GEOTECHNICAL CHARACTERIZATION OF THE VOLCANICLASTIC ROCKS IN AND AROUND TAIZ CITY, YEMEN

Abstract

An attempt has been made to evaluate the geotechnical characteristics of volcaniclastic rocks in Taiz city of Republic of Yemen for slope stabilisation of the existing civil structures and new construction purposes. The aim of this paper is to define the quality of volcaniclastic rock mass into several “classes” based on which the concerned civil engineer can take suitable measures for the stabilisation of the rock mass itself.Classifications are made based on field analysis and laboratory measurements. Finally, the indirect method of assessing the quality of the rock mass is also adopted. Rock quality assessment based on two attributes (1) properties of intact rock and (2) joints has been made by determining the Uniaxial Compressive Strength (UCS), Rock Quality Designation (RQD) index and Rock Mass Rating (RMR) system. The volcaniclastic rocks in the study area are represented by tuffs, ignimbrites, breccias and volcanic agglomerates rocks. All these volcanic materials are mainly of mafic and felsic (basalt and rhyolite) composition and belong to different phases of Tertiary volcanism. Volcanics show wide variation in their colours, thicknesses, degree of consolidation, degree of compactness and homogeneity. The different volcaniclastic rocks are grouped into two geotechnical subunits based on their strengths; strongly welded basaltic/rhyolitic volcaniclastic rocks (SBVc- Tb1/Tb2 /SRVc - Tr1/ Tr2) with strengths more than 25 MPa and weakly welded basaltic/ rhyolitic volcaniclastic rocks (WBVc-Tb1/Tb2 /VRVc -Tr1/ Tr2) which have strengths less than 25 MPa. These rocks are heterogeneous material having low to high porosity and low to moderate density, with high levels of water absorption depending on the size, the shape and petrological composition of the grains/fragments in addition to the type of matrix, the degree of packing between all components and the degree of consolidation and compaction state of these deposits. In some sites, the volcanic pyroclastic rocks (especially, tuffs and ignimbrite flows) are affected by discontinuities aligned principally in NE-SW and NW-SE directions following the trends of the regional structures. Stereographically, at most investigated sites, typically three joint sets are identified in addition to other joints orientated randomly. The study based on several geotechnical parameters brings to light that weakly welded volcaniclastic materials are causing instability to the civil structures in the region