An integral part of the newly constructed, state-of-the-art National English Literary Museum (NELM) in Grahamstown were three composite Kaytech drainage systems; installed under floors, behind retaining walls and under a roof garden.
Of concern during the project planning stages was the retention of groundwater at the building site where investigations revealed highly weathered sandstone and/or silty clay, all with PI’s exceeding ten. As the best possible solution, Kaytech’s economical geocomposite drainage systems were specified to rapidly remove groundwater and thereby maintain atmospheric pressure around the structure. Since the museum was built with the intention of housing priceless documents, (some having been stored for decades in the nearby St Patrick’s Cathedral), it was imperative that temperature and humidity be effectively controlled to safeguard the longevity of the documents.
The first composite drainage system installed by contractor WBHO involved encasing the entire underfloor basement area in a layer of bidim A4 to cover the soil. Bidim is a continuous filament, nonwoven needlepunched geotextile manufactured from 100% recycled polyester processed from discarded cooldrink bottles. Advantages provided by the needlepunching process include appreciable thickness, high porosity and a significant drainage capacity in both transverse and normal to the plane. With its simple installation, environmentally friendly bidim is extremely cost-effective and meets the highest civil engineering and industrial specifications.
Megaflo 300 (1 500m), a high strength panel drain, completely wrapped in bidim, was installed lay flat to provide regular collector drains above the layer of bidim. Compared to conventional 110mm diameter round pipe subsoil drains, Megaflo can be installed in a much narrower trench, thereby reducing both the volume of excavation required and the quantity of commercial drainage aggregate. The Megaflo 300 panel drain was covered with a layer of Flownet (2 200 m2), a heavy-duty, extruded, HDPE drainage net. In this application, bidim will function as a filter between the soil and the Flownet spacer.
To complete the system, a layer of Kaytape S120 (2 280 m2), a slit film, woven polypropylene geotextile was installed and subsequently covered with a damp proof course (DPC). During placement of concrete, the low elongation of porous Kaytape S120 prevented the DPC from bagging into voids within the Flownet. The Bidim A4 was installed first, followed by the Megaflo, then the Flownet and finally Kaytape S120.
The second composite drainage system involved the installation of Flo-Drain (600 metres) between the waterproofed concrete retaining wall and the backfilled in-situ soil. Consisting of a Flownet drainage core covered on both sides with a bidim filter jacket, Flo-Drain is the ready-made solution to all subsoil drainage, and in this application will protect the waterproofed concrete from the backfill. The bidim jacket performs two functions; firstly, cushioning the waterproofing from the ribs of Flownet, and secondly, acting as a filter along the opposite interface between the wall and the soil. To ensure continuous filtration performance against the reworked material, a 100-150 mm layer of clean, free-draining sand was installed between the backfilled in-situ soil and the bidim.
To collect all the filtered groundwater, Geopipe M100R was placed at the invert to the Flo-Drain. The 70% perforated surface area of Geopipe allows for such a significant increase in the infiltration rate of water into the pipe, that it surpasses all other drainage pipes; while the high infiltration capacity allows for smaller diameter Geopipes to be used instead of conventional pipes.
After completion, the National English Literary Museum was awarded a 5-Star Green Building rating for meeting design, construction and operation standards that are undertaken using environmentally sustainable methods. Contributing to the award was the xeriscaping green-roof system that was installed above the concrete deck to provide thermal massing, or heat storage. Xeriscaping is landscaping using carefully selected water-wise plants that either reduce or eliminate the need for supplemental irrigation.
To provide highly effective drainage for the roof garden, a tri-component system was designed, starting with a layer of HDPE Zip Core sheeting installed directly onto the roof waterproofing. With the flat side facing downwards and the cuspated side facing upwards, Zip Core will protect the waterproofing while providing an uninterrupted air space between the concrete roof and growing medium.
TheZip Core sheeting was covered with Geomesh, a dimensioned PVC-coated, multi-filament, woven polyester mesh designed for soil reinforcement and as a high modulus separator in composite drains. A final layer of bidim A4 over the Geomesh completed the system. Geomesh will ensure that the bidim filter layer does not bag into the spaces between the Zip Core cuspations, while the one millimetre gaps in the Geomesh will allow unhindered throughflow of water. Since Geomesh is not attached to Zip Core, any chemical degradation at the contact nodes, or delamination, will be eliminated. Above the drainage system was placed a 600 mm layer of free-draining growing medium.
Without adequate drainage in a roof garden, ponding of water within the root zone can deprive plants of oxygen. This problem was eliminated by the installation of Kaytech’s top quality tri-component drainage system, since the high permeability of bidim surpasses that of all slit-film woven geotextiles as well as heat or chemically treated nonwoven geotextiles.
The Department of Arts and Culture can rest assured that Kaytech has provided the best possible drainage systems for the new National English Literary Museum, enabling the storage facility to ensure the longevity of priceless artefacts.