Tiêu đề 14 Soil Testing.pdf ✅

Geotech 02: Soil Testing 1. Plasticity of Soil This includes the Atterberg limits and geotechnical parameters, such as activity and liquidity index, which are related to Atterberg limits. 1.1. Liquid Limit (LL) The liquid limit is the moisture content at which the transition from plastic to liquid state takes place. It is determined by using the Casagrande apparatus. The device consists of a brass cup and a hard rubber base. Figure 1. Casagrande Apparatus The brass cup can be dropped onto the base by a cam operated by a crank. The soil sample is placed in the cup. A groove is then cut at the center of the soil pat with the standard grooving tool. Using the crank-operated cam, the cup is lifted and dropped from a height of 10mm. The moisture content required to close 12.7mm along the bottom of the groove after 25 blows is defined as the liquid limit. Since it is difficult to adjust the moisture content to meet the requirement at exactly 25 blows, at least three tests for the same soil are conducted at varying moisture contents, with the number of blows varying between 15 and 35. The results are plotted on a semilogarithmic graph.
Figure 2. Sample flow curve for liquid limit determination (Casagrande Method) Another method of determining the liquid limit is the fall cone method. The liquid limit is defined as the moisture content at which a standard cone of apex angle 30଴ and weight of 0.78N will penetrate a distance of d = 20mm in 5 seconds when allowed to drop from a position of point of contact with the soil surface. Figure 3. Fall Cone Test Four or more tests are conducted at various moisture contents to determine the fall cone penetration, d. The results are also plotted on a semilogarithmic graph. The moisture content corresponding to d = 20mm is the liquid limit. Figure 4. Sample flow curve for liquid limit determination (Fall Cone Method)

G௦ = 1 1 SR − SL 100 1.4. Liquidity Index and Consistency Index Liquidity Index (LI) - Ratio defining the relative consistency of a cohesive soil in the natural state LI = ω − PL LL − PL = ω − PL PI Where: ω = in situ moisture content of soil Figure 6. Liquidity Index Soil deposits that are heavily overconsolidated may have a natural moisture content less than the plastic limit, LI < 0. Consistency Index (CI) CI = LL − ω LL − PI If ω is equal to LL, the consistency index is zero. If ω = PI, CI = 1. 1.5. Activity Activity - Slope of the line correlating PI and % finer than 2μm. - Used as an index for identifying the swelling potential of clay soils A = PI % of clay − size fraction, by weight