Geotechnical Considerations for the Design and Construction of Foundations in a Marshy Stream Channel of Iwochang - Ibeno, Eastern Niger Delta, Nigeria

Abstract

A pre-construction site investigation was carried out in a marshy stream channel and adjoining areas for a proposed building site to characterize the subsurface subgrades and recommend foundation design for which proposed structures include a 1-floor 39.7m x 33.7m hostel, a 20.5m x 10.0m 4-bedroom duplex and a 1- floor 2-bedroom block of flats measuring 28.2m x 11.5m with 1.5 factored design dead + live load data as 2700tons, 655tons and 1270 tons respectively. Field investigations include boring of 10 boreholes to a depth of 10m using auger and sounding of 6 cone penetration tests using a 2.5tons mechanical cone penetrometer. The results indicate a soft clay layer existing from ground surface to a depth varying from 1.0m – 1.1m in the stream channel and 0.60m – 0.70m on the adjoining land. These clays are extra-sensitive to sensitive high compressibility Kaolin clays (CH – OH, MH - OH) with undrained shear strength varying from 42 – 75.0KN/m2, angle of internal friction ranging from 0 - 30 with cone resistance values of 3.0 – 11.0 Kg/cm2. Swell potential ranges from 11.45 – 30.64%, swell index from 0.44 – 0.57, activity from 7.0 – 11.0 and swelling pressure 4.776KPa – 4.890KPa. Below this depth a harder clay layer occurs to a depth of between 4.5m – 5.2m and is proposed to found the structures. Pre-consolidation pressures determined from Oedometer test on undisturbed clay samples retrieved from the centre of the second clay layer varies from 125.0KPa – 162.5KPa and Overconsolidation ratios from 2.75 – 6.40 depicting overconsolidation while water table corrected bearing capacities indicates a favourable fully compensated depth of 1.2m for the building foundations. However excessive total settlement determined using Boussinesq’s average vertical stress ranges from 180.1cm - 211.1cm on adjoining land and 160 -111.9cm on the stream channel under the worst case scenario for the structures necessitating further depth compensation to 2.0m. This yielded a reduction in settlement varying from 8.0% to 9.9% on the stream channel and 16.7% - 18.4% on the adjoining land. Rate of settlement depicts that it will take 6.655 and 28.65 years after construction to achieve 50% and 90% settlement under the worst case scenario. Below these clays are loose to medium density sands of varying grain sizes. Load transfer to these sands through pile foundations was considered using the cone penetrometer as a load test to derive unit toe bearing capacities of piles which embedment depth of 11.0m was recommended.