9.3 Dewatering and Pumping

Dewatering and Pumping

Why This Topic Matters for the PE Construction Exam

Controlling groundwater is often one of the highest-risk activities in construction. Excavating below the water table without proper dewatering leads to flooded trenches, unstable slopes, and "boiling" or "heaving" at the bottom of the excavation. The PE exam tests your knowledge of different dewatering techniques and your ability to size pumping systems by calculating flow rates and head losses. Selecting an inadequate pump can bring a project to a standstill, while over-sizing leads to unnecessary costs and inefficiencies.

Dewatering Techniques

Sumps and Trenches

The simplest form of dewatering involves routing water via ditches or trenches to a low point (sump) within the excavation and pumping it out.

• Application: Best for shallow excavations in dense soils or rock where water inflow is minimal.
• Limitations: Does not lower the groundwater table outside the excavation. It can cause soil erosion, instability in fine-grained soils, and poor subgrade conditions.

Wellpoint Systems

A wellpoint system consists of a series of closely spaced, small-diameter shallow wells (wellpoints) connected to a common header pipe, which is connected to a vacuum pump.

• Application: Highly effective in permeable soils like sands and gravels.
• Limitations: Because wellpoints rely on a vacuum to suck water to the surface, they are subject to atmospheric pressure limitations. The maximum theoretical suction lift is about 34 feet, but practically, wellpoints are limited to about 15 to 20 feet of drawdown. For deeper excavations, multiple stages (tiers) of wellpoints must be installed.

Deep Wells

Deep wells consist of large-diameter boreholes equipped with slotted casings and submersible pumps placed at the bottom of the well.

• Application: Used for deep excavations, highly permeable soils, or when lowering artesian pressure in confined aquifers below the excavation.
• Limitations: High initial installation cost. However, because the pump pushes the water up rather than pulling it via suction, there is no practical limit to the depth of drawdown.

Pump Sizing and Head Calculations

To select the correct pump, an engineer must determine two primary parameters: the required flow rate ($Q$) and the Total Dynamic Head (TDH).

Flow Rate ($Q$)

The required flow rate is determined by geotechnical analysis of the soil permeability, the depth of drawdown required, and the radius of influence. On the exam, $Q$ is often provided, or you must calculate it using Darcy's Law or well equations (like the Theis or Thiem equations).

Total Dynamic Head (TDH)

TDH is the total resistance a pump must overcome to move water from the source to the discharge point. It is measured in feet (or meters) of head.

TDH = Static Suction Lift + Static Discharge Head + Friction Head + Minor Losses

1. Static Suction Lift: The vertical distance from the water level at the source to the centerline of the pump. (If the water level is above the pump, this is considered "suction head" and is subtracted).
2. Static Discharge Head: The vertical distance from the pump centerline to the point of free discharge. Note: The sum of Static Suction Lift and Static Discharge Head is the Total Static Head (the total vertical distance the water is lifted).
3. Friction Head ($H_f$): The pressure lost due to the friction of water moving against the pipe walls. It depends on the pipe material (roughness), pipe diameter, pipe length, and flow velocity. The Hazen-Williams equation or Darcy-Weisbach equation is used to calculate this. Friction head increases exponentially as flow rate increases.
4. Minor Losses: Losses due to turbulence at fittings, valves, bends, and entrances/exits. These are usually calculated using loss coefficients ($K$) multiplied by the velocity head ($v^2/2g$).

Pump and System Curves

• System Curve: A graph showing TDH vs. Flow Rate for a specific piping arrangement. As flow increases, TDH increases due to higher friction losses. The curve starts at the Total Static Head (when flow is zero).
• Pump Curve: Provided by the manufacturer, showing the head a specific pump can generate at various flow rates. As flow increases, the head the pump can produce decreases.
• Operating Point: The exact flow rate and head at which the system will operate. It is the intersection point of the System Curve and the Pump Curve.

Worked Example: Pump Head Calculation

Scenario: A construction site requires pumping $Q$ = 500 gallons per minute (gpm) from a sump.

• The water surface in the sump is at elevation 80 ft.
• The pump centerline is at elevation 95 ft.
• The discharge point is at elevation 130 ft.
• The total length of the 6-inch diameter discharge hose is 200 ft.
• At 500 gpm, the friction loss in the 6-inch hose is known to be 2.5 ft per 100 ft of pipe.
• Assume minor losses through valves and fittings total 5 ft of head.

Question: What is the Total Dynamic Head (TDH) required for the pump?

Step 1: Calculate Total Static Head.

• Static Suction Lift = Pump Elev - Water Elev = 95 ft - 80 ft = 15 ft.
• Static Discharge Head = Discharge Elev - Pump Elev = 130 ft - 95 ft = 35 ft.
• Total Static Head = 15 ft + 35 ft = 50 ft. (Alternatively, Total Static Head = 130 ft - 80 ft = 50 ft).

Step 2: Calculate Friction Head.

• The hose is 200 ft long. Friction loss is 2.5 ft per 100 ft.
• Friction Head = (200 / 100) * 2.5 ft = 5.0 ft.

Step 3: Calculate TDH.

• TDH = Total Static Head + Friction Head + Minor Losses
• TDH = 50 ft + 5.0 ft + 5.0 ft = 60 ft.

The pump selected must be capable of delivering 500 gpm at a TDH of 60 ft.