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## Construction Dewatering

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**Construction Dewatering**Lecture 11**Construction Dewatering**• The purpose of construction dewatering is to control the surface and subsurface hydrologic environment in such a way as to permit the structure to be constructed “in the dry.” • Dewatering means “the separation of water from the soil,” or perhaps “taking the water out of the particular construction problem completely.” • This leads to concepts like pre-drainage of soil, control of ground water, and even the improvement of physical properties of soil.**Dewatering: CAISSONS**• Excavation from within the permanent structure. • If the site is on land, the structure is built in place. • If the site is offshore, the structure is floated into position. • To reduce the frictional resistance between the caisson and the surrounding ground: • Add weight • Bentonite clay slurry is injected at the soil-structure interface. • Jetting is used in cohesionless soils.**CAISSONS (Cont’d)**• During unwatering a caisson in cohesionless soils, the upward flow from the surrounding groundwater induces a quick condition which results in loss of strength at the bottom of excavation. • To prevent quick condition, the head difference causing flow should be kept low. • Caissons should not be used in the vicinity of existing structures that can be damaged due to loss of ground from beneath their foundations.**Permeability and SeepageFlow of Water in Soil**• Soils have interconnected voids through which water can flow from points of high energy to points of low energy. • It is necessary to estimate the quantity of underground seepage for investigating problems involving the pumping of water for underground construction, and making stability analysis of earth dams and earth-retaining structures that are subjected to seepage forces.**Permeability Test(Constant Head Test)**• ASTM D2434 • q = Water flowing through the soil at a constant rate • Q = Amount of water collected in a given time period, t • Then:Q = qt • Apparent velocity of the flow Soil area,A q H L Q or**Permeability (Cont’d)**• In 1856, Darcy published a simple equation for the discharge velocity of water through saturated soils: • n: the apparent velocity • k: the coefficient of permeability (aka:Hydraulic conductivitya material’s constant) • i: hydraulic gradient • By definition: • H : the head causing flow over the distance L.**Permeability (Cont’d)**Q = qt => Q = (vA)t => Q = (ki) At Q = k ( ) At Solve fork:**Range of Permeability for Various Soils**• Gravels are 1 million times more pervious than clays**Example for the Constant Head Test**• For a constant head laboratory permeability test on a fine sand, the following values are given: • Length of specimen = 10 in. • Diameter of specimen = 2.5 in. • Head difference = 18 in. • Water collected in 2 minutes = 0.031 in.3 • Determine: a. Hydraulic conductivity, k, of the soil (in./min.) b. Discharge velocity**Permeability in the Fieldby Pumping from Wells**• In the field, the average hydraulic conductivity of a soil deposit in the direction of flow can be determined by performing pumping tests from the well.**k Determined from Pumping Tests**For D10 = 0.3 mm, k = 2000x10-4 cm/sec = 0.2 cm/sec 0.3**Example**• Consider the case of pumping from a well in an unconfined permeable layer underlain by an impermeable stratum. Given: • q = 26 ft3/min • H1 = 15.7 ft at R1 = 100 ft • H2 = 18.0 ft at R2 = 200 ft • Calculate the hydraulic conductivity (in ft/min) of the permeable layer.**Dewatering Methods - Wellpoints**• Small pipes, up to 2.5 inches in diameter, connected to screens at the bottom and to a vacuum header pipe at the surface constitute a wellpoint system.**Dewatering Methods - Wellpoints**• Effective lifts of 15 ft. are quite common at sea level, and under certain circumstances, lifts can be increased to as much as 25 ft. • Effective lifts of 15 ft. are quite common at sea level, and under certain circumstances, lifts can be increased to as much as 25 ft.**Dewatering Methods - Wellpoints**Multistage wellpoint system