Question

Explain how marine clay can quickly loose its strength with the rainfall

Answer 1

Sensitive Soils In some soils the clay minerals are arranged in random fashion, with much void space between the individual grains, which can be referred to as a "house of cards" structure. Often the grains are held in this position by salts (such as gypsum, calcite, or halite) precipitated in the pore space that "glue" the particles togetheir: The Role of Water (Rainfall) Although water is not always directly involved as the transporting medium in mass movement processes, it does play an important role Water becomes important for several reasons Addition of water from rainfall or snow melt adds weight to the slope. Water can seep into the soil or rock and replace the air in the pore space or fractures. Since water is heavier than air, this increases the weight of the soil. Weight is force, and force is stress divided by area, so the stress increases and this can lead to slope instability. Water has the ability to change the angle of repose (the slope angle which is the stable angle for the slope) Water can be adsorbed or absorbed by minerals in the soil. Adsorption, causes the electronically polar water molecule to attach itself to the surface of the minerals. Absorption causes the minerals to take the water molecules into their structure. By adding water in this fashion, the weight of the soil or rock is increased. Furthermore, if adsorption occurs then the surface frictional contact between mineral grains could be lost resulting in a loss of cohesion, thus reducing the strength of the soil. In general, wet clays have lower strength than dry clays, and thus adsorption of water leads to reduced strength of clay-rich soils Liquefaction As we have already discussed, liquefaction occurs when loose sediment becomes oversaturated with water and individual grains loose grain to grain contact with one another as water gets between them. This can occur as a result of ground shaking, as we discussed during our exploration of earthquakes, or can occur as water is added as a result of heavy rainfall or melting of ice or snow. It can also occur gradually by slow infiltration of water into loose sediments and soils. Clay bearing sediments in general require more water because water is first absorbed onto the clay minerals, making them even more solid-like, then further water is needed to lift the individual grains away from each other: