Twin Waterproofing & Ground Gas Protection

The scope of the project was to provide a gas protection system and waterproofing solution for four new build properties in Barnet, North London.

The four new residential (semi-detached and detached) houses were built on an existing brownfield site.  Low levels of methane were detected on site during the site investigation and gas monitoring of the site.

Methane can migrate through subsoil and through cracks and fissures into buildings. A build up of methane to hazardous levels will cause harm to health and compromise safety. Methane (CH4) is an explosive and asphyxiating gas.

Gas protection is a technical solution to prevent or to control gas penetration into properties. The control of gas migration is normally achieved by blocking the pathway or removing the source of the gas generation. There are several methods available to achieve the protection to existing/new build properties.

The Gas Protection measures required should conform to the requirements of BS 8485:2015 – Code of Practice for the Design of Protective Measures for Methane and Carbon Dioxide Ground Gases for New Buildings.

The site investigation and gas monitoring in accordance with Section 5 BS8485:2015 – Site Investigation for Ground Gases and Section 6 BS 8485:2015 – Process for Ground Gas Characterisation and Hazard Assessment – gave a quantitative assessment of gas risk to characterise the site as CS2.

In accordance with table 3 BS 8485:2015 the building type –  private residential dwellings – would be classified as Type A, therefore in accordance with table 4 BS8485:2015 – Gas protection score by CS and Type of building would be 3.5 points.

Gas protection measures to achieve the minimum points score then a combination of two or more of the following protection measures should be used:

• The Structural Barrier
• Ventilation Measures
• Gas Resistant Membrane.

Three of the four properties were relatively straightforward – the construction (structure) was well reinforced (to minimise cracking) cast in situ monolithic ground bearing raft with minimal penetrations, the structure in accordance with table 5 BS8485:2015 would achieve a point score of 1.5. The gas protection barrier (MemTech M1) conforming to table 7 BS 8485:2015, would achieve a point score of 2, the combination providing the required minimum point score of 3.5.

The fourth property and the focus of this case study was earth retaining to approximately 1.5m to two thirds of the ground floor area and therefore required waterproofing as well as gas protection.

The waterproofing requirement in accordance with table 2 BS 8102 (2009) was for a grade 3 habitable space. The building warranty provider on the project in accordance with their standards required two forms of waterproofing for a grade 3 space.

To add to the complications, when Delta Membrane Systems Ltd were asked to develop a robust waterproofing and gas protection solution the property construction had already commenced, thus limiting both the options for waterproofing selection (two forms required) and gas protection measures.

The earth retaining/ground floor was constructed from a cast in situ well reinforced ground bearing RC slab, the walls were constructed from two skins of blockwork with a concrete infill. The site investigation revealed that the water table was low, but generally soil and ground conditions inferred the possibility of a perched water table – therefore in accordance with BS 8102 (2009) section 5.1.3 Water Table classification – this was classified as variable.

If involvement of the project had been prior to construction, then the use of the MemTech Titan Bond and Titan tank to the external of the building could have been utilised as the first form of waterproofing (Type A barrier protection) and gas protection membrane conforming to both BS 8102 (2009) and BS 8485:2015.

Given the type and form of construction, below ground use, water table classification, requirement for two forms of waterproofing and gas protection measures to CS2 for type A building the following robust waterproofing and gas protection design was developed. The waterproofing options available based on current construction were a Type A barrier protection system and a Type C drained protection system along with an additional gas protection barrier conforming to table 7 BS 8485:2015.

Prior to the installation of the waterproofing and under guidance from the structural engineer a 100mm wide by 75mm deep recess was created at the base of the retaining walls, the floor area was flood tested to ensure it was level and that water drained as required to the recesses and to the proposed point of discharge for the drained protection system.

The proposal was to apply Delta’s Koster NB1 barrier protection system to the structure to provide primary waterproofing – this included the use and application of the Koster Polysil TG 500 as surface primer to all walls and floors. At the wall -floor and wall – wall junction – Koster Repair Mortar Plus was installed as a 40mm bottle coved fillet. Once these had been applied and cured then 2 coats of Koster NB1 incorporating Koster SB Bonding Emulsion within the gauging water was applied in a two- coat application. Koster NB1 is a polymer modified in depth crystallizing mineral slurry which has excellent bonding properties and waterproofs against pressurised water to 13bar.

The above provided the first form of waterproofing a type A Barrier Protection system conforming to the requirements of Table 1 BS 8102 (2009).

Once cured, MemTech LGB (Liquid Gas Barrier) was applied as a gas protection membrane, conforming to BS 8485:2015 Table 7. Methane permeability in accordance with the requirements of BS 8485:2015 Table 7 – states permeability to be <40.0ml/day/m2/atm – tested in accordance with BS ISO 15105-1 Manometric method) MemTech LGB Methane permeability is 0.01 ml/day/m2/atm.

MemTech LGB was initially applied as a 150mm band to all wall-floor and wall-wall junctions and around service penetrations, application rate @ 1kg/m2 giving and initial first application thickness to these areas of 0.5mm.

Once these areas had cured, two full coats were applied to all walls up to gas resistant DPM and the entire floor area. Application of the two coats to provide a minimum 1mm thick barrier to provide the required gas protection.

The application of the MemTech LGB was carefully monitored and correct thickness of application and total coverage was inspected and checked prior to the subsequent installation of the type C drained protection system. This was required as part of the verification and integrity testing of the gas protection membrane.

Delta MS500 was installed to all retaining walls and sealed at the top using Delta Cornertstip. The installation of the fixings to hold the membrane in place and to provide a sealed fixing point for lining systems was provided by Delta’s Qwikseal plugs, where these were to be fixed though the applications of the Koster NB1 and MemTech LGB the holes were pre- filled with Koster PU907 – a low modulus polyurethane sealant with excellent sealing and adhesion properties. The Koster PU907 would provide a seal through both the Koster NB1 waterproofing layer and the MemTech LGB gas protection layer.

Delta channel was installed within the formed recess at the base of the retaining walls, this incorporated access points at key changes in direction and approx. every 6m on straight runs, these were included to ensure the system was maintainable and conforms to the requirements of section 10 Drained Protection – BS 8102 (2009). The Delta channel lead to a suitable point of discharge, in this case to an external surface water drain on the external terrace areas on the non- earth retaining elevation.

Delta MS 20 was installed to the floor area and linked and sealed to the MS500 to the walls and also the DPM to the non-earth retaining wall elevation, thus providing a totally continuous waterproofing barrier from DPM to DPM throughout the building.

All joints to the Delta cavity drain membranes were double sealed using double sided butyl sealing tape and 150mm wide Delta cornerstrip.

The Delta cavity drain system was flushed through and commissioned.

As an additional precaution the Delta channel was ventilated to external atmosphere, in the event that, any ground gas or ground gas in solution in groundwater enter the building through both the Koster NB1 and MemTech LGB applications, this could not build up within the cavity drain system. The Delta cavity membranes do have some resistance to Methane however they do not conform to the full requirements of Table 7 BS 8485:2015 – Methane permeability in accordance with BS ISO 15105-1 Manometric test is just above the maximum permitted level of methane permeation.

BS 8102:2009 Code of Practice for Protection of Below Ground Structures Against Water from the Ground

BS 8485:2015 Code of Practice for the Design of Protective Measures for Methane and Carbon Dioxide Ground Gases for New Buildings

The above waterproofing (two forms) and gas protection system was verified and accepted as suitable and appropriate and conforming to the requirements of BS 8485:2015, BS 8102 (2009) and the building warranty providers standards prior to installation and the installation was carried out by Delta registered installer Oaks Damp Proofing Ltd.