MOTORWAY

ABSTRACT

This Report describes an investigation in which the performance of a silty clay fill, placed to complete the approach embankment to one of the abutments of a bridge, was compared directly with the performance of a sandy gravel filling, at the other abutment. The construction of the embankments was carefully observed and records of the settlement of the road surface and the subsoil were obtained.

It was found that although the silty clay fill- ing was particularly well compacted settlements of up to 45mm subsequently occurred within the fill material after the road was completed. After a period of about 4~ years movement within the fill had practi- cally ceased but settlement of the subsoil was however still continuing at a rate of about 5mm per year due to the slow consolidation of the underlying London clay.

In general settlement within the sandy gravel filling was found to have been negligible and con- firmed the performance previously given by this material at other sites.

i. INTRODUCTION

As part of a study into the problem of the serious differential settlement which frequently occurs between bridges and their approach embankments a series of investigations is being made of the performance of different materials when used for filling behind bridge abutments.

This Report describes an investigation in which the performance of a silty clay fill was compared directly with that of sandy gravel behind opposite abutments at the same bridge site.

2. DETAILS OF THE SITE

2.1Bridge

The investigation was carried out at the site of the Harmondsworth Road overbridge to the M4 motorway. The bridge, which was constructed under Contract 4 of the Chiswick-Langley Special Road scheme, also

formed part of a diversion improvement to the original route between Harmondsworth and West Drayton. The general arrangement of the site is shown in Figure I.

The reinforced concrete bridge structure consisted of a single span portal with 'V' frame open abutments on spread footings which were con- nected by prestressed concrete ties passing under the motorway (see Fig.l and Plate i).

The approach embankments were constructed with gradients of about 1 in 30 and had a maximum height of 6.7 metres at the abutments of the bridge.

Transition slabs of reinforced concrete approximately 5 metres long ~were provided at both ends of the bridge.

2.2 Subsoil conditions

The topsoil was removed prior to the commencement of embankment construction. Records obtained from boreholes located near the bridge footings showed that the subsoil consisted of a 1.75 metres thick layer of sandy clay on a stratum of gravel and sand between 4.4 and 5.5 metres thick; this was underlain to a considerable depth by London clays.

2.3 Fill materials

The main parts of the bridge approach embankments were constructed with spent domestic refuse ash more than 30 years old but this material was not permitted in the areas immediately behind the bridge abutments.

Behind the south abutment a sandy gravel filling was used which was specified to have a grading within the limits required for Type 2, Granular sub-base materials.(1) Grading tests on samples of the fill taken during construction showed that two differently graded gravels were employed (Fig. 2). The bulk of the abutment filling however had a grading which was close to the coarse limit of the specification and the results of the B.S. laboratory compaction test (5.5 Ib (2.5 kg) rammer method) on this material are given in Figure 2.

For the purpose of the investigati n the filling behind the north abutment was specified to be a clay soil having a liquid not greater than 50 per cent and not less than 30 per cent. The soil selected was a silty clay similar to that which occurred in the uppermost layer of the subsoil. It had a liquid limit of 37 per cent, a plastic limit of 19 per cent and is often referred to as Harmondsworth brickearth. The mean particle-size distribution of the clay and the results of the B.S. laboratory compaction test are given in Figure 2.

Both types of filling were specified to be placed in accordance with the normal requirements for the Compaction of Earthworks in the Ministry of Transport's Specification for Road and Bridge Works' which was in use at the time the work was carried out.

2

2.4 Construction p r o c e d u r e

To provide working space for the construction of the bridge the " approach embankments had been terminated about lOmetres from each abutment. The remaining areas behind the abutments were filled later when bridge construction had been substantially completed and during this work the ends of the main embankments were cut back to remove any uncompacted material. Both types of selected filling were compacted generally by 1.2Mg smooth-wheeled rollers. However, the open type of abutments required filling to be placed under the ends of the structure and as the height of fill increased the reduced headroom beneath the bridge precluded the use of the large rollers. Compaction in this restricted area was therefore continued with a small tandem vibrating roller and finally completed by plate compactors.

When t h e embankments had r e a c h e d the l e v e l o f t h e u n d e r s i d e o f t h e b r i d g e deck , f i l l i n g was h a l t e d to a l low t h e s u p p o r t i n g beams f o r t h e t r a n s i t i o n s l a b s to be c o n s t r u c t e d . At each a b u t m e n t t h i s i n c l u d e d t h e c a s t i n g on t h e f i l l o f a c o n c r e t e beam 0.9 m e t r e s wide , 11.9 m e t r e s long and a p p r o x i m a t e l y 1.5 m e t r e s deep f o l l o w e d b y t h e c o n s t r u c t i o n o f c r a w l - ways to p r o v i d e a c c e s s b e n e a t h t he t r a n s i t i o n s l a b s to d u c t s c a r r y i n g s e r v i c e s t h r o u g h t h e b r i d g e d e c k . The r e m a i n i n g a r e a s were b a c k - f i l l e d and narrow t r e n c h e s a p p r o x i m a t e l y 1.5 me t r e s d e e p and 0 .6 m e t r e s wide were e x c a v a t e d from t h e crawlway e n t r a n c e s t o connec t w i t h t r e n c h e s in t h e ve rges o f t h e approach embankments, where p i p e s f o r c a r r y i n g t h e e x i s t i n g s e r v i c e s ove r t h e motorway had been i n s t a l l e d . These t r e n c h e s were r e - i n s t a t e d u s i n g the p r e v i o u s l y e x c a v a t e d m a t e r i a l which was r e - compacted wi th a power rammer.

The road pavement was constructed with a 8-in (200mm) thick lean concrete base surfaced with rolled asphalt basecourse and wearing course 2~in (63mm) and l~in (38mm) thick respectively. A gravel sub-base 14 in (350mm) thick was also provided at the north abutment where the silty clay filling was employed. The placing of the fill at the abutments was carried out between September and November 1964; pavement construction was completed and the road opened to traffic in December 1964.

3. EXPERIMENTAL PROCEDURE

3.1 CompaCt i0n lpbse rva t i ons

The s t a t e of compaction of t h e filling a t each abutment was d e t e r m i n e d as work p r o c e e d e d from d r y d e n s i t y t e s t s made u s i n g the sand r e p l a c e m e n t method2. These t e s t s a l s o p r o v i d e d a check on the m o i s t u r e c o n t e n t a t which t h e f i l l was p l a c e d . The samples o f g r a v e l and s i l t y c l a y t aken f o r the d r y d e n s i t y measurements were r e t a i n e d and g r a d i n g and p l a s t i c i t y t e s t s made t o a s s e s s t h e compl iance o f t h e f i l l m a t e r i a l s w i th t h e s p e c i f i c a t i o n .

3.2 S e t t l e m e n t Of subsoil

To record settlement of the subsoil a multi-point mercury-filled

3 settlement gauge was installed in the surface of the area behind each abutment before the filling was placed. Each gauge was equipped with three measuring points which were located on the Centre line of the road beyond the ends of the transition slabs as shown in Figure 1 and Table I.

3.3 Settlement ofroad surfaces • :

The movement of the road surface was recorded by optical levelling on metal studs which 'were driven into the asphalt wearing course immediately after laying. The levels were initially related to temporary bench marks established on the bridge and these bench marks were then referred to a permanent datum installed well clear of any influence from the earthworks. The arrangement of studs on the road surface at each abutment is shown in Figure 3.

4. RESULTS "i ' . ' :

4.1 C0mpac,tion Of fill materials : . . .'

,. The rgsults of the dry density tests made in the sandy gravel and silty clay materials during the filling Of each abutment are given in Figure 4.

In the •sandy gravel filling at the •south abutment 90 per cent of the dry density samples had less than .lOper cent air voids• indicating that the compaction work folr the main bulk• of the fill had been carried out in accordance with the specffication.

The tests made in the silty clay filling at the north abutment indicated that the bulk of the material was very well compacted, 95 per cent of the:dry density samples having less than I0 per cent air voids. The resu.!ts of the dry density tests also showed that the moisture content of the filling did not exceed the specified maximum of plastic limit plus 2 per cent .... •

: . . . .

4.2 Settlement measurements ,

The results of measurements of the settlement of the subsoil and the road surface at the bridge approaches approximately 4~ years after the road was completed are summarised in Table I° From these measurements the settlements which occurred within the two differenttypes of.filling have been evaluated assuming that there was no compression Of the pave- ment materials.

4.2.1 . Distribution of settlement on the road surface.. The longitudinal distributions of settlement onthe approaches to the bridge are shown in Fi~gure 5. ~

The results of measurements on the transverse rows of studs installed, in the road surface are shown in Figure 6. The settlement of the studs in the surfacing on the bridge deck did not differ significantly from that recorded on the temporary bench marks established closeby on the bridge parapets, indicating that no ~ significant compaction of the~bituminous surfacing had occurred under traffic.

TABLE 1 - . . . . . .

Summary of settlement measurements at 4 Years (approx.) after road completed (millimetres)

, (to nearest millimetre) ~.

ABUTMENT

Settlement Point No.

NORTH (silty clay)

SOUTH ( sandy g r a v e I)

3 2 Distance from transition slab*

Road Surface

Maximum on embankment

Above settlement point

(0.6m) (3.7m)

Subsoil

Before paving

After paving

Total

Within fill

Above settlement point

since paving

24

29

53

36

87

65 77

22

38

60

39

(6.7m)

82

17

37

54

45

3 :

(0.6m)

43

41

36

77

2

(2.7m)

51

37

40

38

78

- I

1

(4.9m)

39

33

40

73

-I

* measured from edge of transition slab nearest to the approach embankment.

5

4.2.2 Rate Of settlement. The records obtained from the multi-point settlement gauges installed on the subsoil beneath the filling at each abutment are given in Figure 7 and show the total settlement which has occurred in the subsoil since the start of construction.

The rates of subsoil settlement behind each abutment following com- pletion of the construction are compared in Figure 8 (north abutment) and Figure 9 (south abutment) with the rates of settlement of the road surface immediately above the positions of the multi-point gauges. Assuming that there has been no change in the thickness of the pavement materials ~he differences between the subsoil and road surface settlement-time relations represent the settlements which have occurred within the two types of filling.

Measurements o f t h e l e v e l s o f the t empora ry bench marks on the b r i d g e p a r a p e t s i n d i c a t e d t h a t t h e b r i d g e had i t s e l f s e t t l e d a p p r e c i a b l y s i n c e t h e c o m p l e t i o n o f t h e app roach r o a d s . The r a t e s a t which s e t t l e m e n t o f t h e n o r t h and s o u t h a b u t m e n t s o c c u r r e d a r e shown in F i g u r e 10 t o g e t h e r w i t h t h e r e c o r d e d r a t e o f t h e d i f f e r e n t i a l movements be tween t h e ends o f t he b r i d g e .

5. DISCUSSION OF RESULTS

5.1 PerfOrmance Of the silt~ clayfill

The t o t a l s e t t l e m e n t o f t h e r o a d s u r f a c e b e h i n d t h e n o r t h abu tment , where s i l t y c l a y was u s e d to comple te t h e approach embankments, has amounted t o a maximum o f about 8Smm in t h e 4~ y e a r s which have e l a p s e d s i n c e s u r f a c i n g was c o m p l e t e d . Dur ing t h i s p e r i o d t h e b r i d g e abutment has s e t t l e d by 2Omm and t h e main embankment by 1Smm which has r e s u l t e d in a maximum d i f f e r e n t i a l s e t t l e m e n t o f about 70mm. This s e t t l e m e n t , which i s shown in F i g u r e 5 t o have been d i s t r i b u t e d o v e r a d i s t a n c e o f about 25 m e t r e s from t h e end o f t h e b r i d g e , r e s u l t e d i n a f l a t t e n i n g o f t h e o r i g i n a l v e r t i c a l c u r v a t u r e o f t h e r o a d ' s l o n g i t u d i n a l p r o f i l e and had o n l y a s l i g h t e f f e c t on t he r i d i n g q u a l i t y o f t h e r o a d . The p r e s e n t r a t e s o f s e t t l e m e n t o f t h e b r i d g e and o f t h e road s u r f a c e both on the a r e a o f abutment f i l l i n g and on t h e main p a r t o f t h e a p p r o a c h embankment a r e l e s s t h a n 5mm p e r y e a r and o n l y sma l l changes i n d i f f e r e n t i a l s e t t l e m e n t a re e x p e c t e d to o c c u r in t h e f u t u r e . Most o f t h e p r e s e n t s e t t l e m e n t can be a t t r i b u t e d to c o n t i n u i n g s u b s o i l movement p r o b a b l y r e s u l t i n g from the slow c o n s o l i d a t i o n o f the u n d e r - lying London clay.

Traffic loading was found to have made a considerable contribution to the settlement of the silty clay filling. The influence of traffic on thesettlement only became apparent in the third year after the road was opened and was indicated as shown in Figure 8 by an increase in the rate of settlement of the road surface. Prior to this time the settlement had proceeded at a uniform rate in the areas behind the abutment where there was a similar depth of filling. This is also shown in Figure 5 by the longitudinal settlement profiles obtained on the east and west traffic lanes from levels recorded 657 days after the road was completed. The settlement during this first period occurred mainly from the consoli- dation of the fill and the subsoil under the weight of the embankment and

6

it is clearly demonstrated in Figure 5 that the presence of the tran- sition slab has prevented an abrupt step being formed at the run~on to the bridge. A possible explanation for the subsequent increase in the rate of settlement in the third year after the road was completed could be the occurrence of increased dynamic stresses caused by traffic as a result of the changes in the level profile of the road. These increased dynamic stresses could possibly have produced some slight further densi- fication of the sub-base and the upper layers of the fill. The effect of this increase in dynamic loading appears to have been related to the direction of the traffic. Invariably more settlement was found to occur on the west side of the road (see Fig. 5) where traffic travels in a direction from the bridge towards the approach embankment and this is also shown in Figure 6 which gives the transverse distribution of settle- ment on the road surface immediately behind the transition slab.

The performance of the silty clay filling at the present site was very different from that obtained from a previous investigation in the series which was carried out at a site on the M1 motorway. At this site a medium clay fill was used and only very small movements within the filling were recorded 4. A major difference between the sites exists in the type of abutment employed. At the previous site the bridge was designed with closed abutments whereas at the present site open abutments were constructed. It is possible therefore that the restraint offered by closed abutments may have some effect in reducing the settlement of a road surface carried by a fairly plastic clay fill.

5.2 PerfOrmance of the'sandy gravel fill

At the south abutment, where a sandy gravel fill was used for com- pleting the approach embankment, settlement of the road after 4~ years varied from 25mm on the main embankment to about 50mm on the gravel filled area behind the abutment. As the abutment has also settled by 30mm the maximum differential movement over a length of about 20 metres from the end of the bridge has only amounted to about 25mm. Comparisons of the settlements measured by the subsoil gauges with the total movements recorded on the road surface directly above the gauges indicated that in general there has been negligible settlement within the gravel filling and this is in accordance with the results obtained at other sites 5. However, some disruption of the road surface occurred in a limited area immediately adjoining the end of the transition slab. The transverse distribution of settlement at this position, shown in Figure 6, was appreciably irregular forming two shallow depressions in the central areas of the traffic lanes on either side of the road. Crazing of the road surface was found to extend over a distance of about 2 metres from the end of the transition. The settlement within the fill on these small areas probably resulted from inadequate compaction due to difficulties in back-filling the narrow excavations made during the construction of the crawlway entrances.

6. CONCLUSIONS

Although the silty clay was particularly well compacted and complied with the M;O.T. Specification{ I) current at the time of construction, settle- ments of up to 45mm occurred , after the road was completed, as a result of compression of the abutment filling under the self-weight of the embankment and the dynamic effects of traffic. The compression of the silty clay filling together with subsoil movements produced differential settlements of about 70mm on the road surface behind the abutmen~,e- this ha~ n~t zeriovsly ~rte~ the ri~ing qun!ity of the re~d. After 4~ years movements within the fill have practically ceased but settlements of the subsoil were still occurring at a rate of about 5mm/year due to consoli- dation in the underlying London clay.

In general negligible compression occurred within the sandy gravel filling and this was in agreement with the performance obtained with this type of material at previous sites.

7. ACKNOWLEDGEMENTS

This Report was prepared in the Earthworks and Foundations Section of the Construction Division. The work of the section is directed by Mr. W.A. Lewis and this investigation forms part of the research programme of the Stability of Earthworks Group which is under the supervision of Mr; G. Margason. Members of the group who participated in the investi- gation were Messrs. D. McLaren, M..J. Irwin, R.T. Murray and B. Pimley.

Thanks are due to the Consulting Engineers, Sir Alexander Gibb and Partners and in particular to Mr. J.M. Preece, M;I.C.E., Resident Engineer (Contract 4), for their co-operation during the investigation.

i.

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8. REFERENCES

MINISTRY OF TRANSPORT. Specification for roadand bridge Works. London 1963. (H.M. Stationery Office).

BRITISH STANDARDS INSTITUTION. British Standard 1377 : 1967. Methods of testing soils for civil engineering purposes. London 1967 (British Standards Institution).

IRWIN, M.J. A mercury-filled gauge for measuring the settlement of foundations, Ministry of Transport, RRL Report LR 62, Crowthorne, 1967 (Road Research Laboratory).

McLAREN, D. The performance of a medium-clay fill used to form the approach embankments to a bridge on the MI motorway. Ministry of Transport, RRL Report LR 76. Crowthorne, 1967 (Road Research Laboratory).

MARGASON, G. A study of the settlements at a number of bridge approaches on the Maidenhead By-pass, Chart. Min. Engr., 1963, 90, 134-138.

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ABSTRACT

Settlement behind bridge abutments. The performance of a silty clay fill in an approach embankment on the H4 motorway: D. McLAIlEN: Ministry of Transport, RIlL Report Lil 309: Crowthome, 1969 (Road Research Laboratory). This Report describes an investiga- tion in which the performance of a silty clay fill, placed to complete the approach embank- ment to one of the abutments of a bridge, was compared directly with the performance of a sandy gravel filling, at the other abutment. The construction of the embankments was carefully observed and records of the settlement of the road surface and the subsoil were obtained.

It was found that although the silty clay filling was particularly well compacted settlements of up to 45 mm subsequently occurred within the fill material after the road was completed. After a period of about 4~ years movement within the fill had practically ceased but settlement of the subsoil was however still continuing at a rate of about 5 mm per year due to the slow consolidation of the underlying London clay.

In general settlement within the sandy gravel filling was found to have been negli- gible and confirmed the performance previously given by this material at other sites.

ABSTRACT

Settlement behind bridge abutments. The performance of a silty clay fill in an approach embankment on the H4 motorway: [3. McLAIIEN': Ministry of Transport, RIlL Report LR 309: Crowthorne, 1969 (Road Research Laboratory). This Report describes an investiga- tion in which the performance of a silty clay fill, placed to complete the approach embank- ment to one of the abutments of a bridge, was compared directly with the performance of a sandy gravel filling, at the other abutment. The construction of the embankments was carefully observed and records of the settlement of the road surface and the subsoil were obtained.

It was found that although the s ihy clay filling was particularly well compacted settlements of up to 45 mm subsequently occurred within the fill material after the road was completed. After a period of about 4~ years movement within the fill had practically ceased but settlement of the subsoil was however still continuing at a rate of about 5 mm per year due to the slow consolidation of the underlying London clay.

In general settlement within the sandy gravel filling was found to have been negli- gible and confirmed the performance previously given by this material at other sites.