204280803-Settlement-of-Embankments-on-Soft-Ground.pdf

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Proc. Instn Ciu. Engrs, Part 2, 1975, 59, Dec., 571-593

7877 Settlement and stability of embankments constructed

on soft alluvial soils

This Paper describes the results of an extensive programme of research into the problem of predicting the magnitude and rate of settlement of embankments constructed on compressible subsoils. Field measurements have been obtained from a number of sites. These results have been compared with predictions based on conventional consolidation theory and on a multi-layer analysis using compressibility parameters determined from measurements of the in situ permeability of the subsoil. The problem of stability control is also considered and an approach which has been developed and used successfully at several sites is described. A brief outline is given of some construction expedients for countering problems of settlement and stability and an assessment is made of the cost effectiveness of drainage consolidation techniques.

Introduction Although traditionally road builders have always tried to keep the line of the highway on well drained stable ground, many constraints now restrict the freedom of choice of engineers. As a result, highway engineers are often faced with the necessity of having to site lengths of major new road on poor ground where deposits of soft fine-grained or organic soils exist to considerable depths.

2. Difficult subsoil conditions are frequently encountered near major rivers or estuaries where recent deposits of alluvial soils are common. How- ever, soft clays and silt deposits may be encountered in many areas of the UK. During the past decade about one third of the major motorway schemes involved the construction of significant lengths of road over compressible soils. Serious problems of settlement and instability have had to be overcome, necessitating considerable expenditure in some cases. During the design stage of a major road scheme where compressible soils are likely to be encountered in the subsoil the engineer needs to have reliable information on the

for publication in Proceedings, Part 2. Ordinarymeeting 3 February. 1976, at 5.30p.m. Written discussioncloses 17 February, 1976.

Q Crown copyright. * Transport and Road Research Laboratory, Crowthorne.

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likely magnitude and rate of settlement of the embankments and on the possible development of instability in the subsoil arising from the stresses produced by the weight of the embankments. Armed with this information the engineer can take steps to minimize the effects of the problems. Unreliable information can lead to the adoption of unnecessary and costly expedients or to large claims from contractors who have to cope with unexpected difficulties during the construction work.

3. Since about 1950 the Transport and Road Research Laboratory (TRRL) has been carrying out a comprehensive programme of research into many of the problems posed by road construction on compressible soils. Improved procedures for the prediction of settlement and the control of instability have been developed. A considerable amount of factual information has been obtained from field studies, including the value of some practical techniques for accelerating or minimizing settlement and of controlling the rate of construction to avoid the occurrence of instability. This Paper discusses the problems generally in the light of the TRRL's research.

Sites of full-scale settlement studies 4. The earliest full-scale investigation made by the TRRL was started in

1938 and it concerned studies on chalk and clay embankments on the Mickle- ham, Caterham, Crawley, Winchester and Leatherhead bypasses.l However, these studies were of a fairly simple nature and did not involve any measurements of the compressibility properties of the constituent materials. The first settlement investigation in which detailed measurements were made of the properties of the subsoil was carried out in 1951 at Lackford, West S ~ f f o l k . ~ This was followed in 1952 by a study of the effect of vertical sand drains on the settlement of a road embankment at Leigh on the Thames E s t ~ a r y . ~ Although these two investigations provided a certain amount of useful information it was clear that a more comprehensive programme of field studies was necessary if reliable guidance was to be provided to engineers on the general problem of settlement analysis.

5. With this in mind the opportunity was taken wherever possible, as the programme of new road construction developed, to carry out detailed field settlement studies. The sites were located on the routes of proposed new road works where the ground conditions were particularly poor and where significant settlement was likely to O C C U ~ . ~ - ~ The subsoils at these sites were invari- ably alluvial deposits of recent origin and although the soils varied widely in terms of their texture, plasticity and engineering properties they can generally be described as soft silty clays and clayey silts. Occasionally silt and sand partings and silt inclusions were present together with a considerable amount of organic material. Peat layers were also encountered at several sites.

6. Following the selection of an area for an experiment, a detailed site investigation was carried out to est.ablish the soil profile and to obtain samples for laboratory testing. Table 1 gives the soil descriptions and thicknesses of the main soil layers at the sites.

Laboratory tests on subsoil materials 7. A detailed programme of laboratory tests was carried out on samples

from the main layers of the subsoil at each site. The consolidation properties 572


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LEWIS, MURRAY AND SYMONS

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Fig. 1. Apparatus for recording automatically porewater pressures

were determined from ‘undisturbed’ samples which, in the earlier studies, were obtained using a U4 type sampler. In the later studies a I00 mm dia. piston sampler was used to reduce sample disturbance. Disturbed samples, for classification purposes, were obtained using a Swedish-type peat sampling auger.

8. The consolidation tests were made on 75 mm dia. samples, initially 25 mm thick, and at least five repeat tests were carried out on each soil type to establish the compressibility properties. For those sites where there was a likelihood of significant horizontal drainage occurring, specimens were

tube to allow the vertical and horizontal Coefficients of consolidation to be prepared for testing both concentric and normal to the axis of the sampling

determined. 9. The laboratory consolidation test is time-consuming both in making the

measurements and processing the results. As an extensive programme of laboratory testing was involved a system was developed for logging the data automatically* and a computer programs was used to evaluate the data. Apart from reducing the staff effort involved, this technique produced more consistent results by avoiding the variability introduced by different laboratory technicians in their interpretation of the data

main soil strata at each of the sites, obtained from the laboratory tests, are 10. The coefficients of consolidation and volume compressibility for the

included in Table 1. The values given correspond to the mean of the initial and final effective strews at the centre of each layer for the field loading conditions.

11. It was expected that many of the soils, particularly those with a high organic content, would exhibit significant amounts of secondary consolidation. To enable an assessment to be made of the rate of secondary consoliddion.

for a number of days. From these tests the average coefficients of secondary tests were performed in which each increment of load was maintained constant

consolidation for the full depth of the compressible strata were estimated (Table 1). 574


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Instrumentation 12. At each site a comprehensive system of gauges was installed to measure

the settlement of the compressible layers and the changes in the porewater pressure. Generally the mercury filled gaugelO developed by the TRRL was used for measuring settlement but rod gauges were also used either on their own or to provide a check on the mercury gauges. The advantage of the mercury gauge is that its installation does not interfere with the construction of the embankment and it is less liable to be damaged by construction plant than the more conventional rod gauge. However, considerable care is necessary in both the manufacture and installation of the mercury gauge if a satisfactory performance is to be obtained.

13. The piezometers used for porewater pressure measurements were of a fairly conventional type. They consisted of a 30 mm dia. filter candle, about 80 mm long with two tubes connecting to the piezometer board located in a gauge house near the embankment site. The filter candle was usually installed at the desired depth in a borehole and was surrounded by sand to improve the response. The borehole above the piezometer was sealed with bentonite.

14. It was rather time-consuming to obtain the measurements of porewater pressure using the conventional manometers and apparatus was therefore developed for recording this information a~tomatical ly .~~ This equipment (Fig. 1) was used successfully for several field studies at a stage when staff were not required permanently on site; it proved to be of considerable value in reducing staff effort.

15. At Tickton and Over Causeway measurements were also made of the vertical pressure generated in the subsoil and the lateral movements which

Topsoil Chalk

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Horizontal and verttcal uu

Fig. 2. Cross-section of embankment at Tickton showing location of instruments

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occurred. The vertical pressures were measured using the hydraulic gaugell developed by the TRRL and the lateral movements were obtained by means of an inclinometer12 with its associated tubing installed vertically through the compressible subsoil.

16. A cross-section through the embankment and subsoil at Tickton which shows a typical arrangement of instruments is given in Fig. 2.

In situ permeability testing 17. There is a growing amount of evidence that the consolidation para-

meters determined from small-scale laboratory tests are frequently very different from the values obtained on the basis of an assessment of the settlement behaviour of embankments; These latter values are usually larger and may exceed the laboratory test coefficients by a factor of several hundred.13 The main cause of such differences is probably because the natural drainage structure of the soil is not properly assessed by testing a small sample. Moreover, disturbance of the soil during sampling may further reduce its natural permeability. In situ permeability tests were therefore carried out on piezometers located in the main layers of the subsoil at each site to obtain a more realistic assessment of the drainage characteristics of the compressible strata.

18. The in situ permeability test is performed by establishing the relation between time and volumetric rate of flow into or out of a piezometer while a small differential pressure is maintained between the piezometer probe and

Fig. 3. Apparatus for recording automatically data from in situ permeability tests

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surrounding soil. The measurements were made using apparatusll developed at the TRRL for recording these values automatically (Fig. 3) and the results were evaluated according to the method proposed by Gibson.14

19. Using these coefficients of in situ permeability together with the coefficients of volume compressibility from laboratory tests the ‘field’ values of the coefficients of consolidation were obtained (Table 1).

Calculation of settlement

embankment is made up of four components 20. The settlement of the foundation beneath a loaded area such as an

(a) movement arising from the elastic compression of the subsoil (this may be a combination of vertical and horizontal movement)

(b) movement arising from primary consolidation, i.e. the expulsion of the porewater from the layers of subsoil under the action of sustained loading (this may be a combination of vertical and horizontal consolidation)

(c) movement arising from secondary consolidation which is not associated with the dissipation of porewater pressure developed by the loading

( d ) movement arising from plastic deformation as a result of regions in tbe subsoil approaching a state of failure.

21. The normal method of calculating ultimate settlement and the rate of movement is well established as far as primary consolidation is concerned and is based on Terzaghi’s theory of consolidation. The assessment of secondary consolidation is usually based on a semi-empirical approach first proposed by Buisman.15 From studies of long-term settlement behaviour it was observed that, after excess porewater pressures were largely dissipated, the movements generally continued at a rate which produced a linear relation between settlement and the logarithm of time. This led to the development of a simplified method whereby the linear relations obtained from long-term laboratory consolidation tests are used to estimate the rate of secondary movement. The usual procedure in performing the calculations is to assume that the secondary movements start when 90% of primary consolidation is complete.

22. The assessment of plastic deformation is complex and has not yet been fully resolved. Therefore no allowance has been made for it in the analyses of the field studies.

23. In analysing the settlement of road embankments the calculations will usually be based on two-dimensional theory as the lengths of such embankments are normally much greater than their width or the depth of compressible strata. However, occasionally a simplified one-dimensional settlement analysis may be possible for locations beneath the centre of the embankment if the depth of compressible material is relatively shallow compared with the other dimensions.

24. Computer programs16-18 have been developed at the TRRL to analyse problems of one-dimensional and two-dimensional settlement for multi-layer soil systems. These programs take account of the effects of multi-stage construction, variations of the consolidation parameters with effective stress, non- linear relations between stress and strain and the influence of secondary consolidation.

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€ 6 = ISurfaclng and road - 4 -

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Fig. 4. Oxford Southern and Western bypasses ; recorded settlement in area without sand drains compared with the calculated relations between settlement and time using coefficients of consolidation from both laboratory and in situ tests

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Fig. 5. Donnington Bridge site; recorded settlement compared with the calculated relations between settlement and time using coefficients of consolidation from both laboratory and in situ tests

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SETTLEMENT OF EMBANKMENTS O N ALLUVIAL S O I L S

E - - 10-

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Settlement prediction with surcharge/ --- - _. - I I I I I I F

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Fig. 6. Datchet ; recorded settlement compared with the calculated relations between settlement and time both with and without surcharge

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Fig. 7. Oxford Southern bypass extension ; recorded settlement compared with the calculated relations between settlement and time using coefficients of consolidation from both laboratory and in situ tests

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25. In addition to the programs providing the relations between settlement and time for selected locations on the ground surface, the values of excess porewater pressure at specified points in the subsoil beneath the embankment and beyond the toe are also given at pre-selected intervals of time.

Comparisons between predicted and measured settlements 26. Comprehensive records of settlement were obtained from the sites

during construction and for a number of years afterwards. The settlement with time of the ground surface beneath the centre of the embankment at these sites is shown in Figs 4-10 together with the variation in construction loading. Also shown in the figures are the calculated relations between settlement and time based on the laboratory coefficients of consolidation and the conventional method of settlement analysis. The comparisons between the calculated and measured relations show that in most cases the rates of settlement have been seriously underestimated although, in general, the magnitudes of the predicted ultimate settlements were in reasonably close agreement with the values measured at the sites.

27. Very reasonable assessments of the rates of settlement were obtained from the use of field coefficients of consolidation together with the multi-layer consolidation analysis described in 0 24. Detailed analysis of the results has indicated that the use of more realistic consolidation parameters determined using the in situ permeability measurements had the greater influence on the results and that the computer program for multi-layer analysis contributed

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Fig. 8. Avonrnouth ; recorded settlement compared with the calculated relations between settlement and time using coefficients of consolidation from both laboratory and in situ tests

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E 151 I i5I , ~ - , I e 10

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Fig. 9. Tickton ; recorded settlement compared with the calculated relations between settlement and time using coefficients of consolidation from both laboratory and in situ tests

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Fig. 10. Over Causeway ; recorded settlement compared with the calculated relations between settlement and time using coefficients of consolidation from both laboratory and in situ tests

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only a relatively small improvement in the prediction. However, for stability calculations detailed information on the magnitude and distribution of excess porewater pressure is also a requirement of consolidation analyses and the computer program makes a valuable contribution in this area. This is illus- trated by the comparison between the predicted and measured vertical distributions of excess porewater pressure beneath the centre of the embankment at the site near Avonmouth (Fig. 11). Although there is some disparity between

Excess porewater pressure m of water

Fig. 11. Avonmouth ; comparison of recorded and theoretical vertical distributions of porewater pressure (predicted using multi-layer consolidation analysis with coefficients of consolidation from in situ measurements)

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SETTLEMENT O F E M B A N K M E N T S ON A L L U V I A L S O I L S

the two profiles, the general form of the vertical distributions is the same and the differences are most likely to be the result of minor anomalies between the assumed and actual external boundary conditions.

28. As suggested in 0 17, the disparity between the field behaviour and that predicted solely on the basis of the data from laboratory tests may well be due to the presence of a natural drainage structure in the soil which is destroyed in sampling or unrepresented in the small-scale laboratory test. This hypothesis is further substantiated by the comparisons between the rates of settlement at two other of the TRRL's field sites where vertical sand drains were installed in the s u b ~ o i l . ~ . ~ At both of these sites a section of embankment was also constructed without sand drains to allow their performance to be assessed. The drains should have produced a considerable reduction in the lengths of drainage path and should have greatly improved the rates of settlement. In practice, no significant difference was obtained between the rates of settlement in the areas with and without sand drains (Fig. 12). These results suggest that a

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Fig. 12. Relations between rate of movement and time in areas with and without sand drains at Leigh and Oxford

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natural drainage structure was already present in the subsoil which provided a much shorter drainage path than was offered by the sand drain installation.

29. A point of particular importance concerning the analyses is that in most cases the use of the 'field' coefficients of consolidation predicted that primary consolidation would be largely completed within the construction period and this was in line with what actually occurred. These results are in direct conflict with the predictions obtained by conventional methods which suggested that significant primary settlements would occur after the completion of the works. Thus these latter predictions might well have led to the adoption of alternative and more expensive methods of construction such as viaducts or the use of sand or wick drains.

Problem of stability control 30. Apart from the question of settlement prediction and the problem of

devising methods of minimizing the effects of settlement, engineers may also be faced with the possibility that instability could develop in the subsoil if the materials have insufficient strength to carry the stresses set up by the construction of the embankment. Although the layers in the subsoil will gain in strength with the progress of consolidation, the rate of increase in strength might be less than that of the shearing stresses generated by the loading and if no regard is paid to this a serious stability failure could develop. Failures of this type are usually extremely costly to remedy and can involve a major dis- ruption of the construction work. Engineers engaged in major road construction work are familiar with some of the well publicized examples of this type of problem.

31. One of the simplest methods of dealing with the question of possible foundation instability is to reduce the stresses generated in the foundation by employing lightweight fill. Fly ash is most commonly used for this in the UK, but other materials such as sawdust have been used in other countries where the need for stability is more important than the minimization of settlement. Although fly ash is very effective as it can reduce the stresses by a half or more compared with normal fill, the material can be expensive if it has to be hauled a long way to the site. Apart from using lightweight fill, flatter side slopes and the use of berms might provide a solution but these procedures might not always be possible and are likely to involve significant extra expenditure and more land acquisition.

32. An alternative approach to the problem of avoiding instability is to construct the embankment sufficiently slowly for the increase in the strength of the subsoil to more than balance the increase in the shear stresses. This approach is likely to provide an economic solution in many cases but necessi- tates close control over the rate of construction of the embankment if failure is to be avoided. Although this idea has been used by engineers for many years the TRRL considered that it was very desirable for an approach having an acceptable theoretical basis to be developed which site engineers could specify and employ with confidence.

33. The procedure developed by the TRRL6e7*19 involves a stability analysis and the monitoring of the porewater pressures in the subsoil. By the use of control charts prepared in advance for the particular embankment the factor of safety can be read off by the site engineers at any given time. Construction 584


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Embankment height

Days after construction

Depth of failure 3m below ground l e 4

1 I I I I I I 20 30 40 50 M) 70 80

Dlssipation: Y0

Fig. 13. Stability chart for circular failures

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can then be halted if the factor of safety falls to an unacceptable value. A more comprehensive description of this technique and its application in practice is given in reference 20.

34. To prepare control charts the cross-section of the embankment is divided into a number of zones to allow the distribution of porewater pressure and the variation in soil properties to be defined adequately. Stability analyses are then carried out for a range of embankment heights and for at least three conditions of porewater pressure distribution in the subsoil for each height considered. The results of the analyses can be presented in chart form relating minimum factor of safety for different embankment heights to the porewater pressure condition in the subsoil (Fig. 13).

35. The method will usually involve embankment construction in stages. During the first stage the work can be allowed to proceed at an uncontrolled rate to the safe height calculated on the basis of total stress analysis using undrained strength values from field and laboratory tests. Measurements of porewater pressure in the subsoil are made frequently during construction and in the pause following the end of the first stage of construction. Using the measurements of porewater pressure, the assumptions made in the preparation of the control charts are checked and, if necessary, revised charts are prepared relating the increment of excess porewater pressure to the factor of safety for the second stage. Some control over the rate of placement of the fill will usually be required during this stage of construction.

36. The procedure of monitoring the porewater pressure during the first stage when the rate of construction is uncontrolled allows a more realistic assessment to be made of the response to loading, distribution and dissipation of porewater pressures than would normally be possible on a theoretical basis. The use of stage construction has the advantage that the requirement for control can be easily defined and specified.

Reliability of method 37. As the factor of safety can only be known explicitly when a stability

failure occurs, the reliability of the approach cannot be validated for stable embankments although other observations made at the site might provide some qualitative information supporting the analytical results. At the Over Causeway bypass site, for example, construction was halted when the embankment had reached a height of 8.4 m as the factor of safety obtained from the control charts was 1-25. Corresponding to this stage a significant increase in the rate of lateral movement occurred (Fig. 14) and this behaviour substantiated the stability analysis which indicated that the foundation was approaching a state of limiting equilibrium.

Application of the method of stability control to a road embankment constructed near Tickton, Yorkshire

38. An illustration of the method is given by its application to the construction of one approach embankment to a bridge over the River Hull near Tickton, Yorkshire. The embankment formed part of a new bypass (A1035) which traversed areas of weak subsoil beside the river.6

39. The subsoil conditions at the site consisted of a 7.3 m thick layer of soft and compressible alluvium overlying chalk. The embankment was 586


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SETTLEMENT OF EMBANKMENTS ON ALLUVIAL SOILS

October 1970 September 1971 15. I

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Time after start of construction: days

Fig. 14. Maximum lateral movement related to time beneath toe of embankment

constructed in two main stages to a final height of 6.5 m. Fig. 2 shows the cross-section giving the location of the instrumentation and the soil profile.

40. The control chart which was used at the scheme was based on predictions of excess porewater pressure which corresponded to the increments in vertical stress. In carrying out the analysis it was assumed that consolidation would occur by vertical drainage to ground surface and to the free draining chalk layer located at the base of the soft alluvium.

41. During the first stage of construction, the measurements of porewater pressure showed that the assumptions used in the preparation of the stability chart led to an overestimate of the porewater pressure in the upper layers of the soft alluvium and a corresponding underestimate in the lower layers. Moreover, the predicted values generally overestimated the porewater pressures in the subsoil beneath the side slopes of the embankment. A reassess- ment of the stability was therefore made when the embankment had reached a height of 3.2 m using distributions of porewater pressure in the subsoil which were based on the measured values. For shallow depths, where the analysis indicated that failure was most likely to occur, the reassessed factors of safety were within 10% of the values given by the stability chart. The factors of safety at different times during and immediately following the completion of construction are shown in Fig. 13.

42. The use of control charts based on effective stress analysis used in conjunction with field measurements of porewater pressure provided a rapid method of assessing stability at the site. Construction of the embankment was therefore able to proceed at a rate consistent with its stability and without the need for the adoption of more expensive construction expedients.

Construction expedients for countering problems of settlements and stability

43. Frequently even a realistic assessment of the settlement of an embankment may indicate that the duration of the consolidation process will extend

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beyond the construction period and that significant movements of the completed road structure are likely to occur. Provided that these movements are relatively uniform over the length of the road its riding quality will not be adversely affected and preventative action will be unnecessary. However, if significant differential settlements are expected after completion of the road pavement, say in excess of 25 mm over a 10 m length, an alternative construction technique will usually be required to alleviate the problem. The methods which are most commonly adopted are

(a) excavation of the compressible material including the use of bog blasting techniques

(6) reduction in the settlement by using lightweight fill (c) acceleration of the settlement by the use of surcharge, vertical or hori-

Frequently a combination of two or more of these processes may provide the most economic solution for a given situation. For example, the use of lightweight fill for the main embankments combined with heavier fill to act as surcharge could lead to a significant reduction in the quantity required of this latter material.

zontal drains, or dynamic methods of consolidation.

Excavation 44. Excavation of the compressible material has great appeal as problems

of settlement and stability are virtually eliminated. However, even with relatively shallow deposits such an approach can be extremely costly, particularly if the excavation has to be supported with sheet piling and if, as is often the case, the high groundwater levels necessitate extensive pumping operations to permit the deposition of normal fill in the dry. Moreover, if only those areas where the foundations are considered most troublesome are excavated the problem of differential settlement may be accentuated.

45. Although not always practicable, a more satisfactory procedure would be to excavate only some of the compressible material in the deeper pockets to achieve, as nearly as possible, uniformity in the settlement of the foundation. It is always desirable to eliminate abrupt changes in the thickness of the compressible material by tapering the excavation as necessary to produce a smooth transition of settlement. This method of tapering the excavation could also be of value in contending with the difficulties of approach embankments to structures. Here, problems of differential settlement are likely to be particularly acute because of the usual practice of founding such structures on piles taken through the compressible layers to relatively stiff substrata while the adjoining embankments are founded directly on the soft subsoil. Other techniques which might assist in alleviating the settlement problems in these situations involve the use of transition slabs between the bridge deck and embankment or the installation of additional piles to support the approach embankments directly adjacent to the structure. By increasing the spacing of the piles, or reducing their length, the amount of support given to the approach embankment is gradually diminished until it merges with the unsupported main embankment. Another useful technique adopted sometimes in the UK and often in other countries is to seat the approach spans to the bridge deck directly on the ends of the embankment. 588


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Use of surcharge 46. Provided no problems of stability are introduced, the use of surcharge

will often produce the cheapest and most effective method of accelerating settlement, particularly for low embankments. Although the removal of the surplus material at the conclusion of the work must be included in any assessment of cost, this can be reduced by arranging at the planning stage to use the surplus fill for some other part of the works which can be left to a late stage in the construction programme.

47. In principle the method of surcharging consists of adding a sufficient quantity of extra fill to attain, during the construction period, the predicted ultimate settlement for the required final height of embankment and road pavement. On reaching the desired settlement the excess fill material is removed. Care should be taken with certain soils to ensure that, in relation to the final loading conditions, they are not overconsolidated by the surcharge otherwise there is a danger that swelling may occur which could affect the riding quality of the road in the long term. The technique of surcharge was used by the TRRL at Datchet and the settlement-time relation showing the influence of the surcharge is given in Fig. 6.

Use of sand drains 48. Considerable success is often claimed for the technique of accelerating

the consolidation process by means of vertical sand or wick drains installed through the compressible strata. However, as these apparent successes were based largely on comparisons between the measured rate of settlement and the rate predicted by conventional methods, such claims may not be valid. As discussed in 0 28, studies by the TRRL at two sites indicated that the rates of settlement were not significantly increased by the sand drain installation used.

49. A factor which may have a considerable influence on the performance of sand drains is the amount of disturbance to the soil produced by the installation techniques. Such disturbance has an adverse effect on the drainage properties of the soil and may well counteract any benefit which would otherwise occur from the installation.

50. Sand drains have occasionally been used in attempts to improve the rate of consolidation of peat but they are unlikely to be of much value with these soils. Primary settlements usually occur quickly because of the normal relatively high permeability of peats and much of the settlement will arise from long-term secondary consolidation not associated with the dissipation of porewater pressure. Secondary consolidation is probably associated with the reorientation of soil particles and drains will be of little practical benefit in aiding this process. A further factor is that the continuity of drains in peat is likely to be destroyed as a result of the large deformations which frequently occur. 51. To overcome some of the disadvantages of the original types of sand

drain system, several new forms of drain and techniques of installation have been developed in recent years. These include cardboard wick and sand wick drains.

52. The cardboard wick drain as first described by Kjellman in 1948a1 has a rectangular cross-section of 100 mm by 3 mm with longitudinal drainage

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channels preformed internally. For design purposes the drain is considered equivalent to a sand column of 50 mm dia. The method of installing wick drains is highly mechanized and this has led to rapid installation and relatively low costs. During installation the drain is enclosed within a mandrel which is driven into the subsoil. On extraction of the mandrel the wick remains and it is claimed that the inward collapse of the displaced soil has a beneficial effect in reducing problems of smearing. Other advantages over conventional sand drain installations claimed for this system, in addition to that of low cost, are that vertical continuity of the drains is likely to be better maintained and, because of the very close spacing which can be achieved, a greater degree of acceleration of consolidation is possible.

53. The sand wick drain consists of a preformed sand-filled sleeve about 65 mm in diameter, made of water permeable material. The advantages claimed for this are that only relatively small diameter boreholes are needed and that by containing the sand in a sleeve the continuity of the drain is maintained even when large deformations occur within the subsoil.

Horizontal drains 54. At a road scheme constructed recently a method involving the use of

deep land drains has been used for accelerating the consolidation process.2a The technique involved the use of a machine, originally developed for groundwater lowering operations, which could excavate a trench up to 6 m in depth by 0.23 m wide and backfill it with suitable drainage material in a continuous operation.

Use of dynamic methods of consolidation 55. A dynamic method of consolidation whereby a large mass (8-40 Mg)

is dropped in free fall from heights of 6-30 m has recently been introduced for accelerating con~olidat ion.~~ In soft ground conditions the site has first to be covered with a 1 m layer of hardcore or crushed rock before the dynamic consolidation process is applied. It is claimed that the large local deformations induced in the hardcore and underlying soil by the dropping weight result in locked-in stresses equivalent to a surcharge effect. Moreover, as a result of the shock waves and large dynamic stresses which are produced, partial lique- faction of the soil occurs and cracks and fissures are created through which the excess porewater pressures can dissipate more quickly.

56. Cycles of the tamping process are carried out at regular intervals over the area of poor ground until a stage is reached when in situ shear strength and compressibility tests confirm that the subsoil can support the weight of the proposed structure without a danger of instability or excessive differential settlement. A variant of this method has also been used in the USSR where explosive charges set off in the ground replace the dropping weight process.

Cost effectiveness of accelerating settlement by drainage and dynamic methods 57. Although the relative costs of the various techniques will clearly depend

on the size and nature of the work and on local conditions, it is interesting to compare the costs for the use of the various drainage and dynamic consolidation techniques described. To provide information on this, the TRRL obtained prices from contractors for the use of the techniques on a hypothetical

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S E T T L E M E N T OF E M B A N K M E N T S O N A L L U V I A L S O I L S

0 Wator-@ted 225mm dla. sand b i n s

0 Land drains 225mm wide trench (up to 6 4

A Cardboard wick drams (up to 20m)

A Sand wick drains 65mm dia. Dlsplacemenl drwn by vibration

0 hgored sand drains 300m dia. without casing

H Dynamic consolidation

Depth of soikm

Fig. 15. Relations between cost of treatment and depth of soil comparing the most economic methods of accelerating consolidation

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road embankment 1 km long where the object was to improve the ground such that 90% of the primary consolidation would be achieved within one year of the completion of the embankment. A range of different thicknesses of subsoil was considered in the analysis.

58. Although this study may be considered as being rather artificial, particularly in view of the assumption that the methods are equally effective, the results (Fig. 15) reveal some interesting points. Augered sand drains and sand wick drains appear to be the most expensive of the techniques, particularly where deeper deposits are involved. Lowest prices were obtained for the jetted drains and the dynamic consolidation process. However, as the effectivenesses of the individual processes have been assumed to be equal in the study, the figures should only be taken as indicating general trends.

Conclusions 59. Road embankments constructed over recent deposits of alluvial soils

are likely to induce significant settlements even with comparatively low embankments. Although the conventional method of settlement analysis will provide a realistic assessment of the magnitude of the settlement of such soils, it is unreliable for predicting the rates of movement and usually over- estimates the time required for these movements to be completed.

60. Satisfactory estimates of the rates of settlement at the sites of the full- scale studies were obtained from multi-layer settlement analysis using ‘field’ coefficients of consolidation obtained from measurements of in situ permeability. The use of more realistic consolidation parameters was the most important factor in obtaining the improved predictions.

61. Stability charts based on effective stress analysis and used in conjunction with field measurements of porewater pressure provided a convenient method of controlling construction to avoid instability. Although the safety factor against instability can be known explicitly only when failure occurs, the reliability of the assessments was supported by the measurements of lateral movement which showed significant increases in the rates of movement corresponding to low values of the factors of safety obtained from the stability charts.

62. Various methods are now available for accelerating the consolidation process or for alleviating the problem in some other way. A cost effectiveness study indicates that local site conditions largely govern which method is likely to provide the most economic solution. Generally most of the movements at the sites of the full-scale studies were completed during or shortly after the construction period and, in view of the considerable expenditure involved in using any of these expedients, it is particularly important to ensure that the need for such techniques is established on a realistic basis.

Acknowledgements 63. The Authors gratefully acknowledge the co-operation of the many local

and county highway authorities involved in the studies. They also acknowledge the assistance of their colleagues in the execution of the work described in this Paper which is published by permission of the Director of the Transport and Road Research Laboratory. Any views expressed are not necessarily those of the Department of the Environment. 592


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References 1. LEWIS W. A. An investigation of the settlement of chalk and clay embankments.

Highw., Bdges & Aerodromes, 1947, 14, No. 698, 10-11; 1947, 14, No. 699, 4, 6, 14.

2. LEWIS W. A. The settlement of the approach embankments to a new road bridge at Lackford, West Suffolk. Gdotechnique, 1956, 6, Sept., 106-114.

3. LEWIS W. A. The effect of vertical sand drains on the settlement of a road embankment on the Thames Estuary. Proc. Eur. Conf. Soil Mech. Fdn Engng, Wiesbaden, 1963, 1, 359-366.

4. MURRAY R. T. Embankments constructed on soft foundations: settlement studies near Oxford. Transport and Road Research Laboratory, Crowthorne, 1973, Report LR 538.

5. MURRAY R. T. Embankments constructed on soft foundations: settlement study ut Avonmouth. Road Research Laboratory, Crowthorne, 1971, Report LR 419.

6. MURRAY R. T. and SYMONS I. F. Embankments on soft foundations: settlement and stability study at Tickton in Yorkshire. Transport and Road Research Laboratory, Crowthorne, 1974, Report LR 643.

7. SYMONS I. F. and MURRAY R. T. Embankments on sofi foundations: settlement and stability study of Over Causeway By-pass. Transport and Road Research Laboratory, Crowthorne, 1975, Report LR 675.

8. IRWN M. J. Automatic data-recording for the laboratory consolidation test. Road Research Laboratory, Crowthorne, 1968, Report LR 188.

9. MURRAY R. T. The analysis of consolidation test data by computer. Road Research Laboratory, Crowthorne, 1970, Report LR 344.

10. IRWN M. J. A mercury-filled gauge for measuring the settlement of foundations. Road Research Laboratory, Crowthorne, 1967, Report LR 62.

11. IRWIN M. J. Instruments developed by the TRRL for studying the behaviour of earthworks. Zn Field instrumentation in geotechnical engineering. Butterworths, London, 1974,194-206.

12. MURRAY R. T. and IRWIN M. J. The performance of an inclinometer for measuring lateral movements in soil. Road Research Laboratory, Crowthorne, 1970, Report LR 332.

13. ROWE P. W. The influence of geological features of clay deposits on the design and performance of sand drains. Proc. Instn Ch. Engrs, 1968, suppl., 1-72.

14. GIBSON R. E. An analysis of system flexibility and its effect on time-lag in porewater pressure measurements. Giotechnique, 1963, 13, Jan., 1-11.

15. BUISMAN A. S. K. Results of long duration settlement tests. Proc. 1st Znt. Conf. Soil Mech., Cambridge, Mass., 1936, 1, 103-106.

16. MURRAY R. T. Computer program for the one-dimensional analysis of the rate of consolidation of multi-layered soils. Transport and Road Research Labora- tory, Crowthorne, 1972, Report LR 443.

17. MURRAY R. T. Computer program for multi-layer soil settlement. Civ. Engng Publ. Wks Rev., 1973, 68, No. 809, 1086-1089, 1097.

18. MURRAY R. T. Two-dimensiona! analysis of settlement by computer program. Transport and Road Research Laboratory, Crowthorne, 1974, Report LR 617.

19. MARGASON G. and SYMONS I. F. Use of pore pressure measurements to control embankment construction on soft foundations. Proc. 7th Znt. Conf. Soil Mech., Mexico, 1969, 2, 307-315.

20. SYMONS 1. F. Stability of embankments on soft foundations. MSc thesis, University of Birmingham, 1974.

21. KJELLMAN W. Accelerating consolidation of fine-grained soils by means of cardboard wicks. Proc. 2nd Int. Con$ Soil Mech., Rotterdam, 1948,2,302-305.

22. Land drain stabilisation could save €+M. Contract J. , 1972, Oct., 40. 23. MENARD L. The dynamic consolidation of recently placed fills and compressible

soils. Application to maritime works. Travaux, 1972, No. 452, Nov., 56-60.

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