Geotechnical design and analysis Subject:
residual soils- increase in shear strength & bearing capacity
Author: c.y. abdulla
I'd be glad if i can get answers on the following questions.
1. In firm to medium stiff residual soils, is it possible to get increase in shear strength due to placing of fill on top of the residual soils. (similar to like placing surcharge on soft clay?)
2. If a foundation i.e. footing, is placed on competent residual soils, then followed by fill surrounding the fill, can the fill be taken as contributing to the bearing capacity of the founding soils, as part of the surcharge factor?
I think certainly you can gain some increase in bearing capcity of your foundation according to Terzaghi's equation. But gaining of bearing capacity does not mean increase of the shear strength of soil, it only means that you have a larger "counter weight" to counter the load due to foundation. The gain in shear strength may be the later part when the subsoil has further consolidated. For residual soil they are overconsolidated and thus in order that strength is gain the overburden fill should be greater than the preconsolidation pressure of the soil.