Module 1 Lecture 3 Soil Aggregate -3 Topics 1.4 CONSISTENCY OF COHESIVE SOIL 1.4.1 Atterberg Limits 1.4.2 Liquidity Index 1.4.3 Activity 1.5 SOIL CLASSIFICATION 1.5.1 Unified Soil Classification System 1.5.2 Theory of Compaction and Proctor Compaction Test 1.5.3 Harvard Miniature Compaction Device 1.5.4 Effect of Organic Content on Compaction of Soil 1.4 CONSISTENCY OF COHESIVE SOIL 1.4.1 Atterberg Limits The presence of clay minerals in a fine-grained soil will allow it to remolded in the presence of some moisture without crumbling. If a clay slurry is dried, the moisture content will gradually decrease and the slurry will pass from a liquid state to a plastic state. With further drying, it will change to a semisolid state and finally to a solid state as shown in Figure 1.19. In about 1911, a Swedish scientist, A. Atterberg, developed a method for describing the limit consistency of fine-grained soils on the basis of moisture content. These limits are the liquid limit, and the shrinkage limit. Figure 1.19 Consistency of cohesive soils Dept. of Civil Engg. Indian Institute of Technology, Kanpur 1
The liquid limit is defined as the moisture content, in percent, at which the soil changes from illiquid state to a plastic state. The liquid limit is now generally determined by the standard Casagrande device (Casagrande, 1932, 1948). The moisture contents (in percent) at which the soil changes from a plastic to a semisolid state and from a semisolid to a solid state are defined, respectively, as the plastic limit and the shrinkage limit. These limits are generally referred to as the Atterberg limits. The Atterberg limits of cohesive soil depend on several factors, such as amount and type of clay mineral sand type of absorbed cation. The difference between the liquid limit and the plastic limit of a soil is defined as the plasticity index PI: (1.44) Where LL is the liquid limit and PL the plastic limit. 1.4.2 Liquidity Index The relative consistency of a cohesive soil can be defined by a ratio called the liquidity index LI. It is defined as (1.45) Where is the natural moisture content. It can be seen from eq. (1.45) that, if, then the liquidity index is equal to 1. Again, if the liquidity index is equal to 0. Thus, for a natural soil deposit which is in a plastic state (i.e.,, the value of the liquidity index varies between 1 and 0. A natural soil deposit with will have a liquidity index greater than 1. In an undisturbed state, these soils may be stable; however, a sudden shock may transform them into a liquid state. Such soils are called sensitive clays. 1.4.3 Activity The oriented water (absorbed and double layer) gives rise to the plastic property of a clay soil. The thickness of the oriented water around a clay particle is dependent on the type of clay mineral. Thus, it can be expected that the plasticity of given clay will depend on (1) the nature of the clay mineral present and (2) the amount of clay mineral present. Based on laboratory test results for several soils. Skempton (1953) made the observation that, for a given soil, the plasticity index is directly proportional to the percent of clay size fraction (i.e., percent by weight finer than 0.002 mm in size), as shown in Figure 1.20. With this observation, Skempton defined parameter called activity. Dept. of Civil Engg. Indian Institute of Technology, Kanpur 2
Figure 1.20 Variation of plasticity index with the percent of clay size fraction (1.46) Where C is the percent of clay-size fraction, by weight. It should be noted that the activity of a given soil will be a function of the type of clay mineral present in it. The activities of several sand-clay mineral mixtures have been evaluated by Seed et al. (1964b). They concluded that although PI bears a linear relation to clay-size fractions, the line of correlation may to pass through the origin. For practical purposes, it seems convenient to define activity as (1.47) Activity has been used as an index property to determine the swelling potential of expansive clays 1.5 SOIL CLASSIFICATION Soil classification is the arrangement of soils into various groups or subgroups to provide a common language to express briefly the general usage characteristics without detailed descriptions. At the present time, two major soil classification systems are available for general engineering use. They are the unified system, which is described below, and the AASHTO system. Both systems use simple index properties such as grain-size distribution, liquid limit, and plasticity index of soil. 1.5.1 Unified Soil Classification System The unified system of soil classification was originally proposed by A. Casagrande in 1942 and was then revised in 1952 by the Corps of Engineers and the U.S. Bureau of Reclamation. In its present form, the Dept. of Civil Engg. Indian Institute of Technology, Kanpur 3
system s widely used by various organizations, geotechnical engineers in private consulting business, and building codes. Initially, there are two major divisions in the system. A soil is classified as a coarse-grained soil (gravelly and sandy) if more than 50% is retained on a No. 200 sieve and as a fine-grained soil (silty and clayey) if more than 50% is passing through a No. 200 sieve. The soil is then further classified by a number of subdivisions, as shown in Table 1.7. The following symbols are used: 1.5.2 Theory of Compaction and Proctor Compaction Test Compaction of loose fills is a simple way of increasing the stability and load-bearing capacity of soils, and this is generally achieved by using smooth-wheel rollers, sheepsfoot rollers, rubber-tire rollers, and vibratory rollers. In the compaction process, loose fills are placed in small lifts. Water is then added to the soil to serve as a lubricating agent on the soil particles. With the application of compacting effort, the soil particles slip over each other and move into a densely packed position. The effect of increasing the moisture content is demonstrated in Figure 1.21. A silty clay when compacted dry with a compaction effort of can be compacted to a unit weight of. However, as the moisture content is increased under the same compactive effort, the weight of soil solids in a unit volume gradually increases. A peak is reached with a moist unit weight of about at a moisture content of about 20%. So the dry unit weight attained by adding water is Figure 1.21 The moisture content vs. unit weight relationship indicating the increased unit weight resulting from the addition of water and that due to the compaction effort applied. (Redrawn after A. W. Johnson and J. R. Sallberg, Factor Influencing Compaction Test Results. Highway Research Board, Bulletin 319, 1962) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 4
Table 1.7 unified soil classification system Major divisions Group symbols Typical names Criteria of classification * Course-grained soils (percent passing No. 200 sieve less than 50) Gravels (percent of coarse fraction passing No. 4 sieve less than 50) Gravels with little or no fines GW Well-graded gravels, gravel-sand mixtures (little or no fines) GP Poorly graded gravels, gravel-sand Not meeting the two criteria for GW mixtures (little or no fines) Gravels with fines GM Silty gravels, gravel-sand-silt mixtures Atterburg limits below A line or plasticity index less than 4 GC Clayey gravels, gravel-sand-clay Atterburg limits above A line with mixtures plasticity index greater than 7 Sands (percent of coarse fraction passing No. 4 sieve greater than 50) Clean sands (little or no fines SW Well-grraded sands, gravelly sands (little or no fines) Sands with fines (appreciable amount of fines) Fine grained soils (percent passing No. 200 sieve greater than 50%) Silts and clay (liquid limit less than 50) Silts and clay (liquid limit greater than 50) SP Poorly graded sands, gravelly sands (little or no fines) Not meeting the two criteria for SW SM Silty sands, sand-silt mixtures Atterburg limits below A line or plasticity index less than SC Clayey sands, sand-clay mixtures Atterburg limits above A line with plasticity index greater than 7 ML Inorganic silts, very fine sands, rock flour, silty or clayey fine sands CL Inorganic clays (low to medium plasticity), gravelly clays, sandy clays, silty clays, lean clays OL Organic silts, organic silty clays (low plasticity) MH Inorganic silts. Micaceous or diatomaceous fine sandy or silty soils, elastic silts CH Inorganic clays (high plasticity), fat clays Highly organic soils Pt Peat, mulch, and other highly organic soils *Classification based on percentage of fines: Percent passing No. 200 Less than 5 More than 12 classification GW, GP, SW, SP GM, GC, SM, SC 5 to 12 borderline-dual symbols required Such as GW-GM, GW-GC, GP-GM, GP-SC, SW-SM, SW-SC, SP-SM, SP-SC Dept. of Civil Engg. Indian Institute of Technology, Kanpur 5
Atterburg limits above A line and plasticity index between 4 and 7 are borderline cases. It needs dual symbols. The effect of compactive effort on the dry unit weight vs. moisture content relation is shown in Figure 1.24 With increasing compactive effort the optimum moisture content decreases, and the same time the maximum dry unit weight of compaction increases. Figure 1.22 Nature of the variation of dry unit weight of soil with moisture content in a compaction test Figure 1.23 Moisture content vs. dry unit weight relationships for eight soils according to AASHTO method T-99. (Note: ). (After A. W. Johnson and J. R. Sallberg, Factors Influencing Compaction Test Results. Highway Research Board, Bulletin 319, 1962) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 6
Figure 1.24 Effect of compactive effort on dry unit weight vs. moisture content relation With the development of heavier compaction equipment, the standard Proctor test has been modified for better representation of field conditions. In the modified Proctor test (ASTM designation D-1577 and AASHTO designation T-180), the same mold as in the standard Proctor test is used. However, the soil is compacted in five layers with a 101b (44.5-N) hammer giving 25 bows to each layer. The height of drop of the hammer is 18in (457.2 mm). Hence the compactive effort in the modified Proctor test is equal to Conducting Proctor tests in sandy and a gravelly soil in rather tedious because of lack of control over the moisture content. The nature of the dry unit weight vs. moisture content plot for sand is shown in Figure 1.25. With increasing moisture content, the dry unit weight gradually decreases and then increases up to the optimum moisture content. The decrease of dry unit weights obtained at lower moisture contents is a result of the effect of capillary tension in the pore water. The capillary tension resists the movement of soil particles and thus prevents the soil from becoming densely packed. Dept. of Civil Engg. Indian Institute of Technology, Kanpur 7
Figure 1.25 Proctor compation test results on a sand (AASHTO test designation T-99) 1.5.3 Harvard Miniature Compcation Device The Harvard miniature compaction device is used in the laboratory for compaction and preparation of soil specimens that are mostly used in research work. Unlike the Proctor test, the compaction is achieved by kneading. The volume of the mold of the Harvard miniature compaction device is. A tamper with a calibrated spring delivers the static pressure to the soil layers. The spring pressure may by 20 lb (89 N) or 40 lb (178 N). the number of layers of soil in the mold and the number of tamps can be varied, thus varying the energy of compaction per unit volume of soil. 1.5.4 Effect of Organic Content on Compaction of Soil Soils with high percentage of organic content are often encountered during construction work. Increase of organic content in a soil tends to decrease the maximum dry unit weight of compaction and increase the compressibility of the soil, tendencies which are not desirable in the construction of foundations, embankment, and etc. Franklin et al. (1973) studied the effect of organic contents on the strength and compaction characteristics of mechanical mixtures of inorganic soils and peat and of natural soil samples with the same organic content. The mineralogy of the inorganic fraction of these samples was reasonably the same. Samples for these tests were compacted in the Harvard miniature compaction device with three layers, 40 lb spring force, and 40 tamps per each layer. Figure 1.26 shows the variation of the maximum dry unit weight of compaction with the organic content, and the variation of the optimum moisture content with the organic content is shown 1.27. The organic content O for these soils is defined as Dept. of Civil Engg. Indian Institute of Technology, Kanpur 8
Figure 1.26 Maximum dry unit weight vs. organic content for all compaction tests. (Note: ). (Redrawn after A. F. Fraklin, L. F. Orozco, and R. Semrau, Compaction of Slightly Organic Soils, J.Soil Mech. Found. Div., ASCE, vol. 99, no. SM7, 1973) Two major conclusions can be drawn from Figures 1.26 and 1.27 (1) if the organic content in a given soil is more than about 10%, the maximum dry unit weight of compaction decreases considerably. (2) the optimum moisture content increases with the increase of organic contents of soil. Dept. of Civil Engg. Indian Institute of Technology, Kanpur 9
Figure 1.27 Effect of drying history and organic content on optimum moisture content. (after A. F. Fraklin, L. F. Orozco, and R. Semrau, Compaction of Slightly Organic Soils, J.Soil Mech. Found. Div., ASCE, vol. 99, no. SM7, 1973) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 10