answer a and b please 1. Figure 9.18 is representative of soil conditions at a c
ID: 1765817 • Letter: A
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answer a and b please
1. Figure 9.18 is representative of soil conditions at a construction site. If you are the con- tractor, select a dewatering method you would consider appropriate for each of the projects described below. Consider the information given in this chapter, earlier chapters and any other information that may be appropriate. Describe in detail how your system would be built and operated. Justify clearly any assumptions you make. Discuss what difficulties you may encounter and how you should plan for them a. A circular reservoir 100 ft in diameter is to be built with a bottom slab 25 ft below pre sent ground surface. The groundwater level must be held at least 2 ft below the slab until the reservoir is filled. Standard Depth, feet Soil description 100 Sandy gravel, some cobbles /22/72 20 30 Loose to medium dense fine silty 50 Figure 9.18 Soil profile for Problem I. 184 CHAPTER 9 DEWATERING b. A bridge pier with a driven pile foundation is to be built with the bottom of the pile cap 40 ft below present grade. The cap is 20 ft by 30 ft in plan view.Explanation / Answer
a)for the given construction conditions for circular reservoir and the soil conditions the dewatering methods may be followed are well point system and ejector system.
for the given condition of reservoir the required dewateing depth is about 9 ft.total depth of reservoir below the ground is 25 ft.the min depth of water level below the cap is 2 ft.the water level from the groung level is at arround 18 ft.then the rrrequirred depth of dewatering is 25+2-18 = 9ft.
for this depth of dewatering and the type of given sand and siltysoil the method of dewatering adopted is well point system and ejector systems.
but in this case we are adopting wellpoint system.the method includes porviding perforated pipes of metal or plastic of 5.0to12.0 cm dia whose bottom end is also perforated and connected to bottom of the water level which are rised above (rised pipes) and these are connected to the header pipe.the water from the bottom levelis rised throuh this header pipe and discharged by connecting these header pipe to the pumps.these pumps will discharge thewater from the pits and lowers the water level upto 30 ft in this method.
the main problems which involves in this method includes the stability of the slope and the blocking of the perforatedd pipes due to the debris.and the air is entered into the pipes through the joints of the pipe and it will reduce ethe pump effeciency.these entry of debris can be over come by providing screens at the bottom.and the entry of air can be limted by providing tight seal joints.
b)bridge pier with the bottompile cap at 40 ft below the ground surface. for this condition the depth of dewatering is equal to 40-18 =22ft. and for this depth and the typeoffoundation the dewatering method that employed is cassion method.as the project is on the land and off shore the well is constructed and set in position.after this the well is sunked into the ground by excavating the earth below it.in this process if water is infiltrates then dewater is done by means of pumping.and this pumpin is done bymeansof pumps which are injected into the well and placedd their bottom below the required dewater surface and the water us discharged.
the problems which can be faced during this processis tilting of wellfrom its original position as dewatering may cause instability of the slopes.and blockage of pipes and pumps during discharge.this can be overcome by making the ends stiff by proviting sheeting to the ends below the well and also providing screens below the pipes to overcome th entry of debrisin to the pipe and pump.
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