Anisotropic strength characterization of compacted Dhaka clay

Abstract

A series of direct shear testing program was undertaken in the laboratory conditions upon specially treated and prepared (i.e., reconstituted) soil samples collected from different locations/ depths of Dhaka clay (i.e., one from 2m depth at Mohammedpur and the other from 6 m depth at Bijoynagar, Dhaka) to study the anisotropic strength characterization of compacted Dhaka clay. The collected soil samples were air dried, powdered by grinding and tested for the determination of index properties, optimum moisture content and maximum dry , density under different compaction energy. Soil samples were reconstituted by mixing the powdered soil with predetermined amount of water. It was then thoroughly mixed and compacted in a mold by using various energies. Test samples, thus prepared as compacted soil cakes were extruded from the compaction mold. The test specimens were then retrieved at different vertical angular orientations relative to its compaction direction. The major principal stress direction along with the application of compaction energy . during preparation of the compacted soil sample was considered as the 0° (i.e., a horizontally oriented specimen). Each set of soil specimens comprising 0°, 30°, 45°, 60° and 90° orientations were prepared at different moisture contents such as optimum minus two percent moisture content (OMC-2) %, optimum plus two percent moisture content (OMC+2) % and optimum moisture content OMC %. Again each set of soil specimens was reconstituted at different compaction energy such as Standard Proctor, Modified Proctor and Intermediate compaction. Where, Intermediate compaction is defined as the energy in between Standard Proctor and Modified Proctor compaction. Direct Shear Test was conducted for each set of specimen at normal pressure of 50 kPa, 100 kPa and 150 kPa. The existence of strength anisotropy was observed for both the soils (Dhaka Clay) collected from two locations of Dhaka city. The coefficient of anisotropy (i.e. I = So/So), defined as the ratio of strength of specimens in inclined orientation (i.e., 8 is other than 0°) to that of a horizontally oriented specimen (i.e., 8 = 0°) at OMC%, varied in the range of 0.95-1.04 for Standard Proctor compaction, 1.00-1.11 for Modified Proctor and 0.96-1.13 for Intermediate compaction for Dhaka clay type-l under normal stress of crv = 50 kPa. The same was varied in the range of 0.90-1.00 for Standard Proctor, 0.99-1.11 for Modified Proctor and 0.93-1.03 for Intermediate compaction for Dhaka clay type-2 under otherwise similar conditions. The coefficient of anisotropy I varied in the range of 0.95-1.04, 0.92-1.03 and 1.00-1.05 at OMC%, (OMC+2)% and (OMC-2)%, respectively, for Dhaka clay type-I tested under normal stress of cry = 50 kPa and reconstituted under Standard Proctor compaction. The value of I varied in the range of 0.95-1.04, 0.93-1.00 and 0.98-1.03 for specimens tested under normal stress cry = 50 kPa, 100 kPa and ISO kPa, respectively, at OMC% of Dhaka clay type-I reconstituted under Standard Proctor compaction. The same was varied in the range of 0.91-1.00, 0.90-1.00 and 0.85-1.00 for Dhaka clay type-2 under otherwise similar conditions. In short, strength of reconstituted compacted Dhaka clay is anisotropic. However, the degree of anisotropy is not substantial. Anisotropy is observed to increase slightly with the increased compacted energy. Moisture content and cry (i.e., normal stress) have little influence in the anisotropic response. Keywords: anisotropy, clay, strength, compaction, reconstitution, moisture content.