Principles of Geotechnical Engineering (MindTap Course List)
9th Edition
ISBN: 9781305970939
Author: Braja M. Das, Khaled Sobhan
Publisher: Cengage Learning
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Chapter 9, Problem 9.7P
To determine
Find the safe depth of excavation before the bottom heaves.
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An exploratory drill hole was made in a stiff saturated clay, as shown in Figure 1, having a moisture content of 25% and Gs = 2.56. The sand layer underlying the clay was observed to be under artesian pressure. Water in the drill hole rose to a height of 3 m above the top of sand layer. If an open excavation is to be made in the clay.
a.What is the maximum depth of excavation before the bottom heaves?
b.What would be the required height of water inside the cut in order to ensure a factor of safety of 1.5?
An exploratory drill hole was made in a stiff saturated clay having a moisture content of 29% and Gs = 2.68 (Figure 9.27). The sand layer underlying the clay was observed to be under artesian pressure. Water in the drill hole rose to a height of 6 m above the top of the sand layer. If an open excavation is to be made in the clay, determine the safe depth of excavation before the bottom heaves.
A clay layer of 30 m thick with a unit weight of 20 kN/m3 overlies a sand layer. The groundwater table is at the ground surface initially. A 20 m deep excavation is made in the clay layer. Piezometric measurements show artesian pressure at the bottom of the clay layer, amounting to a water level of 8 m above the excavation base. Calculate the factor of safety against the bottom heave for the excavation.
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Principles of Geotechnical Engineering (MindTap Course List)
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- 8. An exploratory drill hole was made in a saturated stiff clay having a water content of 32%. It was observed that the sand layer underlying the clay was under artesian pressure water in the drill hole rose to a height of 4 m. above the top of the sand layer. If an open excavation is to be made in the clay. (See picture below) Determine the void ratio of clay. (Answer: 0.864) Determine the saturated unit weight of clay. (Answer: 18.76 kN/m3) How deep can the excavation proceed before the bottom of the saturated clay heaves or when its effective stress is zero. (Answer: 5.91 m)arrow_forwardAn exploratory drill hole was made in a stiff saturated clay having a moisture content of 29% and G, = 2.68 (Figure 9.27). The sand layer underlying the clay was observed to be under artesian pressure. Water in the drill hole rose to a height of 6 m above the top of the sand layer. a. If an open excavation is to be made in the clay, determine the safe depth of excavation before the bottom heaves. b. How much is the pressure at the interface of the clay and sand? c. If a factor of safety of 2.0 is to be ensured, how deep should be the excavation from the ground surface?arrow_forwardA sand deposit of 12 m thick overlies a clay layer. The water table is 3 m below the ground surface. In a field permeability pump-out test, the water is pumped out at a rate of 540 liters per minute when steady state conditions are reached. Two observation wells are located at 18 m and 36 m from the Centre of the test well. The depths of the drawdown curve are 1.8 m and 1.5 m respectively for these two wells. Determine the coefficient of permeability. Include FBD.arrow_forward
- A sand deposit of 12 m thick overlies a clay layer. The water table is 3 m below the ground surface. In a field permeability pump-out test, the water is pumped out at a rate of 540 liters per minute when steady state conditions are reached. Two observation wells are located at 18 m and 36 m from the Centre of the test well. The depths of the drawdown curve are 1.8 m and 1.5 m respectively for these two wells. Determine the coefficient of permeability. Answer must be: 4.5 × 10–1 mm/sarrow_forwardA soil profile consists of a clay layer of 10 m thick between a permeable sand layer above and an impermeable rock layer below, the water table being in the sand layer. Before the construction of a reservoir raft foundation, piezometers were installed in the clay layer.The whole process of filling the reservoir may be approximated to applying rapidly a single pressure increment of 100 kN/m2 uniformly throughout the clay layer. One year after filling the reservoir, the settlement was 20 cm and the piezometers indicated the distribution of excess pore pressure as shown in the figure below. Estimate the final settlement and the time to complete 90 % of primary consolidation.arrow_forwardA sand deposit of 12 m thick overlies a clay layer. The water table is 3 m below the ground surface. In a field permeability pump-out test, the water is pumped out at a rate of 540 litres per minute when steady state conditions are reached. Two observation wells are located at 18 m and 36 m from the centre of the test well. The depths of the drawdown curve are 1.8 m and 1.5 m respectively for these two wells. Determine the coefficient of permeability. a. 625 X 10^-2 b. 2.15 x 10^-2 c. 4.5 x 10^-1 d. 1.6 X 10^-1arrow_forward
- An exploratory drill hole was made in a stiff saturated clay having a moisture content of 38.9% and Gs = 2.34 as shown in the figure. The sand layer underlying the clay was observed to be under artesian pressure. Water in the drill hole rose to a height of 13.8 ft above the top of sand layer. If an open excavation is to be made in the clay, determine the (a) unit weight of the clay, and the (b) maximum cut before the bottom heaves.arrow_forwardA recently filled up fill was 10.54 m thick and its average void ration was 1.0. The fill was loaded on the surface by constructing an embankment covering a large area of the fill. Some months after the embankment was constructed, measurements of the fill indicated an average void of 0.8. Calculate the compression of the fill.arrow_forwardA layer of sand 6 m thick lies beneath a clay stratum 5 m thick and above abed thick shale. In order to determine the permeability of sand, a well was driven to the top of the shale and water pumped out at a rate of 0.01m3/sec. Two observation wells driven through the clay at 15m and 30 m from the pumping well and water was found to rise to levels of 3m and 2.4 m below the ground water surface. Calculate the permeability of soil.arrow_forward
- Saturated silty clay encountered in a deep excavation is found to have a water content of 15%. Determine the unit weight of clay. G=2.7arrow_forwardA double drain consolidation test was performed on a sample of marine clay. The thickness of the clay layer is 50mm and the hydraulic conductivity during the test was 1,753x10-8 m/min. During the test the following data were determined. σ '1= 300 KN/m2 e= 0.42 σ '2= 500 KN/m2 e=0.30 Determine: The time to reach 75% consolidation The excess water pressure value after 50 min of starting the test at 10 mm deptharrow_forwardIn a clay layer with a thickness of 16 m, bidirectional drainage conditions exist. The consolidation coefficient for the clay layer is given as CV = 6.2 * 10^-81) At the end of 10 years, find the percentages of consolidation at 4 , 8, 12 and 16 m in the clay layer.2) If Δσ = 150 kpa find the pore water pressure at 4.8, 12 and 16 m after 10 years.arrow_forward
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