The use of waterproof membranes for underground construction is becoming more and more common, with a range of products now available on the market. Spray applied membranes can offer a range of benefits compared to the sheet membrane alternatives, but with any product there are key points to note for its successful application and lessons to be learnt to improve future membrane systems and applications.

The following is based on experiences obtained from both the design and site supervision of three tunnelling projects in the UK. These projects illustrate the range of applications of a sprayed waterproof system, covering large shafts, small-scale tunnelling schemes, as well as major infrastructure projects.

The experience gained on these projects is documented and lessons learnt put forward to try and assist the industry in successful use of sprayed waterproof membranes in the future.

The Hampton Pump Out Shaft (HPOS), south-west London, is a 40m-deep, 15m-diameter shaft and constructed between 2007 and 2009 using a combination of precast concrete segments and sprayed concrete. The initial section of the shaft was constructed using precast concrete segments using underpinning techniques until the shaft was within London Clay. For the second half of the shaft a permanent primary and secondary sprayed concrete lining (SCL) was constructed utilising a sprayed waterproof membrane. A waterproof membrane was required for the scheme, despite the relative impermeability of the ground, in order to provide the client the assurance that the end product would be waterproof for the entire design life.

The Dorchester Service Link, London, consists of two 12m-deep, 3.7m-diameter shafts and one 117m-long connection tunnel. The two shafts were excavated within the existing basements of 45 Park Lane and the Dorchester Hotel.

The primary lining of the shafts utilized a combination of steel liner plates and SCL with a 200mm thick cast in situ concrete secondary lining. The tunnel primary lining was designed as 150mm-thick SCL without any reinforcement. A 200mm-thick, cast in situ concrete secondary lining completed the tunnel lining. The client’s requirement was for a leak-free, undrained scheme that minimises the influence on the water table. A sprayed waterproof membrane system was chosen to fulfil this requirement as it was accepted by all parties within the scheme and was seen to be a simple and swift solution for the waterproofing issues present in a challenging construction environment.

The A3 Hindhead road improvement scheme comprises 6.7km of new dual carriageway in Surrey, south-east England. Of this, 1.8km is twin tunnel bores constructed during 2008-2009 using SCL. The tunnel alignment was through the Hythe Beds, which comprise highly interbedded sandstone with sand lenses, and was kept above the water table at all times.

Both primary and secondary linings at Hindhead included fibre reinforcement. The primary sprayed concrete lining was designed to be permanent and was applied direct to the exposed material by robotic sprayng equipment.

Spray-applied waterproofing was placed using the same equipment as the primary lining. Secondary lining walls were cast using single-sided shutters and finally the newly sprayed secondary crown was installed.

The spray-applied waterproofing system at Hindhead was chosen for a number of reasons:
The primary linings were mostly dry during excavation due to the alignment being above the water table.

The limited reinforcement in the secondary linings prevented the need for many penetrations through the sprayed membrane.

The system offers great construction flexibility by removing the need for large installation staging.

A spray-applied crown could be used which removes the need for large shuttering, again improving construction flexibility.

A distinct advantage of the sprayed waterproof membrane system is the ease and flexibility of its application through robotic application. This gave particular benefits to the schemes at Hindhead and Hampton. For the Hampton HPOS, robotic application of the SCL and membrane was proposed utilising the same robot throughout the project. While the plant is more expensive than ‘hand’ spraying, the use of a robot allows the operator to be away from the face of the excavation and provides more control over the thickness of the sprayed concrete applied with less rebound (thereby saving material costs). The robot used at Hampton proved its flexibility and was used without problems for both the 15m-id shaft as well as the 3.7m-id SCL connection tunnel required from the HPOS. In practice the robot swiftly managed to apply the membrane due to its large operating reach and certainly proved its worth in terms of quality, safety and programme benefits.

For the successful application of any construction product, preparation is key. Preparation must begin with the training of the staff that will be applying the membrane. From all three project experiences, successful application of waterproof membranes was related to the learning curve of the site labour. Consequently it is strongly recommended that for future projects a significant length of time is allocated for trial spraying of any membrane with testing of the consistency of the membrane mix to ensure that it suitable for the application proposed.

Going beyond the preparation of the site team, good preparation of the concrete substrate is vital for effective application of a sprayed waterproof membrane. All of the projects mentioned either benefitted or would have benefitted from the use of a smoothing layer. Through the use of a smoothing layer all large uneven parts of the substrate can be overcome as well as preventing any protrusions caused by fibres in the substrate that could jeopardise the membrane performance. For the Hampton scheme, the absence of a smoothing layer led to a substrate that was insufficiently smooth in certain areas for successful membrane application and therefore required respraying.

Given the large difference in the cost of a smoothing layer compound versus the waterproof membrane, it would be recommended that smoothing layers are specified for SCL substrates to minimize material wastage.

Unlike the other projects described in this article, the application of the waterproof membrane for the HPOS was applied in the shaft while being exposed to the elements, which presented some additional challenges. The success of the curing of a waterproof membrane can be affected by both low temperature and large temperature ranges. For the HPOS project it was required for the membrane to be applied in a particularly harsh winter month. In order to cope with the low temperature, space heaters were used to ensure that ambient temperatures were kept to acceptable levels, but had mixed results. While the extent and duration of British winters have been difficult to predict in recent years, from the work undertaken at Hampton it would be recommended for any future projects that application of waterproof membranes should be undertaken in summer months if at all possible within construction programming.

Managing water
Active water ingress (eg dripping) prevents the currently available spray-applied membranes from curing. Proactive water management may therefore be required in order to provide a dry substrate during the curing process. Unlike the other projects described in this article, the application of the waterproof membrane for the HPOS was applied in the shaft while being exposed to the elements. In order to prevent water flow down the surface of the curing membrane, local guttering at the transition of the segmental and SCL sections of the shaft was used to ensure water ingress down the surface of the shaft could be controlled and managed. These measures are quite simple but provided an additional set of activities to be undertaken which could have impacted upon the scheme programme.

At the A3 Hindhead project, water ingress was prevented through by a number of means (see flow-chart):
Application of natural cement: This fast curing product could be placed over active water ingress, and provided a dry substrate for a short time (allowing the spray-applied waterproofing to cure). In practice this solution reduced the flexibility of spray-applied waterproofing because of the short timescale required to apply the waterproofing over the cement.

Application of dimpled drainage membrane: This was fixed to the spray-applied waterproofing, as a water-path for permanent drainage. Care is required to ensure sealing of the product to the substrate in order to prevent side leakage (particularly if not installed vertically). A second coat of spray-applied waterproofing was applied over the product.

Application of fast-curing spray-applied waterproofing: This product had the ability to be applied to areas of active water ingress but had a significant cost impact.

Injection of water path: Water was diverted to temporary drain pipes whilst grout injection halted the water ingress. Once the spray-applied waterproofing had cured, the temporary pipes were sealed.

Quality assurance
An area for improvement with spray-applied waterproofing is the ability of the system to be checked prior to application of the secondary lining. The system used on the three projects featured cannot be completely checked for watertightness, with testing being limited to:
visual inspection and spot checks of film thickness and bond, depth gauges used during sprayed membrane application , removing 50mm x 50mm patch off the membrane from the SCL surface, to check for the required minimum 3mm thickness.

While these measures are simple to undertake on site, they offer little assurance of the integrity of the membrane composite for the entirety of the application. Consequently, whilst the bonding of the secondary lining to the waterproofing will minimise water ingress, it is not possible for the construction team to guarantee the quality and integrity of the installed system. With remedial work after installation of the secondary lining being costly, a solution to allow checking of the waterproofing would be welcomed.

Whilst penetrations through the waterproofing may be minimised by design (eg through removing rebar and equipment fixings), they will inevitably be required during construction. At A3 Hindhead penetrations were limited to small quantities of rebar around openings and support for duct banks buried within the secondary lining.

Each of the items passing through the membrane were specified to be non-corroding (stainless steel or GRP) and membrane was hand applied around the items once installation was complete. In practice it is difficult to ensure a complete waterproofing seal around penetrating items. As a mitigation, the use of resin fixings is recommended as the resin acts as a backup waterproof barrier.

The preceding points are brought to the industry’s attention with the intention of improving the use and design of future sprayed waterproof membrane systems and products.

By improving the usage and design of waterproof membranes it is hoped that they can be successfully used in many future UK and worldwide schemes.

It is worth noting additionally that the successful use of these products is also down to growing client acceptance of sprayed waterproof systems and contractors appreciating the benefits that they can offer.

All the authors thank the contractors and clients of the schemes willingness to accept the benefits of a sprayed waterproof membrane solution.

Applying BASF Meyco Masterseal in Hindhead Tunnel using standard robotic spraying equipment Connecting tunnel for the Dorchester Hotel service link Examining the sprayed membrane applied in the Hampton pump-out shaft Decision flowchart to deal with water penetration of primary lining with sprayed membrane Uneven substrates such as this can cause problems in applying sprayed membrane Proximity of reinforcement wire etc to the membrane may result in accidental penetration Some planned penetration of the membrane can be allowed if sealed as with reinforcing mesh Other membrane penetrations such as duct fixings may require non-corroding fixings and hand-applied sealing