GEOTECHNICAL AND ENGINEERING GEOLOGICAL INVESTIGATION OF
MEKELE TOWN, NORTHERN ETHIOPIA

ABSTRACT

In this work geological, geotechnical and engineering geological, groundwater condition, classification of soils and rocks of the town area and its environ is investigated in relation to their effects to civil engineering structures and related developments. The engineering geological and geotechnical condition of the rocks and soils is studied on the bases of field description, simple in-situ geotechnical test and laboratory analysis.

Geological and engineering geological map of Mekele town is prepared at a scale of 1:10,000. The dominant lithologic units are quaternary sediment, dolerite, sandstone, limestone-marl-shale intercalation and well-bedded limestone. Three systems of faults, WNW-ESE, N-S and NNE-SSW trending, and two major joint sets are dominant with some other minor sets (NE and NW strike).

Soils in Mekele town area and its environ are dominantly composed of fine fractions. Four soil types are identified in the field: clay, silt, sandy clay/silt and clayey/silty sand soils. Moisture content of soils varies from 15.8 to 40.9% for clay; 21.7 to 34.7% for silt; 6.6 to 20.5% for sandy silt/clay and 14.2 to 23% for clayey/silty sand soils. Silt and clay fractions are dominant in most soil samples. The specific gravity of the soils is not highly variable (2.45-2.9). pH and electrical conductivity of the soils vary from 7.1-8.5 and 180-1930?S/cm respectively. The liquid limit (LL) in percent varies from 29-59, 50-67.4, 37.5-70.8 and almost non-plastic to 66 for clay, silt, sandy clay/silt and clayey/silty sand respectively. Similarly the plasticity index in percent (PI) of the soils varies from 14-36.6, 13.3-37.4 and non-plastic -38.6 for clay, silt, sandy clay/silt and clayey/silty sand soils respectively.

The shrinkage limit in percent (SL) of the soils varies from 9.3 for clayey/silty sand to 30.9 for silt. The free swell of the soils is highly variable, from 0 to as high as 70%. High values are recorded in clay and silt soils, indicating their potential expansive nature and this property is counter checked using correlation tables and potential expansion classification charts. The internal friction and cohesion of the soils varies from 130 to 300 and 30.4 to 86 KN/m2 respectively, while the unconfined compressive strength of the soils of the area ranges from 88 to 441 KN/m2.

The soils of the town are classified based on two classification systems: Unified Soil Classification System (USCS) and International Association of Engineering Geologists (IAEG). The rocks of the area are classified based on Schmidt strength and degree of weathering and fracturing. The dolerite shows variable engineering property in terms of strength from weak rock mass strength to very high rock mass strength; the sandstone and limestone-marl-shale intercalation show low or weak rock mass strength and the well-bedded limestone have generally high rock mass strength. The well-bedded limestone is better for general building stone and aggregates, while the weak or dolerite with low rock mass strength is better for pavement material (for road sub-base, base and sub-grade materials).

The main geotechnical problems that affect any design and related development in the town are found to be presence of expansive soil, cyclic weak and strong rock units with depth, flood hazard, variable weathering profile, presence of corestones, fluctuation in groundwater and its damaging constituents and presence of considerable amount of sulfate and chloride content in soils, rocks and groundwater in the area. Generally in this research basic engineering geological maps and systematic engineering geological and geotechnical data, useful for future infrastructure development, have been prepared.

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