## Explain what is meant by a flow net. With the aid of a simple example describe how an approximate flow net can be drawn. Also describe briefly an experimental method of deriving a flow net using the electrical analogy.

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## A layer of sand 6m thick lies beneath a clay stratum 5m thick and above a bed of thick shale. In order to determine the permeability of the sand, a well was driven to the top of the shale and the water pumped out at the rate of 10 x 10-3m3/s. Two observation well were driven through the clay at 15m and 30m from the pump well and the water was found to rise to levels of 3m and 2.5m below the ground surface respectively. Calculate from first principles the coefficient of permeability of the soil (assuming Darcy’s law).

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## Under what circumstances might it be justified to carry out determinations of permeability in the field? An undisturbed sample was taken from a borehole made in a stratum of soil, and was subjected to a falling head permeability test in the laboratory. The test details were: Diameter of sample 100mm, Length of sample 100mm, Initial head 450mm, Final head 380mh, Standpipe diameter 3mm, Time interval 4min. A field test in the same soil was made, in which water was pumped from a well which penetrated the full depth of the soil stratum. Under equilibrium conditions water was pumped from the well at the rate of 8.5m3/day. The observed height of the water level above the horizontal base of the stratum (which rested on impervious rock) was 4.5m and 5.5m in boreholes made at distance of 15m and 30m from the well, respectively. Calculate the coefficient of permeability (in m/day) from each test. Comment on the results

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4 weeks 0 Answers 15 views

## A stratum of sandy soil overlies a horizontal bed of impermeable material, the surface of which is also horizontal. In order to determine the in-situ permeability of the soil, a test well was driven to the bottom of the stratum. Two observation boreholes were made at distances of 12.5 and 25m respectively from the test well. Water was pumped from the test well at the rate of 3 x 10-3m3/s until the water level became steady. The heights of the water in the two boreholes were found to be 4.25m and 6.50m above the impermeable bed. Find the value, expressed in m3 per day, of the coefficient of permeability of the sandy soil, deriving any formula used.

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4 weeks 0 Answers 11 views

## Describe briefly with a sketch the variable head permeameter. A permeameter of this type has a diameter of 75mm and the length of the soil sample is 15mm. The diameter of the standpipe is 15mm. During the test the head decreased from 1300mm to 800mm in 135s. Calculate the coefficient of permeability of the soil in mm/s. prove the formula used assuming Darcy’s law.

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4 weeks 0 Answers 11 views

## Describe with a neat sketch the constant head permeameter and assuming Darcy’s law, derive an expression for the coefficient of permeability. Calculate the coefficient of permeability of a sample of sand given the following data: Diameter of permeameter 75mm, Loss of head on a 200mm length 83.2mm, Water collected in 1 min 66.8 ml

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4 weeks 0 Answers 9 views

## Excavation is being carried out in a soil with porosity n=0.35 and grain specific gravity Gs = 2.65. Define what is meant by critical hydraulic gradient, and evaluate it for this soil, A 1.25m layer of the soil is subject to an upward seepage head of 1.85m. What depth of coarse sand would be required above the soil to provide a factors of safety against ‘piping’? (Assume that the coarse sand has the same porosity and grain specific gravity as the soil, and that there is negligible head loss in the sand).

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4 weeks 0 Answers 16 views

## (a) Define the terms effective pressure and pore water pressure, (b) A layer of sand 4.5m deep overlies a thick bed of clay. The water table is 2m below the top of the sand. Above the water table, the sand has an average void ratio of 0.52. The clay has a water content of 42%. Calculate the total, effective and pore water pressure on a horizontal plane 9m below the ground surface and draw pressure distribution diagrams down to this level. Assume the grain specific gravity =2.65 for both the sand and the clay.

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4 weeks 0 Answers 9 views

## The results of a sieve analysis on a soil contained: Dia. Of particles 0.06 – 0.02mm 43g, 0.02 – 0.006mm 17g, 0.006 – 0.002mm 7g, Less than 0.002mm 3g, Plot the standard grading curve for the soil, determine the effective size and coefficient of uniformity for the soil, and describe the soil.

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4 weeks 0 Answers 14 views

## Define the liquid and plastic limits of a soil, and explain their significance as a means of soil identification and classification. For a saturated soil derive the expressions for: (a) dry density; (b) bulk density; (c)submerged density. An undisturbed sample of a saturated soil has a volume of 143 x 103mm3 and a mass of 260g. Determine its void ratio, water content, and dry density. A sample of a different soil was found to have a bulk density of 1.92Mg/m3 and a water content of 30%. Find the degree of saturation of the soil. (Take the grain specific gravity = 2.7 for each soil)

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