**2. Geological and geotechnical investigations**

Thick overburden layers generally refer to quaternary materials deposited over river beds, including boulders, cobble, gravel, sand, silt and clay constituents. Mixtures of these complex overburden materials are often much more compressible and permeable than an intact rock foundation. Adequate geological and geotechnical investigations on the distribution, thickness and other relevant properties of the soil strata are necessary for the design of impermeable systems and for the preparation of required foundation treatments during dam planning stages. In particular, weak layers, such as sand lenses, soft clays and collapsible loess, should be revealed in these investigations and then properly treated to eliminate safety risks to both the foundation itself and to the overlying dam.
