Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
SOIL ENGINEERING (EENV 4300)
Chapter 5
Classification of Soil
2
Why Classification?
Soil Engineering EENV 4300
Classification systems provide a common language to concisely express the general characteristics of soils, which are infinitely varied, without detailed descriptions.
3
Two Main Groups of Classification
Soil Engineering EENV 4300
4 Soil Engineering EENV 4300
(A) Textural
classification
(B) Classification by engineering behavior
5 Soil Engineering EENV 4300
• The textural classification systems developed by the U.S. Department of Agriculture (USDA) will be used here.
• This classification method is based on the particle-size limits as described under the USDA system in Table 2.3;
• Table 2.3 divided soils into gravel, sand, silt, and clay categories on the basis of particle size.
• What is texture? In a general sense, texture of soil refers to its surface appearance. Soil texture is influenced by the size of the individual particles present in it.
• Figure 5.1 (next slide) shows the textural classification systems developed by the U.S. Department of Agriculture (USDA)
A- Textural Classification
6 Soil Engineering EENV 4300
7
8 Soil Engineering EENV 4300
• Textural classification of soil is relatively simple (it is based entirely on the particle-size distribution).
• The amount and type of clay minerals present in fine-grained soils dictate to a great extent their physical properties. Hence, the soils engineer must consider plasticity, which results from the presence of clay minerals, to interpret soil characteristics properly.
• Because textural classification systems do not take plasticity into account and are not totally indicative of many important soil properties, they are inadequate for most engineering purposes.
• AASHTO and USCS systems are used.
B- Classification by Engineering Behavior
9 Soil Engineering EENV 4300
10 Soil Engineering EENV 4300
AASHTO Required parameters are grain-size analysis, LL, PI. Using the known parameters & the AASHTO table, one
proceeds to classify the soil into type A-1 or A-2 or … A-7
A soil can be further described using a group index (GI) This index utilizes the % of soil passing No. 200 sieve,
LL & PI
11 Soil Engineering EENV 4300
12 Soil Engineering EENV 4300
…
To classify a soil according to Table 5.1, one must apply the test data from left to right. By process of elimination, the first group from the left into which the test data fit is the correct classification.
General Guidance for AASHTO Method 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).
A4 ~ A7 A1 ~ A3
Granular Materials
≤ 35% pass No. 200 sieve Silt-clay Materials
≥ 36% pass No. 200 sieve
Using LL and PI separates silty materials from clayey materials
Using LL and PI separates silty materials from clayey materials (only for A2 group)
13
The original purpose of this classification system is used for road construction (sub-grade rating).
14 Soil Engineering EENV 4300
• It is used to evaluate the quality of a soil as a highway subgrade material.
• This index is written in parentheses after the group or subgroup designation.
Group Index (GI)
15 Soil Engineering EENV 4300
Rules for Determining the Group Index (GI)
16
Soil Engineering EENV 4300
17
Soil Engineering EENV 4300
18
Soil Engineering EENV 4300
• The original form of this system was proposed by Casagrande in 1942 for use in the airfield construction works undertaken by the Army Corps of Engineers 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 (ASTM Test Designation D-2487).
19
Soil Engineering EENV 4300
20
Soil Engineering EENV 4300
21
Soil Engineering EENV 4300
22
Soil Engineering EENV 4300
23
Soil Engineering EENV 4300
24 Soil Engineering EENV 4300
25 Soil Engineering EENV 4300
Fig. 5.4: Flowchart group names for gravelly soil
26 Soil Engineering EENV 4300
Figure 5.4: Flowchart group names for sandy soil
27 Soil Engineering EENV 4300
28 Soil Engineering EENV 4300
29 Soil Engineering EENV 4300
30
31
32
33
34
35
36
37