CIV2403 : Geology And Geomechanics - The Sheetpile - Impermeable Block - Assessment Answer

Geology And Geomechanics Assignment

Assignment Task1

A long sheetpile wall is embedded 3 m into a permeable soil, as shown in Fig 1. Dimensions are in meters. The soil is silty sand. A falling head test was performed to find the hydraulic conductivity of the soil. In this test, the head in the standpipe dropped from 450mm to 270mm in 120sec. The soil sample is 200mm long and has a diameter of 40mm. The diameter of the standpipe is 7.1 mm.

1. Determine the hydraulic conductivity of the soil in mm/min (per meter run).
2. Produce a flow net on the provided sketch sheet (Fig 2).
3. Calculate the seepage rate in lit/min (per meter run).
4. Calculate the porewater pressure at point A (the downstream water level is at +0.5 m).
5. Assuming that we are going to use one pump in every 25 m along the sheet pile in downstream side, determine the capacity of the pumps in lit/min to dry the downstream side (keep water level at the ground level).

Introduction

• Marble may be fine to coarse grained, and is a massive, even textured rockresulting from complete recrystallization of the calcite crystals of limestone. The rock istypically white but may be coloured due to impurities. Marble consists almost entirely ofCalcite. It may be formed by either contact or regional metamorphism.
• Hornfels is a fine-grained, massive, even textured rock resulting from contact metamorphism of various sedimentary rocks. The rock usually shows completere crystallization to a granulose structure.
• Gneisspossesses a crude or rough banding with light and dark minerals partially segregated into layers. This is also a form of foliation. Gneisses are generally not fissile.Mineral grains are visible and minerals can usually be identified.
• Schistsare foliated and fissile along the foliation. Mineral grains are visible, there forethe type of foliation present is schistosity. The schistosity may be planar or undulating.As grains are visible, the minerals are identifiable.The name of the most prominent mineral is normally placed before the name, schist inorder to distinguish between different types of schist e.g. Mica Schist or Chlorite Schist.
• Slateis a very fine grained rock, usually dark in colour. The mineral grains are too smallto be visible. It is fissile, splitting into thin layers. On these planes the slate has a sheen,indicating the presence of minute grains of mica. The planes along which it splits arecalledslaty cleavage, one of the forms of Foliation.Slate is derived from the Regional Metamorphism of a shale or mudstone. Sometimesremnants of the original bedding are still visible on the surfaces of the slaty cleavage.
• Quartz Porphyryis a shallow intrusive equivalent of granite. It is porphyritic with phenocrysts of quartz and orthoclase in a fine grained, light coloured groundmass.
• Orthoclase is the main constituent of syenite. Minor minerals may be quartz (<10%), plagioclase (<30%), biotite and hornblende. Syenites have very low colour index, usually not more than 5% to 10%. They are equigranular with medium to coarse grainsize. Syenite occurs in plutonic intrusions.
• Pegmatitieshave pegmatitic texture, i.e. extremely coarse grain size from about 3 cm to several metres. They may have the composition of any plutonic rock but granite pegmatites are the most common consisting of quartz and orthoclase with some muscovite and other less common minerals.
• Rhyoliteis very fine grained and light in colour. It may contain some crystals of quartz and orthoclase and may show flow banding. It may contain volcanic glass and chalcedony, a form of cryptocrystalline silica.
• Limestones consist of calcite (CaCO3). The diagnostic tests for calcite are a hardness of 3 and reaction with Hydrochloric acid.

Many limestones are accumulations of shells, pieces of coral or other fossils composed of calcite. The shapes of these fossils are visible on either cut or broken surfaces of the rock.

OoliticLimestones consist of spheres of calcite about 2 mm in diameter. On broken surfaces it is possible to see the spherical shape and on cut surfaces it is usually possible to see the nucleus around which the calcite precipitated.

• Dolerite is the shallow intrusive equivalent of gabbro. It has medium to fine grain size. In the finer grained dolerites, identification of the minerals is difficult. Dolerites are very dark in colour and occur in dykes and volcanic vents.

B) Cross Sections:

Two cross sections, with no vertical exaggeration, need to be constructed that best highlighted the geology of the region, particularly focusing on where the potential damsites could be located (1 page):

1. An initial cross section needs to be constructed running east - west.
2. A second cross section running north – south should be constructed.

C) Potential Dam Sites:

Identify three potential dam sites along any of the water ways found within the map for adam of approximately 10000 ML. Each potential dam site should be on a different

geological unit to ensure that a range of geological factors are considered and the best potential site is chosen. An analysis of each site should be conducted and a comparison

of the various sites should detail the strengths and weaknesses of each site (2 pages - appendices may be included; Geology Map; Topographic Map; Appendices 1 & 2).

There needs to be a final selection of the best dam site based on an evaluation of the threeselected sites that considers all of the data presented. Further site investigation recommendations to verify the viability of the site need to be included (1 page) prior to

seeking tenders to actually construct the dam.In order to estimate the dam costs for this project, a pre-feasibility / concept design

process was undertaken for a roller compacted concrete dam for FSL’s EL75m, 80m,

and85m. As the potential dam height is of the order of 20m it may be feasible that an earthfill dam would be economic at this site, however foundation condition ssuggest that it may be relatively difficult to control seepage if the dam were to be constructed on the alluvium. It may also be difficult to design an earthfill dam for earthquake loading. For this reason a preliminary estimate of an RCC dam constructed on rock foundations has been assumed for this cost estimate. Review ofthese assumptions will be necessary should this option be considered further.

Stripping depths of 20m were assumed for flood plain areas, decreasing to 5m on each abutment where the abutment steepness suggests that there is only minimal or no alluvium cover over the normal weathered rock profile.

For the purposes of these cost estimates, it has been assumed that materials for the construction of the dam embankment would be available.

In the absence of flood hydrology or spillway flood routing for this site, assumptions regarding the peak outflow were made as follows:

• The peak outflow was assumed to be equal to the peak inflow; and,
• The maximum design flood value was assumed based on a catchment area of 2,110 km2 with the maximum design inflow flood assumed to be 10 m³/s/km².

The concept design was therefore developed to pass a peak outflow for the maximum design flood of 21,100m³/s.

A spillway, 600m long was assumed to discharge directly into the river via a dissipater. This length of spillway has been adopted to minimise the impact. If full supply levels are adopted that result in flooding of cost savings associated with the spillway may be possible.

These hydrological and hydraulic assumptions, including spillway length and peak outflow, should be reviewed, should this project be taken further.

An amount equal to 3% of the total contract price for the work has been allowed to provide for implementation and management of environmental works. This includes provision for fish lifts, erosion control works etc.

The estimated costs of the dams for these full supply levels are indicated in Figure. The optimum development was not able to be determined within the range ofstorage capacity and yield information available and so this information wasextrapolated a small amount to enable the optimum development to be assessed. Thisoptimum far exceeds the practical limit for the development, which is about a fullsupply level of EL75m as discussed above. The extrapolation therefore has no impacton the project costs at the critical full supply level of about EL 75m.