Construction Dewatering

[WBell]

I am working with a contractor to design a dewatering system for an open excavation 44 feet x 40 feet x 30 feet deep which will be shored on 3 sides with H-piles and timber lagging. The contractor has proposed a line of eductor well points behind the shoring and I am tasked with designing the system. I am using the equations for gravity flow for a fully penetrating well that will extend 45 feet from existing grade. The water table is 1 foot below grade and the soil borings indicate a hydraulic conductivity = 3E-5 m/s is appropriate.

Reference: J. Patrick Powers, et al., Construction Dewatering and Groundwater Control, 3rd Edition, 2007

I would appreciate comments on the following aspects of the design:

1. Page 340 of Powers text shows an example calculation of the water and brake horsepower for a case where the water level is being lowered 60 feet for a given flowrate, Q = 500 gpm. Am I correct in assuming that I can calculate the water and brake horsepower for the dewatering system based on the site data for my design? I have calculated the flow rate for the entire system, Q_sys = 87 gpm, which would require a water horsepower, WHP = Q_sys x h / 3960 = 0.99. The reference suggests calculating the brake horsepower for an eductor system assuming 10% efficiency, which indicates a required BHP = 9.9.

Question: Is the required WHP/BHP based on the system Q or the pump rate of the individual well?

2. The design submittal requires a plot of the drawdown curve. I am planning to plot the curve for drawdown at the center of the excavation using Eq, (5) from ARMY TM-518-5 Dewatering and Groundwater Control. Although the excavation is open at one end to allow for construction access, the dimensions of the excavation (40'x44') and the well spacing around the perimeter (10') should not affect the calculation significantly.

Question: Are there other methods of calculating the drawdown curve for the configuration?

Thank you for your assistance. I appreciate any comments and suggestions.

 

[1503-44]

If you have 1 pump, then the pump must handle the sum of all well point flows.

You should include pipe flow loss and add that to "h", if you have not already done that.

I have always found the TM methods to be spot on, practical, easy to follow and implement. If there was another method, I probably would stick with the TM anyway. I have not used that particular calculation.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[WBell]

Powers, et al., Construction Dewatering and Groundwater Control, Section 6.2, pp. 68-69, discusses radial flow to a well in a water table aquifer, which is the condition I am modeling. In Section 6.1 the equation for drawdown is presented for a confined aquifer; however, Section 6.2 states that when the distance to the well is less than the original saturated thickness, H, Eq. (6.4) from the text does not give satisfactory results and a numerical analysis is suggested, e.g., MODFLOW.

I would follow the suggestion but time does not permit getting familiar with the software. The wells are arranged in an open ended rectangle with two sides 40 feet in length and one side 50 feet long. I would like to calculate the drawdown in the center of the excavation and TM 5-818-5, Dewatering and Groundwater Control, Figure 4-13, p. 4-14, presents the equations for multiple wells for an irregular array of wells. I am considering this model, but the statement by Powers regarding the distance to the well being less than the saturated thickness may apply.

Question: For a rectangular configuration of wells, are there accepted methods of analysis for the drawdown curve or modifications to the methods outlined in Powers' text or the TM document?

Thank you for responses and comments.