GEOTECHNICAL PROPERTIES OF SUB-SOIL AND GEOTECHNICAL MAPPING OF AL-KHOBAR- DHAHRAN-DAIVHVIAM AREA, KSA

Abstract

ABSTRACT The geotechnical properties of subsurface soil and rock formations of Khobar - Dhahran and parts of Dammam city , covering an area of about 675 sq.km were investigated during this study. ~ The geotechnical properties were determined through field and laboratory tests. Field testing was conducted by SPT; standard penetration tests, CPT, cone penetration test and PMT, pressuremeter tests. The- index properties as grain’ size analysis , moisture content and plasticity limits were determined in the laboratory . Strength tests as unconfined compressive strength , undrained shear strength as well as one dimensional consolidation properties were also determined . The chemical properties of soil and water samples were also analyzed . Soil types encountered within the study area were determined depending on their geotechnical properties. The study area is divided into four zones of similar properties . ' I Zone I , is a very loose to loose, saturated, sabkha soil, that consist mainly of poorly graded sand, silty sand, sandy silt and sandy clay. Vertical and horizontal extent of this material indicated that soils within zone I are mainly characterized by low bearing capacity and higher settlement. These loose bearing strata , needs to be improved before constructing even light structures as one to two storied villas , on them. Soil improvement within zone I can be done by preloading, dynamic compaction or use of stone columns. The method of soil improvement should be selected based on soil type and thickness , type of structure and loads as well as economy .

Zone II , covers the coastal plain of AL-Khobar city and parts of Dammam city. Soils encountered in this stripvare, backfill and reclaimed soil ‘in the upper 2m, followed by loose sand till a depth of about 7m and dense sand till a depth of 15m to 18m. Stifl' to hard clay layers were encountered below dense sand till a depth of about 50m below existing ground surface. Zone HI covers an area of about 300sq.krn. Two major soil types were encountered in this area, the residual calcareous soils and the sand deposits. Residual calcareous soils are the product of weathering of parent calcareous rocks in the area. Sand deposits are mainly wind-blown sand that deposited on outcropping rocks, wadis or covering residual soils. Both residual soil and sa.nd deposits are dense/hard and rarely loose or sofi. Zone IV, is mainly covered by outcropping calcareous rocks of the central part of Dammam Dome. Most of these rocks belong to Rus and Dammam formation. The major rock types encountered in Zone IV are limestone , marlstone and shale. Limestone i_s generally light cream, moderately weathered, fractured, voided, partly cavemous and moderately weak to strong. Marlstone is highly to moderately weathered, fractured and voided rock with very poor to poor RQD and moderately weak to moderately strong . Shale is mostly belonging to Salia and Medra members of Dammam formation. It is highly weathered, fractured, less voided with very poor to poor RQD and weak to moderately weak strength. Groundwater was encountered at an average depth of 1.0m within sabkha flat , at a depth of 1.74m in Zone II and a depth of 3.15m in Zone III . Groundwater is generally below 10m in Zone IV . The chemical analysis of both soil and water fiom different zones , show a high to very high concentration of salts ( TDS , chloride and sulphates).

These high concentrations of salts, give rise to a very aggressive and corrosive ‘environment whereby good quality concrete using sulphate resisting cement, pozzolanic materials , corrosion inhibitors and suitable water proofing membranes and sutable concrete covers are required. A multipurpose, comprehensive, medium scale geotechnicall engineering geological map is prepared. This map is considered useful, valuable and l'16lpfi1lfO1' planners, developers, designers as well as researchers interested in the area. " It is highly recommended to extent this work to cover other parts of Dammam city and its tenitories as Sihat , Qatif and Ras Tanura . A hazardous map showing locations of cavernous rocks , uncontrolled fills and liquefiable soil is also prepared . Since settlement is the controlling design criteria for cohesionless soils, other than bearing capacity , it is estimated following Burland & Burbridge (1985) , semi-empirical procedure which gives reliable values for design of shallow foundations in most areas . Standard penetration test results obtained from some -‘sites , were correlated with modulus of elasticity from pressuremeter tests conducted on the same sites . The correlated values were used in estimation of settlement and values obtained are very close to those of Burland & Burbridge method .