1. Figure 9.18 is representative of soil conditions at a construction site. Assu
ID: 1765826 • Letter: 1
Question
1. Figure 9.18 is representative of soil conditions at a construction site. Assuming that you are the contractor, select a dewatering method you would consider appropriate for each of the projects described below. Consider the information given in this and earlier chapters and any other information that may be appropriate. Describe in detail how your system would be built and operated. Justify clearly any assump- tions you make. Discuss what difficulties you may encounter and how you should plan for them. a. 1200 ft of an 18-in sewer line is to be placed in a trench about 16 ft below ground surface. b. A 30-ft-diameter concrete pump station is to be founded on the basalt rock at 65 ft in depth. c. Conditions as in a except that the pipeline is 28 ft below the ground surface. EWATERING Soil description Depth feet penetration resistance, 50 100 0 Medium stiff to soft brown clayey silt 10 6/16/72 20 Medium dense silty fine sand Medium dense silty clayey 40 Stiff to very stiff gray clay 60 Basalt rock 70 100 50 Water content, wExplanation / Answer
The methods of dewatering used for the various projects are as follows:
a) Wellpoint method of dewatering can be used for the case. This is because sewer line needs to be placed at a depth of 16ft below ground level hence wellpoint method would be sufficient to create proper drawdown as this method is restricted to create drawdown around 5 to 6 metres below the wellpoint pump level. Since the soil layer encountered has low permeability hence wellpoint method will be preferred. Since the geologic conditions encountered also permit close spacing of devices.
b) Deep well method of dewatering can be used in this case. This is because the pump station is to be founded on basalt rock which is 65ft deep and for the deep well method the depth of the wells could reach upto 30m.
c) Open sump pumping method of dewatering can be used in this case. This is because the pipeline has to be laid at 28ft below ground surface in sandy soils.
The difficulties that may be encountered and the steps that could be taken to counter with the difficulties are:
1) Creating deep well upto the level of basaltic rock may be difficult.
The drilling bit should be tipped with very hard cutting material (Tungsten Carbide or industrial diamond) attached to the drill rod through a hammer mechanism in between. Compressed air is used both for rotating the drill rod as well as generating a constant hammering action on the attached drill bit.
2) Installation of wellpoint system could be little expensive.
Supporting conditions needs to be improved to prevent failure of the dewatering system.
3) For deep well systems the amount of power required to operate the pump could not be properly available in certain areas where access to powergrid is not proper.
The alernative standby option for power should be arranged beforehand for the locations of deep well dewatering.
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