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Ability of clay fill to support construction plant
Journal oJ Terramechanics, 1965,Vol. 2, No. 4, pp. 51 to 68. PergamonPress Ltd.
Printed in Great Britain.
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT* S. RODIN~" IN THE DESIGN of earthworks many factors must be considered, not the least important of which are the nature of the works, the physical properties of the available materials and their relative costs. Consideration must be given to the problems of the construction teams in transporting and placing the materials, and to the types and costs of plant available for this work. The natural moisture contents of earth fill materials in the British Isles are usually relatively high and, except in special circumstances, watering of the fill is unnecessary: nor is it normally practicable in this country to dry out clayey soils to any significant extent. Thus, the moisture content of the fill when placed will generally be at or near the natural moisture content. At the same time, the degree of compaction assumed in the design must not be higher than can reasonably be achieved by modern plant. All too often these considerations, if studied at all, are considered as individual factors. Most designs for earthworks--large jobs as well as small ones--are approached by starting with a "standard" specification for the moisture content and density limits of the compacted fill, and in many cases little thought is given as to whether this specification should be modified to suit the particular site and as to whether this specification should be modified to suit the particular site and construction conditions. All too rarely are the design, construction methods and economic design [ 1]. In practice, it is desirable to make the fullest use of available excavation material provided its strength is adequate for the design criteria and the earthmoving plant can operate satisfactorily and economically. Thus, with the relatively wet conditions in the British Isles, a realistic basis for the specification would be to relate the placement limit of wet material to the shear strength requirements, and so provide an easily measured common factor between design and construction considerations. The object of this article is to consider the basis for such a method and to discuss the limited amount of available information relating the principal factors. Primary attention is given to cohesive soils, since these are usually the critical materials in relation to specification and construction problems in the British Isles. SHEAR STRENGTH OF COMPACTED SOIL Although a considerable number of earthworks have been built in recent years, surprisingly little information has been published on the relation between shear *Reproduced from Civil Engineering and Public Works Review, February, 1965. tCentral Laboratory, Messrs. George Wimpey & Co., Ltd., Southall, Middlesex, England Communicated by A. R. Reece.
51
52
S. RODIN
strength, moisture content and dry density for different soil types. One of the few such publications [2] gives this information for a heavy clay and a sandy clay, although the information obtained for these soils covers a moisture content range somewhat drier than the values more usually encountered in practice. It is of -
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FIG. 1. Relation between dry density, moisture content and shear strength with a compacted heavy clay (LL=75%, PL=23%). Key to Fig. 1. Curve 1 indicates B.S. laboratory compaction tests. Curve 2 indicates B.S. heavy laboratory compaction tests. ( A > indicates average moisture content range for British Isles. denotes density achieved with different pneumatic-tyred rollers, viz. Roller Q •
12-ton 20-ton
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45-ton
Wheel load Tyre inflation pressure (tons) (lh/in ~) 1'33 2-22
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ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT I~
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FIG. 2. Relation between dry density, moisture content and shear strength with a compacted sandy clay (LL=40%, PL--20%). Key to FiI. 2.
Curve 1 indicates B.S. laboratory compaction tests. Curve 2 indicates B.S. heavy laboratory compaction tests. < A ~ indicates average moisture content range for British Isles. denotes density achieved with different pneumatic-tyred rollers, viz. Roller O •
12-ton 20-ton
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36 80
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interest to re-plot this information in the form shown in Figs. 1 and 2, against the background of the compaction curves obtained in the field compaction trials. For a given moisture content, the shear strength of the compacted soil increases as the dry density increases, until the zero air voids curve is approached. There does not appear to be any test data to cover the latter stage, when the air voids content is only 1 or 2 per cent, although the available information suggests an upturn in the constant strength curves.
54
S. RODIN
It would be valuable to engineers to have data similar to that shown in Figs. 1 and 2 for different types of compacted soil, if only to obtain a general appreciation of the relation between shear strength, dry density, moisture content and the classification characteristics of the soil. It should be pointed out that the densities shown in Figs. 1 and 2 are for 32 passes of the roller. Since the number of passes normally used is much smaller than this--for practical and economic reasons-the range of maximum densities obtained in the field would generally be about 2 to 5 lb/fP lower than the values shown.
Relationship between shear strength, moisture content and plasticity indices of compacted clay Black [3] describes an empirical method of estimating the California Bearing Ratio (CBR) of a remoulded cohesive soil from a knowledge of the suction of the remoulded soil and its true angle of friction, which can be inferred from a knowledge of the plasticity index (P/), the liquid and plastic limits (LL and PL) and moisture content; in the derivation of these relations the following equations were used.
CBR=q./10
(1) where qu is the bearing capacity of a circular footing on clay,
(PI) = 0.838 ( L L ) - 14.2
(2)
Equation (2) appears to be characteristic of British cohesive soils in general; it is shown plotted in Fig. 3. At this stage, only clays or plastic soils are considered in this and subsequent sections dealing with the shear strength and wheel supporting capacity of the fill, since the analysis for granular soils is more complex. Also, plastic soils are 60
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FIG. 3. Plasticity characteristics of soils used in analysis. Key
• London clay (A) + Sandy clay (B) Q London clay (C)
x •
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90
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ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
55
usually the critical materials in relation to specification and construction problems in the British Isles. Taking q. = 6.2c for a circular on clay, from equation (1). CBR = 6.2c/10
(3)
where c is the apparent cohesion (lb/inS). Black's Fig. 9 gives the relation between CBR and plasticity index at various consistency indices. In view of the use of plastic limit as the present basis for specifying placement moisture content limits, it would be useful to determine the u,O0
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Relation between shear strength and moisture content at a range of plastic limits for soil compacted to 10 per cent air voids. Key to Fig. 4. London clay (A) PL=23 LL=75 c measured Sandy clay (B) P L =2 0 L L = 4 0 c measured London clay (C) PL--27 LL=75 c converted from C.B.R. Brickearth (D) P L = 2 0 L L = 3 0 c converted from C.B.R. Silty clay (E) PL=22 LL=42 c measured
56
S. R O D I N
relation between shear strength and moisture content relative to the plastic limit. This can be done through the relationship. Consistency Index (C/) = ( L L - w ) / ( L L - P L )
(4)
C I = 1 + [ ( P L - w)/(5.16 ( P L ) - 87.7)]
(5)
which becomes
From the above equations and Black's curves, the relation between estimated shear strength and moisture content has been calculated for different values of plastic limit, for conditions corresponding to 5 per cent and 10 per cent air voids tLO0
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Relation between shear strength and moisture content at a range of plastic limits for soil compacted to 5 per cent air voids. Key to Fig. 5. Q London clay (A) P L = 2 3 L L = 7 5 c measured Sandy clay (B) P L = 2 0 L L = 4 0 c measured @ London clay (C) P L = 2 7 L L = . 7 5 c converted from C.B.R. Brickearth (D) P L = 2 0 L L = 3 0 c converted from C.B.R. x Silty clay (E) P L = 2 2 L L = 4 2 c measured
FIG. 5.
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
57
respectively. These relationships are given in Figs. 4 and 5. Air voids contents of 5 per cent and 10 per cent have been selected as they are representative of the degree of compaction usually achieved in practice. Measured values of shear strength, or converted from CBR values, are compared with the estimated values in Figs. 4 and 5. The sources of these measured values are as follows. Soils A and B: [2]. Soils C and D: [3]. Soil E: unpublished data obtained from site investigation in Iraq (due to the lack of any other relevant published information for British soils, this example is included further to illustrate the relationship). Although the relations between shear strength and moisture content followed the estimated pattern, there are differences between the measured and estimated values. In the two cases of London clay, the difference between the measured and estimated shear strengths is greater in one case than in the other. This is no doubt due to the various assumptions and generalizations made in the different steps leading to the theoretical relation for estimating the shear strength from a knowledge of the moisture content and the plasticity index, and also to natural variations in the physical characteristics of a particular soil deposit. Hence, on any earthworks of importance, these theoretical relations should not be relied upon as a basis for design without carrying out some testing to determine the actual shear strength of the compacted soil. Nevertheless, bearing in mind the simplifying and very generalized nature of the theoretical relation, Figs. 4 and 5 provide a useful general guide on the effect of moisture content on the shear strength of the compacted soil. Perhaps of greater importance in the context of this paper. Figs. 4 and 5 illustrate the significance of the moisture content relative to the plastic limit of the soil. Interposed on these graphs are two curves corresponding to: (a) (b)
w=PL. w = P L + 2 per cent.
The curve corresponding to (b) represents the upper limit of placement moisture content, which has been commonly used in road work specifications. It shows that at this relative moisture content, the strength of the compacted clay increases very significantly as the plastic limit of the soil increases. It should be noted that the experimental points shown in Figs. 4 and 5 represent somewhat drier conditions than usually found in the British Isles. ABILITY OF THE GROUND TO SUPPORT EARTHMOVING PLANT As earthmoving plant have become bigger, their loads on the ground have also increased. The high tyre contact pressures of standard earthmoving tyres require good ground conditions for their satisfactory operation. The working surface in the borrow pit or on the fill must be strong enough to support the wheels, or tracks, without their sinking excessively. The stability of the ground surface under a moving wheel or track is a much more complex subject than the stability of the ground under static foundations, such as for buildings. The interrelation between the physical properties of soils and their load carrying capacity for wheeled and tracked types of vehicle systems is a vital consideration in the design and efficient use of earthmoving plant, but, unfortunately, research
58
S. RODIN
on the subject has been sadly neglected. The voluminous proceedings of the 1st International Conference on the Mechanics of Soil-Vehicle Systems, held in Turin in 1961, record the work of several organizations in different countries who are trying to develop soil-wheel relationships, some of which are highly theoretical and complex, others empirical. However, a great deal more work is required before a satisfactory soil-wheel relationship is developed and which can be applied to practical earthmoving problems. There is no procedure for determining a limit of trafficking on earthworks, nor has this factor been defined in physical terms. A practical criterion, from the civil engineer's point of view, is that the loading on the wheel or track should not cause excessive rutting or sinkage of the wheel or track into the soil. That is, the wheel should not overstress the soil to the extent that excessive plastic deformation of the soil occurs, since at any higher loading the wheel would begin to sink into the soil and the remoulding action would tend to weaken the soil. There is a very wide range of sizes of earthmoving tyres. Most of the earthmoving (off-the-road) tyres are designed for maximum inflation pressures of between 45 and 70 lb/inL The wide base tyres, which are designed to provide greater flotation, have maximum inflation pressures of 45 to 55 lb/inL Smaller inflation pressures can be used in all cases, but only at the expense of reducing the maximum load on the wheel, and hence the capacity of the machine. The tyre manufacturers do not publish data relating wheel load, tyre inflation pressures and tyre contact pressure for the standard earthmoving tyres. Since some of the load on a wheel is carried by the walls of the tyre, the tyre contact pressure (p~) is not necessarily the same as the tyre inflation pressure (P3. A rule of thumb given by Heiple [4] for a condition of no penetration, i.e. tyre resting on a hard surface, is as follows : Area of contact (A1)=90 per cent weight on tyre/tyre inflation pressure =0.9W/p,
(6)
where the weight (W) on the tyre is the maximum permissible load on the tyre at the particular inflation pressure (from the Tyre and Rim Association tables). For a particular tyre, the permissible load decreases as the inflation pressure decreases. From this equation, the mean tyre contact pressure can be obtained as follows. Mean tyre contact pressure (pc) =Permissible load/Area on ground = W / A 1 = W x (pal0.9 W)= l.lp,
(7)
These equations are based on the tyre resting on a hard surface. There does not appear to be any published information on the contact area and pressure of standard earthmoving tyres resting on soils, in which some settlement or sinkage occurs. In this case, the contact area and pressure will be dependent on the extent to which the wheel sinks into the soil; the contact area against the soil will be greater than the contact area given by equation (6) for the same load on a hard surface. To obtain some guidance on the significance of different amounts of sinkage, Fig. 6 has been produced to indicate the estimated contact area for different
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT I
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AI = Normal g r m s contact area (hard ground) A2 = 6roes contact area at sinkage
Relation between estimated contact area (A 3) after sinkage = A in and contact area (A1) on hard ground for some typical earthmover tyros.
FzG. 6.
values of sinkage of some typical earthmoving tyres, basezl on the assumptions shown. Maximum allowable tyro contact pressure
This section discusses the relation between the shear strength of the ground and the maximum tyro contact pressure that can be supported without excessive sinkage. As seen from Fig. 6, the value of actual contact area for a particular w h ~ l varies with the amount of sinkage. To determine when sinkage of the wheel is becoming too large, it is necessary to select an arbitrary limit of sinkage.
60
S. RODIN
As a first approximation, assume that 2 in. is the maximum sinkage desirable when plant is running over, say previously compacted fill, in order to avoid excessive disturbance of the compacted material. At this limit Ao.=I.4A1 (approximately). The vertical pressure distribution over the area A2 will be non-uniform; the vertical pressure on the soil in contact with the sloping sidewalls of the tyre will presumably be much less than over the area A1. Also, any permanent rut formation behind the tyre would reduce the amount of the contact area A2 shown in Fig. 6. Owing to the lack of adequate experimental information on the actual pressure distribution, and to obtain some guidance on the effect of the larger contact area on the bearing capacity of the soil, it is assumed that the total load on the soil is equivalent to a mean contact pressure, p',, over an area A =½ (A1+A2)= 1.2A1. Hence, at a sinkage limit of 2 in. the assumed mean tyre contact pressure becomes p'c= W~ 1.2Al=0"9p, (approx.)
(8)
Static condition The condition of complete bearing capacity failure for a static circular or square footing resting on the surface of a clay soil having a shear strength c (lb/in2) is given by the equation ultimate bearing capacity, q,=6.2c
(9)
Before complete failure occurs, local overstressing in shear takes place and results in some plastic deformation of the soil. In the case of a flexible loaded area, the condition of overstressing begins when the pressure on the footing is approximately p -- 1rc (10) There is no satisfactory and simple method available for calculating the contact pressure (p'¢) required to produce a particular amount of sinkage of a rubber tyre, especially as a sinkage of, say, 2 in. involves relatively large shear deformation in the soil. The value of the pressure lies somewhere between the pressures given by equations (9) and (10). If an average value is taken, say
p'c=4.5c
(11)
then from equation (8), the following approximate relation is obtained for the assumed limit of sinkage -- 2 in., p,=5c (12) For the standard earthmoving tyres with the range of inflation pressures between 45 and 70 lb/in z, this limit of sinkage would occur when the corresponding shear strength is between 9 and 14 lb/in 2. In the case of the wide base tyres with inflation pressures ranging from 45 to 55 lb/in 2, the limit of sinkage would occur when the corresponding shear strength of the clay is about 9 to 11 lb/in ~. The effect of the wheel dimensions has not yet been included in this simplified analysis. In practice, the stresses under the larger wheels penetrate to greater depths and, therefore, for a given contact pressure produce greater deformation of the soil, i.e. a particular value of sinkage will be produced by a lower contact pressure than in the case of small wheels. However, this has not been taken into account
ABILITY OF CLA~ FILL TO SUPPORT CONSTRUCTION PLANT
61
in the foregoing discussion, the purpose of which being to obtain a reasonable approximate and general relation.
Dynamic condition A moving wheel travelling in an excavation or on fill applies a dynamic load to the ground. There are two factors to be considered: the stresses in the soil produced by the moving load and the bearing capacity of the soil under a dynamic loading condition. The values of these two factors are not necessarily the same as occur under static loading conditions. The very limited information published on the shear strength of soils under transient loading [5,6] indicates that their measured strengths increase with decreasing time of loading. Also, so far as the author is aware, there is very little information on the stresses in the soil caused by a wheel load moving on the soil. Nor is there any published information on the overall effect of the dynamic stresses produced in the soil and the dynamic strength of the soil under a moving load. However, there are two general observations which suggest that the stability of the soil under a moving wheel load is, if anything, better than that under a static wheel load. The first is that, when a vehicle moves over a patch of soft ground it can often cross it at speed, where it would otherwise get bogged down if it travelled slowly or stalled. TXeL~ I. APPROXIMATE STANDARD CLASSIFICATION OF TIlE CONSISTENCY OF CLAYS
Consistency Very soft Soft Finn Stiff Very stiff Hard
Shear strength of clay (lb/in~) <2"5
2"5-5 5-10 10-20 20--40 >40
The second observation relates to the compaction of soils. Experience with rollers travelling at speeds of up to 10 m.p.h, or faster, indicate that, with this range, an increase in the speed of travel has little effect on the compacted dry density--usually a slight reduction in density. Since the process of compaction involves local shear failure and plastic deformation under a moving wheel load, this suggests that the stability of soil under a wheel load is, if anything, better when the wheel is moving than when it is static. So that, the limit as indicated by equation (12) for a static loading condition probably provides a conservative guide to the limit for the same load when moving.
General To assess the significance of the above, consider the shear strengths of the clays usually encountered in earthworks in the British Isles. Most of the clays encountered in cutting and fills in this country are heavily overconsolidated "fat" clays and boulder clays. As a generalization, the upper weathered zones of these deposits are more frequently of a firm consistency (see Table I), becoming stiff with depth,
62
s. RODIN
so that in relatively shallow cuttings--say to depths of 10 to 20 ft below ground level--firm to stiff clays are likely to be encountered more frequently than stiff to very stiff clays. Soft clays are encountered immediately around estuaries, rivers and streams, and to a certain extent in the boulder clays and weathered parts of the overconsolidated clays. The foregoing simplified analysis of the relation between the shear strength of the soil and the maximum tyre pressure that it can support is approximate, But it provides a useful guide on the relation between the strength of the soil and the wheel loads and contact pressures that it can support without excessive deformation of the soil. It is evident that standard earthmoving tyres with inflation pressures of the order of 50 1b/in ~ or more should be used on stiff, or stronger, clays if +10
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FIG. 7. Relation between moisture content and plastic limit for a range of shear strengths at 10 per cent air voids. K e y to F i g . 7.
c = Shear strength (lb / in2) p~=Maximum permissible tyre pressure 0b/in z) =5c they are to operate without causing large sinkage into the clay. For rubber tyres to operate satisfactorily on firm clays, it is necessary to reduce the inflation pressures to between about 25 to 50 lb/in ~, depending on the shear strength of the soil. To further illustrate its significance, the relation between the relative moisture
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
63
content of the compacted soil and its plastic limit for a range of maximum allowable tyre pressures (based on p, = 5c) is indicated in Figs. 7 and 8 (which are based on Figs. 4 and 5). These diagrams show that at an arbitrary specification moisture content limit such as w = P L + 2 per cent, the shear strength of the soil and hence the wheel load that it can support, varies considerably over the range of plastic limits. For example, to support a wheel with a tyre inflation pressure of 50 lb/in s on soil compacted to 5 per cent air voids, the theoretical moisture content ranges from approximately (PL=2.5 per cent) at P L = 2 0 per cent to (PL+4.5 per cent) at PL = 32 per cent. It is emphasized that this comparison should only be used/or illustrative purposes at the present stage since, as shown in Figs 4 and 5, some o/
the measured shear strengths differ from the theoretically estimated values. +10
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FIG. 8. Relation between moisture content and plastic limit for a range of shear strengths at 5 per cent air voids. Key to Fig. 8. c=Shear strength (lb/in~) p~---Maximum permissible tyre pressure Ob/ins) =5c
STRENGTH OF COMPACTED FILL AS A BASIS FOR SPECIFICATION, COMPACTION A N D WHEEL LOADING REQUIREMENTS
The shear strength of the fill is a basic consideration in the design of earthworks The minimum design shear strength could provide a practical basis for the
64
S. RODIN
specification, compaction and wheel loading requirements, and so provide an easily measured criterion common to the various phases of the design and construction of the earthworks. At present, there is no direct link between these phases. For example, for road works the design and specification have usually been based on the arbitrary placement moisture content limit of PL + 2 per cent; but the engineer is unable to assess the significance of this limit in relation to the trafficability of different types of earthmoving plant and to the design and economics of the earthworks for the particular soil conditions at a site. Material is specified as unsuitable if its moisture content is greater than PL+ 2 per cent, but no study is usually made of the feasibility and the practical and economic considerations of utilizing some of the wetter material as fill. The suitability and significance of using the shear strength criterion in the various phases of earthworks design and construction are discussed in the following sections.
Design For the design it is necessary to study the strength of the compacted soil in order to assess a minimum value for stability analysis. It is also necessary to consider the variability of the soil conditions at the site and the extent to which available material can be utilized for the fill; the latter will depend on the ability to operate earthmoving plant and the cost of such work. As discussed earlier, the ability to operate earthmoving plant on compacted fill is dependent on the shear strength of the fill For the general range of earthmoving wheels, the standard inflation pressures for maximum load varies between 45 and 70 lb/in 2. The simplified analysis mentioned earlier indicates that, to support tyres with inflation pressure of 45 lb/i#, the minimum shear strength of the fill should be about 9 l b / i # if excessive sinkage is to be avoided. The corresponding value for an inflation pressure of 70 lb/in ~ is a minimum shear strength of about 14 lb/in ~. For comparison, a firm clay has a shear strength in the range of 5 to 10 lb/in 2. This suggests that the general range of earthmoving plant, when working at their maximum tyre inflation pressures, cannot operate satisfactorily on firm clays, or at least the lower part of the firm range. Yet, from the design point of view, firm clays having a minimum shear strength of 5 lb/in ~ could probably be placed in most earthworks without serious consequences. Most road and railway embankments built with clay fills are designed with side slopes of 2½ to 1 or 3 to 1. Taking, as an example, a bank 30 ft high, with side slopes of 3 to 1, and built entirely with a clay fill having a shear strength of 5 lb/in ~ the factor of safety against slipping would be 1.9. Thus, where a large amount of the available material is relatively weak and the compacted shear strength is inadequate to support present type earthmoving plant, a study should be made of the possibility of using plant with lower tyre pressures and relating the operating cost of doing this with the design (cross-section) requirements for a stable fill. It may be worthwhile to introduce thin drainage layers in order to accelerate consolidation, and hence 'an increase in shear strength, as the height of fill is being increased. The alternative would be to take the relatively
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
65
wet material to spoil and replace it with imported material--usually an expensive operation. The compressibility, and hence settlement, of a bank is also basic design consideration and would depend on the consistency of the fill material. The significance of using weaker fill than might be allowed in present practice is a matter of investigation and will depend on the circumstances in each case. For a road embankment, the differential settlement over a short length of road is more important than the total settlement; in this respect, good construction practice in the selective placing of the fill material should reduce the degree of differential settlement over short lengths of embankment. The shear strength of the compacted soil depends on its moisture content and dry density. The dry density varies with the type and size of the compactor, thickness of layer and number of passes. The consideration of all these factors can be simplified by selecting a particular degree of compaction from past field experience. For example, for cohesive soils (which generally exist in a nearly saturated condition in the British Isles), it is now fairly well established that compaction to a relative density corresponding to 10 per cent air voids or less, produces a satisfactory fill for the general run of road embankments. This value of maximum air voids is used to illustrate this article, but whatever value is selected to be most suitable for the fill at a particular job, the general procedure discussed herein remains the same. At the investigation stage, representative samples of the fill material could be compacted in the laboratory at the natural moisture content and to the selected minimum relative density (e.g. corresponding to 10 per cent air voids content), and the shear strength measured in the unconfined or undrained triaxial compression test. As seen from Figs. 1 and 2, this would represent the minimum shear strength of that soil sample after compaction at the natural moisture content, since any higher degree of compaction would produce a higher shear strength (the upturn of the curves of constant shear strengths as they approach the zero air voids curve--which is due to high pore pressures set up by overcompacting--might qualify this statement, but it does not significantly affect the suggested procedure).
Specification Having established the economic design requirements from laboratory tests on representative samples compacted at the natural moisture contents or any other moisture contents relevant to the particular construction, the minimum shear strength of the compacted fill can be specified. Control tests on the earthworks could then consist of unconfined compression tests or in situ CBR tests, using the relation in equation (3). Either test can be carried out quickly and immediately following compaction; in this respect, they provide a more satisfactory control test than the relative compaction or air voids tests which rely basically on moisture content determinations and involve a long time lag in the standard test procedure. Field control based entirely on a particular value of minimum shear strength may not by itself ensure that the soil has been adequately compacted to a particular minimum degree of compaction and that excessive consolidation of the fill will not occur. As can be seen from Figs. 1 and 2, as the moisture content of the fill decreases, a particular value of shear strength can be obtained at an increasing
66
S. RODIN
value of air voids content. To check the degree of compaction by, say, the air voids method it is necessary to determine the moisture content of the sample; from the time point of view, this would nullify the time advantage of the shear strength control test mentioned in the previous paragraph. An alternative approach to the control of degree of compaction is to specify the compaction procedure, i.e. type and size of compactor, minimum thickness of layer and minimum number of passes. A great deal of experience has already been gained with the commonly encountered soils to enable the minimum compaction procedure requirements to be established within reasonable practical limits. On any important job or fill involving a material on which there is inadequate experience, the compaction procedure can be obtained by field compaction trials prior to commencement of construction. Such trials are desirable in any case, whatever method of control is used. By thus establishing the minimum compaction procedure required to produce the minimum degree of compaction for the particular soil over the range of moisture contents that will be encountered, it will not be necessary to have routine moisture content control tests. This leaves the strength test as the only or principal routine control test necessary to check the quality of the compacted fill. Taking this a stage further, the wheel loadings of the earthmoving plant and of the pneumatic-tyred rollers, where these are used, provide a check on the strength of the compacted fill. As discussed previously, the compacted fill must have a sufficient shear strength to enable it to support the wheel loads. The wheel loads therefore provide a proof loading test over virtually the entire area of each layer of compacted fill. The relation between the shear strength of the soil and its wheel carrying capacity, as discussed earlier, is indicated by equation (12). That is, where a wheel is supported by the fill and the sinkage is not greater than 2 in., the shear strength of the fill is given by c >1 pi/5.
For example, if a minimum shear strength of 6 l b / i # is specified for a particular job, the compacted fill should be capable of supporting wheels with a minimum inflation pressure of 30 lb/in 2. If, the earthmoving plant actually used on that job have higher inflation pressures, say, 50 Ib/in 2, they act as proof loading for a minimum shear strength of 10 lb/in 2 and excessive sinkage would be expected if the fill has lower shear strengths. The latter statement should perhaps be qualified in that, if the pressures of the wheels on the earthmoving plant were greater than the pressures of the compactor wheels, excessive sinkage could occur as a result of further compaction under the earthmoving #ant. This will largely depend on whether the moisture content of the fill lies on the dry or wet side of the optimum moisture content on the field compaction curve. If further compaction can occur because of higher contact pressures of the earthmoving plant, this could be undesirable, because of the effects of excessive sinkage of the wheels on trafficability and tractive resistance, Consequently, from a practical point of view, it is preferable for the pressure of the compaction wheels to be equal to or greater than the pressure of the wheels on the earthmoving plant.
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
(;7
Plant requirements
The foregoing analysis provides the contractor with a method for assessing the wheel loading requirements to suit the soil conditions. It should be appreciated that the supporting capacity of the ground is only one of the criteria affecting the performance of the plant from the soil mechanics point of view--other important factors are traflickability and tractive resistance. A discussion on tractive resistance is outside the scope of this paper but the subject of trafficability is largely covered by the discussion on sinkage. It is necessary for the contractor to consider the range of materials that will be encountered in the cuttings and embankments and the basis of the specification (i.e. design). With earthworks in the British Isles, cohesive soils with a wide range of consistencies are likely to be encountered on a large earthworks project. At present, the contractor's choice of plant is based on experience of previous work, and there is no method whereby the wheel loading requirements can be calculated in advance from a knowledge of the soil conditions. At present, neither the contractor nor the engineer knows whether the usual specification moisture content limit corresponds to the limit of satisfactory operation of the available plant. From the engineer's point of view, having selected a minimum design strength, he would like to continue working up to this limit, providing it is economical compared to the alternative of running the weaker material to spoil. The relation between the shear strength of the fill and its wheel carrying capacity provides both the engineer and the contractor with a means of assessing the tyre pressures required for the particular conditions. It is worthwhile, therefore, to consider some possible methods of working rubber-tyred plant on firm clays with a shear strength of say 5 lb/in ~, viz. : 50,000
f
jr1'
37.5 x SS
.,~,,,,L-30.O0 x 33
~,000
30,000
/ Earthmover tyre
~- 2o,oo02
Wide base earthmover tyre I
I
30 qO 50 Tyre i n f l a t i o n pressure (Pi) " Ib/sq. in. I
*
s
6
8
I0
6
12
Shear Strength (c ,QPi ) . Ib/sq. in. 5
Relationbetween maximumtyre load and inflationpressure for some typical earthmovertyres (Data from Tyre and Rim Association Handbook).
FiG. 9.
68
~. RODIN
(a) Reduce the tyre inflation pressure on the standard wheels of the particular plant to the value required by the shear strength of the fill. This would reduce the m a x i m u m load that could be carried by the wheels, as illustrated in Fig. 9 for three of the heavier earthmoving tyres. (b) Replace the standard wheels on the particular plant with larger wheels and operate them at the required lower inflation pressure, so that the load that can be carried by the larger wheels at the lower inflation pressure is of the same order as the load rating of the plant. The increase in size of the wheel is probably more effectively carried out by increasing the width of the tyre rather than the rim diameter. However, with the bigger earthmoving plant, the present wheels are already considerable in size and are very heavy to handle. On the other hand, increasing the size of a wheel generally reduces the rolling resistance and improves traction. (c) Use dual wheels with lower tyre pressures. Dual wheels were, in fact, used on the older scraper plant but gave trouble because of clogging between the wheels; however, this might not be an insurmountable problem and may be worth reconsidering. Since rubber tyres are a large item in earthmoving costs, any of the above medifications may involve a higher plant cost. But the principal advantage would be the ability to work with relatively weaker soils than is practicable at present and which would otherwise have to be taken to spoil and replaced with imported material. In addition, the use of a lower tyre inflation pressure would reduce the adverse effect of wet weather on the plant working on the present range of fill material due to the rainfall wetting the surface layer of soil and reducing its shear strength. Acknowledgements--The author is indebted to Mr. J. T. Finey, B.SC., A.C.G.I., for his assistance in the preparation of this article.
[1] [2] [3] [4] [5] [6] [7]
REFERENCES S. RODIN, Muck Shifter and Bulk Handler, April, pp. 34-39 and May, pp. 34-41 (1964). W.A. LEWm. R. Res. Tech. Pap. No. 45 (1959). W.P.M. BLACK. Geotechnique, 12, (1962). D.K. HEIPLE. Constr. Methods and Equip., Aug., pp. 84-90 (1951). A. CASAGR~rDE and W. L. SHANNON. Trans. Amer. Soc. Civ. Eng., 114, p. 775 (1949). H.B. SEED and R. LUNt~t~N. Proc. Arner. Soc. Test Mater. 54, p. 288 (1954). University of Birmingham, Graduate School of Highway and Traffic Engineering, Research Bulletin 1964--65, pp. 168-174. Note A d d e d in Proof
Subsequent to the preparation of this article, C. S. Du.nn [7] has collected CBR results for several clays and compared them to the values predicted using Black's method. There is a wide scatter in the results obtained from different clays; the strength of the softer clays tends to l~e underestimated by Black's method. However, Black's method, or a modification of it, represents a useful guide to the strength properties which can be expected under varying moisture contents. "Ihe principal reason for using it in this article was to illustrate that at an arbitrary moisture content limit (e.g.P.L. +2 per cent) the strength of clay fill can vary o v e r a very wide range. Therefore, such arbitrary placement limits bear no relation to the ability of a clay fill to support a particular wheel load.
Printed in Great Britain.
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT* S. RODIN~" IN THE DESIGN of earthworks many factors must be considered, not the least important of which are the nature of the works, the physical properties of the available materials and their relative costs. Consideration must be given to the problems of the construction teams in transporting and placing the materials, and to the types and costs of plant available for this work. The natural moisture contents of earth fill materials in the British Isles are usually relatively high and, except in special circumstances, watering of the fill is unnecessary: nor is it normally practicable in this country to dry out clayey soils to any significant extent. Thus, the moisture content of the fill when placed will generally be at or near the natural moisture content. At the same time, the degree of compaction assumed in the design must not be higher than can reasonably be achieved by modern plant. All too often these considerations, if studied at all, are considered as individual factors. Most designs for earthworks--large jobs as well as small ones--are approached by starting with a "standard" specification for the moisture content and density limits of the compacted fill, and in many cases little thought is given as to whether this specification should be modified to suit the particular site and as to whether this specification should be modified to suit the particular site and construction conditions. All too rarely are the design, construction methods and economic design [ 1]. In practice, it is desirable to make the fullest use of available excavation material provided its strength is adequate for the design criteria and the earthmoving plant can operate satisfactorily and economically. Thus, with the relatively wet conditions in the British Isles, a realistic basis for the specification would be to relate the placement limit of wet material to the shear strength requirements, and so provide an easily measured common factor between design and construction considerations. The object of this article is to consider the basis for such a method and to discuss the limited amount of available information relating the principal factors. Primary attention is given to cohesive soils, since these are usually the critical materials in relation to specification and construction problems in the British Isles. SHEAR STRENGTH OF COMPACTED SOIL Although a considerable number of earthworks have been built in recent years, surprisingly little information has been published on the relation between shear *Reproduced from Civil Engineering and Public Works Review, February, 1965. tCentral Laboratory, Messrs. George Wimpey & Co., Ltd., Southall, Middlesex, England Communicated by A. R. Reece.
51
52
S. RODIN
strength, moisture content and dry density for different soil types. One of the few such publications [2] gives this information for a heavy clay and a sandy clay, although the information obtained for these soils covers a moisture content range somewhat drier than the values more usually encountered in practice. It is of -
,, ',
\~/7
Zero air voids I s~. air voids
xX/~
I0,~ air voids
",
\
~
,,,,
"~
~ noo
g
/ 'k/
~//
Shear ;trength
"
1
~
V~._
,,o
I
"~ "'~ ~
-~ /
Ib/sq. i n . ) l ~
/
__.
/
//,
/ x
90
/: I2
,
20
2q Noisture Content - per cent
28
FIG. 1. Relation between dry density, moisture content and shear strength with a compacted heavy clay (LL=75%, PL=23%). Key to Fig. 1. Curve 1 indicates B.S. laboratory compaction tests. Curve 2 indicates B.S. heavy laboratory compaction tests. ( A > indicates average moisture content range for British Isles. denotes density achieved with different pneumatic-tyred rollers, viz. Roller Q •
12-ton 20-ton
•
[] •
45-ton
Wheel load Tyre inflation pressure (tons) (lh/in ~) 1'33 2-22
36 80
5-0
9O 90 140
10"0 10"0
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT I~
/ r ~,
', / \, Z i 20
/ ~'
'
,
53
I
~
/-Zero air voids
\VI
I
\~.f'-5~
~
air void, I
i
10~ sir voids
,/
',// ',,
,.Q I o_
g 4
II0 105
~~v~~ .'
lOO 8
/.'/ I , ' // I ', /"#
~'#/
,
"
/~'
I
',,
!~~i
Y~Strength ,/ (lb/.~
'.
12 16 20 t4oisture Content - per cent
2~
FIG. 2. Relation between dry density, moisture content and shear strength with a compacted sandy clay (LL=40%, PL--20%). Key to FiI. 2.
Curve 1 indicates B.S. laboratory compaction tests. Curve 2 indicates B.S. heavy laboratory compaction tests. < A ~ indicates average moisture content range for British Isles. denotes density achieved with different pneumatic-tyred rollers, viz. Roller O •
12-ton 20-ton
A
[] •
45-ton
Wheel load Tyre inflation pressure (tons) lb/in. 1.33 2"22
36 80
5'0
9o 90 140
10"0 10.0
interest to re-plot this information in the form shown in Figs. 1 and 2, against the background of the compaction curves obtained in the field compaction trials. For a given moisture content, the shear strength of the compacted soil increases as the dry density increases, until the zero air voids curve is approached. There does not appear to be any test data to cover the latter stage, when the air voids content is only 1 or 2 per cent, although the available information suggests an upturn in the constant strength curves.
54
S. RODIN
It would be valuable to engineers to have data similar to that shown in Figs. 1 and 2 for different types of compacted soil, if only to obtain a general appreciation of the relation between shear strength, dry density, moisture content and the classification characteristics of the soil. It should be pointed out that the densities shown in Figs. 1 and 2 are for 32 passes of the roller. Since the number of passes normally used is much smaller than this--for practical and economic reasons-the range of maximum densities obtained in the field would generally be about 2 to 5 lb/fP lower than the values shown.
Relationship between shear strength, moisture content and plasticity indices of compacted clay Black [3] describes an empirical method of estimating the California Bearing Ratio (CBR) of a remoulded cohesive soil from a knowledge of the suction of the remoulded soil and its true angle of friction, which can be inferred from a knowledge of the plasticity index (P/), the liquid and plastic limits (LL and PL) and moisture content; in the derivation of these relations the following equations were used.
CBR=q./10
(1) where qu is the bearing capacity of a circular footing on clay,
(PI) = 0.838 ( L L ) - 14.2
(2)
Equation (2) appears to be characteristic of British cohesive soils in general; it is shown plotted in Fig. 3. At this stage, only clays or plastic soils are considered in this and subsequent sections dealing with the shear strength and wheel supporting capacity of the fill, since the analysis for granular soils is more complex. Also, plastic soils are 60
!
~o
/
Ill
• .<
iiii'!::iiii:iiii i::i!i
J
f/ Y
•.. 30 U
~
2o
I0
I0
20
~
~0 50 60 Liquid L i m i t -
70
80
FIG. 3. Plasticity characteristics of soils used in analysis. Key
• London clay (A) + Sandy clay (B) Q London clay (C)
x •
Brickearth (D) Silty clay (E)
90
I00
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
55
usually the critical materials in relation to specification and construction problems in the British Isles. Taking q. = 6.2c for a circular on clay, from equation (1). CBR = 6.2c/10
(3)
where c is the apparent cohesion (lb/inS). Black's Fig. 9 gives the relation between CBR and plasticity index at various consistency indices. In view of the use of plastic limit as the present basis for specifying placement moisture content limits, it would be useful to determine the u,O0
.20o --
so
\\\\
60 J ~
I
'
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-
a
20
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,
X)\
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60
N o i s t u r e Content (w) -
1~o. 4.
• + O × A
Relation between shear strength and moisture content at a range of plastic limits for soil compacted to 10 per cent air voids. Key to Fig. 4. London clay (A) PL=23 LL=75 c measured Sandy clay (B) P L =2 0 L L = 4 0 c measured London clay (C) PL--27 LL=75 c converted from C.B.R. Brickearth (D) P L = 2 0 L L = 3 0 c converted from C.B.R. Silty clay (E) PL=22 LL=42 c measured
56
S. R O D I N
relation between shear strength and moisture content relative to the plastic limit. This can be done through the relationship. Consistency Index (C/) = ( L L - w ) / ( L L - P L )
(4)
C I = 1 + [ ( P L - w)/(5.16 ( P L ) - 87.7)]
(5)
which becomes
From the above equations and Black's curves, the relation between estimated shear strength and moisture content has been calculated for different values of plastic limit, for conditions corresponding to 5 per cent and 10 per cent air voids tLO0
200
o C
I00
,,..,..
,,,
80
--
60
I
-~
q.o
0 z,,. a$
2O
~r~ ,~u
I0
g
e
~
6
\ 2".\ ,'
w=
PL
\\
\/.\A
iA\ I'0
0"8 0"6 I0
20
30 Hoisture
q,O Content
50
60
(wI -
Relation between shear strength and moisture content at a range of plastic limits for soil compacted to 5 per cent air voids. Key to Fig. 5. Q London clay (A) P L = 2 3 L L = 7 5 c measured Sandy clay (B) P L = 2 0 L L = 4 0 c measured @ London clay (C) P L = 2 7 L L = . 7 5 c converted from C.B.R. Brickearth (D) P L = 2 0 L L = 3 0 c converted from C.B.R. x Silty clay (E) P L = 2 2 L L = 4 2 c measured
FIG. 5.
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
57
respectively. These relationships are given in Figs. 4 and 5. Air voids contents of 5 per cent and 10 per cent have been selected as they are representative of the degree of compaction usually achieved in practice. Measured values of shear strength, or converted from CBR values, are compared with the estimated values in Figs. 4 and 5. The sources of these measured values are as follows. Soils A and B: [2]. Soils C and D: [3]. Soil E: unpublished data obtained from site investigation in Iraq (due to the lack of any other relevant published information for British soils, this example is included further to illustrate the relationship). Although the relations between shear strength and moisture content followed the estimated pattern, there are differences between the measured and estimated values. In the two cases of London clay, the difference between the measured and estimated shear strengths is greater in one case than in the other. This is no doubt due to the various assumptions and generalizations made in the different steps leading to the theoretical relation for estimating the shear strength from a knowledge of the moisture content and the plasticity index, and also to natural variations in the physical characteristics of a particular soil deposit. Hence, on any earthworks of importance, these theoretical relations should not be relied upon as a basis for design without carrying out some testing to determine the actual shear strength of the compacted soil. Nevertheless, bearing in mind the simplifying and very generalized nature of the theoretical relation, Figs. 4 and 5 provide a useful general guide on the effect of moisture content on the shear strength of the compacted soil. Perhaps of greater importance in the context of this paper. Figs. 4 and 5 illustrate the significance of the moisture content relative to the plastic limit of the soil. Interposed on these graphs are two curves corresponding to: (a) (b)
w=PL. w = P L + 2 per cent.
The curve corresponding to (b) represents the upper limit of placement moisture content, which has been commonly used in road work specifications. It shows that at this relative moisture content, the strength of the compacted clay increases very significantly as the plastic limit of the soil increases. It should be noted that the experimental points shown in Figs. 4 and 5 represent somewhat drier conditions than usually found in the British Isles. ABILITY OF THE GROUND TO SUPPORT EARTHMOVING PLANT As earthmoving plant have become bigger, their loads on the ground have also increased. The high tyre contact pressures of standard earthmoving tyres require good ground conditions for their satisfactory operation. The working surface in the borrow pit or on the fill must be strong enough to support the wheels, or tracks, without their sinking excessively. The stability of the ground surface under a moving wheel or track is a much more complex subject than the stability of the ground under static foundations, such as for buildings. The interrelation between the physical properties of soils and their load carrying capacity for wheeled and tracked types of vehicle systems is a vital consideration in the design and efficient use of earthmoving plant, but, unfortunately, research
58
S. RODIN
on the subject has been sadly neglected. The voluminous proceedings of the 1st International Conference on the Mechanics of Soil-Vehicle Systems, held in Turin in 1961, record the work of several organizations in different countries who are trying to develop soil-wheel relationships, some of which are highly theoretical and complex, others empirical. However, a great deal more work is required before a satisfactory soil-wheel relationship is developed and which can be applied to practical earthmoving problems. There is no procedure for determining a limit of trafficking on earthworks, nor has this factor been defined in physical terms. A practical criterion, from the civil engineer's point of view, is that the loading on the wheel or track should not cause excessive rutting or sinkage of the wheel or track into the soil. That is, the wheel should not overstress the soil to the extent that excessive plastic deformation of the soil occurs, since at any higher loading the wheel would begin to sink into the soil and the remoulding action would tend to weaken the soil. There is a very wide range of sizes of earthmoving tyres. Most of the earthmoving (off-the-road) tyres are designed for maximum inflation pressures of between 45 and 70 lb/inL The wide base tyres, which are designed to provide greater flotation, have maximum inflation pressures of 45 to 55 lb/inL Smaller inflation pressures can be used in all cases, but only at the expense of reducing the maximum load on the wheel, and hence the capacity of the machine. The tyre manufacturers do not publish data relating wheel load, tyre inflation pressures and tyre contact pressure for the standard earthmoving tyres. Since some of the load on a wheel is carried by the walls of the tyre, the tyre contact pressure (p~) is not necessarily the same as the tyre inflation pressure (P3. A rule of thumb given by Heiple [4] for a condition of no penetration, i.e. tyre resting on a hard surface, is as follows : Area of contact (A1)=90 per cent weight on tyre/tyre inflation pressure =0.9W/p,
(6)
where the weight (W) on the tyre is the maximum permissible load on the tyre at the particular inflation pressure (from the Tyre and Rim Association tables). For a particular tyre, the permissible load decreases as the inflation pressure decreases. From this equation, the mean tyre contact pressure can be obtained as follows. Mean tyre contact pressure (pc) =Permissible load/Area on ground = W / A 1 = W x (pal0.9 W)= l.lp,
(7)
These equations are based on the tyre resting on a hard surface. There does not appear to be any published information on the contact area and pressure of standard earthmoving tyres resting on soils, in which some settlement or sinkage occurs. In this case, the contact area and pressure will be dependent on the extent to which the wheel sinks into the soil; the contact area against the soil will be greater than the contact area given by equation (6) for the same load on a hard surface. To obtain some guidance on the significance of different amounts of sinkage, Fig. 6 has been produced to indicate the estimated contact area for different
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT I
Kl~If:-
. . . . . . .
59
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600
700
, /
[--$inkage,A,~ ~ ! W l
.
\
:
AI = Normal g r m s contact area (hard ground) A2 = 6roes contact area at sinkage
Relation between estimated contact area (A 3) after sinkage = A in and contact area (A1) on hard ground for some typical earthmover tyros.
FzG. 6.
values of sinkage of some typical earthmoving tyres, basezl on the assumptions shown. Maximum allowable tyro contact pressure
This section discusses the relation between the shear strength of the ground and the maximum tyro contact pressure that can be supported without excessive sinkage. As seen from Fig. 6, the value of actual contact area for a particular w h ~ l varies with the amount of sinkage. To determine when sinkage of the wheel is becoming too large, it is necessary to select an arbitrary limit of sinkage.
60
S. RODIN
As a first approximation, assume that 2 in. is the maximum sinkage desirable when plant is running over, say previously compacted fill, in order to avoid excessive disturbance of the compacted material. At this limit Ao.=I.4A1 (approximately). The vertical pressure distribution over the area A2 will be non-uniform; the vertical pressure on the soil in contact with the sloping sidewalls of the tyre will presumably be much less than over the area A1. Also, any permanent rut formation behind the tyre would reduce the amount of the contact area A2 shown in Fig. 6. Owing to the lack of adequate experimental information on the actual pressure distribution, and to obtain some guidance on the effect of the larger contact area on the bearing capacity of the soil, it is assumed that the total load on the soil is equivalent to a mean contact pressure, p',, over an area A =½ (A1+A2)= 1.2A1. Hence, at a sinkage limit of 2 in. the assumed mean tyre contact pressure becomes p'c= W~ 1.2Al=0"9p, (approx.)
(8)
Static condition The condition of complete bearing capacity failure for a static circular or square footing resting on the surface of a clay soil having a shear strength c (lb/in2) is given by the equation ultimate bearing capacity, q,=6.2c
(9)
Before complete failure occurs, local overstressing in shear takes place and results in some plastic deformation of the soil. In the case of a flexible loaded area, the condition of overstressing begins when the pressure on the footing is approximately p -- 1rc (10) There is no satisfactory and simple method available for calculating the contact pressure (p'¢) required to produce a particular amount of sinkage of a rubber tyre, especially as a sinkage of, say, 2 in. involves relatively large shear deformation in the soil. The value of the pressure lies somewhere between the pressures given by equations (9) and (10). If an average value is taken, say
p'c=4.5c
(11)
then from equation (8), the following approximate relation is obtained for the assumed limit of sinkage -- 2 in., p,=5c (12) For the standard earthmoving tyres with the range of inflation pressures between 45 and 70 lb/in z, this limit of sinkage would occur when the corresponding shear strength is between 9 and 14 lb/in 2. In the case of the wide base tyres with inflation pressures ranging from 45 to 55 lb/in 2, the limit of sinkage would occur when the corresponding shear strength of the clay is about 9 to 11 lb/in ~. The effect of the wheel dimensions has not yet been included in this simplified analysis. In practice, the stresses under the larger wheels penetrate to greater depths and, therefore, for a given contact pressure produce greater deformation of the soil, i.e. a particular value of sinkage will be produced by a lower contact pressure than in the case of small wheels. However, this has not been taken into account
ABILITY OF CLA~ FILL TO SUPPORT CONSTRUCTION PLANT
61
in the foregoing discussion, the purpose of which being to obtain a reasonable approximate and general relation.
Dynamic condition A moving wheel travelling in an excavation or on fill applies a dynamic load to the ground. There are two factors to be considered: the stresses in the soil produced by the moving load and the bearing capacity of the soil under a dynamic loading condition. The values of these two factors are not necessarily the same as occur under static loading conditions. The very limited information published on the shear strength of soils under transient loading [5,6] indicates that their measured strengths increase with decreasing time of loading. Also, so far as the author is aware, there is very little information on the stresses in the soil caused by a wheel load moving on the soil. Nor is there any published information on the overall effect of the dynamic stresses produced in the soil and the dynamic strength of the soil under a moving load. However, there are two general observations which suggest that the stability of the soil under a moving wheel load is, if anything, better than that under a static wheel load. The first is that, when a vehicle moves over a patch of soft ground it can often cross it at speed, where it would otherwise get bogged down if it travelled slowly or stalled. TXeL~ I. APPROXIMATE STANDARD CLASSIFICATION OF TIlE CONSISTENCY OF CLAYS
Consistency Very soft Soft Finn Stiff Very stiff Hard
Shear strength of clay (lb/in~) <2"5
2"5-5 5-10 10-20 20--40 >40
The second observation relates to the compaction of soils. Experience with rollers travelling at speeds of up to 10 m.p.h, or faster, indicate that, with this range, an increase in the speed of travel has little effect on the compacted dry density--usually a slight reduction in density. Since the process of compaction involves local shear failure and plastic deformation under a moving wheel load, this suggests that the stability of soil under a wheel load is, if anything, better when the wheel is moving than when it is static. So that, the limit as indicated by equation (12) for a static loading condition probably provides a conservative guide to the limit for the same load when moving.
General To assess the significance of the above, consider the shear strengths of the clays usually encountered in earthworks in the British Isles. Most of the clays encountered in cutting and fills in this country are heavily overconsolidated "fat" clays and boulder clays. As a generalization, the upper weathered zones of these deposits are more frequently of a firm consistency (see Table I), becoming stiff with depth,
62
s. RODIN
so that in relatively shallow cuttings--say to depths of 10 to 20 ft below ground level--firm to stiff clays are likely to be encountered more frequently than stiff to very stiff clays. Soft clays are encountered immediately around estuaries, rivers and streams, and to a certain extent in the boulder clays and weathered parts of the overconsolidated clays. The foregoing simplified analysis of the relation between the shear strength of the soil and the maximum tyre pressure that it can support is approximate, But it provides a useful guide on the relation between the strength of the soil and the wheel loads and contact pressures that it can support without excessive deformation of the soil. It is evident that standard earthmoving tyres with inflation pressures of the order of 50 1b/in ~ or more should be used on stiff, or stronger, clays if +10
/
, .•' X*
-,..~ ,8
,
/
~,~Y, /
~. /~ _ q y
:y
/
/
;/
/o,.',J
I /
~.
.
I/
/
o..
4
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-I
FIG. 7. Relation between moisture content and plastic limit for a range of shear strengths at 10 per cent air voids. K e y to F i g . 7.
c = Shear strength (lb / in2) p~=Maximum permissible tyre pressure 0b/in z) =5c they are to operate without causing large sinkage into the clay. For rubber tyres to operate satisfactorily on firm clays, it is necessary to reduce the inflation pressures to between about 25 to 50 lb/in ~, depending on the shear strength of the soil. To further illustrate its significance, the relation between the relative moisture
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
63
content of the compacted soil and its plastic limit for a range of maximum allowable tyre pressures (based on p, = 5c) is indicated in Figs. 7 and 8 (which are based on Figs. 4 and 5). These diagrams show that at an arbitrary specification moisture content limit such as w = P L + 2 per cent, the shear strength of the soil and hence the wheel load that it can support, varies considerably over the range of plastic limits. For example, to support a wheel with a tyre inflation pressure of 50 lb/in s on soil compacted to 5 per cent air voids, the theoretical moisture content ranges from approximately (PL=2.5 per cent) at P L = 2 0 per cent to (PL+4.5 per cent) at PL = 32 per cent. It is emphasized that this comparison should only be used/or illustrative purposes at the present stage since, as shown in Figs 4 and 5, some o/
the measured shear strengths differ from the theoretically estimated values. +10
+8
I
!
B ,..J
._o +q. m o o
+2
i.
g
FIG. 8. Relation between moisture content and plastic limit for a range of shear strengths at 5 per cent air voids. Key to Fig. 8. c=Shear strength (lb/in~) p~---Maximum permissible tyre pressure Ob/ins) =5c
STRENGTH OF COMPACTED FILL AS A BASIS FOR SPECIFICATION, COMPACTION A N D WHEEL LOADING REQUIREMENTS
The shear strength of the fill is a basic consideration in the design of earthworks The minimum design shear strength could provide a practical basis for the
64
S. RODIN
specification, compaction and wheel loading requirements, and so provide an easily measured criterion common to the various phases of the design and construction of the earthworks. At present, there is no direct link between these phases. For example, for road works the design and specification have usually been based on the arbitrary placement moisture content limit of PL + 2 per cent; but the engineer is unable to assess the significance of this limit in relation to the trafficability of different types of earthmoving plant and to the design and economics of the earthworks for the particular soil conditions at a site. Material is specified as unsuitable if its moisture content is greater than PL+ 2 per cent, but no study is usually made of the feasibility and the practical and economic considerations of utilizing some of the wetter material as fill. The suitability and significance of using the shear strength criterion in the various phases of earthworks design and construction are discussed in the following sections.
Design For the design it is necessary to study the strength of the compacted soil in order to assess a minimum value for stability analysis. It is also necessary to consider the variability of the soil conditions at the site and the extent to which available material can be utilized for the fill; the latter will depend on the ability to operate earthmoving plant and the cost of such work. As discussed earlier, the ability to operate earthmoving plant on compacted fill is dependent on the shear strength of the fill For the general range of earthmoving wheels, the standard inflation pressures for maximum load varies between 45 and 70 lb/in 2. The simplified analysis mentioned earlier indicates that, to support tyres with inflation pressure of 45 lb/i#, the minimum shear strength of the fill should be about 9 l b / i # if excessive sinkage is to be avoided. The corresponding value for an inflation pressure of 70 lb/in ~ is a minimum shear strength of about 14 lb/in ~. For comparison, a firm clay has a shear strength in the range of 5 to 10 lb/in 2. This suggests that the general range of earthmoving plant, when working at their maximum tyre inflation pressures, cannot operate satisfactorily on firm clays, or at least the lower part of the firm range. Yet, from the design point of view, firm clays having a minimum shear strength of 5 lb/in ~ could probably be placed in most earthworks without serious consequences. Most road and railway embankments built with clay fills are designed with side slopes of 2½ to 1 or 3 to 1. Taking, as an example, a bank 30 ft high, with side slopes of 3 to 1, and built entirely with a clay fill having a shear strength of 5 lb/in ~ the factor of safety against slipping would be 1.9. Thus, where a large amount of the available material is relatively weak and the compacted shear strength is inadequate to support present type earthmoving plant, a study should be made of the possibility of using plant with lower tyre pressures and relating the operating cost of doing this with the design (cross-section) requirements for a stable fill. It may be worthwhile to introduce thin drainage layers in order to accelerate consolidation, and hence 'an increase in shear strength, as the height of fill is being increased. The alternative would be to take the relatively
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
65
wet material to spoil and replace it with imported material--usually an expensive operation. The compressibility, and hence settlement, of a bank is also basic design consideration and would depend on the consistency of the fill material. The significance of using weaker fill than might be allowed in present practice is a matter of investigation and will depend on the circumstances in each case. For a road embankment, the differential settlement over a short length of road is more important than the total settlement; in this respect, good construction practice in the selective placing of the fill material should reduce the degree of differential settlement over short lengths of embankment. The shear strength of the compacted soil depends on its moisture content and dry density. The dry density varies with the type and size of the compactor, thickness of layer and number of passes. The consideration of all these factors can be simplified by selecting a particular degree of compaction from past field experience. For example, for cohesive soils (which generally exist in a nearly saturated condition in the British Isles), it is now fairly well established that compaction to a relative density corresponding to 10 per cent air voids or less, produces a satisfactory fill for the general run of road embankments. This value of maximum air voids is used to illustrate this article, but whatever value is selected to be most suitable for the fill at a particular job, the general procedure discussed herein remains the same. At the investigation stage, representative samples of the fill material could be compacted in the laboratory at the natural moisture content and to the selected minimum relative density (e.g. corresponding to 10 per cent air voids content), and the shear strength measured in the unconfined or undrained triaxial compression test. As seen from Figs. 1 and 2, this would represent the minimum shear strength of that soil sample after compaction at the natural moisture content, since any higher degree of compaction would produce a higher shear strength (the upturn of the curves of constant shear strengths as they approach the zero air voids curve--which is due to high pore pressures set up by overcompacting--might qualify this statement, but it does not significantly affect the suggested procedure).
Specification Having established the economic design requirements from laboratory tests on representative samples compacted at the natural moisture contents or any other moisture contents relevant to the particular construction, the minimum shear strength of the compacted fill can be specified. Control tests on the earthworks could then consist of unconfined compression tests or in situ CBR tests, using the relation in equation (3). Either test can be carried out quickly and immediately following compaction; in this respect, they provide a more satisfactory control test than the relative compaction or air voids tests which rely basically on moisture content determinations and involve a long time lag in the standard test procedure. Field control based entirely on a particular value of minimum shear strength may not by itself ensure that the soil has been adequately compacted to a particular minimum degree of compaction and that excessive consolidation of the fill will not occur. As can be seen from Figs. 1 and 2, as the moisture content of the fill decreases, a particular value of shear strength can be obtained at an increasing
66
S. RODIN
value of air voids content. To check the degree of compaction by, say, the air voids method it is necessary to determine the moisture content of the sample; from the time point of view, this would nullify the time advantage of the shear strength control test mentioned in the previous paragraph. An alternative approach to the control of degree of compaction is to specify the compaction procedure, i.e. type and size of compactor, minimum thickness of layer and minimum number of passes. A great deal of experience has already been gained with the commonly encountered soils to enable the minimum compaction procedure requirements to be established within reasonable practical limits. On any important job or fill involving a material on which there is inadequate experience, the compaction procedure can be obtained by field compaction trials prior to commencement of construction. Such trials are desirable in any case, whatever method of control is used. By thus establishing the minimum compaction procedure required to produce the minimum degree of compaction for the particular soil over the range of moisture contents that will be encountered, it will not be necessary to have routine moisture content control tests. This leaves the strength test as the only or principal routine control test necessary to check the quality of the compacted fill. Taking this a stage further, the wheel loadings of the earthmoving plant and of the pneumatic-tyred rollers, where these are used, provide a check on the strength of the compacted fill. As discussed previously, the compacted fill must have a sufficient shear strength to enable it to support the wheel loads. The wheel loads therefore provide a proof loading test over virtually the entire area of each layer of compacted fill. The relation between the shear strength of the soil and its wheel carrying capacity, as discussed earlier, is indicated by equation (12). That is, where a wheel is supported by the fill and the sinkage is not greater than 2 in., the shear strength of the fill is given by c >1 pi/5.
For example, if a minimum shear strength of 6 l b / i # is specified for a particular job, the compacted fill should be capable of supporting wheels with a minimum inflation pressure of 30 lb/in 2. If, the earthmoving plant actually used on that job have higher inflation pressures, say, 50 Ib/in 2, they act as proof loading for a minimum shear strength of 10 lb/in 2 and excessive sinkage would be expected if the fill has lower shear strengths. The latter statement should perhaps be qualified in that, if the pressures of the wheels on the earthmoving plant were greater than the pressures of the compactor wheels, excessive sinkage could occur as a result of further compaction under the earthmoving #ant. This will largely depend on whether the moisture content of the fill lies on the dry or wet side of the optimum moisture content on the field compaction curve. If further compaction can occur because of higher contact pressures of the earthmoving plant, this could be undesirable, because of the effects of excessive sinkage of the wheels on trafficability and tractive resistance, Consequently, from a practical point of view, it is preferable for the pressure of the compaction wheels to be equal to or greater than the pressure of the wheels on the earthmoving plant.
ABILITY OF CLAY FILL TO SUPPORT CONSTRUCTION PLANT
(;7
Plant requirements
The foregoing analysis provides the contractor with a method for assessing the wheel loading requirements to suit the soil conditions. It should be appreciated that the supporting capacity of the ground is only one of the criteria affecting the performance of the plant from the soil mechanics point of view--other important factors are traflickability and tractive resistance. A discussion on tractive resistance is outside the scope of this paper but the subject of trafficability is largely covered by the discussion on sinkage. It is necessary for the contractor to consider the range of materials that will be encountered in the cuttings and embankments and the basis of the specification (i.e. design). With earthworks in the British Isles, cohesive soils with a wide range of consistencies are likely to be encountered on a large earthworks project. At present, the contractor's choice of plant is based on experience of previous work, and there is no method whereby the wheel loading requirements can be calculated in advance from a knowledge of the soil conditions. At present, neither the contractor nor the engineer knows whether the usual specification moisture content limit corresponds to the limit of satisfactory operation of the available plant. From the engineer's point of view, having selected a minimum design strength, he would like to continue working up to this limit, providing it is economical compared to the alternative of running the weaker material to spoil. The relation between the shear strength of the fill and its wheel carrying capacity provides both the engineer and the contractor with a means of assessing the tyre pressures required for the particular conditions. It is worthwhile, therefore, to consider some possible methods of working rubber-tyred plant on firm clays with a shear strength of say 5 lb/in ~, viz. : 50,000
f
jr1'
37.5 x SS
.,~,,,,L-30.O0 x 33
~,000
30,000
/ Earthmover tyre
~- 2o,oo02
Wide base earthmover tyre I
I
30 qO 50 Tyre i n f l a t i o n pressure (Pi) " Ib/sq. in. I
*
s
6
8
I0
6
12
Shear Strength (c ,QPi ) . Ib/sq. in. 5
Relationbetween maximumtyre load and inflationpressure for some typical earthmovertyres (Data from Tyre and Rim Association Handbook).
FiG. 9.
68
~. RODIN
(a) Reduce the tyre inflation pressure on the standard wheels of the particular plant to the value required by the shear strength of the fill. This would reduce the m a x i m u m load that could be carried by the wheels, as illustrated in Fig. 9 for three of the heavier earthmoving tyres. (b) Replace the standard wheels on the particular plant with larger wheels and operate them at the required lower inflation pressure, so that the load that can be carried by the larger wheels at the lower inflation pressure is of the same order as the load rating of the plant. The increase in size of the wheel is probably more effectively carried out by increasing the width of the tyre rather than the rim diameter. However, with the bigger earthmoving plant, the present wheels are already considerable in size and are very heavy to handle. On the other hand, increasing the size of a wheel generally reduces the rolling resistance and improves traction. (c) Use dual wheels with lower tyre pressures. Dual wheels were, in fact, used on the older scraper plant but gave trouble because of clogging between the wheels; however, this might not be an insurmountable problem and may be worth reconsidering. Since rubber tyres are a large item in earthmoving costs, any of the above medifications may involve a higher plant cost. But the principal advantage would be the ability to work with relatively weaker soils than is practicable at present and which would otherwise have to be taken to spoil and replaced with imported material. In addition, the use of a lower tyre inflation pressure would reduce the adverse effect of wet weather on the plant working on the present range of fill material due to the rainfall wetting the surface layer of soil and reducing its shear strength. Acknowledgements--The author is indebted to Mr. J. T. Finey, B.SC., A.C.G.I., for his assistance in the preparation of this article.
[1] [2] [3] [4] [5] [6] [7]
REFERENCES S. RODIN, Muck Shifter and Bulk Handler, April, pp. 34-39 and May, pp. 34-41 (1964). W.A. LEWm. R. Res. Tech. Pap. No. 45 (1959). W.P.M. BLACK. Geotechnique, 12, (1962). D.K. HEIPLE. Constr. Methods and Equip., Aug., pp. 84-90 (1951). A. CASAGR~rDE and W. L. SHANNON. Trans. Amer. Soc. Civ. Eng., 114, p. 775 (1949). H.B. SEED and R. LUNt~t~N. Proc. Arner. Soc. Test Mater. 54, p. 288 (1954). University of Birmingham, Graduate School of Highway and Traffic Engineering, Research Bulletin 1964--65, pp. 168-174. Note A d d e d in Proof
Subsequent to the preparation of this article, C. S. Du.nn [7] has collected CBR results for several clays and compared them to the values predicted using Black's method. There is a wide scatter in the results obtained from different clays; the strength of the softer clays tends to l~e underestimated by Black's method. However, Black's method, or a modification of it, represents a useful guide to the strength properties which can be expected under varying moisture contents. "Ihe principal reason for using it in this article was to illustrate that at an arbitrary moisture content limit (e.g.P.L. +2 per cent) the strength of clay fill can vary o v e r a very wide range. Therefore, such arbitrary placement limits bear no relation to the ability of a clay fill to support a particular wheel load.