FOUNDATION ANALYSIS AND DESIGN
Module 1 (Lecture 2) GEOTECHNICAL PROPERTIES OF SOIL AND OF REINFORCED SOIL Topics 1.1 SOIL CLASSIFICATION SYSTEMS 1.2 AASHTO System ο Unified System ο Examples and Solutions
1.3 HYDRAULIC CONDUCTIVITY OF SOIL 1.4 STEADY-STATE SEEPAGE 1.5 FILTER DESIGN CRITERIA 1.6 EFFECTIVE STRESS CONCEPT
FOUNDATION ANALYSIS AND DESIGN SOIL CLASSIFICATION SYSTEMS Soil classification systems divide soils into groups and subgroups based on common engineering properties such as grain-size distribution, liquid limit, and plastic limit. The two major classification systems presently in use are (1) the AASHTO (American Association of State Highway and Transportation Officials) Systems and (2) the Unified Soil Classification System (also ASTM), the AASHTO classification system is used mainly for classification of highway subgrades. It is not used in foundation construction. AASHTO System The AASHTO Soil Classification System was originally proposed by the Highway Research Boardβs Committee on Classification of Materials for Subgrades and Granular Type Roads (1945). According to the present form of this system, soils can be classified according to eight major groups, A-1 through A-8, based on their grainsize distribution, liquid limit, and plasticity indices. Soils listed in groups A-1, A-2, and A-3 are coarse-grained materials, and those in groups A-4, A-5, A-6, and A-7 are fine-grained materials. Peat, muck, and other highly organic soils are classified under A-8. They are identified by visual inspection. The AASHTO classification system (for soils A-1 through A-7) is presented in table 8. Note that group A-7 includes two types of soil. For the A-7-5 type, the plasticity index of the soil is less than or equal to the liquid limit minus 30. For the A-7-6 type, the plasticity index is greater than the liquid limit minus 30. For qualitative evaluation of the desirability of a soil as a highway subgrade material, a number referred to as the group index has also been developed. The higher the value of the group index for a given soil, the weaker will be the soilβs performance as a subgrade. A group index of 20 or more indicates a very poor subgrade material. The formula for group index, πΊπΊπΊπΊ, is πΊπΊπΊπΊ = (πΉπΉ200 β 35)[0.2 + 0.005(πΏπΏπΏπΏ β 40)] + 0.01(πΉπΉ200 β 15)(ππππ β 10) [1.23] Where πΉπΉ200 = percent passing no 200 sieve, expressed as a whole number πΏπΏπΏπΏ = liquid limit
ππππ = plasticity index
FOUNDATION ANALYSIS AND DESIGN Table 8 AASHTO Soil Classification System General classification
Granular materials (35% or less of total samples passing no. 200 sieve) A-1
Group classification
A-1-a
A-2
A-1-b
A-3
A-2-4
A-2-5
A-2-6
35 max
35 max
35 max
40 max
41 min
40 max
10 max
10 max
11 min
Sieve analysis (%) passing) No. 10 sieve
50 max
No. 40 sieve
30 max
50 max
51 max
No. 200 sieve
15 max
25 max
10 max
For fraction passing No. 40 sieve Liquid limit (LL) Plasticity (PI)
index 6 max
Usual type material
Nonplastic
of Stone fragments, Fine sand gravel, and sand
Subgrade rating
Silty or clayey gravel and sand Excellent to good
General classification
Silt-clay materials (More than 35% of total sample passing no. 200 sieve)
Group classification
A-4
A-5
A-6
A-7 A-7-5a A-7b
FOUNDATION ANALYSIS AND DESIGN
Sieve analysis (% passing) No. 10 sieve No. 40 sieve No. 200 sieve
36 min
36 min
36 min
36 min
40 max
41 min
40 max
41 min
index 10 max
10 max
11 min
11 min
For fraction passing No. 40 sieve Liquid limit (LL) Plasticity (PI)
Usual types of Mostly silty soils material Subgrade rating
Mostly clayey soils
Fair to poor
a
If PIβ€LL-30, it is A-7-5. b
If PI>LL-30, it is A-7-6. When calculating the group index for a soil belonging to groups A-2-6 or A-2-7, us only the partial group index equation relating to the plasticity index: πΊπΊπΊπΊ = 0.01(πΉπΉ200 β 15)(ππππ β 10)
[1.24]
The group index is rounded to the nearest whole number and written next to the soil group in parentheses; for example, οΏ½οΏ½οΏ½ π΄π΄ β4 (5) οΏ½ | Soil group Group index
FOUNDATION ANALYSIS AND DESIGN Unified System The Unified Soil Classification System was originally proposed by A. Casagrande in 1942 and was later revised and adopted by the United States Bureau of Reclamation and the Corps of Engineers. This system is presently used in practically all geotechnical work. In the Unified System, the following symbols are used for identification. Symbol
πΊπΊ
Descripti Grav on el
ππ
San d
ππ
πΆπΆ
Sil Cla t y
ππ
Organ ic silts and clay
ππππ
Peat and highl y organ ic soils
π»π»
πΏπΏ
ππ
High Low Well plastici plastici grad ty ty ed
ππ
Poorl y grad ed
The plasticity chart (figure 1.7) and table 9 shows the procedure for determining the group symbols for various types of soil. When classifying a soil be sure to provide the group name that generally describes the soil, along with the group symbol. Tables 10, 11 and 12, respectively, give the criteria for obtaining the group names for coarsegrained soil, inorganic fine-grained soil, and organic fine-grained soil. These tables are based on ASTM Designation D-2487. Table 9 Group Symbol for Soil According to the Unified Classification System [Based on Material Passing 3-in. (75-mm) Sieve] Major division Coarse-grained soil
π
π
200 > 50
Gravelly soil π
π
4 > 0.5π
π
200
Criteria πΉπΉ200 < 5, πΆπΆπ’π’ β₯ 4, 1 β€ πΆπΆπ§π§ β€3
πΉπΉ200 < 5, πΆπΆπ’π’ < 4, and /or πΆπΆπ§π§ not between 1 and 3
Group symbol GW
GP
πΉπΉ200 > 12, ππππ < 4, or GM Atterberg limits plot be low A line (figure 1.7) πΉπΉ200 > 12, ππππ < 7, and GC Atterberg limits plot on or above A line (figure 1.7)
FOUNDATION ANALYSIS AND DESIGN
GC-GM a πΉπΉ200 > 12, πΏπΏπΏπΏ < 50, 4 β€ ππππ β€ 7 and Atterberg limits plot on or above A line
5 β€ πΉπΉ200 β€ 12; meets the GW-GM a gradation criteria of GW and the plasticity criteria of GM 5 β€ πΉπΉ200 β€ 12; meets the GW-GC a gradation criteria of GW and the plasticity criteria of GC 5 β€ πΉπΉ200 β€ 12; meets the GP-GC a gradation criteria of GP and the plasticity criteria of GM 5 β€ πΉπΉ200 β€ 12; meets the GP-GC a gradation criteria of GP and the plasticity criteria of GC Sandy soil
π
π
4 β€ 0.5π
π
200
πΉπΉ200 < 5, πΆπΆπ’π’ β₯ 6, 1 β€ πΆπΆπ§π§ β€3
SW
πΉπΉ200 < 5, πΆπΆπ’π’ < 6, and/or πΆπΆπ§π§ SP not between 1 and 3
πΉπΉ200 > 12, ππππ > 4, or SM Atterberg limits plot below A line (figure 1.7) πΉπΉ200 > 12, ππππ > 7, and SC Atterberg limits plot on or above A line (figure 1.7) SC-SM a πΉπΉ200 > 12, πΏπΏπΏπΏ > 50, 4 β€ ππππ β€ 7 And Atterberg limits plot on or above A line (figure 1.7) 5 β€ πΉπΉ200 β€ 12; meets the SW-SM a gradation criteria of SW and the plasticity criteria of SM 5 β€ πΉπΉ200 β€ 12; meets the SW-SC a gradation criteria of SW and
FOUNDATION ANALYSIS AND DESIGN the plasticity criteria of SC 5 β€ πΉπΉ200 β€ 12; meets the SP-SM a gradation criteria of SP and the plasticity criteria of SM 5 β€ πΉπΉ200 β€ 12; meets the SP-SC a gradation criteria of SP and the plasticity criteria of SC Fine-grained π
π
200 β€ 50
soil
(inorganic), ππππ < 4, or Atterberg limits ML plot below A line (figure 1.7)
Silty and clayey soil πΏπΏπΏπΏ < 50
ππππ > 7 or Atterberg limits CL plot on or above A line (figure 1.7)
4 β€ ππππ > 7, and Atterberg CL-ML a limits plot above A line (figure 1.7) Silty and clayey soil πΏπΏπΏπΏ β€ 50
Atterberg limits plot on or MH above A line (figure 1.7) Atterberg limits plot on or CH above A line (figure 1.7)
Fine-grained soil (organic) Organic silt and clay) πΏπΏπΏπΏ < 50 Organic silt and clay πΏπΏπΏπΏ β₯ 50
πΏπΏπΏπΏnot oven dry < 0.75 πΏπΏπΏπΏoven dry πΏπΏπΏπΏnot oven dry < 0.75 πΏπΏπΏπΏoven dry
OL
OH
Note: πΉπΉ200 = percent finer than no. 200 sieve; R 200 = percent retained on no. 200 sieve; R 4 = perent retained on no. 4 sieve; Cu = uniformity coefficient; Cz = coefficient of gradation; πΏπΏπΏπΏ = liquid limit; ππππ = plasticity index Atterberg limits based on minus no. 40 fraction Borderline case; dual classification
FOUNDATION ANALYSIS AND DESIGN Example 4 Classify the following soil by the AASHTO classification system: Percent passing no. 4 sieve = 82 Percent passing no. 10 sieve = 71 Percent passing no. 40 sieve = 64 Percent passing no. 200 sieve = 41 Liquid limit = 31 Plasticity index = 12 Solution Refer to table 8. More than 35% passes through a no. 200 sieve, so it is a silt-clay material. It could be A-4, A-5, A-6, or A-7. Because LL = 31 (that is, less than 40) and PI = 12 (that is, greater than 11) this soil falls in group A-6. From equation (23), πΊπΊππ = (πΉπΉ200 β 35)[0.02 + 0.005(πΏπΏπΏπΏ β 40)] + 0.01(πΉπΉ200 β 15)(ππππ β 10) π π π π
πΊπΊπΊπΊ = (41 β 35)[0.02 + 0.005(31 β 40)] + 0.01(41 β 15)(12 β 10) = 0.37 β 0 Thus the soil is A-6(0). Example 5 Classify the following soil by the AASHTO classification system. Percent passing no. 4 sieve = 92 Percent passing no. 10 sieve = 87 Percent passing no. 40 sieve = 65 Percent passing no. 200 sieve = 30 Liquid limit = 22 Plasticity index = 8
FOUNDATION ANALYSIS AND DESIGN Solution Table 8 shows that it is granular material because less than 35% is passing a no. 200 sieve. With πΏπΏπΏπΏ = 22 (that is, less than 40) and ππππ = 8 (that is, less than 10), the soil alls in group A-2-4. From equation (24), πΊπΊππ = 0.01(πΉπΉ200 β 15)(ππππ β 10) = 0.01(30 β 15)(8 β 10) = β0.3 β 0 The soil is A-2-4(0).
Table 10 Group Names for Coarse-Grained Soils (Based on ASTM D-2487) Criteria Group symbol GW
GP
GM
GC
GC-GM
GW-GM
Gravel fraction (%)
Sand fraction (%)
Group name
< 15
Well-graded gravel
β₯ 15
Well-graded gravel with sand
< 15
Poorly gravel
β₯ 15
Poorly graded gravel with sand
< 15
Silty gravel
β₯ 15
Silty gravel with sand
< 15
Clayey gravel
β₯ 15
Clayey gravel with sand
< 15
Silty clayey gravel
β₯ 15
Silty clayey gravel with sand
< 15
Well-graded gravel with silt
β₯ 15
Well-graded gravel with silt and sand
graded
FOUNDATION ANALYSIS AND DESIGN
GW-GC
GP-GM
GP-GC
SW
SP
SM
SC
SM-SC
SW-SM
< 15
Well-graded gravel with clay
β₯ 15
Well-graded gravel with clay and sand
< 15
Poorly graded gravel with silt
β₯ 15
Poorly graded gravel with silt and sand
< 15
Poorly graded gravel with clay
β₯ 15
Poorly graded gravel with clay and sand
< 15
Well graded sand
β₯ 15
Well-graded with gravel
< 15
Poorly graded sand
β₯ 15
Poorly graded sand with gravel
< 15
Silty sand
β₯ 15
Silty sand gravel
< 15
Clayey sand
β₯ 15
Clayey sand with gravel
< 15
Silty clayey sand
β₯ 15
Silty clayey sand with gravel
< 15
Well-graded with silt
sand
with
sand
FOUNDATION ANALYSIS AND DESIGN
SW-SC
SP-SM
SP-SC
β₯ 15
Well-graded sand with silt and gravel
< 15
Well-graded with clay
β₯ 15
Well-graded sand with lay and gravel
< 15
Poorly graded sand with silt
β₯ 15
Poorly graded sand with silt and gravel
< 15
Poorly graded sand with clay
β₯ 15
Poorly graded sand with clay and gravel
sand
Note: sand fraction = percent of soil passing no. 4 sieve but retained on no. 200 sieve= π
π
200 β π
π
4 ; gravel fraction = percent of soil passing 3-in. sieve but retained on no. 4 sieve = π
π
4 Table 11 Group Names for Inorganic Fine-Grained Soils (Based on ASTM D2487) Criteria Group symbol
π
π
200
CL
< 15 15 to 29
β₯ 30
Sand fraction
Gravel fraction
Gravel fraction
Sand fraction
Group name Lean clay
β₯1
Lean clay with sand