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| Geotechnical Assessment of Subgrade And Sub-Base Quality for The Design And Construction of Road Pavement In Parts of the Coastal Section of Kasese District, Western Region, Uganda. |
LION ENGR. ITAFE VICTOR ADACHA, HND (Civil), PGDPM, MNATE, MNSE, COREN
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Abstract:
Pre-design and construction geotechnical assessment of subgrade and sub-base materials was carried out for optimal performance and pavement sustainability. Method involved boring of 59holes and trial pits, sampling and geotechnical testing. Soil profile indicates a top inorganic clay layer (CL) from surface to an average depth of 0.55m, a silty sand layer (ML and MH) to 1.1m and a poorly graded sand layer (SP) to 2.0m. Subgrades classify as USCS CL - CI clays, AASHTO A3, A-2-4, A-2-6, A-2-7, A-7-6, A-7-5 with 58.85% of the subgrades rated as poor to fair and 41.15% good to excellent. Subgrades clay activity varies from 0.8 to 41.25 and expansivity from 4.83 to 68.90 indicating presence of extra - sensitivity and quick and expansive clays, 48hours soaked CBR from 7% to 38% and surface modulus (Es) from 61.5 MPa to 168.14 MPa. Subbase fills have moisture content, liquid limit, plastic limit and plasticity index of 15 - 20%, 24 - 34%, 13 - 22% and 9 - 15 respectively, composed of 29.8% fines (clay and silt classes) and 70.2% sands; AASHTO classified as A-2-6 and rated excellent to good; 48hours soaked CBR of 17.0 - 39.0% and un-soaked CBR of 30 - 58 depicting a 10.0% - 62.0% reduction in strength. The resilient modulus vary from 206.84 - 537.78 (MN/m2), grading modulus from 0.67 - 1.04, and plasticity modulus from 603.0(MPa) - 1395.0(MPa). Pavement thickness has been recommended to be 300 - 350mm based on CBR and group index and coarse granular subbbases of sand and gravely materials for egress of moisture to improve road bed effective stress.
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