Hoover Dam Construction

Geological Context Canyon’s tilted layers of andersite breccias Extrusive igneous rock Minerals - plagioclase, pyroxene & hornblende Withstand +200mpa in compression Formed around 15 million years ago (cenozoic period) Thick layer of sand and gravel on the canyon floor

On the Lake Mead fault system Primary reason for crustal deformation is strike-slip faulting Strata dips North to North -East, cut by South to South- West faults Geological Context

Problems Inactive fault system – before construction Increase in seismic activity corresponds with rise in water level Dam blocks river sediment 1947 – 30m of sediment behind the dam wall River Channels downstream increased by up to 3 metres

Alternative Engineering Solutions to the Hoover Dam Project The Initial location chose by the government was at Boulder Canyon; - Fault Lines - Lower Density Rock - Poor Strength of Bed-rock - Topography meant smaller reservoir could be made. - Further away from population centres. Hoover Dam Boulder Canyon Lake Mead

Alternative Engineering Solutions to the Hoover Dam Project Due to the St. Francis collapse the dam was heavily over-engineered - Could have been higher with larger reservoir. - Less concrete required for a slender taller structure. Sediment build up on rear of dam becoming a problem. - Tunnel to divert sediment around the dam. - Very expensive to remove to dredge lake. - River in front of the dam eroding river bed at alarming rate Hoover Dam Proposed Sediment Tunnel 220m 620m

Location Black Canyon: Creates a larger reservoir Better neighbouring geology Nearer to facilities/resources Huge ridges formed natural abutments Surveying work carried out, core samples of bed rock were drilled

Design Gravity-Arch Design  Ultra Conservative Recent seismic forces have had a negligible effect on the dam’s structural integrity Designed with a huge heavy bottom similar to a pyramid design. The Arch routes the water towards the two huge natural abutments.

Construction Diverted the course of river with 4 tunnels Wooden tunnel supports were deemed redundant due the strength of the rock Excavated the to the bed rock, clearing the canyon walls of loose rock 3.4 million m 3 of concrete poured into interlocking 18m 2 blocks, 1.5 metres deep. Two spillways built either side of the dam to safely traverse water

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