Main Considerations Covered by BS8102
1. Has the waterprooﬁng system been designed by a specialist
2. Has a site investigation taken place by a geotechnical specialist
3. Is it possible to design out the faults in the workmanship and materials
4. Repairability should be taken into account and the feasibility of remedial measures assessed
To achieve the correct grade of waterproofing for your below ground structure the waterproofing design and installation is essential.
There are 4 grades of waterproofing ranging from Grade 1 basic utility to Grade 4 Document storage.
To achieve the correct grade of waterproofing for your below ground structure the waterproofing design and installation is essential.
Can be achieved simply by using waterproof concrete and detailing joints in concrete correctly (to BS8110 & BS8007) but can be easily upgraded to a full internal Cavity Drain System if necessary.
Can be achieved using correctly detailed concrete (to BS8110 & BS8007) and the installation of an internal Cavity Drain System.
Is easily achieved using a full Cavity Drain System.
Is only possible if the concrete is detailed correctly (to BS8110 & BS8007) and a full Cavity Drain System is installed. The environment also needs to be controlled using an air conditioning system.•
BS8102 (the British Standard for Waterproofing) recommends that in designing the waterproofing system for your basement, you should expect that water pressure will come to bear against the structure at some time in the future. This is because no matter how dry the excavation is now or has been in the past you are designing for the future and it is not possible to predict future rainfall levels, changes in watercourses or even possible burst water mains in the street adjoining the property.
As a specialist waterproofing company we see many examples of basements / cellars that have been built into dry drained chalk or stone excavations that suddenly fill with water.
Types of waterproofing
A Type A waterproofing system can be applied in a Varity of ways to the structure. They can be used to waterproof walls, slabs and decks, some are sheet membranes some are cementitious slurries and some are liquid membranes. A specialist will only ever design and install a system that is fit for purpose. Type A system’s must always have a suitable external land drain, to remove water pressure from the structure. If this cannot be accommodated, then we would not recommend this type of system. All land drains must be fully serviceable
Pre Applied Membranes;
Fosroc Proofex Engage
Post Applied Membranes;
Fosroc Proofex 3000
Vandex BB75, BB75e, Super
Liquid Spray Applied Membranes;
A Type B waterproofing is an integrally waterproof structure. This can be achieved with waterproof concrete admixtures, but these are limited and still rely on perfect workmanship to be of any benefit.
PVC Water bars
Are used to seal construction joints within concrete structures. They are largely considered the best form of construction joint sealing, but are particularly difficult to install. They work by forming a physical barrier to prevent water from ingressing past them.
Are used to seal construction joints, in the same way as PVC water bars to. They are used more widely than PVC water bars as they are mush easier to install. However because they swell in the presents of water and they take time to swell, they will always let an amount of water through before they form a stop. And because if there is no water present they shrink back, this amount of water will ingress through the structure ever time this cycle occurs.
Resin injection can be used in two ways, either by installing an injection tube into a construction joint. Which can then be injected with resin after construction is complete. Or it can be used post construction to seal joints or cracks. The benefit of using resin injection for joint sealing when using the injection tube, is that if there is any honey combed concrete it will seal it. The resin injection system as a wide Varity of use’s such as sealing leaking construction joints, cracks and expansion joints and is one of the only systems that can be used for remedial actions for these problems.
Type C waterproofing systems work by allowing water that ingresses through the structure to move freely and unhindered to either a surface water drainage system or to a sump and pump system where it is pumped from the structure.
The system is basically an internal lining which provides a cavity through which waster can pass. The system consists of a dimpled wall membrane, a drainage channel (sited at the wall floor junction) and a dimpled floor membrane. Further to this sumps and pumps will also be required if natural drainage is not available.
All membrane joints and fixings are sealed, using specialized sealing products to prevent any ingress of water or moisture. The system can also be used as protection against contaminants such as salts, carbon deposits, fertilizers, oil etc. Because it is not possible to determine the amount of potential water pressure surrounding the structure, BS8102 tells the designer to expect a head of water to ¾ the depth of the structure for a structure up to 4m below the ground, and for deeper structures, a head of water to 1m below the depth of the structure.
The advantages of a Type C system are that they are easily repaired, easily maintained, quickly installed (and largely unaffected by weather conditions), They do not impose any load onto the structure (particularly important when dealing with older structures).
A wide range of finishes can be adopted to walls, floors and soffits which also provides protection to the membrane system.
Typical wall finishes include:
A) Block work wall built in front of the system
B) Independent timber framing and plasterboard
C) Independent metal framing systems
D) Conventional plasters and renders
Installing waterproof concrete as a method of waterproofing, as classified under ‘Type B waterproofing by BS8102, is very effective at making the concrete more waterproof. However correctly installed concrete is waterproof without any additives.
Water proof additives do not protect the structure from water penetration through the construction joints. There are a great many methods of joint waterproofing available. However, they frequently fail and water will normally ingress through joints in the concrete.
The system will conform to BS8102:1990 Protection of Structures Against Water from the Ground, and is termed as Type C Drained Protection. Our method of waterproofing is used on a daily basis to waterproof both new build and existing structures and is used extensively to waterproof structures where Tanking has failed.
Because water will ingress at the weakest point in the structure, System 500 provides depressurization air spaces to intercept the water that may leak into the structure through the junction between the wall and floor. Utilizing the wall-floor junction Drainage Channels then remove this water. It is a condition of insurance companies that the plastic Drainage Channels are used and for good reason. Water will almost certainly only ingress through defective joints in the wall-floor junctions, floor slab joints or defects in existing masonry. Water should be picked up at these points by the Drainage Channels rather than across the concrete to a drainage point. Water running over new concrete picks up free lime and when it dries calcification can occur which eventually causes blockages. Drainage Channels should always be accessible and serviceable.
Internal Waterproofing Vs External Waterproofing
We are often asked by our customers to help in the design of an external waterproofing system and have found that in most occasions we recommend internal waterproofing as being the best option. This leaflet has been produced to clarify our reasons for specifying internal systems rather than external waterproofing systems.
External waterproofing is only really an option in new build construction, and even then, a complete system to both the walls and floor should be used. However, in many cases the floor slab/raft has already been constructed making external waterproofing problematic.
If external waterproofing is being considered, the removal of ground water by the use of French drains/land drains should be incorporated into the design of both tanking and cavity drainage waterproofing. Removal of water from the structure is a method of complying with BS 8102 that states that the designer should assume that less than adequate workmanship will been employed during the installation of the waterproofing.
Removal of water to prevent a build up against structure means that the structure and its waterproofing will not be tested. However, the lifespan of the water removal system is finite and can be problematic and asks too many unanswered questions.
It is not possible to ascertain exactly how much water will need to be removed to prevent a build up of water pressure against the structure. In many cases watercourses are arriving at the structure from many miles around and it is possible to be in a scenario where you are trying to remove unlimited sources of water. This can create an enormous strain on the system, especially where pumps are used to remove the water.
As well as being extremely costly, the life expectancy of the pumps is dramatically reduced. If the designer is asked how much water he expects to have to remove, the honest answer is that it is not possible to accurately determine the volume. Therefore, what size pumps do you need?
Natural drainage is only a safe means of drainage providing it is impossible for the drainage to back up. Therefore unless the drainage or soak-away are below the level of the basement floor, pumps will be required. If natural drainage is available further down hill from the property, the question still has to be asked as to how much water is to be removed by the system, and the same vague answers will result. Without knowing how much water is to be removed, it is difficult to determine the capacity of the drainage system that will be required.
The potentially large volumes of water being removed by the system can bring relatively large amounts of fines from the ground into the system. External drainage systems utilize geotextile membranes as a filter to prevent the fines blocking the perforated pipe. However, like all filters they will eventually block up, and once the geotextile is completely blocked it will no longer remove water from around the structure. It is not possible to determine how long this process could take but evidence of complete blockage on external drainage systems within months has been documented.
As well as having the potential to block the drainage system, the removal of fines can undermine surrounding structures. It is vitally important that a geological engineer assesses the design of an external system to determine that neighbouring structures will not be undermined by the system.
It is difficult for any designer to guarantee that the external drainage system will remain in working operation for the whole of the life expectancy of the building and in our opinion it is not a viable option for these reasons.
Internal waterproofing of new build structures is in our opinion a much safer option. It is possible to design the system that will outlive the life expectancy of the structure, without having the problems that are associated with external drainage systems.
It is likely that the new structure will be built to BS 8110, or possibly to BS 8007. These structures are very strong and will only leak at joints within the construction. Measures to prevent leakage at the construction joints should be taken using hydrophobic or hydrophilic water stops, leaving a structure that should not leak. However, we should assume that the structure would leak if subjected to a head of water pressure because of the possibility of less than adequate workmanship.
This may be a slightly leaking water stop at a construction joint, for example, but wherever the weakest point of the structure lies, the amount of water entering the structure will be minimal, and significantly less than the amount of water removed by the external waterproofing system. Therefore, the internal waterproofing system is only dealing with relatively small amounts of water, and so the system can work well within its designed performance.
Water entering a strong structure is virtually crystal clear, as the structure filters out the fines. Because the movement of water through the structure is massively smaller than the amount of water potentially being moved by an external system, the volume of fines being removed from the ground is also much less. However, because the structure filters the fines they are not actually being removed at all and so in a worst-case scenario you have small volumes of fines being moved towards, but not removed from the structure.
Internal drainage systems will still require pumps to remove any water that may enter the structure. However, the size of pumps is much easier to determine and these will be dramatically smaller pumps will be operating much less than in an external waterproofing system and so power consumption and pump replacement will be significantly less with an internal waterproofing system.
BS8102 also states that the designer should consider the form and feasibility of remedial words in the event of failure of the waterproofing system. Repair of an external system is in many cases just not possible. Where it is possible to excavate to show the system, identification of the problem and repair is almost just as difficult, especially if the ground is, as is often the case, wet and muddy.
Repair of an internal cavity drainage system is much less of a problem, as in the main, the problem will be with the drainage system. Inspection ports make the identification and cleaning of the system quick and easy.
Legal – The Outwing Case
HIGH COURT JUDGEMENT
OUTWING CONSTRUCTION –V- THOMAS WEATHERALD
A recent ruling in the High Court of Justice, Queens Bench Division of the Technology and Construction Court, is likely to have significant implications for the waterproofing industry.
Thomas Weatherald Ltd. was the main contractor responsible for the design and construction of a new nursing home at Bramley Hill in Croydon. The structure included the construction of basement built into the well drained chalk on the site.
The floor was constructed in reinforced concrete. The walls were constructed out of two skins of concrete block, sandwiching a layer of concrete in between. The structure was waterproofed externally using Bitite, a bonded sheet membrane.
A land drain was positioned approximately one third of the way up the wall, and discharged to a soak away a little distance from the building.
The construction of the basement structure, waterproofing and land drain was subcontracted to Outwing Construction. Shortly after completion, leaks occurred internally after a period of prolonged and heavy rainfall. Thomas Weatherald withheld money from Outwing Construction, on the basis that they had incurred additional expense in applying Sika render internally.
Outwing sued Thomas Weatherald for the balance of money owing, and Thomas Weatherald counterclaimed for the cost of the Sika installation together with other damages.
THE OUTWING CLAIM
Phil Hewitt, Expert Witness for Outwing Construction, argue that there was a significant fault with the design, for the following reasons:
1. Clause 3.3 of BS 8102, Code of Practice for the Protection of Structures Against Water from the Ground, states that the designer should i) Consider the consequence of less than adequate workmanship, ii) Consider the consequence of leaks and iii) Consider the form and feasibility of remedial work.
2. By installing the land drain in the position shown, the designers created a head of water that would bear against the membrane. In these circumstances, any defect would constitute less than adequate workmanship, as the consequence of those defects would be flooding through the membrane into the basement.
3. It is not realistic or reasonable to expect a bonded sheet membrane to be applied without any defects at all.
4. Clause 3.1.1 of BS 8102, Pre-Design Considerations, recommends that basements should include provision for resisting a pressure equivalent of 1m head of water at least.
5. The interpretation of the above was that a design team must anticipate that defects will occur in a membrane, and so must design a system in such a way that water pressure is removed before it comes to bear against the membrane. If they are unable to achieve this, it is implied that an alternative form of waterproofing must be used.
6. Furthermore, a bonded sheet membrane is only one element within an overall waterproofing system. The membrane, together with the drainage and the structure, all form part of the system and must be considered together.
No one element should be considered in isolation.
THE THOMAS WEATHERALD DEFENCE AND COUNTERCLAIM
John Mawditt, Expert Witness for Thomas Weatherald Ltd., argued as follows:
1. Clause 3.1.1 of BS 8102 says that the membrane alone must be capable of withstanding a head of water of at least 1m without leaking.
2. The installation of the land drain above the floor slab did not induce a water head in excess of 1m, and so the design complied with BS 8102.
3. In the absence of a design fault, the problem has to lie with the installation of the membrane, by default.
Recorder Colin Reese QC found for Outwing Construction without any qualification. In his 25 page judgement, the following exact extracts are pertinent:
1. Having read their reports and heard their oral evidence, I unhesitatingly prefer Mr. Hewitt’s evidence and reject Mr. Mawditt’s views that a self-adhesive tanking system of waterproofing such as that which was installed could be expected to resist water penetration in the event of a build up of hydrostatic pressure.
2. I agree with Mr. Hewitt that overlapping self-adhesive membranes cannot be expected to achieve a total or absolute watertight bond capable of resisting penetration by water pressure. If this were thought to be something realistically achievable, then the guidance given, in relation to the use of tanked protection systems where high or perched water tables exist (permanently or from time to time), in publications such as BS 8102 and the Basement Waterproofing Design Guide (see in particular, the discussion under the heading “Guide to Assessing Basement Designs” beginning on page 17 of the document -page 84 of the trial bundle) would be unnecessary. Furthermore, if this were thought to be something realistically achievable, then it is difficult to understand why those responsible for the Standard and/or the Design Guide should so clearly and consistently contemplate the
3. Provision of perimeter land drainage below the lowest level of the tanking system.
4. In my judgment, as Mr. Hewitt said, the waterproofing system consisted of both the tanking membrane and the subsoil drainage.
5. For all the reasons put forward by Mr. Hewitt (with which I agree) this design did carry with it such a risk after periods of heavy rainfall when a perched water table or perched water tables might come into existence, and the factual evidence was that the water penetration problem only became apparent after heavy rain.
This is a High Court judgment, and now sets a precedent for future similar cases. As a result, designers of waterproofing systems should now give serious consideration to:
1. Ensuring that, where there is any risk of water pressure building up against a membrane, (whether it is permanent or temporary) the water is removed before it comes into contact with the membrane.
2. This could be done by incorporating drainage membranes leading down to land drains at the base of and running around the perimeter of the structure, from where the water is either drained or pumped away.
3. If the above is not practical, then an alternative form of Waterproofing/Construction must be selected.
A free copy of the judgment can be obtained from the Mechanical Recording, Royal Courts of Justice, by faxing 020-7936-6662 and quoting the following identifying information:
Technology and Construction Court
Outwing Construction v. Thomas Weatherald
Date: 13 September 1999
Recorder: Colin Reese.
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