Figure 2 shows a typical cross-section of the proposed earth dam, in which the e

Question

Figure 2 shows a typical cross-section of the proposed earth dam, in which the earthwork construction is principally divided into 3 different zones: a) Stability Zone b) Core Zone c) Filter Zone Full Reservoir Level Core Zone Stability Zone Stability Zone Figure 2: Typical Cross-Section of Proposed Earth Dam The Tasks 2. You are asked to investigate the purpose and design function of each of the 3 earth zones as shown in Figure 2, and suggest the desirable engineering soil properties required for each zone.

Explanation / Answer

The most important variables affecting the construction of earthfill embankments are the distribution of soils, the method of placement, water content, and compaction.Soils may be classified by engineering properties into various groups. These groups fall into two main divisions, the coarse grains and the fine grains. Coarse grains are those larger than a number 200 sieve size and include gravels and sands. Fine grains are smaller than a number 200 sieve size and include silts and clays. Course grain material is used for the outer zones of an earth fill embankment, and fine grain material is used for the impervious core or central portion of the dam. A sieve analysis test will determine the percent of material passing a given sieve size.

The soil material must be placed in horizontal layers not more than 15 cm. thick after being compacted. The soil should be homogeneous and free from lenses, pockets, organic material, or other imperfections. Prior to placement, the material should have the optimum moisture content required for the purpose of compaction. The optimum moisture content, or the water content that produces the maximum density, may be obtained by a laboratory Proctor test.

Good compaction of a cohesive soil reduces permeability and increases shear strength and the stability of the dam. Compaction equipment includes sheep-foot rollers, pneumatic rollers, and hand tampers. The dry density of the soil should not be less than 95 percent of standard Proctor test.

For a core zone in a dam, A compaction grouting method was selected as the remediation technique. In addition, the reduction and prevention of leakage and settlement were assessed. Compaction grouting was successful for filling voids, closing channels, and compacting the disturbed core soils. The loose or voided zones were properly filled, and the leakage was reduced after compaction grouting. Verification of the compaction grouting work was performed by evaluating the grouting pressures and volumes injected internally and by monitoring the dam leakage rate and tracer externally. All of these factors provide a good indication of changes inside the core of the dam, including reduction or closure of the leakage channels in the dam core.

For a filter zone, the narrow zones of dark brown filter material between the clay core and the rock fill both upstream and downstream of the core. When the dam is completed the core and filter zones are normally hidden from view inside the dam so that only the rock fill zones can be seen on the upstream and downstream faces of the dam wall. In foreground drilling rigs are being used to drill grout holes in the conglomerate, sandstone and shale foundation. Grouting is used to fill open cracks in the foundation rock so water cannot leak out of the dam and is carried out by pumping grout (a mixture of cement and water) under pressure into holes drilled in the foundation rock.

A homogeneous dam with a height of more than about 6 m to 8 m should have some type of downstream drain. The purpose of a drain is:

The effectiveness of the drain in reducing pore pressures depends on its location and extent. However, piping is controlled by ensuring that the grading of the previous material from which the drain is constructed meets the filter requirements for the embankment material.

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