Influence of Variation in Moisture Content to Soil Bearing Capacity in Nairobi Area and Its Environs

The increasing human population in cities and urban areas continues to raise the demand for housing and other infrastructure in developing nations. Stability of structures is critical for sustainable development to ensure longer useful life of structures and reduction in the rate at which natural resources for construction purposes are extracted from the environment. Foundation of buildings infrastructure plays a key role of transferring the loading from the structure to the soil underneath. In foundation design, the ultimate bearing capacity of soil under normal circumstances assumes that the water table is located well below the foundation. Variation in soil moisture content during construction and during the structure’s lifespan affect the soil bearing capacity. Information on the extent to which variation in soil moisture content affect the soil bearing capacity was lacking. This paper presents findings of a research that sought to establish the extent to which variation in soil moisture content affects the soil bearing capacity. Seven soil samples collected from Nairobi area and its environs were subjected to 30%, 50% and 75% moisture content variation. The soil bearing capacity was tested using Direct Shear method and Undrained Triaxial method in accordance to British Standard 1377 of 1990 Part 7 and Part 8 respectively. Test results determined that the insitu moisture content for the collected 7 soil samples from Nairobi area and its environs varied from 21.9% to 55.4% implying the diverse characteristics of soil samples and sites studied. Increasing the soil moisture content from 30% to 50% and to 75% all other factors held constant contributed to reduction in soil bearing capacity as illustrated by a linear equation y = -170.89x + 565.64 using direct shear method. y is the resultant soil bearing capacity (kN/mm2) while x is the soil moisture content in percentage. This shows that variation in soil moisture content contributes to a significant reduction in soil bearing capacity by a factor of -170.89x. To mitigate the negative effect of reduction in soil bearing capacity as a result of changes in soil moisture content, a factor of safety should be applied at design stage by adjusting the allowable soil bearing capacity to take cognisance of the contribution by changes in soil moisture content. This is critical to ensure that all structures are designed to withstand variation in moisture content at the foundation throughout their lifespan and avoid potential structural failure.