Geotechnical characteristics of Quaternary Sediments in Mahshar Port

Regarding to the importance of the industrial region in engineering project execution, assessment of Geotechnical characteristics is essential to prevent undesired problems. Because of the uprising number of oil and gas industry structures, the Mahshahar Special Industrial Region is very important. So, the Geotechnical investigation in this region is necessary. Executing these investigations guaranties the implementation of any construction projects and prevents any unwanted detrimental events.
The study area is located in Mahshahar Port near in southwest of Iran. This port is one of the important industrial ports of Iran. Mahshahr port is located in area between 49º04´-49º06´E and 30º25´-30º31´N.
Based on Sedimentary-structural division of Iran, this area is located in Zagros zone. This zone is limited to Sanandaj-Sirjan zone from the northeast, to the Makran Zone from the southeast, to the highlands of northeastern Iraq and southeast Turkey from the northwest and to Arabian Plate from the south and southwest. Based on geological data, the properties of Zagros zone varies in different areas and this zone is divided to some subzones that Abadan Subzone, in which the study area has located. This subzone is located in the southwestern part of Zagros.
To investigate the Geotechnical characteristics of Quaternary sediments in the study area, eight boreholes data, performed by continuous coring method, with depth 11 to 25m were studied. The seismic downhole test and Standard Penetration Test (SPT) were done in all boreholes. The laboratory tests are included the index tests on soil samples (particle size distribution, Hydrometer, Atterberg test), consolidation, direct shear, uniaxial and triaxial tests. Strata were classified using index tests.
To reveal the stratification of site, the Seismic Downhole test was performed in all boreholes, in which the source was in surface (blow on a plate placed in 2-3m distance from borehole) and the P-wave was measured in geophones installed in different depth of boreholes (1m intervals), so whole depths of borehole were investigated. To measure the S-waves, the blows were stroked in a horizontal direction on timber to generate S-waves. Finally, the relationship between elasticity modulus in downhole and SPT tests and also the relationship between friction angles in triaxial tests with NSPT values were assessed.
The plasticity of soil has investigated using Atterberg test results, performed applying standard ASTM D4318 on all samples. The results show that the Liquid Limit and plasticity index vary between 25-40% and 10-20%, respectively, showing the low to medium plastic characteristic of soil.
The elastic modulus and also the relationship between the elastic modulus on downhole and NSPT tests were investigated in the study area. Based on the results, the elastic modulus of quaternary sediments of Mahshahar varies between 100-350 Mpa. In general the relative density and the seismic wave velocity of soil strata were increased with depth in this area.
Also, the fitting curve of NSPT values and elastic modulus in downhole tests show the R2=0.80, in which the NSPT values were corrected to N60.
Soil classification of the site, regarding the average S-wave velocity of soil up to 30m depth, was performed based on the Iranian Building Code (IBC) 2800. The recorded mean S-wave velocity in all boreholes is less than 375m/Sec so the ground type III (based on IBC 2800) is recommended to be taken into account in the design.
Estimating of friction angle of soils, some researchers like Hunt (1984) have been published some reference tables. In this study, to have a simple equation to estimate the friction angle of quaternary sediments, the results of NSPT and consolidated undrained triaxial test (CU) have been used. The result shows the R2=0.85 for fitting curve. Regarding the results, it can be concluded that because of overestimating of friction angles, the presented tables by Hunt (1984) is not applicable to Mahshahr quaternary sediments. It seems that the main reason comes from remarkable existence Clay soils in the sediments.
In this research, based on data from boreholes, the general characteristics of quaternary sediments were revealed and also the strata were classified based on the Unified System as ML, CL and SM. To determine the elastic modulus of soil, the results of the seismic downhole test were used. Based on these results the elasticity of soil varies from 100 to 350MPa. The relationship between mentioned above elasticity and NSPT was assessed and an equation was determined. In general the relative density and seismic velocity increase with depth in this site. S-wave velocity in Seismic downhole test results were used to determine the site or ground type. This parameter was less than 375 m/Sec in all boreholes. So, the ground type III (IBC2800) is recommended for the whole site. Finally, an equation was concluded for relationship between NSPT and UC triaxial friction angle which can be more useful than other published relationships (like those by Hatanaka & Uchida) and Hunt’s tables, because these published ones overestimate the friction angle; the main reason of this case is the existence of the great amount of Clay in soil in this site.