Soil Classification PDF

Soil Classification Purpose Classifying soils into groups with similar behavior, in terms of simple indices, can provide geotechnical engineers a general guidance about engineering properties of the soils through the accumulated experience. Why do we need to classify soils ? – To describe va...

Soil Classification Purpose Classifying soils into groups with similar behavior, in terms of simple indices, can provide geotechnical engineers a general guidance about engineering properties of the soils through the accumulated experience. Why do we need to classify soils ? – To describe various soil types encountered in the nature in a systematic way and gathering soils that have distinct physical properties in groups and units. Extent and properties of the soil have to be determined. Cheap and simple tests are required to give an indication of engineering properties, e.g. stiffness, strength, for preliminary design Classification systems are based on simple index properties such as particle-size distribution and plasticity Unified Soil Classification System (USCS) Origin of USCS: This system was first developed by Professor A. Casagrande (1948) for the purpose of airfield construction during World War II. Afterwards, it was modified by Professor Casagrande, the U.S. Bureau of Reclamation, and the U.S. Army Corps of Engineers to enable the system to be applicable to dams, foundations, and other soils.  Definition of Grain Size No specific grain size-use Atterberg limits Gravel Sand Silt and Boulders Cobbles Clay Coarse Fine Coarse Medium Fine 300 mm 75 mm No.4 No.200 4.75 mm 0.075 mm 19 mm No.10 No.40 2.0 mm 0.425 mm General Guidance 50 % Coarse-grained soils: Fine-grained soils: Gravel Sand Silt Clay 50% NO. 4 NO.200 4.75 mm 0.075 mm Grain size distribution PL, LL LL>50 LL 50) S: Sand L: Low LL (LL12%, use plasticity chart USCS Plasticity Chart LL = 35, PI = 15 Soil fines are Clay Soil: SC (clayey sand) Borderline Cases (Dual Symbols) For the following three conditions, a dual symbol should be used. Coarse-grained soils with 5% - 12% fines. About 7 % fines can change the hydraulic conductivity of the coarse-grained media by orders of magnitude. The first symbol indicates whether the coarse fraction is well or poorly graded. The second symbol describe the contained fines. For example: SP-SM, poorly graded sand with silt. Fine-grained soils with limits within the shaded zone. (PI between 4 and 7 and LL between about 12 and 25). It is hard to distinguish between the silty and more claylike materials. CL-ML: Silty clay, SC-SM: Silty, clayed sand. Soil contain similar fines and coarse-grained fractions. possible dual symbols GM-ML American Association of State Highway and Transportation Officials system (AASHTO) Origin of AASHTO: (For road construction) This system was originally developed by Hogentogler and Terzaghi in 1929 as the Public Roads Classification System. Afterwards, there are several revisions. The present AASHTO (1978) system is primarily based on the version in 1945. (Holtz and Kovacs, 1981). Currently uses seven major groups of soils, A1 to A7.  General Guidance – 8 major groups: A1~ A7 (with several subgroups) and organic soils A8 – 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). A1 ~ A3 A4 ~ A7 Granular Materials Silt-clay Materials  35% pass No. 200 sieve  36% pass No. 200 sieve Using LL and PI separates silty materials Using LL and PI separates silty materials from clayey materials (only for A2 group) from clayey materials Group Index 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) use the second term only F200: percentage passing through the No.200 sieve GI: group index (evaluate the quality of a soil as a highway subgrade material). Higher values of GI means a less suitable soil.  AASHTO Plasticity Chart Low Compressibility, High Compressibility, Shrink-Swell Shrink-Swell Clays Silts  AASHTO Example 100 200 40 10 P10=80% 90 80 P40=52% 70 % Passing 60 50 P200 = 20% 40 30 LL = 35 PL = 20 20 10 0 0.001 0.01 0.1 1 10 Grain Size, mm  AASHTO Example P10=80% P40=52% P200=20% LL = 35 PL = 20 PI = 15 Because P20050%, not A-1-a Because P40>50%, not A-1-b Because P200>10%, not A-3 So must be A-2 soil, use plasticity chart AASHTO Plasticity Chart LL = 35, PI = 15 A-2-6  AASHTO Classification A-2-6 Soil GI  0.01(F200  15)(PI  10) GI=0.01(20-15)(15-10)=0.25=1 So soil is A-2-6 (1) Clayey Sand & Gravel  Review Passing No.200 sieve 30 % Passing No.4 sieve 70 % LL= 33 PI= 12 Problems Passing No.200 sieve 30 % Passing No.4 sieve 70 % LL= 33 PI= 12 PI= 0.73(LL-20), A-line PI=0.73(33-20)=9.49 SC Clayey sand with gravel Highly (Santamarina et al., 2001)  Review Problems Classify the following soils by AASHTO classification system: Description A B C D E % finer than No.10 sieve 83 100 48 90 100 % finer that No.40 sieve 48 92 28 76 82 % finer that No.200 sieve 20 86 6 34 38 LL 20 70 - 37 42 PI 5 32 - 12 23 A: A-1-b(0), B: A-7-5(33), C: A-1-a(0), D: A-2-6(0), F: A-7-6(4)