Beverly Hias 246 Slopes and Subsidence The Baldwin Hills, California, reservoir

Question

Beverly Hias 246 Slopes and Subsidence The Baldwin Hills, California, reservoir disaster The Baldwin Hills dam and reseveir were built in 1950 as a backoup facality during peak hours of municipal water Figure 13.21 The site of the Baldwin Hill active Newport-Inglewood Fault is reservois sits astride only a few hundred yards from the dam, two faalts and was demand The site chosen for this reservoir could hardly have been worse The 1962 actively subsiding at the time of it and two faults with questionable activity run directly through the site (Fig 13.21). Te structural picture is compounded by subsidence caused by onstruction Palos Verde od extraction at the adjacent Inglewood ol field. Measured subsidence during the period 1917-1962 is 9 ft Planners were aware of the tenucus elaborate reservoir liner, with drain Figure 13.22 An This, they believed, could contend with rescrvoir liner (not to geologic setting, so they designed an claborale four-layer pipes beneath to handle any leakage. scale here) was designed, with leak drains beneath it 10 feet compacted earth 4 inches cemented pea-gravel ground motions (Fig. i3.22). At 11:15 am on Saturday, December 14, 1963 tha liner failed along one of the faults (Fig. 13.23), releasing 65,000,000 galloas down the canyon to the sorth (Fig 13.24). The wall of water swept away five people and destroyed scores of homes. Now only Figure 13.23 The he shell of what once was marks that empty reservoir. The fateful spot. rupture (shown with the hachured gash) was along one of the 01324 The Baldwin Hills reservoir faults was completed in 1950, and it failed in 1963. Given this time line, which is more logical, (a) the reservoir was not designed sufficiently strong to hold 65,000,000 gallons of water, or (b) there was a geologic process going on here? 400 ft 13.25 Review: What were the two features associated with the prospec- tive site that should have been Figure 13.24Water rushes through the breach in the dam. The high-water mark sulficient to dissuade thinking peopleThe high-water mack from constructing a water reservoir on that site? Hint: Both features are presented in the first paragraph on this page and are illustrated in Figure 13.21. $gnals what once was. The reservoir has been abandoned, and grass now grows through the tell-tale cracks in the failed liner

Explanation / Answer

Slope –

Definition - A surface that is inclined relative to the horizontal (or) a surface with two hypothetical points located at different elevations define a Slope.

Characteristics - Downslope is subjected to Gravitational forces and material keeps on moving down the slope. The movement of the material depend on the degree of slope and Slope-stability. If the slope is steep, then the movement is faster. Slopes also vary according to Climate and lithology or the underluying rock types. Well jointed lithologies with extreme climate have steep slopes whereas those with soft rocks have gentle slopes. It is important to understand slope characteristics while under-taking any construction project. Slopes give rise to Mass movements. These are extremely dangerous to human kind if not studied appropriately. Slope stability analysis caters this study and thus saves many lives.

Slope instability leads to subsidence. The safety factor (SF) of a slope is determined by the ratio of the resisting force versus the driving forces. The latter includes water saturation level, porosity values of soil, Lithological drainage and the corresponding permeability values, climate, slope angle etc. Whereas the resisting forces include growth of vegetation, Shear strength of the material etc.

The ratio of resisting forces to driving forces is the safety factor (SF):

SF = Resisting Forces

      Driving Forces

If SF > 1 then the slope is safe

If SF < 1 then it is unsafe

Subsidence -

Subsidence is the process of slow or rapid sinking of the land surface. Subsidence occurs due to presence of solutional lithology like Limestone or Dolomite in Karst. It can also be caused due to movement of underlying faults or changes in Volcanic material (magma) volumes forming a caldera due to subsidence. Also, Chemical weathering at an extensive scale may also create voids which coalesce and at one point and become large enough to cause largescale saubsidence. Underground or subsurface mining as observed mainly in Coal production may also cause subsidence due to loss of support for the overluying rockmass. Similarly, extraction of groundwater cause solutional depression which may also trigger subsidence. Such cases are frequent in oil exploration areas.

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