Membranes and the use of Table

The membrane requirements in Table 8.1 are only indicahve and may be changed subject to clear and rational justification. The purpose of Table 8.1 is to show common solutions for each characteristic situation. The solutions are based on the assumption that achieving a good membrane installation is more important with leaky floor slabs such as block and beam. Thus block and beam slabs require a more robust membrane than a cast in situ reinforced concrete floor slab (see Chapter 7 for more discussion on membranes). Regardless of material type all membranes should be inspected after installation and protected from damage (see Chapter 9). If there is likely to be any differential movement between the floor slab and the wall foxmdations this must be allowed for in the design and specification of the gas membrane. 

The membrane requirements in Table 8.1 are consistent with the advice in Chapter 7. The NHBC guidance requires membranes that are installed on Amber 2 sites to be integrity tested. It is recommended that this requirement is also applied to membranes on sites that fall in situation 3 and above in the modified Wilson and Card approach (Situation A in CIRIA, 2006). 

For low-risk sites if 1200 g DPM is used below a substantial floor slab it needs very careful installation and possibly protection from follow on trades using protective boards or fleeces. It will also require very close independent inspection. Thus it can often be cost effective to use a more robust membrane with less protection. 

A site is proposed for a shopping centre development and has a maximmn gas concentration of 26% and borehole flow rate of 0.11/h. The source of the gas is a tiiick layer of alluvium with peat lenses and groundwater levels in flie area are known to rise and fall quickly in response to rainfall events. The development will have a suspended cast in situ reinforced concrete floor slab fliat is 200 mm thick. 

Eight sets of gas monitoring results are available but these have been taken when there has been little rainfall and groundwater levels have remained constant. 

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