Dewatering Case Study – Detention Tank, Haverigg.
Introduction
The scope of work was to design, install, maintain and remove on completion of dewatering, equipment, capable of dewatering and maintaining a dry and stable conditions for the construction of a 15mØ x 16m deep Detention Tank as part of alterations of Haverigg Pumping Station, Cumbria.
Analytical methods where used in dewatering design followed by well testing that enabled a final dewatering design to be made. An 8no deep well dewatering system was successfully installed and maintained required drawdown over a 12 week construction period.
Information
The storage tank to be constructed was 15m Ø x 16m deep, with a base slab at 10.0m BEGL. The tank was built using a diaphragm wall support method (16m).
Soil Information: Allied Exploration Report
Sketch of Tank Section
Drawing No: BRW0005/90012634/01/12/2001
Drawing No: BRW0005/90012634/01/12/2002
Determining Aquifer Characteristics
The BGS solid geology map of the area (Ulverston, sheet 48. 1997) indicates Quaternary Storm Beach Deposits. The location next to a river known as Haverigg Pool which drains into the tidal Millom Bay approximately 200m. Study of the borehole information shows a typical profile:
| 0.0m – 0.4m | Made Ground |
| 0.4m – 3.5m | Superficial Deposits – grey silty SANDS with rootlets |
| 3.5m – 12.5m | Beach Deposits - UPPER brown f-c SAND & GRAVEL c/w cobbles |
| 12.5m – 14.0m | Stiff Boulder CLAY |
| 14.0m – 24.0m | LOWER Dense brown gravely SAND |
Groundwater Level in upper SAND & GRAVEL at 3.0m BEGL
Sub-artesian head within lower SAND Deposits at 3.0m BEGL
Permeability Assessment
Particle Size Distribution (PSD) curves where included in the SI documentation. Bulk permeability for design was gained using Hazens’ formula (D102/100 = k (m/sec) and Prugh methods using uniformity coefficient (D60/10).
The permeability was calculated using both methods (see Appendix ). A comparison of permeabilities is plotted (see Appendix 3) to given good illustration of bulk permeability, but not values which depth.
Permeability Range for UPPER Sand & Gravel - 4 x 10-6 m/sec to 1 x 10-3 m/sec
Permeability Range for LOWER Sand - 1 x 10-4 m/sec to 9 x 10-4 m/sec
Design of Dewatering System
With desk-top data a dewatering design can be made, starting with a conceptual model that includes the following information. (Appendix 4)
The shaft terminates ~ 1.0m into the Boulder Clay and assessment on base heave and critical depth using equilibrium equation (D?s = H?s) was made (Appendix 4 ) It predicted the lower sub-artesian head in the LOWER Sands needed to be reduced to 6.55m BEGL to balance with the weight of underlying Boulder CLAY. The critical depth of the excavation was 9.0m should it go beyond this without pressure relief of the underlying SANDS the base was at risk is heaving over time.
Dewatering required for pressure relief of lower SAND aquifer.
No specialist dewatering of UPPER Sands & Gravels required, as within diaphragm wall residual water can be removed by sump pumps. Wells would have an open section in the upper Sand & Gravels to assist in some under-drainage.
Properties of the ground for Conceptual Model
| Aquifer type & properties - |
Upper Unconfined aquifer with groundwater level at 3.0m BEGL. Lower confined aquifer with sub-artesian head at 3.0m BEGL |
| Aquifer type & properties - |
9.0m thick (3.5m – 12.5m) Beach Deposits - UPPER brown f-c SAND & GRAVEL c/w cobbles |
| Presence of Aquitards & Aquicludes | Stiff Boulder Clay & Silts of varying thickness acting as an aquitard. Assumed no mechanism for seepage face over pumping period. |
| Radius of influence - | River Haverigg Pool runs approximately 20m to the West of the site. Millom Bay ~ 200m to South expected tidal influence. |
| Groundwater Level - | 3.0m BEGL in both upper & lower aquifer. |
Other factors for Conceptual Model
| Geometry of Works - | 15m Ø diaphragm wall x 16.0m deep shaft with 6.0m base slab. |
| Dewatering Technique - | Given the depth required to dewater to and the permeability the only feasible groundwater lowering technique is Deep wells. |
| Time - | ~ 1 week to achieve drawdown and maintain for 12 weeks |
| Depth of Wells - | Sufficient depth to effectively dewater lower aquifer SI bore terminated 24m BEGL. |
| Environmental - | Discharge into river. Settlement Tanks, wells designed with appropriate filter pack to allow no removal of fines. |
Calculating Discharge (Q) on Drawdown (radial flow analysis)
To estimate steady-sate flow using established analytical methods Mansur & Kaufman (1962).
Full penetration of equivalent single well of confined aquifer fed by circular source
Q = 34 lts/sec (steady state) – F.O.S & allowance for storage 1:25 = 42.0 lts/sec
Conceptual Design: 8no x 24m deep wells (250mmÆ bore) equi-distance around perimeter of shaft.
Well Test Analysis
The dewatering system was tested by means of a well test during commissioning. The testing gives comfort in enabling a final design to be confirmed and highlight any problems early in the contract.
There are two ways to analysis the data, with time drawdown data, or distance drawdown data. However well interference caused by their close spacing, means no sensible results can be obtained using distance drawdown method. The time/drawdown method uses Jacobs method. Using this method the Transmissivity of the aquifer can be determined:
T = 0.183Q/Δs
Where Q is the pumping rate in m3/day and ?s is the difference which occurs over any one log cycle.
Transmissivity and Permeability values calculated from the time drawdown curves for lower aquifer:
| Well No. | Transmissivity (m2/day) | Permeability (m/s) |
|---|---|---|
| 3 | 1084 | 1.05 x 10-3 |
| 4 | 940 | 9.07 x 10-4 |
| 6 | 2417 | 2.33 x 10-3 |
| Average | 1480 | 1.42 x 10-3 |
The average permeability is higher than that used in the conceptual model. However using the distance/drawdown data with well superposition concepts, together with extraction rates a judgement was made, with the proposed 8no deep well design being acceptable.