Ashcroft Coast

Isle of Sheppey

 

Project Value: £220,000

Subject: Groundwater depressurisation to increase slope stability, Ashcroft Holiday Park, Isle of Sheppey

 

Introduction

Project Dewatering Ltd. (PDL)  was commissioned by h2oK Ltd. on behalf of A&M architects to design a permanent dewatering scheme for the purposes of reducing groundwater pore pressure to prevent instability of parts of a coastline at Ashcroft Coast Holiday Park, Minster-on-Sea, Isle of Sheppey, ME12 4JH.

 

Background

H2oK Ltd investigated the instability of the coastline and interpret the instability at the site as largely a result of saturation of the Bagshot Formation due to elevated groundwater levels at the site. The elevated groundwater levels result in elevated pore pressures in the formation with a consequential reduction of effective stress, so shear strength of the Bagshot Formation with an overall decrease in the angle of slope.

The Environment Agency’s Shoreline Management Plan (SMP) Policy Unit 4a 04: Minster Slopes to Warden Bay, for this part of the coastline is currently designated as ‘No active intervention’. That is to say, no investment is earmarked for coastal erosion prevention measures along this stretch of coastline.

The site is located adjacent the Sheppey Cliffs and Foreshore Site of Special Scientific Interest (SSSI). The designation of this SSSI is due to its geological and geomorphological interest.

The client recommended that a reduction in pore water pressure is likely to reduce instability of the coastline at the site and recommended installation of French drains along the inland perimeter of the site to control surface waters and reduce groundwater levels. The cost of these works is understood to be prohibitive and so a well dewatering scheme was proposed by PDL.

At the request of the site owners, the dewatering scheme was designed to concentrate on the protection of two areas of the site within which the most valuable site infrastructure is situated. These areas include the existing sewage treatment plant and the amenity building.

Ashcroft 2   Ashcroft

Ashcroft Map

Figure 4 Comparison of cliff lines from 1940 and 2014

 

Objectives

  • review, analyse and interpret the test pumping undertaken at the site;
  • propose a design for a permanent dewatering scheme;
  • highlight issues associated with the proposed dewatering scheme; and
  • make recommendations for further investigation and study.

 

Hydrogeological Conceptual Model

The site is currently occupied by a holiday park comprising caravans and associated infrastructure. The surrounding area is occupied by open fields and arable land. It is understood that a caravan site has been at the site since the 1960s. Ground elevation is around 53 to 54 maOD in the area of the amenity building and around 42 to 45 maOD in the area of the sewage treatment plant. The sewage treatment plant lies within a shallow valley depression which runs approx.. NNE in which a small brook (called Boarer’s Run) flows. The sewage treatment plant discharges treated water in to a culverted section of Boarer’s Run which then flows down the side of the cliff. There is evidence of scouring of the cliff by Boarer’s Run and flow from the sewage treatment plant.

Ashcroft Graph

Figure 1 Hydrographs showing groundwater elevation at the site and tidal elevation recorded at Sheerness tidal gauge.

 

Pump testing has been undertaken in the area around the amenity building only. A pump well and 2 monitoring wells were installed by PDL specifically for test pumping. The pump well encountered the London Clay Formation at 10.2 mbgl.

A 72-hour constant head pump test in a well screening the Head, Bagshot Formation, Claygate Member and London Clay Formation has been undertaken.

Anecdotal information from site indicates that cliff collapse occurs at a higher frequency 1 to 2 days after rainfall. The park maintenance manager believes there to be 2 to 3 “springs” at the site which discharge water across the surface of several parts of the holiday park. It is highly likely that these springs occur as a result of the underlying geology. Where the Bagshot Formation thins and the underlying Claygate Member (mainly clay) outcrops (i.e. geological boundary between Bagshot Formation and underlying Claygate Member) is where springs and groundwater seepage is likely to occur.

It is recommended that an ejector-based system is used.

Twin-pipe ejector-based dewatering schemes are proposed due to the low bulk permeability of the geological materials beneath the site. Should large volumes of groundwater be abstracted continuously from the wells then submersible pumps could be trialled.

It is proposed to use a series of deep wells to dewater the areas around the amenity building and the sewage treatment plant. Proposed locations of the dewatering well arrays are presented in Figure 5a and Figure 5b.

The higher ground elevation to the south of the site is likely to be the main source and driver of groundwater flow under the site. There is considered to be negligible flow coming from the direction of the cliff face. Once dewatering commences and groundwater levels are depressed it is expected that there will be very little, if any, groundwater flow in between the dewatering array and the cliff face. However, some groundwater may still flow in this area due to other recharge sources such as leaking water supply and drainage pipework and incident rainfall.

The final design of the dewatering wells in the area of the sewage treatment plant will depend on the geology encountered and will be finalised in the field. It is expected that wells will be around 7m deep in the sewage treatment plant area and around 14m deep in the amenity building area. A generic design for all wells is presented as follows:

  • boreholes should be drilled to 1 m below the base of the Claygate Member (i.e. 1 m below the required drawdown elevation);
  • boreholes should be drilled at a diameter of 250 mm and installed with 125 mm diameter slotted well screening from depth to 3 mbgl;
  • solid casing should be installed from 3 mbgl to ground level;
  • filter pack should be installed from depth to 2.5 mbgl. This filter pack will act as a high permeability drainage route for layers which are hydraulically isolated due to interbedded clays and will help reduce loss of fines during dewatering thus reducing subsidence;
  • a bentonite seal should be installed from 2.5 mbgl to ground level.

 

Each well will be fitted with an individual ejector and will be connected to supply and return ring mains. Supply pumps and generators will be located adjacent to the well array. The discharge route could be to the existing storm sewers on site.

Groundwater levels need to be depressed by 8m in the area of the amenity building. For the purposes of dewatering, a conservative assumption that groundwater levels need to be depressed to 43 maOD is assumed.

Groundwater levels need to be depressed by an average of 3 m in the area of the sewage treatment plant. For the purposes of this dewatering design, a conservative assumption that groundwater levels need to be depressed to 43 maOD is assumed.

Amenity building area deep wells 15 m depth (depths may vary and base of borehole should be 5m in to London Clay Formation) along a 170 m line at 10 m spacings – total 17 wells proposed.

Sewage treatment plant area deep wells 5 m depth (depths may vary and base of borehole should be 5 m in to London Clay Formation) along a 130 m line at 5 m spacings – total 26 wells proposed.

  • boreholes should be drilled to 1 m below the base of the Claygate Member (i.e. 1 m below the required drawdown elevation);
  • boreholes should be drilled at a diameter of 250 mm and installed with 125 mm diameter slotted well screening from depth to 3 mbgl;
  • solid casing should be installed from 3 mbgl to ground level;
  • filter pack should be installed from depth to 2.5 mbgl. This filter pack will act as a high permeability drainage route for layers which are hydraulically isolated due to interbedded clays and will help reduce loss of fines during dewatering thus reducing subsidence;
  • a bentonite seal should be installed from 2.5 mbgl to ground level.

 

Abstraction estimates:

  • amenity building area assuming bulk permeability of 0.2 m/d = estimated abstraction rate of around 30 m3/d to 42 m3/d; and
  • sewage treatment plant area assuming bulk permeability of 0.02 m/d = estimated total abstraction rate of around 2 m3/d.

 

Site works

Site layout: See drawing J-5190-CG-3201H in photos

Chamber Details: See drawing Geoquip Manhole. The manhole is placed over the well with a supply and return line making up the headworks for the ejector unit

Well Details: 17 wells drilled upto 16mBGL

Monitoring & Adjustment:

Ashcroft Graph 2

The Dewatering system was commissioned on the 3rd of February 2016, the system was tested for correct functioning. Over a period of weeks the system was left to pump, data was logged from 5 pumped wells evenly spaced across the system.

On the 2nd of March the system was turned off to allow water levels to recover, all site equipment was inspected. A leak on one of the supply/return tanks had been reported so it was repaired whilst the system was turned off.

On re-commissioning of the dewatering system, well 9 was turned off and used as a monitoring well. Well 9 was chosen as it is central to the system. The graph below is a snap shot of water levels during pumping and recovery. The blue and green lines indicate the water level in the pumped wells with the black line showing the effect the system is having on the ground water level.

During persistent pumping the water level is reduced by around 4.5m in the centre of the system, around 1.8m of water remains in the un-pumped well. With system adjustment this can be reduced further to gain a greater drawdown across the site. At present every monitored pumped well is emptying completely indicating that the system is functioning correctly in its current set up.