4.2 Foundation Conditions
Poor foundation conditions are cited as the most frequent cause of dump slope instability. The foundation of a dump may have a variety of rock formation from saturated soil to competent bedrock. The presence of water within the voids of a dump seriously affects the dumps stability depending on the quantity or elevation of the phreatic surface (limit of saturation). Inflows of water to a dump include precipitation, snow melt, surface runoff, and foundation seepage.
Presence of weak saturated soil or similar zones in the foundation affects stability of a dump by allowing deformation of the toe material. Similar conditions may arise where material is placed on previous failure debris. Debris often exhibits a much lower strength than in its original state due to changes in particle size and shape, and moisture content.
Surface runoff is controlled to a large degree by the physiography of the site, however manmade diversion systems can be constructed to minimize this amount. Foundation seepage can occur in the form of groundwater springs which are often controlled by the geologic structure.
The potential presence of groundwater springs needs to be investigated prior to dumping. It is not uncommon for these features to be located in gullies or creek beds where contact with the natural groundwater table can be expected. Dump configurations such as valley fills or cross valley fills require an assessment of the drainage characteristics of the foundation and fill material, to prevent built up of water within the dump.
Competent foundations are formed of highly competent bedrock or soil of equal or greater strength than the dump materials, and which is insensitive to pore pressure generation or strength reduction due to loading. Intermediate foundation is formed of intermediate material which consolidate and gain strength with time, but may be subjected to pore pressure generation and strength loss if loaded and sheared too rapidly. Weak foundations are formed of weak materials which cannot safely be loaded beyond a limiting level of shear stress, and which does not gain strength at a significant rate by consolidation. This is frequently the case where clay layers occur within the foundation soils. Such foundations are also subjected to potential liquefaction or high pore pressures.