Client: Gaelic Athletic Association (GAA)
Consulting Engineers: Malachy Walsh & Partners
Civil Engineering Contractor: John Sisk & Son LTD
The site is located on the south bank of the River Lee to the east of Cork City Centre. The site was reclaimed from the mud flats in the Lee Estuary and is predominantly flat. There is an embankment along the existing River Lee and the site is bounded by two water courses which flow into the Atlantic Pond, a water feature to the east of the site. The existing Pairc Ui Chaoimh stadium is located in the northern portion of the site, with the ground investigation undertaken around the stadium and in the Agricultural Society Show Grounds to the south of the stadium. It is proposed to redevelop the stadium and surrounding grounds to increase the capacity and to install a new second pitch to the south of the stadium. The new second pitch has dimensions of 150m *100m, which equates to a surface area of 15000m2.
A comprehensive site investigation was carried out by Ground Investigations Ireland LTD. The ground conditions encountered during the investigation are summarized below with reference to in-situ and laboratory test results. The sequence of strata generally encountered across the site consisted of:
• Made Ground
• Upper Soft Cohesive Alluvial/Estuarine Deposits
• Lower Soft Cohesive and Loose Granular Alluvial/Estuarine Deposits
• Glaciofluvial Granular Deposits
• Lower Stiff Cohesive Deposits
Made Ground: The cable percussion and trial pits exploratory holes encountered cohesive and granular Made Ground deposits consisting of grey brown gravelly Clay and clayey sandy Gravel to a depth of 2.0m BGL. The depth of the Made Ground deposits was generally deeper around the existing stadium and shallower in the trial pits completed at the south of the site.
Upper Cohesive Alluvial/Estuarine Deposits: Soft cohesive deposits were encountered beneath the Made Ground. They were generally described as very soft and soft to firm grey and brown laminated (slightly) sandy clayey SILT with shell fragments present to depths of between 3.5m and 4.8m BGL. These deposits were slightly organic to organic and an organic odor was noted in this stratum on the majority of the trial pit logs.
Lower Alluvial/Estuarine Deposits: Loose and soft interstratified sand, gravel and silt deposits were present below the upper alluvial deposits. These were present in layers between 0.5m and 3.0m thick and were typically described as slightly silty sandy sub- rounded to rounded fine to medium gravel.
THRACE Design & Results:
The design called for removal of all Made Ground. On-top of the Soft Silty Clay (with an assumed CBR of 1%), a ground improvement granular base was to be constructed, as follows (from bottom to top):
• One layer of PB1000/S8NW nonwoven geotextile.
• One layer of TG4040S biaxial extruded PP geogrid
• A 300mm thick layer of 6F2 capping.
The granular base was required to have a final CBR of minimum 2% to support an attenuation water system and eventually the astro-turf training pitch.
To evaluate the effectiveness of the design proposal it was decided to carry out tests in accordance with BS1377-9:1990 in order to determine the elastic modulus of the subgrade reaction, k, and CBR values. The test involves the stage loading of a circular steel plate, placed on the test material surface while recording the settlement of the plate for each load stage. The back pressure was obtained using an excavator as a counter weight. The plate was loaded using a pre-calibrated hydraulic pump and jacking system. A 300mm diameter plate was used, which provides a 450mm deep pressure bulb. (or 1.5 times plate diameter).
The results, expressed in terms of modulus of subgrade reaction, k, indicated the efficiency of THRACE geogrids as the structure stabilized with the biaxial extruded geogrid presented a modulus of subgrade reaction, k, equal to 50MN/m2/m, which corresponds to an estimated Bearing capacity of 540kPa and a 8.4% CBR value of the subgrade, which is well above the required 2% CBR value.
*The site investigation data provided, have been extracted from a report by “Ground Investigations Ireland LTD”.