ROAD RESEARCH LABORATORY
Ministry of Transport
RRL REPORT LR 310
composer C.4 JEI.
SETTLEMENT BEHIND BRIDGE ABUTMENTS.
THE PERFORMANCE OF A UNIFORMLY-GRADED SAND FILL
IN AN APPROACH EMBANKMENT ON THE M4 MOTORWAY
by
J. E. Cross
Road Research, Laboratory,
Crowthorne, Berkshire
1970
CONTENTS
Abstract
i.
2.
.
.
.
6.
7.
Introduction
Details of the site
2.1 The bridge
2.2 Subsoil conditions
2.3 Fill materials
2.4 Construction procedure
Experimental procedure
3.I Compaction observations on the fill materials
3.2 Settlement measurements on the subsoil
3.3 Settlement measurements on the road surface
Results
4.1 Compaction of the fill materials
4.2 Settlement measurements
4.2.1 Rate of settlement
4.2.2 DistribUtiOn of settlement on the road surface
Conclusions
Acknowledgements
References
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O CROWN. COPYRIGHT 1969
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 uniformly-graded sand fill in an approach embankment on the M4 Motorway)
ABSTRACT
This Report describes an investigation which is one of a series to determine whether it is necessary to place restrictions on the types of fill material to be used in constructing approach embankments behind bridge abutments.
In this investigation the use of a uniformly- graded fine to medium sand was examined and compared with the performance of a well-graded sandy gravel. Measurements were made of the state of compaction of the fill materials and records obtained of the settle- ments of the road surface and the subsoil. After four years the settlements within both the 7 m high sand and gravel fills were negligible (of thesame order as the accuracy of measurement). However settlements of up to 40 mm were recorded in the subsoil beneath the fill materials due to consoli- dation of the underlying clay. Thishad lead to the occurrence of small differential settlements of up to 26 mm spread over a minimum length of 17 m. ~c3c ~iffu~-ential ~ettlcmcnts __~_ ~n ~mnll that th@-~4A-~_ .... quality ~f thb rcaL
The investigation showed that the uniformly- graded fine to medium sand performed as well as the well-graded sandy gravel and when well compacted, can be used as a satisfactory fill material behind bridge abutments.
i. INTRODUCTION
As part of a study into the problem of the serious differential settlement which often 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. The primary object of this work is to establish whether it is necessary to place restrictions on the type of fill used behind bridge abutments when the compaction work is carried out in accordance with the standards required for mass-fill for embankments.
• This Report describes an investigation in which a uniformly-graded fine to medium sand was used at an approach embankment to an overbridge on the M4 Motorway. The performance of this sand was compared with that of sandy gravel which had previously been shown to be acceptable I.
2. DETAILS OF THE SITE
2.1 The Bridge
The work was carried out at the site of an overbridge which carries Sutton Lane (unclassified road) over the M4 Motorway near Slough. The location of the site is shown in Fig. I. Relevant details concerning the construction of the bridge are given in Fig. 2.
The bridge has 'open' abutments comprising three vertical columns founded on 0.6 m diameter bored piles. The deck is supported on trans- verse beams capping the columns and overhangs the abutment-fill area as can be seen in Plate i.
At each end of the bridge the transition from the bridge deck to the wedge filling was 'bridged' by a three-metre-long slab of reinforced structural concrete.
2.2 Subsoil conditions
Underneath the whole site, at depths of between 4 and 5 m, there is a considerable thickness of London Clay. Overlying the clay is sandy gravel between 2.5 and 4.5 m thick, forming part of the Thames Valley alluvial deposits. The gravel is overlain by up to 1.2 m of sandy-silty clay (brickearth). There was also up to 0.6 m of topsoil and made up ground which was removed prior to the commencement of filling.
2.3 Fill materials
The south abutment wedge was filled with a uniformly-graded fine to medium sand. The average grading of the sand together with the results of laboratory compaction tests, carried out on the sand prior to its use, are shown in Fig. 3. The specification for the compaction of the sand fill was based on the results of the laboratory compaction tests and a field compaction trial carried out prior to the commencement of the work. Because of the uniform grading of the sand it was decided that the M.O.T. Specification 2 then current would not be appropriate. The specification that was employed required that the sand be compacted to a minimum dry density of 1.6 Mg/m 3 (i00 ib/cu.ft.) for at least 9 out of every i0 con- secutive samples and that vibratory compaction plant be employed. No limits were specified for the moisture content as the results of laboratory and field trials indicated that the sand could be compacted to the specified minimum dry density over a fairly wide range of moisture contents.
For comparative purposes a well-graded sandy gravel was used as fill in the wedge on the north side. The average grading of the sandy gravel used, which was specified as the same as that used for Type 2 granular sub-base materials, is shown in Fig. 4. The results of laboratory com- paction tests and the average of specific gravity determinations are also shown inFig 4. The specification required that this gravel be compacted in accordance with the normal requirements of Zhe M.O.T. Specification 2 current at the time for mass earthworks (Clause 711).
2.4 Construction procedure
Prior to the completion of the bridge structure the approach embank- ments had been constructed to within 12 m of the south abutment and 16 m of the north abutment as shown in Fig. 2.
Behind the south abutment the sand fill was spread with a small bull- dozer and was compacted using a 1Mg tandem-vibrating roller and a 340 kg pedestrian-operated vibrating roller. A small vibrating-plate compactor was also used beneath the bridge when space became too restricted to use the larger machines. As the height of the fill increased the loose material on the face of the completed approach embankment was cut back and benched to avoid leaving a zone of uncompacted material between the two sections. The mean moisture content of the sand delivered to the site was approximately 7 per cent, which was drier than the optimum moisture content for compaction by the two vibrating rollers. Some additional wetting of the sand was therefore carried out by spraying the loose layers before compaction.
Behind the north abutment generally the same procedure for spreading and compacting the gravel was adopted although at times a 4 Mg towed- vibratingroller and an 8 to iO Mg smooth-wheeled roller were also used.
The earthworksat this site were carried out during September and October 1964 and the road was opened to traffic in November 1964.
3. EXPERIMENTAL PROCEDURE
During construction a team from the Laboratory was present on the site making determinations of the state of compaction, the rate of construction, thiekness~of layers and moisture content of the fill materials. Measure- ments were also made of the settlement of the subsoil and the road surface to enable settlement within the fill to be determined.
3.1 Compaction observations on the fill materials
The state of compaction of both the uniformly-graded sand and the gravel fill was determined using the sand-replacement method 3. In the south wedge a total of 137 determinations were made on the 5.100 m 3 of Compacted sand and 48 determinations were made on the 5350 m ~ of gravel used behind the north abutment. These tests were also used to provide representative samples for the determination of the grading and the specific gravity of the two fill materials.
3.2 Settlement measurements on the subsoil
Behind each abutment a three-point version of the mercury-filled settlement gauge 4 was installed at subsoil level before construction commenced. The points were located on the centre line of the road; 3 m apart on the south side of the bridge and 5 m apart on the north side as shown in Fig. 5. The settlement of these points was measured relative to datum plates which, together with the gauge indicator units, were
3
housed in metal cabinets clear of the embankments. The levels of these datum plates were checked periodically relative to two permanent datum points located each side of the motorway to the west of the bridge, well clear of any influence from the embankments. These permanent datum points consisted of 32 mm diameter free-standing mild-steel rods driven to refusal into the gravel stratum and protected by 64 mm diameter steel casings with screw caps.
3.3 Settlementmeasurements on the road surface
The movement of the road surface was recorded by levelling with an automatic precise level on metal studs which were driven into the asphalt wearing course as it wasbeing constructed and before it had hardened. These levels were related to temporary bench marks on the bridge which were in turn related to the permanent datum points. The layout of the studs at each abutment is shown in Fig. 5. These studs were levelled periodically and this data enabled the rate and distribution of settle- ment of the road surface to be determined. The settlement of the road surfacewas compared with that of the subsoil and thus the settlement within the fill materials could be calculated.
4. RESULTS
4.1 CompaCtion of the fill materials
The results of the individual dry density determinations made during the investigation are shown in, Figs. 3 and 4.. Although of the 137 determinations made on the sand 24 were below the specified lower limit, many of these results were only very slightly lower than 1.6 Mg/m 3 and in general the sand was compacted to a satisfactory state.
Fig. 4 shows that many of the dry density results obtained on the gravel fill lie above the zero air voids line and must therefore be in error. From previous work at the Laboratory S it has been shown that when making sand replacement method determinations of bulk density in granular materials with low cohesion, errors can be caused by the soil around the sample hole moving inwards during the excavation. It was therefore not possible to check the air void content effectively. However observations of the thickness of layers of fill and the number of passes of compaction plant employed and moisture content of the gravel showed that the work would have complied with the new M.O.T. methods specification 6 for compaction.
4.2 Settlement measurements
A summary of the settlements measured on the road surface and in the subsoil behind both abutments up to April 1969 (about 4 1/3 years after the construction was completed) are given in Table I.
4
TABLE 1
Summary o f s e t t l e m e n t m e a s u r e m e n t s up t o A p r i l 1969 C m i l l i m e t r e s ) *
Abutment
Height of bank
Fill material
Location of settlement measurement
S o u t h N o r t h
7 . 5 m 6 . 7 m
S a n d S a n d y g r a v e l
Settlement gauge No.
Settlement gauge No.
Settlement Of road surface Mean on bridge deck Mean on approach embankment Maximum in wedge area Mean above settlement gauge
Settlement of subsoil Before paving After paving
Total
Settlement within fill Mean above settlementgauge after paving
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30 37
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3! 41
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~3 74
- 2 1
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38 37
75
O
*N.B. Settlement figures quoted to nearest millimetre. All measurements are related to the south and north side datum points. Settlement within the fill above each gauge is calculated by simple difference.
4.2.1 Rate of Settlement. Fig. 6 shows the relations between settle- ment and time for the surface of the subsoil beneath the fill behind each abutment. The results for the three gauge points behind each abutment were very similar and for simplicity the mean values are shown on the curves. The curves show that approximately half the settlement recorded at subsoil level had taken place during the first 2-3 months before the road surfacing was laid and the rate of movement after about 4 years has become negligible.
The relations between settlement and time for the subsoil compared with the road surface since the surfacing was completed are shown in Fig. 7. The settlement of the subsoil was t~ken as the mean of the three gauge points and the settlement of the road surface as the mean value of three groups of three levelling studs directly above the gauge points.
5
The settlement within the fill materials, determined as the difference between the measurements on the road surface and on the subsoil, was so small that separate curves could not be drawn. The results obtained for each gauge location are shown numerically in Table i. Almost all the settlement recorded was due to the compression of the subsoil. The settlement within the fill materials behind both bridge abutments was less than 4 mm. (The negative values in Table 1 are not significant as they are within the accuracy of measurement.)
The rate of settlement of the transverse rows of studs on the bridge deck and on the approach embankments are shown in Fig. 8. The rates of settlement after 4 years were very similar in all cases and did not differ significantly from those shown in Fig. 7. Movement of the road surface, bridge and approach embankments is occurring at a rate of approximately 3 ram per year and is probably due to the slow consolidation of the underlying clay.
4.2.2 DistribUtiOn of Settlement Of the road surface. Measurements on the transverse rows of studs on the bridge deck and the embankments showed that there was no significant variation in settlement with respect to transverseposition across the road. This indicates that the measurements were not affected by compression in the surfacing materials and that traffic loading had no influence on the settlement of the fill materials.
The longitudinal profiles in Fig. 9 show the settlement of the road surface as a mean figure obtained from the three longitudinal rows of studs behind each abutment relative to the site datum points. Also shown is the subsoil settlement recorded on the three settlement gauge points behind each abutment in their relative positions. The maximum differential settlement recorded on the road surface on the approach to the south abutment was 26 mm over a minimum distance of 17 m. On the north side the measure- ment was 22 mm over a minimum distance of 20 m.
Thus the settlements relative to the bridge structure were relatively small and produced a similar long shallow depression at each side of the
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S. CONCLUSIONS
The results show that a uniformly-graded sand, when compacted to a standard corresponding to that likely to be produced by the current M.O.T. methods specification 6, performed as well as a well-compacted, well-graded sandy gravel.
The results of this investigation also show that it would be un- necessary to import high quality granular material, which may be difficult and expensive to obtain, when uniformly-graded sand is available for use as filling behind bridge abutments.
Almos t a l l t h e s e t t l e m e n t r e c o r d e d was due t o t h e c o m p r e s s i o n o f t h e s u b s o i l b e n e a t h t h e embankments and t h e b r i d g e s t r u c t u r e . m o v ~ th~ erder ef 26 mm ~:hcn cpr~A ^,,÷ cvcr a !en~th of 17 m ~ ~-aplga~-ent effcct on--the ridi~g__~ty of the road,.
6
6. ACKNOWLEDGEMENTS
This r epo r t was p repared in the E a r t h w o r k s a n d Foundat ions S e c t i o n of t h e Cons t ruc t ion D i v i s i o n . The work-of the Sec t i on i s d i r e c t e d by Mr. W.A. Lewis and t h i s i n v e s t i g a t i o n forms par t o f the r e s e a r c h programme o f the S t a b i l i t y of Earthworks Group which w a s u n d e r t h e s u p e r v i s i o n of Mr. G. Margason. The au thor was a s s i s t e d in t h i s i n v e s t i g a t i o n by Mr. A . I . Booth.
Thanks<are due to the Resident~iEngineer and s i t e s t a f f of the Consu l t ing Engineers , S i r A lexanderGibb and P a r t n e r s , f o r t h e i r co- o p e r a t i o n du r ing the work d e s c r i b e d i n t h i s r e p o r t .
7. REFERENCES
i.
.
.
.
.
MARGASON, G. A study of the settlements at a number of bridge approaches on the Maidenhead By-pass, Chart. Min. Engr., 1963, 9 0 , 1 3 4 - 1 3 8 . . . . . . . .
MINISTRY OF TRANSPORT. Specification for road and bridge works. London, 1963 (H.M, Stationery Office).
BRITISH STANDARDS INSTITUTION. Methods of testing soils for civil engineering purposes. BritiShStandard1377:1967. London, 1967 (The I n s t i t u t i o n ) . ~
IRWIN, M.J. A m e r c u r y - f i l l e d gauge fo r measuring the s e t t l e m e n t o f f ounda t i ons . M i n i s t r y of Transpor t , Road Research Labora tory , Report LR62. Crowthorne 1967:o (Road Research Laboratory).
LEWIS, W.A. and PARSONS, A.W. An analysis of the states of compaction measured during the Construction of embankments in six major road schemes. LaboratoryNote No. 399/WAL.AWP. 1963 (Unpublished).
. MINISTRY OF TRANSPORT. Specification for road and bridge works. London, 1969 (H.M. Stationery Office).
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ABSTRACT
Settlement behind bridge abutments. The performance of a uniformly-graded sand fill in an approach embankment on the M4 Motorway: J. E. CROSS: Ministry of Transport, RRL Report LR 310: Crowthorne, 1969 (Road Research Laboratory). This Report descr ibes an investigation which is one of a series to determine whether it is necessary to place restrictions on the types of fill material to be used in constructing approach embankments behind bridge abutments.
In this investigation the use of a uniformly-graded fine to medium sand was examined and compared with the performance of a well-graded sandy gravel. Measurements were made of the state of compaction of the fill materials and records obtained of the set t le- ments of the road surface and the subsoil. After four years the se t t lements within both the 7 m high sand and gravel fills were negligible (of the same order as the accuracy of measurement). However sett lements of up to 40 mm were recorded in the subsoil beneath the fill materials due to consolidation of the underlying clay. This had lead to the occur- rence of small differential sett lements of up to 26 mm spread over a minimum length of
. . . . . . . . . 1 "~ . . . ~,C ~.~,~ 17 m. T ~ e l e n t i a l settleinent~ . . . . . . o,.,~..--^11 ~l.~..~. ~h~..~ ..~...":'~:-g '~ . . . . . 7 . . . . ,,. r e . a c t ~ m~,-gee/r-aff'ee tcd.
The investigation showed that the uniformly-graded fine to medium sand performed as well as the well-graded sandy gravel and when well compacted, can be used as a satis- factory fill material behind abutments.
ABSTRACT
Settlement behind bridge abutments. The performance of a uniformly-graded sand fill in an approach embankment on the H4 Hotorway: J. E. CROSS: Ministry of Transport, RRL Report L,q 310: Crowthorne, 1969 (Road Research Laboratory). This Report describes an investigation which is one of a series to determine whether it is necessary to place restrictions on the types of fill material to be used in constructing approach embankments behind bridge abutments.
In this investigation the use of a uniformly-graded fine to medium sand was examined and compared with the performance of a well-graded sandy gravel. Measurements were made of the state of compaction of the fill materials and records obtained of the sett le- ments of the road surface and the subsoil. After four years the set t lements within both the 7 m high sand and gravel fills were negligible (of the same order as the accuracy of measurement). However settlements of up to 40 mm were recorded in the subsoil beneath the fill materials due to consolidation of the underlying clay. This had lead to the occur- rence of small differential settlements of up to 26 mm spread over a minimum length of
The investigation showed that the uniformly-graded fine to medium sand performed as well as the well-graded sandy gravel and when well compacted, can be used as a satis- factory fill material behind abutments.
