Compaction In construction of high way embankments, earth dams and many other engineering structures, loose soils must be compacted to improve their strength by increasing their unit weight. Compaction – Densification of soil by removing air voids using mechanic equipment. (for unsaturated soil) The degree of compaction is measured in terms of its dry unit weight. Consolidation is another kind of densification with fluid flow away. Consolidation is primarily for clayey soils. Objectives for Compaction Increasing the bearing capacity of foundations. Decreasing the undesirable settlement of structures. Control undesirable volume changes. Reduction in hydraulic conductivity. Increasing the stability of slopes. Effect of Water on Compaction Water acts as a softening agent on the soil particles. The soil particles move into a densely packed position Water takes up the spaces that would have been occupied by the solid particles. Same compactive effort Optimum Moisture Content Standard Proctor Compaction Test The standard was originally developed to simulate field compaction in the lab Purpose: Find the optimum moisture content at which the maximum dry unit weight is attained. Compaction curve Compaction curve plotted gd vs w. The peak of the curve is the maximum compaction at optimum moisture content. Zero-air-void Unit Weight At certain water content, what is the unit weight to let no air in the voids Effect of Compaction Energy As the compaction effort increases, The maximum dry unit weight of compaction increases The optimum moisture content decreases Effect of soil type Fine grained soil needs more water to reach optimum; and coarse grained soil needs less water to reach optimum. Modified Proctor Compaction Test The modified proctor compaction test was developed to simulate larger compaction effort for more serious loads and bigger equipment. Mold Test Rammer No. of layers Energy of Blows compaction (kNper layer m/m3) Capacity (cm3) Diameter (mm) Height (mm) Mass (kg) Drop (mm) Standard Proctor 943.3 101.6 116.43 2.5 304.8 3 25 591.3 Modified Proctor 943.3 101.6 116.43 4.54 457 5 25 2696 Empirical Relationship wopt vs PL Gurtug and Sridharan (2004) proposed correlations for optimum moisture content, wopt, and maximum dry unit weight, gdmax, with the plastic limit, PL, of cohesive soils. wopt(%) = [1.95-0.38(log CE)](PL) gdmax(kN/m3) = 22.68e0.0183wopt(%) where CE = compaction energy (kN-m/m3) For modified Proctor test, CE = 2700 kN-m/m3 wopt(%) ≈ 0.65(PL) gdmax(kN/m3) ≈ 22.68e0.012(PL) Relative density Relative density is commonly used to indicate the in situ denseness or looseness of granular soil. where Dr = relative density, usually given as a percentage e = in situ void ratio of the soil emax = void ratio of the soil in the loosest state emin = void ratio of the soil in the densest state Relative density (%) Description of soil deposit 0 – 15 Very loose 15 – 50 Loose 50 – 70 Medium 70 – 85 Dense 85 – 100 Very dense Engineering Classification of Soil Objective Classifying soils into groups with similar behaviour, in terms of simple indices, can provide geotechnical engineers a general guidance about engineering properties of the soils through the accumulated experience. Communicate between engineers Simple indices GSD, LL, PI Classification system Use the accumulated experience Estimate engineering properties Achieve engineering purposes Classification Systems Two commonly used systems for soil engineers based on particle distribution and Atterberg limits: • American Association of State Highway and Transportation Official (AASHTO) System • Unified Soil Classification System (USCS) (preferred by geotechnical engineers) U.S. Standard Sieve Sizes Soil particles Fine grained soil Gravel, Sand, Silt, Clay Coarse grained soil No.10 No.40 Sieve no. Opening (mm) 4 4.75 10 2.00 20 0.850 40 0.425 60 0.250 100 0.150 200 0.075 No.200 (mm) No.4 No.10 No.40 No.200 Atterberg Limits Atterberg limits are the limits of water content used to define soil behaviour. Increasing moisture content V ol u m e, V Solid Semisolid Plastic Liquid S = 100% Plasticity Index PI = LL-PL SL Shrinkage Limit PL Plastic Limit LL Liquid Limit Moisture content, w % AASHTO Classification System This system was developed in 1929 as the Public Road Administration classification system. Afterwards, there are several revisions. The present version is primarily based on the version in 1945. 1. Grain size No.10 No.40 No.200 (mm) If boulders are encountered, they are excluded from the portion of the soil sample from which classification is made. However, the percentage of such material is recorded. 2. Plasticity The term silty is applied when the fine fractions of the soil have a plasticity index of 10 or less. The term clayey is applied when the fine fractions have a plasticity index of 11 or more. Classification for Granular Materials (Proceeding from left to right against the columns) By process of elimination, the first group from the left into which the test data fit is the correct classification. Classification for Silt-clay Materials (Proceeding from left to right against the columns) By process of elimination, the first group from the left into which the test data fit is the correct classification. Range of LL and PI for soils in groups A-2, A-4, A-5, A-6 and A-7 1. 2. 3. 4. Group Index, GI The first term is determined by the LL GI = (F200-35)[0.2+0.005(LL-40)] +0.01(F200-15)(PI-10) The second term is determined by the PI For Group A-2-6 and A-2-7 GI = 0.01(F200-15)(PI-10) where F200 = percentage passing through the No. 200 sieve If equation gives a negative value then GI = 0 The calculated GI is rounded off to the nearest whole number There is no upper limit for GI The GI of soils belong to groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3 is always 0. The quality of performance of a soil as a subgrade material is inversely proportional to the GI Summary AASHTO ● ● ● 8 major groups: A-1 – A-7 (with several subgroups) and organic soils A-8 The required tests are sieve analysis and Atterberg limits. The group index, an empirical formula, is used to further evaluate soils within a group (subgroups). A-1 – A-3 Granular Materials ≤ 35% pass No. 200 sieve Using LL and PI separates silty materials from clayey materials (only for A2 group) A-4 – A-7 Silt-clay Materials > 35% pass No. 200 sieve Using LL and PI separates silty materials from clayey materials ● The original purpose of this classification system is used for Example 3.1 Passing No. 10 sieve =100% LL = 30, PI =10 No. 40 sieve = 80% No. 200 sieve = 58% A-4 – A-7 GI = (F200-35)[0.2+0.005(LL-40)]+0.01(F200-15)(PI-10) = (58-35)[0.2+0.005(30-40)]+0.01(58-15)(10-10) = 3.45 ≈ 3 A-4(3) Example 3.2 Passing No. 200 sieve = 95% LL = 60, PI =40 A-4 – A-7 GI = (F200-35)[0.2+0.005(LL-40)]+0.01(F200-15)(PI-10) 40 > 60-30 = (95-35)[0.2+0.005(60-40)]+0.01(95-15)(40-10) = 42 A-7-6(42) Unified Soil Classification System This system was first developed by Casagrande in 1942 for use in the airfield construction work during World War II. In cooperation with the U.S. Bureau of Reclamation, this system was revised in 1952. At present, it is used widely by engineers. Grain size (mm) No.4 No.10 No.40 No.200 Two broad categories 1. Coarse-grained soils that are gravelly and sandy in nature with less than 50% passing through the No. 200 sieve. 2. Fine-grained soils are with 50% or more passing through the No. 200 sieve. Table 3.2 Group Symbol Soil symbols: ● G : Gravel ● S : Sand ● M : Silt ● C : Clay ● O : Organic ● Pt : Peat Plasticity symbols: ● H : High plasticity (LL ≥ 50) ● L : Low plasticity (LL < 50) Gradation symbols: ● W : Well graded ● P : Poor graded 1 < Cc < 3 and Cu ≥ 4 (for gravels) 1 < Cc < 3 and Cu ≥ 6 (for sands) Prefix Examples: ● SW : Well-graded sand ● SC : Clayey sand ● SM : Silty sand ● MH : Elastic silt Typical USCS: SM – Silty sand with gravel Group symbol Group name Classification The coarse-grained soils are designated by group symbols such as GW, GP, GM, GC, SW, SP, SM and SC. For proper classification, the following factors are to be considered: 1. Percent passing through the No. 200 sieve 2. Percent of coarse fraction passing through the No. 4 sieve 3. Cu and Cc (for soils with 0-5% passing through the No. 200 sieve) 4. LL and PI of that portion of soil passing through the No. 40 sieve (for soils with 5% or more passing through the No. 200 sieve) When the percent passing through the No. 200 sieve is between 5 and 12% dual symbols, such as GW-GM, GP-GM, GW-GC, GP-GC, SWSM, SW-SC, SP-SM and SP-SC are needed. Fine-grained soil classifications with symbols ML, CL, OL, MH, CH, and OH are obtained by plotting the LL and PI of soil on the plasticity chart. The A-line generally separates the more claylike soils from silty soils, and the organics from the inorganics. The U-line indicates the upper bound for general soils 7 4 Dual Symbols For the following three conditions, a dual symbol should be used. 1. Coarse-grained soils with 5-12% passing through the No. 200 sieve. The first symbol indicates whether the coarse fraction is well or poorly graded. The second symbol describes the contained fine fraction. For example, SP-SM, poorly graded sand with silt. 2. Soils with limits within the shaded area of the plasticity chart. (PI between 4 and 7, and LL between about 12 and 25). It is hard to distinguish between the silty and clayey soil. CL-ML: Silty clay, SC-CM: Silty, clayey sand. 3. Soil contain similar fine and coarse-grained fractions. For example, GM-ML. Organic Soils Highly organic soils – Peat (Group symbol, Pt) A sample composed primarily of vegetable tissue in various stages of decomposition and has a fibrous to amorphous texture, a dark-brown to black colour, and an organic odor should be designated as a highly organic soil and shall be classified as peat, Pt. Organic clay or silt (Group symbol, OL or OH) “The soil’s liquid limit (LL) after oven drying is less than 75% of its liquid limit before oven drying.” If the above statement is true, then the first symbol is O. The second symbol is obtained by locating the values of PI and LL (not oven dried) in the plasticity chart. Coarse-grained A Soils Group symbol Group name Group symbol Fine-grained Soils Group name Summary USCS Example 3.3 Passing No. 10 sieve =100% No. 40 sieve = 80% No. 200 sieve = 58% LL = 30, PI =10 A fine-grained soil CL 7 4 Example 3.4 Soil A LL = 30, PL = 22 1. About 8% of the soil pass through No. 200 sieve. Hence this is a coarse-grained soil and, since this is within 5 to 12%, dual symbols needed to be used. 2. 100% of the soil pass through No. 4. Hence this is a sandy soil. 3. Cu = 1.59 < 6, Cc = 1.25 > 1 poorly graded 4. LL = 30, PI = 30-22 = 8, it plots above the A-line. Hence this is a clayey soil. SP-SC – Poorly-graded sand with clay (or silty clay) Example 3.4 (cont.) Soil B LL = 26, PL = 20 1. About 61% of the soil pass through No. 200 sieve. Hence this is a finegrained soil. 2. LL = 26, PI = 26-20 = 6, it falls inside the shaded area of the plasticity chart . CL-ML – Sandy silty clay Homeworks 3.1) The sieve analysis of ten soils and the liquid and plastic limits of the fraction passing through the No. 40 sieve are given below. Classify the soils by the AASHTO classification system and give the group indexes. Soil no. 1 2 3 4 5 6 7 8 9 10 Sieve Analysis % Finer No. 10 No. 40 No. 200 Liquid limit 98 100 100 85 92 97 100 94 83 100 80 92 88 55 75 60 55 80 48 92 50 80 65 45 62 30 8 63 20 86 38 56 37 28 43 25 40 20 70 Plastic limit 29 23 22 20 28 16 NP 21 15 38 3.2) Classify soils 1 through 6 given in Problem 3.1 by the Unified classification system.
