Experimental Study Of Soil Under Static And Cyclic Shear Loading: A Case Of Awash Melkasa Town

Abstract

The dynamic properties of the soil are widely studied parameters for the analysis and design of geotechnical structures subjected to dynamic loads (such as machine vibration, earthquakes, vehicle traffic, pilings, blasting and nuclear explosions). These loads can cause problems in the geotechnical structure. When structures were subjected to these loads they could stand large strain levels. For all geotechnical problems, the dynamic properties of the soil must be determined, especially in earthquake-prone areas. Awash Merkasa is located in the Great Rift Valley of East Africa. The area is prone to seismic activity and is classified as a magnitude four seismic zone. For earthquake-prone areas; it is important to determine the dynamic characteristics of the soil. In this study, an attempt was done to determine the dynamic properties of soils found at Awash Melkasa town. The soil samples were collected from Awash Melkasa town at three different locations, namely; Primary School, High School, and Mesino. Some index soil laboratory tests were carried out on the collected samples, and based on the result they were classified according to USCS and AASHTO. The soil from P @ 1.75 m, M @ 2 m and M @ 3 m were classified as elastic silt soil (MH, A-7-5), P @ 3 m was classified as lean clay soil (CL, A-7-6), H @ 1.5 m was classified as lean silt soil (ML, A-5), and H @ 3 m was classified as (none plastic silt soil, A-5). The elastic silt soils dominate the area, having the liquid limit value ranging from 58.73 % to 63.3 % and plasticity index value of 12.77 % to 30.36 %. The specific gravity value ranges from 2.35 to 2.53. The shear modulus and damping ratio values were determined using cyclic simple shear machine. The tests were conducted as a function of cyclic strain amplitude of 0.01 %, 0.1 %, 1 %, 2.5 % and 5 % under axial stress of 150 kPa, 275 kPa and 400 kPa. The value of shear modules was in the ranges of 0.292 MPa to 15.998 MPa and damping ratio values from 0.146% to 30.851%. The normalized shear modulus values of Awash Melkasa soils have good agreement with curve of sand developed by Seed and Idriss. The shear modulus reduction values at high strain values (≥1%) also agree with the curve of saturated clay of Seed and Idriss (1970) and Vucetic and Dobry (1991). But, the damping ratio values of the studied soils are generally lower than the curves developed for saturated clay by Seed and Indriss (1970) and for different PI values by Vucetic and Dobry (1991). EXPERIMENTAL STUDY OF SOIL UNDER STATIC AND CYCLIC SHEAR LOADING: A CASE OF AWASH MELKASA TOWN 2021 ASTU xvi The major influencing factors that affect the dynamic properties of soils are confining pressure, shear strain amplitude and soil plasticity. The method of sample preparation and testing condition also have significant effect on the damping ratio values. To note the effects of method of sample preparation, frequency of cyclic loading, degree of compaction, degree of saturation and particle size; the test needs to be conducted by considering this factors. In order to obtain complete information about the dynamic properties of the soil, it is recommended to conduct both field and laboratory test methods. Further consideration of strain rate other than the strain rate considered in this study are recommended