Estimating of dynamic modulus of elasticity for top soil layers by borehole seismic survey in industrial area of Sulaimani city, NE-Iraq

Abdulla Karim Amin, Omar Qadir Ahmed, Omar Qadir Ahmed

Abstract


        A near-surface geophysical study was conducted at Sulaimani city, NE-Iraq, using up-hole seismic technique in order to image shallow subsurface in terms of main dynamic elastic properties of soil and rock layers covering the study area. The arrival times of the propagated body (P-and S-) waves are measured within (1–2 m) interval in four boreholes in successive positions and the time-depth curves were plotted. The boreholes No.1 and No.2 were at western part and boreholes No.3 and No.4 at eastern part of the studied area.  In the boreholes (1,2) the arrival times were measured down to approximately(12 m) but in the boreholes (3,4) the measurements had completed down to (16 m) depth. Then, the primary and shear wave velocities (Vp,Vs) and the dynamic modulus of elasticity including the poisson’s ratio (σ) shear modulus (G) and elastic modulus (E) for the subsurface soil in the four boreholes were determined.

      The results of the up-hole seismic survey in the boreholes (No.1) and (No.2) at the western part of the studied area showed the presence of two layers. The first layer extended down to the depth range (0–5 m) through this layer the Vp values ranged from (737 m/s – 870 m/s) and Vs from (400 m/s – 480 m/s), with relatively high values of (σ) ranged from (0.28 to 0.29) and relatively low values of both (G) from (3.50×108 N/m2 – 5.01×108 N/m2) and (E) from (8.91×108 N/m2 –12.83×108 N/m2). These values revealed to a non-cohesive soil layer.

      while, the first layer in the boreholes (No.3) and (No.4) at the eastern part within the depths (0–10 m) showed the values of Vp, ranged from (1033 m/s–1127 m/s) and Vs from (665 m/s –705 m/s) and relatively lower values for (σ) ranged from (0.15) in borehole (No.3) to (0.17) in borehole (No.4). Also, the values of both (G) from (11.01×108 N/m2) in boreholes (No.4) to (22.46×108 N/m2) boreholes (No.3) and (E) from (10.62×108 N/m2–25.77×108 N/m2) in the boreholes (No.3) and (No.4) respectively referred that first layer in the eastern part more cohesive and consolidated sediments.    

     Furthermore, based on the derived seismic velocities and relative dynamic elastic values, the first layer in the eastern part of the study area was relatively cohesive and more suitable for engineering project, where as the soil composition of this layer in the western part varied and considered as non-cohesive sediment up to (5 m) depth and considered as non suitable for constructing high buildings. Also, the results revealed that the second layer in both parts of the area was specified as a consolidated and more cohesive soil layer for the borehole penetrated depths.


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