Construction Dewatering. Lecture 11. Construction Dewatering

Transcription

1 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. Spring Quarter 018 1

2 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. 3 Dewatering: 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. 4 Spring Quarter 018

3 Permeability and Seepage Flow 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 earthretaining structures that are subjected to seepage forces. 5 Permeability Test (Constant Head Test) ASTM D434 q = Water flowing through the soil at a constant rate q Q = Amount of water collected in a H L given time period, t Then: Q = qt Apparent velocity of the flow q v A or q va Soil area, A Q 6 Spring Quarter 018 3

4 Permeability (Cont d) In 1856, Darcy published a simple equation for the discharge velocity of water through saturated soils: v : the apparent velocity v ki k : the coefficient of permeability (aka: Hydraulic conductivity a material s constant) i : hydraulic gradient By definition: i H L H : the head causing flow over the distance L. 7 Permeability (Cont d) Q = qt => Q = (va)t => Q = (ki) At q va v ki i H L Q = k ( H L ) At Solve for k : k QL HAt 8 Spring Quarter 018 4

5 Range of Permeability for Various Soils Soil Permeability Coefficient, k (cm/sec) Relative Permeability Coarse gravel Exceeds 10-1 High Sand, clean 10-1 to 10-3 Medium Sand, dirty 10-3 to 10-5 Low Silt 10-5 to 10-7 Very low Clay Less than 10-7 Impervious Gravels are 1 million times more pervious than clays 9 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 =.5 in. Head difference = 18 in. Water collected in minutes = in. 3 Determine: a. Hydraulic conductivity, k, of the soil (in./min.) b. Discharge velocity QL a. k in./min. HAt b. v ki in./min 10 Spring Quarter 018 5

6 Permeability in the Field by 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. kh H1 q R ln R 1 k q R ln R H H1 1 k q R ln H H W R W 11 k Determined from Pumping Tests For D 10 = 0.3 mm, k = 000 x 10-4 cm/sec = 0. cm/sec Spring Quarter 018 6

7 Example Consider the case of pumping from a well in an unconfined permeable layer underlain by an impermeable stratum. Given: q = 6 ft 3 /min H 1 = 15.7 feet at R 1 = 100 feet H = 18.0 feet at R = 00 feet Calculate the hydraulic conductivity (in feet/min) of the permeable layer. 13 Example (Cont d) q = 6 ft 3 /min Ground Surface Natural GWT Permeable Layer Impermeable Stratum k H q H 1 6ln R ln R ft/min 14 Spring Quarter 018 7

8 Dewatering Methods - Wellpoints Small pipes, up to.5 inches in diameter, connected to screens at the bottom and to a vacuum header pipe at the surface constitute a wellpoint system. 15 Dewatering Methods - Wellpoints Effective lifts of 15 feet are quite common at sea level, and under certain circumstances, lifts can be increased to as much as 5 feet. 16 Spring Quarter 018 8

9 Dewatering Methods - Wellpoints 17 Dewatering Methods - Wellpoints 18 Spring Quarter 018 9

10 Dewatering Methods - Wellpoints Multistage Wellpoint System 19 Dewatering Methods - Wellpoints 0 Spring Quarter

11 Dewatering Methods Wellpoints 1 Dewatering Methods Wellpoints Typical Wellpoint System Spring Quarter

12 Dewatering Methods Wellpoints Groundwater control by Pumping 3 Questions? Kamran M. Nemati nemati@uw.edu Architecture Hall Room 130J Spring Quarter 018 1