road research laboratory · 38 mm. in diameter and 76 mm. in height. four specimens were prepared...
TRANSCRIPT
ROAD RESEARCH LABORATORY
Department of the Environment
RRL REPORT LR 406
A LABORATORY STUDY OF THE USE OF WET FILL IN EMBANKMENTS
D.M. Farrar M.Sc., M.Inst.P.
Earthworks and Foundations Section Road Research Laboratory
Department of the Environment • Crowthorne, Berkshire
1971
CONTENTS
Abstract
1. Introduction
2. Softs used in the investigation
.
.
°
6.
7.
8.
9.
10.
Test procedures and results
3.1 Compaction tests
3.2 Consolidation tests
3.2.1 Total consolidation
3.2.2 Coefficient of consolidation 3.3 Triaxial tests
Evaluation of results
4.1 Calculation of settlement and stability
4.1.1 Heavy clay embankments
4.1.2 Intermediate clay embankments
4.2 Handling .of wet fill
Discussion
Conclusions
Acknowledgements
References
Appendix 1
Appendix 2
Page
2
5
5
5
7
9
9
10
10
10
12
14
( • C R O W N COPYRIGHT 1971
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 ! st 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.
A LABORATORY STUDY OF THE USE OF WET FILL IN EMBANKMENTS
ABSTRACT
The expense of removing wet fill regarded as unsuitable for earthworks,
and replacing it by imported common fill, can form a significant part of
the cost of earthwork construction. There could be a useful saving if the
upper moisture limit on the use of fill could be raised. This Report
describes laboratory consolidation and triaxial tests on three cohesive soils
and the use of the results to predict the behaviour of wet fill in embank-
ments.
It is concluded that the soils tested could be used as fill at a
moisture content in excess of the commonly used maximum 1.2 times
the plastic limit in circumstances where settlements of 100 mm. are
acceptable. Shallow side slopes, and the use of more stable fill near
final formation level, may also be desirable. These restrictions could be
eased by the use of horizontal drainage layers in the fill. It is likely that
pneumatic-tyred scrapers will be unable to operate at these moisture
contents, and that alternative methods of earthmoving would have to be
employed. In spite of these limitations, it may prove cheaper to use the
wet fill and avoid the cost of importing common fill as a replacement.
I. INTRODUCTION
The cost of removing wet fill materials regarded as unsuitable for earthworks, and replacing them by
imported common fill, can form a significant part of the cost of motorway construction. An examination
of a number of tenders showed that on average an allowance of about 20 per cent o f the total cost of
earthworks is made for the replacement of unsuitable fill materials other than peat.
There is, therefore, a possibility of appreciable savings in money and in the quantity of imported fill
if these 'unsuitable' materials could be used without incurring excessive additional expense in handling them,
and without significantly lowering the performance of the embankments. A full-scale trial with Boulder Clay 1
has shown that a stable embankment could be constructed to a height of 6 m., using material at the highest
moisture content consistent with the ability of track-laying tractors to operate. However, the construction
of trial banks to provide information on the upper limits for the moistiare content of fill can be expensive
and is not always possible. It is highly desirable, therefore, that a more practicable procedure for selecting
the limits of moisture content of fill should be developed, preferably using the results of laboratory tests.
(1)
The upper limit for the moisture content of fill will be governed by three main factors:
the need to reduce the settlement of the f'lll after the embankment is compacted to acceptable limits;
(2) the need to provide embankments which have sufficient strength to avoid the possibility of instability; and
(3) the ability of earthmoving plant to operate on the wet fill.
This Report describes laboratory studies of the compressibility and strength properties of three broad
types of cohesive soft. The results are used to provide relations between the settlement and stability of the embankments and the moisture content of the materials.
2. SOILSUSED IN THE .INVESTIGATION ,
The three soils used in this study consisted of a marine heavy clay (L0ndon Clay from Heathrow), an alluvial intermediate clay (Taplow Terrace brickearth from Harmondsworth), artd a glacial intermediate clay
(Glacial deposits near Durham). The resultsof B. S. classification tests2are given in Table I, and the particle- size distributions are shown in Fig. 1.
TABLE I
Classification tests on soils used in the investigation
Intermediate Intermediate
Liquid limit (per cent)
Plastic limit (per cent)
B.S. compaction test
(2.4 kg. rammer);
Max. dry density (Mg/m 3)
Optimum moisture
content ~per cent)
Specific gravity
Heavy clay clay
(Alluvial)
76
26
39
: 18
1 .64 1 .86
23
2.75
15
2.72
clay
(Glacial)
39
21
1.86
16
2.73
The Softs were prepared for testing by air drying or mixing with water• tO give the desired moisture
content, and then leaving them for at least 24 hours in a sealed container. Unless otherwise stated, the soil
was passed through a mincer immediately before use, to give fragments of disturbed soil about 3 mm. by 12 mm. in cross-section.
"3. TEST PROCEDURES A N D RESULTS
3.1 Compaction. tests . . . .
Some preliminary static compaction tests were made on samples of the soils. These tests were intended
to provide data for the preparation of the specimens for the consolidation tests, and for use in the evaluation
of the results.
For these tests, loose soil was placed in a mould 76 rnm. in diameter, and a Series of load increments
applied through a consolidation loading frame. Each increment of load was allowed to act for one hour. The
results obtained for the heavy clay are shown in Fig. 2. A statistical analysis of the results showed a linear
relation between air voids, moisture content and the logarithm of the compaction pressure, for all the soils.
These relations are given in Appendix 1.
3.2 Consolidation tests
The object of the consolidation tests was to provide data from which the settlement of the compacted fill in an embankment could be estimated. The relation between the applied pressure and the voids ratio
after the completion of primary consolidation was determined for specimens having different initial moisture
contents and states of compaction. The coefficient of consolidatiOn was also determined.
The moisture contents employed covered the range between about 0.9 and 1.4 times the plastic limit,
and the range of air voids was 3 - 15per cent. Each specimen was compacted statically or dynamically into
a consolidation ring 76 mm. in diameter and 25 mm. in height. (For dynamic compaction, the soil was given
eight blows of the B.S. 2.5 kg. compaction-test rammer, i.e. the same ratio of blows to volume of soil as in the
B.S. test 2.)
The consolidation tests were carried out in accordance with B.S. 1377:19672, except that the initial
pressure (pi) was fixed at either 13 or 107kN/m 2. That is, the initial pressure was not increased to prevent
swelling. If swelling was expected to occur, the specimen was compacted with the upper soil surface below
the top of the ring.
Some additional tests were carried out using a modified consolidation cell in which both faces of the
specimen could be maintained at a constant negative pore-water pressure (Fig. 3). The specimen was first
consolidated as in the normal test with the faces maintained at atmospheric pressure. A negative pore-water
pressure equivalent to 1 m. of water was then applied, and the additional consolidation of the specimen
measured. Finally, the negative pore-water pressure was increased to 5 m. of water and the consolidation
again measured.
The results obtained from all these tests are discussed below.
3.2.1 Total consolidation
104 specimens of the three soils were tested, and Fig. 4 shows some examples of the results obtained.
To enable interpolations to be made between the results in a way suitable for a computer program, a
set of empirical equations was obtained which would fit the measured results. These equations are described
in Appendix 2, and the calculated relations obtained from these equations are shown in Fig. 4 for comparison
with the measured values. The difference between calculated and measured voids ratio did not exceed 0.030
for any specimen or applied pressure, and within the range of overburden pressures likely to be encountered
in practice the mean difference between calculated and measured voids ratio did not exceed 0.010 for any
specimen. No significant difference was observed between samples compacted statically and dynamically.
3.2.2 Coefficient of consolidation
The coefficients of consolidation were determined for specimens of each soil by the square-root-of-time
(X/i-) and logarithm-of-time (log t) fitting methods 2
Apart from the initial increments of pressure, both methods gave generally similar results. The values
obtained for each soil are given in Table 2.
TABLE 2
Coefficients of consolidation obtained by the square-root-
of-time and logarithm-of-time fitting methods
Soil
Heavy clay
Intermediate clay (alluvial)
Intermediate clay (glacial)
, I Coefficient of consolidation (m2/year)
Mean I Range
0.2
1
0.18 - 0.37
0.6 1.7
2.6 - 4.9
At low increments of pressure (13 - 27 and 27 - 54kN/m 2), and for specimens with high air voids,
the initial rate of compression was very rapid. A plot of compression againstx/-~ did not give a straight line 3.
Although a plot of compression against log t gave a straight line, the point of zero compression could not be
accurately located.
3.3 Triaxial tests
Triaxial tests were carried out to provide data on the shear strength of compacted fill. The data were
used to estimate the stability of embankments constructed of fill, and also to provide some indication of the
ability of earthmoving plant to operate on the fill.
Undrained triaxial tests were made on specimens formed by the static compaction of soil into a mould
38 mm. in diameter and 76 mm. in height. Four specimens were prepared for each test, at as nearly identical
moisture content and dry density as possible, and tests were carried out at a range of moisture contents for
all three soils. The values of cohesion (Cu) and angle of shearing resistance (~tu) obtained are shown in Figs. • 5 and 6.
4
Curves corresponding to soil compacted to 3 per'cent air voids have been drawn through the measured
points. The curve for the heavy clay was extrapolated from the measured points by Black's method 4, s
To provide information on the stability of consolidated embankments, some consolidated drained tests 6
were carried out on the glacial intermediate clay. The cohesion and angle of shearing resistance, in terms of
effective stress (c', ~' ), are shown in Fig. 6.
4. EVALUATION OF RESULTS
4.1 Calculation of settlement and stability
The total settlement of an embankment, due to primary consolidation within the fill, was predicted by
dividing the embankment into horizontal layers and determining the overburden pressure and settlement
within each layer in turn using the relations in Appendix 2. The overburden pressure produced by the
pavement was taken as 15kN/m 2.
The maximum height of embankment which would be stable after rapid construction was estimated by
total stress analysis from the undrained shear strength measurements using Taylor's stability charts 7. The
embankment was assumed to be built on a stable foundation such that any potential slip surface would be
contained within the fdl. A factor of safety of 1.5 was used.
The maximum height of embankment which would be stable when built at a controUed rate of
construction was also estimated, from the results of the drained shear-strength measurements, by effective-
stress analysis using Bishop and Morgenstern's stability charts, a As before, an embankment built on stable
ground was considered. The choice of a factor of safety is discussed later.
4.1.1 Heavy-clay embankments
Table 3 gives the total settlement within an embankment constructed of heavy-clay fill, compacted at
a range of moisture contents in accordance with the method specification for compaction in the current
Specification 9. It was assumed that the rate of construction was such that no settlement would take place
:during construction, and the pore water pressure was assumed to be atmospheric throughout the embank-
ment when final equilibrium was reached.
5
TABLE 3
Predicted movements of heavy-clay fill in embankments
Moisture content of fill
(PL = plastic limit)
Air voids of fill (per cent)
after compaction
Height of embankment (m)
I .OPL 1.1PL
2
4
6
8
10
12
+90
+140
+150
+150
+120
+70
1.2PL
4
1.3PL 1.4PL
Swelling (+) or settlemenl (-) (mm)
+30
+30
0
-50
-100"
-180"
+10 -20
-20 -60
-60 -130"
-110": -230
-200 .1 -360*
-300* -520*
+70
+90
+100
+80
+50
+10
* Side slopes flatter than 1 in 2 would have to be employed if instability within the fill is to be avoided.
Table 3 shows that embankments of about 10 m. in height can be built using heavy clay at a moisture
content in the range 1.0 - 1.15 times the plastic limit, without significant settlement. Appreciable swelling
is predicted for some heights of embankment, but these figures almost certainly over-estimate the effect on
the road surface in practice. Swelling will take place much more rapidly than settlement 10, and part is,
therefore, likely to occur during construction. In addition, the effect of swelling within the fill will be partly balanced by the effect of settlement in the subsoil beneath the embankment.
With increasing moisture content of the heavy-clay fill, there is an increasing amount of settlement
and continuing reduction in the stability of the side slopes of the embankment. Wet heavy clay can, however,
be used to build stable embankments of limited height in circumstances where settlements of 100 mm. are
acceptable (i.e. away from fixed structures such as bridge abutments). The settlement would be accelerated
by the use of horizontal drainage at layers within the fill. The application of conventional theory shows that,
with drainage layers 2 m. apart, about 60 per cent of the total settlement would take place within a
construction period of one year. The limitations on embankment height arising from stability considerations
could be eased by the use of shallower slopes. Thus, provided these measures are used, heavy-clay fill with
a moisture content of as much as 1.3 times the plastic limit can be employed in embankments of up to 10 m. height without introducing serious problems of settlement or stability.
6
4.1.2 Intermediate-clay embankments
The alluvial and glacial intermediate clays are more permeable than the heavy clay, with considerably
higher coefficients of consolidation (Table 2). This affects the use of these soils as f'tll in embankment
construction. The application of conventional theory shows that during the period of construction a
significant proportion of the total settlement will occur, and that part of the excess pore pressure will
dissipate with a corresponding gain in stability. In addition, these soils are likely to develop negative pore-
water pressures beneath the road pavements, after dissipation of excess pore-water pressures. TO allow for
the additional degree of stability gained, a low factor of safety of 1.3 can be used in Bishop and Morgenstern's
stability charts.
Tables 4 and 5 give the total settlements, after completion of the construction of the pavement, of
embankments built of alluvial and glacial intermediate-clay fill. It is assumed that the time between com-
pletion of the embankment and final surfacing is one year, that there are drainage layers at the top and
bottom of the embankment, and that after dissipation of excess pore-water pressures there is a water-table
at the base of the embankment.
TABLE 4
Predicted settlement of alluvial intermediate-clay fill in embankments
Moisture content of fill
(PL = plastic limit)
Air voids of fill (per cent)
afier compaction
Height of embankment (M)
2
4
6
8
10
12
1.0PL 1.1PL 1.2PL 1.3PL
4 3 3 3
Settlement (mm)
0 0 0
10 20 30
30 50 80
70- 90 130
110 140 200
150 190 290*
0
40
110
-210"
330*
440*
1.4PL
0
70
180"
320*
490*
670*
* Side slopes flatter than I in 2 would have to be employed if instability
within the fill is to be avoided.
TABLE 5
Predicted settlement of glacial intermediate-clay fill in embankments
Moisture content of fill
(PL = plastic limit)
Air voids of fill (per cent)
after compaction
Height of embankment (m)
2
4
6
8
10
12
1.0PL
4
0
10
40
80
150
220
1.1PL 1.2PL
Settlement (m)
1.3PL 1.4PL
0
10
50
110
190
300
0
10
70
170
290*
430*
0
20
110"
240*
410"*
600**
0
30
160"*
330**
540**
770**
* Side slopes flatter than 1 in 2 would have to be employed to avoid instability
within the flU after rapid construction.
** Side slopes flatter than 1 in 2 would have to be employed to avoid instability
within the fill after dissipation of excess pore pressures.
Tables 4 and 5 show that when these intermediate clays are used as fill there is no appreciable
swelling, but significant settlement takes place at all moisture contents.
For both the alluvial and glacial intermediate clays at moisture contents near the plastic limit, the
equations in Appendix 2 show that the soils are likely to increase in moisture content after the construction
Of the embankment. The rate of settlement is, therefore, likely to be governed by the coefficient of
swelling, and to be more rapid than indicated using the coefficient of consolidation 10. Tables 4 and 5,
therefore, almost certainly over-estimate the effect on !he road surface in practice.
With increasing moisture content of these intermediate clays, there is an increasing amount of
settlement and a continuing reduction in the stability of the side slopes of the embankment. As with the
heavy clay, however, the intermediate clays can be used to build stable embankments of limited height in
circumstances where settlements of up to 100 mm. are acceptable (e.g. a height of 5 m. at a moisture
content of 1.4 times the plastic limit). The amount of settlement after completion of the pavement can be
very substantially reduced by the use of horizontal drainage layers within the frill. With drainage layers 2 m.
apart, over 90 per cent of the total settlement would take place within a construction period of one year.
Stability would also be improved to some extent by the use of drainage layers and a controlled rate of
8
construction, although the use of shallower slopes would be necessary for higher embankments. Provided
these measures are employed, therefore, very wet intermediate clays can be used as fill even with relatively
high embankments.
4.2 Handling of wet fill
Although the soils tested could be used to build stable embankments, with suitable restrictions on
height and slope when placed at high moisture contents, the decrease in soil strength with increase in moisture
content might cause difficulties during construction.
The soil may not permit the passage of heavy earthmoving plant. Rodin s has suggested that the
undrained shear strength of the soil must exceed one-fifth of the tyre pressure if pneumatic-tyred equipment
is to operate satisfactorily. This limit is shown plotted in Fig. 7, and shows that pneumatic-tyred scrapers of
current design (minimum tyre pressures about 240kN/m 2 (351bf/in 2 )) are likely to be inefficient on fill at
moisture contents in excess of about 1.2 times the plastic limit= Alternative means of earthmoving (for
instance low pressure tyres, tracked vehicles or belt conveyors) may, therefore, be required for wet fill.
Apart from any problems of earthwork construction, the soil may be too weak to provide a satisfactory
formation for the plant used to construct the pavement. The California Bearing Capacity (C.B.R.)of the
soil immediately after compaction can be estimated from the undrained shear strength s , and is shown in
Fig. 7. For the soils tested, the C.B.R. falls below 2 per cent at moisture contents in excess of about 1.3
times the plastic limit. Thus to avoid an uneconomic thickness of sub-base, wet fill should not be used in
the top layer of an embankment, unless it can be strengthened by drainage or by stabilisation.
5. DISCUSSION
It has been shown that the soils used in the present study could be employed as fill at high moisture contents,
but only with some restrictions on the design of embankments and perhaps reduced efficiency of earthmoving.
Thus, although increasing the upper limit of moisture content will reduce the amount of wet fill to be removed
and replaced by common fill and thus produce savings, it is also likely to result in increased unit costs for
handling the material and for using drainage layers. The upper limit must therefore be selected, for each site,
as the optimum value to minimise the total cost of these items. The upper limit is at present often fixed as
the limit at which pneumatic-tyred scrapers can operate (moisture content about 1.2 times the plastic limit),
but the selection of a higher limit could well result in a reduction in the total cost of the earthworks. This
has proved to be the case on the Killington - Tebay section of the M6 Motorway 11
• F6r all the soils tested, the settlement, stability and efficiency of:earthmoving are broadly similar for
wet soils having a similar ratio of moisture content to plastic limit (the ratio of moisture content to optimum
moisture content could also be used, with some loss of accuracy). The data given in this Report should
therefore give a useful guide to the principal factors governing the choice of an upper moisture limit for
other cohesive fills. Two factors may, however, affect the accuracy of the present studies. First, the use of
conventional consolidation theory may under-estimate the actual rate of settlement. Second, the ability of
the fill to support construction traffic was assessed using a fairly simple approach. Further research and
practical experience on these points would be desirable.
9
6. CONCLUSIONS
To assist in obtaining a procedure for selecting the upper limit of moisture content for fill in earthwork
construction, laboratory studies have been carried out of the performance of three cohesive soils. It has
been shown that these soils could be used as fill at moisture contents in excess of 1.2 times the plastic limit,
if restrictions on embankment design and reduced efficiency of earthmoving operations can be accepted.
The upper limit of moisture content should, therefore, be an optimum value for each site, chosen to minimise
the costs imposed by these restrictions and the cost of removing and replacing the wet fill.
For the soils tested, the limitations on embankment design and earthmoving operations (but not the
rate of settlement) are broadly similar for wet softs having a similar ratio of moisture content to plastic
limit. The data given in this Report can, therefore, be used as a guide to the performance of other wet fill materials and the selection of an appropriate upper moisture limit.
Further research and practical experience is needed to improve laboratory tests for predicting the rate
of settlement of compacted ffdl, and the ability of the formation to support construction traffic.
7. ACKNOWLEDGEMENTS
This work was carried out under the general supervision of Mr. W.A. Lewis, Head of the Earthworks and
Foundations Section of the Construction Division. Those participating in the work were Mr. D.Mi Farrar, Mr. A.D. Marsh and Mr. B. Pimley.
8. REFERENCES
1. McLAREN, D. M6 trial embankment at KiUington. Ministry of Transport R.R.L ReportNo. LR 238. Crowthorne, i 968 (Road Research Laboratory).
. BRITISH STANDARDS INSTITUTION. British Standard No. 1377:1967. Methods of testing soils for civil engineering purposes. London, 1967 (British Standards Institution).
3. BARDEN, L. Consolidation of compacted and unsaturated clays. Geotechnique, 1965, 15, 267 - 286.
. BLACK, W.P.M. A method of estimating the California bearing ratio of cohesive soils from plasticity data. Geotechnique, 1962, 12, 271 - 282.
. RODIN, S. Ability of clay fill to support construction plant. Or. Engng. publ. WksRev., 1965, 60, 197 - 202.
.
.
8.
BISHOP, A.W., and D.J. HENKEL. The measurement of soil properties in the triaxial test. London, 1962 (Edward Arnold Ltd.), 2nd Edition.
TAYLOR, D.W. Fundamentals of soil mechanics. New York, 1967 (John Wiley and Sons).
BISHOP, A.W., and N. MORGENSTERN. Stability coefficients for earth slopes. Geotechnique, 1960, 10, 129- 150.
10
9. MINISTRY OF TRANSPORT. Specifications for road and bridge works. London, 1969 (H.M. Stationery Office), 4th Edition.
10. JUMIKIS, A.R. Soil mechanics. Princeton, 1962 (Van Nostrand Co., Inc.), p. 379.
11. LINDSAY, J.F. Thirty-six miles of M6 Motorway completed. Cir. Engng publ. l~ks Rev., 1970, 65, 1285 - 1289.
11
A P P E N D I X I
Preliminary compaction tests
Tests on the effect o f time of application of load on the compression of the soil showed that the greater
part of the compaction took place within the first minute after the load was applied, although some further
compaction took place at a decreasing rate for at least 24 hours. It was, therefore, decided that each load
increment would be applied for one hour, so enabling a complete series of load increments to be applied to a specimen during a working day.
For the compaction tests, sufficient loose soil, prepared as described in Section 2, was placed in a mould
76 mm. in diameter to obtain a compacted specimen about 20 ram. thick. A series of load increments was
applied to the specimen through a consolidation loading frame. Each increment was maintained for one hour
and the compression produced by each increment was noted. This procedure was repeated for all three soils, and for specimens having different initial moisture contents.
The results obtained for the heavy clay are shown in Fig. 2, as a plot of air voids (Va) against the
logarithm of the compaction pressure (Pc) for each moisture content (m). A statistical analysis was made of
the results obtained for the three soils tested, and the relations given in Table 6 were obtained. The relation
for heavy clay is shown plotted in Fig. 2 for comparison with the measured values.
T A B L E 6
Relation between air voids ( V ) , moisture content (m) and
compaction ( P ) for three soils (V a >/ 3 per cent)
Soil
Heavy clay
Intermediate clay
(alluvial)
Intermediate clay
(glacial)
Range of
m (%)
19 - 37
16 - 26
18 - 25
Regression equation
V a = 183.2 - 2.77m - 40.7 log10 Pc
V a = 145.0 - 3.45 m- 30.1 logloPc
V a = 159.5 - 3.84m - 31.7 logloPc
Limits of error
(19 tests in 20)
+2 .2
+ 2.3
+ 3 . 2
(Pc is measured in kN/m 2)
An additional test was carried out using heavy clay which had been rubbed through a coarse sieve to
produce lumps about 20 mm. in diameter. The results of this test, shown in Fig. 2, also agreed with the
relation in Table 6. It was found difficult, however, to keep the loading plate horizontal. It was, therefore,
1 2
concluded that, although the size and shape of the soil fragments did not appear to affect the results, the use
of the smaller fragments was preferable.
13
APPENDIX 2
Empirical relations between voids ratio and applied pressure
Some examples of the measured voids ratio and logarithm of the applied pressure (p) obtained from the
consolidation tests are shown plotted in Fig. 4. No single relation could be found which would fit the
measured values over the entire range of applied pressures. Analysis of the relation could, however, be
simplified by dividing it into three parts; a straight line at pressures greater than the pressure applied during
compaction (Pc), a second straight line at pressures less than Pc, and a third part at low applied pressures which is separately considered for the heavy clay and for the other soils.
For the heavy clay, the voids ratio could be taken as constant up to a critical pressure given by Pi +
0.07Pc (where Pi is the initial pressure. See Section 3.2). The complete relation between voids ratio and
applied pressure can, therefore, be defined by the voids ratio at pressure Pc and at two other pressures (for
convenience these were taken as 0. l Pe and 858kN/m 2). It was found that these values of voids ratio could
be defined in terms of the initial moisture content (m) and air voids (Va) at which the sample was prepared, as given in Table 7 below.
TABLE 7
Voids ratio of consolidated specimens of heavy clay
Applied pressure (p)
0.1p c
Pc
858kN/m 2
Voids ratio
0.30 + 0.0204(m + Va)
-0.015 + 0.0273(m + 0.4Va)
0.60 + 0.035m
For the intermediate clays, the voids ratio was found to be a linear function of the applied pressure,
up to a pressure of 0.25pc. The relation between voids ratio and applied pressure can, therefore, be defined
by one equation, and by the voids ratio at applied pressures pc and one higher pressure (for \ " • convenience 858kN/m2). The equation, and the voids ratios at the two pressures, could be defined as in Table 8 below.
14
TABLE 8
Voids ratio of consolidated specimens of two intermediate clays
Applied pressure (p)
0- 0.25Pc
Pc
858kN/m 2
Voids ratio of intermediate clays
(Alluvial)
VR + 0.016 - 0.15p Pc
0.37 + 0.00175(m-12)(Va+ 10)
0.44 + 0.002m
(Glacial)
VR + 0.015 - 0.12p/pc
-0.11 + 0.030(m+0.5Va)
0.425 + 0.004m
(where VR is the voids ratio at which the specimen was prepared)
The values shown in Tables 7 and 8 are app!icable to the range of moisture contents and air voids
discussed in this Report, but are less accurate for very wet and loosely compacted heavy clay ((V a + m ) > 4 6 )
or alluvial intermediate clay (Pc ( 3 5 k N / m 2 ) •
For the conditions discussed in this Report, the following empirical relation was found to h01d for all the soils;
(Va) p = (900pL " 24)(VRo " VRsss ) (~< Va ) ..... ...(1)
where V a is the initial air voids
(Va) p is the air voids at an applied pressure p
VRp is the voids ratio at an applied pressure p'
VRas 8 is the voids ratio at an applied pressure 858kN/m 2
PL is the plastic limit
The corresponding moisture content estimated from this relation was within 1 per cent of the measured value for all specimens.
The values shown in Tables 7 and 8 were also found to be applicable to samples consolidated with their
ends maintained at pore-water pressure other than atmospheric, provided that a modified pressure was
substituted for the applied pressure (p) as shown in Table 9.
15
TABLE 9
Effect of pore-water pressure on consolidation of softs tested
Soil Modified pressure
Heavy clay
Intermediate clay (alluvial)
Intermediate clay (glacial)
p - 0.55u (m ~>PL)
p - 0.4u
(where u is the pore-water pressure, p and u in kN/m z)
16
z.
171.
'~ 9~
00~
"G
• E'~,
0 0 0 0 0 0"~ ~0 I~
_o ° \ o
7-
\
cl
c-
0 0 0 0 0 0
6u lss~d a 6~.ua~Ja,=l
W > <
W ffl
< 0
IE
0
• k~l
W Z
b.
V
ILl t~ n- ,< 0 U
• 9 IE
J 121 ~ w
• 0
9 W Z
• . . o o
A
E >- E
(0 N
Q;
U ,--
t. 0 n
z
w
z
O w
B
z
N w t . f ~
!
° a
A
t -
U
L
Q.
"lO
0 >
f -
<
40
30
20
10
0 10
i
(2.0 m~ lumps) ~
V Q • .~ 0
100 1 000 Compaction pressure (kN/m 2)
Fig. 2. RELATION BETWEEN AIR VOIDS AND COHPACTIO~ PRESSURE FOR HEAVY CLAY
To lower porous plate From manometer
/ /I ]~/77_/i ~ (Yl~nn tubing
ring ~I - ~ I ~ 04 mm clearance between and plate
(5 mm O.D.)
~er porous plate
From upper porous plate
To manometer
Lower porous plate
Fig. 3. CONSOLIOATION CELL WITH CONTROLLED PORE-WATER PRESSURE
o
o I _
-o
1,1
1"0
0.9
0"8
0.7
O'G
" • 3 ,
H e a v y c lay
10 100
0"9
0"8
0"7
O'G
0 '5
0"4 1000 10
I n t e r m e d i a t e c lay ( a l l u v i a l )
100
Appl ied p r e s s u r e ( k N / m 2) 1 0 0 0
0"9
0 8
o
o-7
0-6
0
I n t e r m e d i a t e c l a y ( . .glacial)
0"5 10 100 1 0 0 0
App l ied p r e s s u r e ( k N / m 2)
S o i I Compacted at MC(%) Vo(°Ig
Heavy clay o 36.1 5 • 25.2 6
I n t e r m e d i a t e clay o 24.5 11 ( a l l u v i a l ) • 14.5 16
I n t e r m e d i a t e clay o 27.8 9 (g lac ia l ) • 20.6 12
Fig. t,.. YOlOS RATIO ANO APPLIEO PRESSURE OF CONSOLIOATEO SOIL SPECIMENS EXPERIMENTAL RESULTS ANO CALCULATEO RELATIONS
E Z
tJ
40
20
0
Heavy clay
0 2
o
Per cent air votds of specimen
10
0 28 30 32 34 36
Moisture con ten t (pe r cent) 38
80
60
E z 40
U
20
0 20
8~00202
Intermediate clay (al luvial) I
10
10 B
0 18 20 22 24
Moisture content (per cent)
~2i 26
Fig. 5. COHESION (C u) AND ANGLE OF SHEARING RESISTANCE (gu) FOR COMPACTED SPECIMENS FROM UNDRAINED TRIAXIAL TESTS
E
v
L~
80
60
• 40
20
0
5
0
9~
20
/ P e r cent oir voids of specimen
11
~ 0 3
22 24 26 28 Moisture content (per cent)
30
&- E
v
b
20
10
0
30
4
2 5
)4
20 20 22 24 26 28 30
Moisture content (per cent)
Fig. 6. COHESION AND ANGLE OF SHEARING RESISTANCE OF COMPACTED SPECIMENS OF GLACIAL INTERMEDIATE CLAY, FROM UNORAINEO TRIAXIAL TESTS (Co,~u) AND DRAINED TRIAXIAL TESTS (C', g')
400
t -
O
E c"
L
0
I.- i~l
o E
v 0
t3. C- O
>,,
O') ¢-
E d
200
0
Lower l imit for pneumatic-tyred scrapers
~ Intermediate clay ~ ~ ~ . ~ (alluvial)
Intermed iate-clay ~ ~ (glacial) "~
t - O
t
O_ v
t - O
0
E 0
m £)
1.0
4
2
1.2 Moisture content-plastic limit
1'4
0 1"0
Fig. 7.
• \ c o , , u v i o , X ~ ' Intermediate cloy
Heavy cloy /
Intermediate clay (g lac ia l )
1'2 Moisture content + plastic l imit
.4
PREOICTEO EFFECT OFNOISTURE CONTENT OH TRAFFICABILITY AND C.B.R. OF THREE CLAYS
(1152) Dd635271 3M 8/71 H.P.Ltd. G1915 PRINTED IN ENGLAND
ABSTRACT
A laboratory study of the use of wet fill in embankment~s: D M FARRAR MSc, AInstP: Department of the Environment, RRL Report LR 406: Crowthorne, 1971 (Road Research Laboratory). The expense of removing wet fill regarded as unsui table for earthworks, and replacing it by imported common fill, can form a significant part of the cost of earthwork construction. There could be a useful saving if the upper moisture limit on the use of fill could be raised. This Report describes laboratory consolidation and triaxial tes ts on three cohesive soils and the use of the results to predict the behaviour of wet fill in embankments.
tt is concluded that the soils tested could be used as fill at a moisture content in excess of the commonly used maximum 1.2 times the plas t ic limit in c i rcumstances where sett lements of 100 mm are acceptable. Shallow side slopes, and the use of more stable fill near final formation level, may also be desirable. These restr ict ions could be eased by the use of horizontal drainage layers in the fill. It is likely that pneumatic-tyred scrapers will be unable to operate at these moisture contents, and that alternative methods of earthmoving would have to be employed. In spite of these limitations, it may prove cheaper to use the wet fill and avoid the cost of importing common fill as a replacement.
ABSTRACT
A laboratory study of the use of wet fill in embankments: D M FARRAR MSc, AInstP: Department of the Environment, RRL Report LR 406: Crowthorne, 1971 (Road Research Laboratory). The expense of removing wet fill regarded as unsuitable for earthworks, and replacing it by imported common fill, can form a significant part of the cost of earthwork construction. There could be a useful saving if the upper moisture limit on the use of fill could be raised. This Report describes laboratory consolidation and triaxial tes ts on three cohesive soils and the use of the results to predict the behaviour of wet fill in embankments.
It is concluded that the soils tested could be used as fill at a moisture content in excess of the commonly used maximum 1.2 times the plast ic limit in c i rcumstances where settlements of 100 mm are acceptable. Shallow side slopes, and the use of more stable fill near final formation level, may also be desirable. These restr ict ions could be eased by the use of horizontal drainage layers in the fill. It is likely that pneumatic-tyred scrapers will be unable to operate at these moisture contents, and that alternative methods of earthmoving would have to be employed. In spite of these limitations, it may prove cheaper to use the wet fill and avoid the cost of importing common fill as a replacement.