Mowlem plc is constructing a new £20 million retail park with 490 underground car parking spaces for Henry Boot Development in the centre of Ayr.  In the past at least part of the site had been used as a railway works and as such there are areas of contamination typical of brown field sites. 

A groundwater management system was required to lower the water table to a level that provided both safe and stable excavation conditions.  Groundwater specialist, OGI was sub-contracted to design and install a system to achieve these goals. 

There were a number of constraints which had an impact on the approach and design of the groundwater management system. The principle constraints were as follows: 

1.  The phased nature and tight deadlines of the construction programme. 

2. The installation of the groundwater management system had to be installed at the same time as the installation of deep load bearing piles. 

3.  The presence of hydrocarbon contamination from the past use of the site. 

4. A requirement to discharge non contaminated silt free water. 

5. A requirement to prevent the spread of the existing contamination to other geological layers or water courses. 

6. The groundwater management system was not to hamper the site operations, or restrict access to the site. 

7.  The groundwater control system was required to reduce the water table and provide dry ground conditions quickly to allow sub-contractors working for Mowlem plc to start installing building foundations and underground structures. 

8. To avoid excessive lowering of the water table beneath the site or under adjacent buildings to the site. 

9. To overcome the dynamic behaviour of the groundwater caused by periods of intense rainfall, combined with the tidal influence of the River Ayr. 

The geology on the site consisted essentially of stiff boulder clay overlying sand with the water table level close to the top of the sand layer. 

Alternative dewatering techniques were considered which included the drilling and installation of deep dewatering wells, both around the boundary and in the centre of the site. Neither of these options was feasible as either the water table would have taken too long to draw down, the wells would restrict site working or there would have been a potential for the spread of contamination. 

The use of sump pumping to reduce the water table was also discounted, as this would have removed sand and silt as well as groundwater.  This would result in ‘ground loss’ and dirty water discharge, which was not acceptable. 

The groundwater management system selected consisted of a network of horizontally installed dewatering pipes.  A geosynthetic filter membrane was installed around the dewatering pipework and, depending on the ground conditions encountered; a fine gravel filter pack was also installed as required. 

The pipework network passed under the haulage road and was connected to a series of vertical wells.  The groundwater pumped from the wells was clear of silt particles.  As a precautionary measure the pumped water passed through a settlement tank before final disposal. 

The groundwater management system proved very successful, despite the presence of considerable quantities of surface water brought about by a period of sustained heavy rainfall.  The groundwater control system provided the following benefits to Mowlem plc: 

1. Dry conditions for the installation of foundations and underground structures. 

2. A safe and stable excavation to below the level of the foundations. 

3. Improved conditions for construction vehicles. 

4. Because the excavated material was not saturated with water, its removal was more cost effective and energy efficient to manage. 

5. An environmental friendly dewatering operation which produced clean silt free water. 

6. A phased, flexible dewatering system installation which caused minimum delay or interruption to Mowlem plc installation programme. 

7. Assistance with surface water control and disposal. 

 Project manager John Kane of Mowlem plc said “The selected system has proved to be very satisfactory and contributed greatly to the progress of the ground works”.