Loading...

Investigation the Effect of Content and Plastisity of Plastic Fines on Liquefaction Resistance of Sand Using Cyclic Simple Shear Tests

Hassani, Mehrdad | 2013

730 Viewed
  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 44082 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ahmadi, Mohammad Mehdi
  7. Abstract:
  8. Regarding the existing contrary in the literature in relation to the effect of plastic fine on sand liquefaction behaviour, we decided to study this effect. Amounts of 5, 15, and 25 percent of two types of plastic fine, Kaolinite with PI=19 and Bentonite with PI=116 added to Firoozkooh Sand and 68 stress controlled tests were conducted using cyclic simple shear apparatus. In all teste the frequency of shear loading and consolidating vertical effective stress were constant and 0.1 Ht and 100 kPa respectively. Reaching excess pore water pressure ratio to the maximum value (r_u=Max) and reaching double amplitude shear strain to the value of 5% have been considered as liquefaction critera. In investigation the effect of each of the fine types, tests results have been studied in three different viewpoints including constant relative density (D_r), constant total void ratio (e), and constant sand skeleton void ratio (e_s). Among these, constant void ratio viewpoint leads to more reasonable results. In soils containing Kaolinite, decreasing trend of liquefaction resistance is obviously noticeable when Kaolinite content increases to 25%, and there is no limiting Kaolinite content (an amont of fine content which leads to least liquefaction resistance). But In soils containing Bentonite and in the constant void ratio viewpoint, a limiting Bentonite content equal to 5% were observed. In both soils, in constant sand skeleton void ratio viewpoint, soil liquefaction resistance increases with fine content. This is due to striking decrease in void ratio or in other words striking increase in relative density with sand skeleton void ratio holding constant. In presence of 5% fine content, results depict negative effect of plasticity on liquefaction resistance. The cause of this behavior is probably related to the specific surface of fines. There is also a threshold percentage of fine content which increasing effect of fine content plasticity on liquefaction resistance starts at. This threshold value is definitely between 5% and 15%. At void ratio equals to 0.6 to 0.65, and for a soil with a P.I. between 85% and 100%, it can be predicted that the amount of fines (whatever between 5% to 25%) will not have any significant effects on liquefaction resistance. Moreover it has been observed that liquefaction resistance increases with soil plasticity (plasticity of whole soil). Increasing effect of soil plasticity on liquefaction resistance decreases with void ratio decreasing (relative density increasing). More accurate investigation of effect of soil plasticity needs more researche. The effect of fine type and content on pore pressure generation has also been studied in this research in addition to the investigation their effect on liquefaction resistance. In dense Firoozkooh sand, r_u does not increase considerably any more after cycle ratio reaches to the value approximately equals to 0.7 (N/N_L =0.7). At lower relative densities, for example 60%, this behaviour never observed. Existence of a little amount of low plasticity fine, increases soil tendency toward flow liquefaction behavior substantially. Also as fines’ plasticity increases, the shape of generated pore pressure curves transforms from flow liquefaction into cyclic mobility. Based on the results of this research, plasticity index limit equal to 7 guarantees cyclic mobility behavior rather than flow liquefaction
  9. Keywords:
  10. Sands ; Liquefaction Resistance ; Plastic Fine ; Fine Content Plasticity ; Pore Pressure Generation

 Digital Object List

 Bookmark

No TOC