Geotechnical Evaluation and Deep Foundation Design Strategy in Gypseous Soils: A Case Study of the Airport Intersection Bridge Project in Najaf, Iraq

Abstract

The paper discusses the integrated geotechnical evaluation of the sub-surface conditions at the proposed site of Najaf Airport bridge, Iraq, based on the field investigation data, standard penetration tests (SPT), laboratory soil characterisation, shear-strength testing and chemical analysis of soil and ground water. The data obtained indicate heterogeneous alluvial soil profile with prevalent sandy deposits interbedded with isolated silty and clayey horizons, with considerable accumulation of gypsum in the near-surface zone. The laboratory tests on soil revealed that the tested materials were mainly poorly graded sand (SP) and silty sand (SM), while the fine-grained deposits at depth were classified as MH and MI. The corrected SPT resistance generally increased with depth, presenting maximum values of 64, 59 and 58 for BH1, BH2 and BH3 respectively, indicating denser and stronger deposits with depth. The friction angle ranged from 30° to 41.2° as shown by the direct shear tests indicating that sandy soils behave essentially in a frictional manner. Groundwater was encountered at a depth of about 4.2 m below ground surface and chemical analyses indicated significant levels of sulphate, chloride, total dissolved solids and gypsum indicative of potentially aggressive subsurface conditions. The average calculated allowable bearing capacity of the investigated raft foundation condition was 14.5 ton/m2 with a range of allowable values of 14.2-14.8 ton/m2. The estimated allowable axial capacity of a 1.2-m diameter bored pile increased from approximately 145 tons at a pile length of 15 m to 310 tons at 20 m. The results show that generally the engineering properties of the site vary significantly in the spatial and vertical directions. The results provide a preliminary basis for foundation selection and indicate that foundation design should consider the groundwater conditions, the presence of gypsum in soils, the chemical aggressiveness, and the variation of soil resistance with depth. Full-scale pile load testing is recommended to confirm analytical pile capacity estimates prior to the final foundation design.