Landslides and Infastructure

Topics: Puget Sound, Pierce County, Washington, Tacoma, Washington Pages: 3 (792 words) Published: February 24, 2013
In the Puget Sound landslides are fairly common, mostly due to a very wet climate and ever changing environment. The definition of a landslide is: A gravity driven down slope movement of a mass composed of soil, rock, and vegetation. A landslide generally happens when the material in the down slope is heavier then the supporting soil. There are many contributing factors that cause landslides, steepness of slope, composition of soil and rock, groundwater condition, recent precipitation patterns, slope aspects vegetation on slopes, and anthropomorphic activities (land clearing). There are generally four types of landslides; Shallow bluff, deep seated Debris flows, and submarine landslides. In a shallow bluff landslide they generally occur on the steep Puget Sound marine bluff. Generally they are caused by the clearing of vegetation to improve views but in turn can lead to soil instability and slope erosion. Deep seated landslides can be as large as thousands of acres but can also be relatively small and confines to a smaller area, and usually occur when the slope is lower than 15% Debris flows are considered to the most hazardous to life. Debris flows are generally fast moving water saturated masses of soil, rock, debris and tree trunks. These landslides can be triggered by intense rainfall and are mostly found in the mountains of Peirce County. Submarine landslides primarily occur underwater along the delta of the Puyallup River. These slides are triggered by rapid sedimentation caused by and unstable slope, the loss of soil strength due to static liquefaction, loss of slope support because of bottom current erosion of material at the base of the delta slope. These slides are also triggered by the additional loading at the top of the delta slope with artificial fill, this caused increased weight on the slope. Liquefaction caused by earthquakes is also a huge factor in submarine earthquakes. Submarine landslides in the Pacific Northwest generally occur on the...
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