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23 causing the layers to open like pages in a book. With mudstones, slaking usually results in cracks forming throughout the mass of rock eventually causing the rock to break up into small cubes. Not all shale deposits and mudstones behave this way. Investigations must be performed on these materials to test how they will behave and perform as a dam foundation. Depending on the chemical composition and minerals that form the rock, the effects of slaking can be severe or hardly noticeable. The Bureau of Reclamation has a procedure that can be used to test if a material is prone to slaking. (Bureau of Reclamation, Engineering Geology Manual, page 80). Rocks whose samples show signs of slaking behavior during testing will need to have special treatments during construction to prevent slaking. Figure 14 shows a sign of a significantly slaked mudstone. Figure 14 - Effects of Slaking on Mudstone Soil Foundations The fundamental characteristics of soil deposits that form dam foundations include plasticity, density, and gradation, which in turn influence the foundation’s strength, permeability, compressibility (including settlement and collapse), and dispersiveness/ erosiveness. (See our previous article, “Soil Characterization – Here’s the Dirt (Part 1)” for more information.) Problems associated with dams founded on deposits of alluvial, colluvial, eolian and glacial soils are related to the soils’ permeability and initial degree of consolidation or density. Low density, soft, or loose soils of all types can settle when extra weight is added on top of them. Differential settlement across a valley with irregular steps in the foundation, or between materials with different densities, can result in cracks in the embankment, and cracks in rigid structures such as outlet works, spillways, valve structures, and vaults. Settlement can also gradually lower the crest elevation, which would reduce the available freeboard. Removal of soft soils or densification of soft soils may be needed at sites where excessive settlement is expected to occur. Stability is also a concern when constructing a dam on soil. The shear strength of very soft to soft soil is less than that of the compacted material used to build the embankment. This condition results in shear surfaces that will pass through the foundation causing slides, slumps, and cracks in the embankment. To mitigate for this situation, soft foundation soils should be removed. The slopes of the embankment can also be flattened, increasing the footprint of the dam, which moves greater shear strength into the foundation, thereby improving stability. Deposits of sand, gravel, and cobble are often adequate foundation materials with respect to support of the embankment; however, the major problems associated with these materials include their higher permeability and potentially open matrix. High permeability foundations can lead to excessive seepage, high gradients, and uplift pressures. Open matrix deposits are similar to open fractures in rock and must be isolated from or made filter compatible with the embankment materials they support to prevent internal erosion and/or piping from damaging the embankment or leading to failure. Seepage and water loss through the foundation can also be an economic problem. Dams that store water intended for irrigation, drinking water, industrial, or commercial uses may not be able to tolerate high water losses. In these cases, porous foundation materials should be removed or treated with low permeable synthetic or clay liners or cutoffs. Loose, low density, saturated sand, and silt deposits subjected to earthquake loading have the potential to liquefy and lose strength. Liquefaction of foundation soils can lead to slope stability and settlement issues that are similar to the issues related to soft, low density soil foundations. These issues include settlement and loss of crest height that could result in a breach of the dam. Near surface liquefiable material should be removed from the dam foundation. These