BEARYS INSTITUTE OF TECHNOLOGY

DEPARTMENT OF CIVIL EGINEERING

Subject Handled by,

Dr. Pooja Kumble

Assistant Professor

Civil Engineering Department

BIT, Mangalore

MODULE -2

DRAINAGE AND DEWATERING

Subject : Applied Geotechnical Engineering (BCV613C)

MODULE -2

DRAINAGE AND DEWATERING

DRAINAGE AND DEWATERING

• Dewatering involves controlling groundwater by pumping, to locally lower groundwater levels in the vicinity of the excavation. The simplest form of dewatering is sump pumping, where groundwater is allowed to enter the excavation where it is then collected in a sump and pumped away by robust solids handling pumps. Sump pumping can be effective in many circumstances, but seepage into the excavation can create the risk of instability and other construction problems. To prevent significant groundwater seepage into the excavation and to ensure stability of excavation side slopes and base it may be necessary to lower groundwater levels in advance of excavation.

OBJECTIVES OF DEWATERING OR DRAINAGE

• • To keep working place dry like excavation for dams, building foundations and tunnels.
• • To stabilize natural or constructed slopes
• • To treat granular soils by reducing their compressibility
• • To decrease lateral pressures on retaining walls or foundation
• • To improve bearing capacity of foundation soils
• • To reduce liquefaction potential due to seismic activity
• • To prevent migration of soil particles by groundwater (phenomenon of piping)
• • To reduce surface erosion

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METHODS OF DEWATERING

• • DRAINAGE AND SUMPS

• WELLPOINT METHOD

1. MULTISATGE WELL POINT SYSTEM

2. VACUUM METHOD

3. ELECTRO-OSMOSIS METHOD

ESTIMATION OF DEPTH OF GWT (HVORSLEV’S METHOD)

• As per the Hvorslev’s method, water table level can also be located in a borehole used for soil investigation. That type of bore hole should have a casing to stabilize the sides. It uses almost the same technique; the rise in water level determines the water level locations. However, there is a slight difference. Unlike Casagrande piezometer method, this method consists of hailing the water out of the casing and observing the rate of rise of the water level in the casing at different intervals of time until the rise in Water level becomes negligible.

• Figure shows the rise of water level in the borehole at different time intervals. The height of water above the levels 0 – 0, 2 – 2 and 3 – 3 is calculated from the following equations.
• Ho = Height above level 0 – 0 =
• H2 = Height above level 2 – 2 = H3 = Height above level 3 – 3 =
• Let the corresponding depth of water level below the ground surface be hw1, hw2, hw3 hw1 = HW – HO
• hw2 = HW – (h1+h2+H2)
• hw3 = HW – (h1+h2+h3+H3)
• Here HW is the depth of water level in casing from the ground surface at the beginning of the test.
• Normally, hw1 = hw2 = hw3…...
• If not, take the average of the same.

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FLOW NET

A flow net is a graphical representation of the flow of groundwater through a porous medium, such as soil or rock. It consists of a network of flow lines and equipotential lines that intersect each other at right angles. Flow nets are commonly used to analyze seepage and evaluate the flow characteristics in geotechnical and hydrogeological engineering. They provide insights into the direction, velocity, and flow patterns of groundwater within a given area.

Flow nets are constructed using mathematical equations, numerical methods, or graphical techniques. They are particularly useful in designing and assessing the stability of structures, such as Dams, Retaining walls, and Foundations, where seepage control is critical. Flow nets help engineers understand the potential for seepage-related issues and develop appropriate remedial measures. By analyzing a flow net, engineers can estimate seepage rates, identify areas of high groundwater flow, and evaluate the effectiveness of drainage systems. Flow nets are essential tools in groundwater management, ensuring the safe and efficient utilization of water resources.

IMPORTANCE OF FLOW NET

• Flow nets are used to quantify seepage and upward lift pressure beneath hydraulic structures.
• Once a flow net is drawn, it can be applied to different flow rates.
• The continuity equation can be used with the flow net to determine velocity at any point, given the velocity of a reference point.
• Flow net analysis aids in designing optimal boundary shapes for efficient flow.
• Flow nets help prevent separation and identify points of stagnation when designing boundary shapes.

PROPERTIES OF FLOW NET

1. Flow lines and equipotential lines in a flow net are always orthogonal to each other, with a 90-degree angle of intersection.
2. Flow lines and equipotential lines within a flow net do not intersect or cross each other.
3. Each flow channel, which is the space between two adjacent flow lines, carries an equal amount of seepage.
4. The head loss is consistent between two adjacent equipotential lines in a flow net.
5. Flow nets are constructed based solely on the given boundary conditions and are not affected by the soil permeability or the magnitude of the hydraulic head driving the flow.
6. The space formed between two flow lines and two equipotential lines is called a flow field. It should be in a square form.
7. Flow lines and equipotential lines in a flow net are typically depicted as smooth curves.

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APPLICATIONS OF FLOW NET

• • Determination of the Quantity of seepage
• • Obtaining the Seepage pressure at a point
• • Estimation of velocities and flow directions at various points within the flow domain.
• • Determining the Hydrostatic pressure at a point
• • Design and evaluation of efficient boundary shapes for structures.
• • Obtaining the Exit gradient
• • Evaluation of seepage-related risks and design of remedial measures.

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PHREATIC LINE

• The phreatic line or seepage line is defined as the line within a dam section below which there are positive hydrostatic pressures in the dam. The hydrostatic pressure on the phreatic line itself is atmospheric. The phreatic line can be located by
• (i) Analytical Method,
• (Ii) Graphical Method,
• (Iii) Experimental method.

THE END