What are the Principles of Waterproofing?

As I advance through the training on the CSSW course in my endeavour to become a Certified Surveyor of Structural Waterproofing (yes I have been burning midnight oil!), I am pleased to reflect on some learnings about the key Waterproofing Principles, and hopefully share some insights for others who may be interested in this sector. Water never sleeps, and neither should we, so lets keep learning! :)

Ground Conditions

When we consider ground conditions in the context of Waterproofing or other forms of substructure protection such Gas Protection, it is important we understand the Source of Moisture or Gas. The following elements are particulary important to identify

• Water table level
• Potential for water table
• Topography of area
• Type and nature of Soil
• Contamination or Ground Gases

This information can be found within Hydrological Reports, or Soil Investigation reports, often reffered to in the construction industry as an 'SI Report' (Site investigation) or 'GI report' (Ground Investigation)

Lets explore some of these elements further;

Sources of Water or Moisture: Water Table

The water table is an underground boundary between the soil surface and the area where groundwater saturates spaces between sediments and cracks in rock. Water pressure and atmospheric pressure are equal at this boundary. There are two main types of Water table which have an important distintcion in the field of waterproofing

• Natural water Table - This occurs where the soil is saturated right down to bedrock
• Perched Water Table - otherwise known as a false water table, this occurs when there are lenses of impermeable material in the ground make up (ie: a seam of clay) which would be otherwise permeable which prevents the water from draining into the soil. These allow small localised water tables to develop which can overlay a regional water table in the surrounding rock mass or landscape. (see example in diagram below)

As specialist Waterproofing Designers, we are expected to take all of these elements into account whilst building the water table risk profile, and conclude whether the water table is High, Low or Variable

Other sources of Water or Moisture

The water table is not the only source of water outside of our structure. The presence of water can also arise from less predictable sources such as;

• Leaking mains and drains
• Overloaded soakaways
• Leaking rainwater goods

All of these result in some hydrostatic pressure, which is determined by the head of water that comes to bear.

What is Hydrostatic Pressure?

Hydrostatic Pressure is the pressure exerted by a fluid at equilibrium at any point of time due to the force of gravity. It is proportional to the depth measured from the surface (the head of water) as the weight of the fluid increases when a downward force is applied.

Capillary Moisture and Ground water

The soil above the water table is known as the Capillary moisture zone, and exerts no pressure. Capillary moisture is that moisture above the water table which is held in the capillaries of the soil.

Types of Rock/Soil

Remember the Rock Cycle from school? There are varying soil types to consider when understanding the waterproofing requirements for a new or existing structure. Here are some common types of Rocks or Soils and its always good to know the difference, so lets take a look...

• Igneous rocks - These are of volcanic origin or cooling Magma such as Granite or Basalt. These are very hard rocks and not layered
• Sediments such as Clay, sands and Gravels - These are usually derived from weathering and erosion of igneous or Metamorphic rock.
• Sedimentary rock - This occurs where clay and/or sands have been compressed or compacted, sometimes resulting in cementation. Examples of this are Sandstone, Limestone or Shale Soils. These rocks are usually crumbly and layered.
• Metamorphic rocks - These are usually derived from sedimentary rock and sometimes Igneous rock, where they have been compressed and heated. Examples of this are Marble or Slate which are relatively hard and may or may not have layers.

Soil Cohesiveness & Stability

Its important to observe how cohesive or stable the soil is when assesing a project.

• Cohesive soils - The soil particles bond together such as clays and soil with clay content
• Cohesionless soils - No bond between soil particles such as sands, gravels and soils without clay content

The cohesiveness of the soil has a direct impact on how stable the soil is likely to be when excavated. Cohesive soils will have steeper excavations in comparison to more sloping sides seen within a Cohesionless soil excavation.

By understanding the soil type, and how cohesive (or not) the soil might be this not only informs the strcutural engineer on how to construct the basement, but also how well drained or poorly drained the site might be, which brings us to permeability.

Soil Permeability

Where there is a hydraulic gradient (pressure difference) water will flow through the voids and capillaries of soil/rock. Permeability will be a function of the number and size of voids in a soil.

This will, in turn, be a factor of particle size and grading of the soils. The ease with which it can flow defines the permeability of the soil/rock. Lets remind ourselves of the permeability characteristics of varying soils and rocks;

• Pure clay soils have very low permeability
• Fine sands, or course sands with clay content, will have low permeability.
• Course sands will have medium permeability
• Gravels will have high permeability
• Solid (Igneous) rocks usually have a low permeability
• Fissured rocks will be more permeable, depending on the number and continuity of the fissures
• Sedimentary and metamorphic rocks tend to be relatively impermeable across the grain, but more so with the grain.
• Chalk is a sedimentary rock, but is actually one of the 'odd ones out', as it is very permeable and pourous in nature, whilst maintaining a high cohesion.

Soil Grading affects Permeability

Soil grading, which refers to the variety and proportion of different particle sizes in soil, also impacts its permeability, therefore a well graded or poorly graded soil should be determined.

Soils composed of single-sized sands are deemed to have poor grading. In contrast, well-graded sands exhibit a diverse range of particle sizes that interlock well.

The grading influences the soil's permeability: a fine, single-sized soil or a coarse, well-graded soil tends to have lower permeability. On the other hand, soils with uniform particle sizes typically exhibit higher permeability which mean a higher rate of Percolation, and possible faster Saturation

Percolation and Saturation

Percolation is the movement of free water through soil or rock from upper layers, such as from rain, broken mains, or drains, as it travels downward.

This process is influenced by the size of voids and capillaries within the soil or rock, which essentially relates to the level of permeability previously discussed.

Saturation happens when the soil reaches its capacity and can no longer absorb additional free water, indicating that all its voids and capillaries are completely filled.

Waterproofing Design & BS 8102:2022

Once we have established the Ground Conditions, as discussed above, we need to move to the Waterproofing Design Phase.

The 'British Standard BS 8102:2022 - Code of practice for protection of below ground structures against water ingress' is the most commonly recognised standard within the construction industry, whether a design is being complied for an existing basement requiring remediation or a new build project.

Lets take a closer look at these standards and how they apply in practice. Amongst other things, it gives us guidance on the following;

BS 8102:2022 - The Design Team

Firstly the design team must be defined, and in place. The question 'Who is taking responsibility for the Waterproofing design?' At Premcrete we are increasingly getting involved early on in the project (Riba stage 2 & 3), in collaboration with the client, Main contractor/developer, architect and structural engineer. Heres what the British Standard covers;

• We are encouraged to seek the advice of a geotechnical specialist, on the geology, hydrogeology, external drainage options and groundwater conditions.
• It is also emphasised that a waterproofing specialist should be included as part of the design team at an early RIBA stage so that an integrated waterproofing solution is created.

• The waterproofing specialist should:a) be suitably qualified and experienced commensurate with the type and size of the proposed project; b) be capable of devising solutions that accommodate the various project constraints and needs with an understanding of construction forms and sequencing; andc) provide the design team with information and guidance that assists with and influences the design, installation and future maintenance of the waterproofed structure.

BS 8102:2022 - Grades of Waterproofing Protection

Onec we understand the ground conditions we must begin to define the grades of waterproofing within the Building.

Table 2 of BS 8102:2022 defines four grades of waterproofing performance for below ground structures (600mm below ground or more) as follows;

• Grade 1a - Seepage(B) and damp areas(C) from internal and external sources are tolerable, where this does not impact on the proposed use of below ground structure. Internal drainage might be necessary to deal with seepage.
• Grade 1b - No seepage(B). Damp areas(C) from internal and external sources are tolerable.
• Grade 2 - No seepage(B) is acceptable. Damp areas(C) as a result of internal air moisture/condensation are tolerable; measures might be required to manage water vapour/condensation(D).
• Grade 3 - No water ingress or damp areas(C) is acceptable. Ventilation, dehumidification or air conditioning necessary; appropriate to the intended use(D),E).

Where A to E are referred to within the Grades above, please see below to expand the meaning of some of the definitions

A) The agreed grade should meet with the client's expectations for the intended use of the below ground space. Reducing the grade could increase the risk of not meeting the expectations of the client for the intended use of the below ground space.

B) Seepage (sometimes referred to as weeping) is defined as a low transmission of water through discrete pathways of a structure. If there is seepage, there is a possibility of mineral deposits forming.

C) Damp area is defined as an area which is slightly wet but no seepage

D) The scope of this document is limited to detailing the process and best practices that can be followed when creating a waterproof or water-resistant structure below ground. The additional considerations that are required to achieve the required environment are beyond the scope of this document.

E) See BS5454 for recommendations for the storage and exhibition of archival documents.

It is important to ask the question (as a CSSW designer), as to the intended use of the environment and to inform the client of the risk. Is it High End? What is acceptable to the client?

At Premcrete, typical environments we might see these Grades play out in, are as follows;

• Grade 1a - Low Grade Car Park with Surface Drainage. Premcrete would rarely, if ever, design for this grade, as allowing seepage, would assume no forms of waterproofing are in place.
• Grade 1b - Typical Car Park. We would usually expect to see a single form of waterproofing in this scenario
• Grade 2 - Plant room, Lift Pit or Medium Grade staircore. No seepage is expected by the client/building operator but condensation is OK. We would usually expect to use two forms of waterproofing in this case.
• Grade 3 - Bedroom or Office - No seepage, Dampness or Condensation permitted, and ventilation installed. Esentially this a completely dry and habitable environment and also would usually have two forms of waterproofing or sometimes 3 forms if the client has chosen to go belt and braces over and above the British standard or Warranty providers requirement.

It should be noted, that nowhere in the standards does it define how many forms of waterproofing are required for each grade of waterproofing. This is down to the Waterproofing designer to assess the risk, and specify, using the availible guidance within the standards, and also the conditions put forward by the Building Warranty provider, such as NHBC, LABC or Premier Guarantee.

BS 8102:2022 - Forms of Waterproofing

When specifying the inclusion of various forms of waterproofing, the British standards give particular emphasis to the requirement of Continuity of Waterproofing right up to the Damp proof course membrane, forming a 'tank'.

There 3 Types of Waterproofing to select from which are referenced in Table 1 of the BS8102:2022 as follows;

Type A Barrier Protection;

• Pre-Applied Bonded Sheet Membranes. These often incorporate a fleece or woven texture backed membrane to encourage the bonding of the membrane to the surface of the concrete. Some preapplied membranes include Combined Self healing & Fully bonded technology ie; Maxiprufe Plus or Combiseal. The bonding nature resists lateral movement and migration or tracking of the water, and therefore localising any defect which may occur.
• Post Applied Bonded or Self Adhesive membranes. These can be heat bonded, but are more commonly availible as a cold applied product, and self adhering, such as Hydroprufe 3000
• Bentonite Membranes - This is a somewhat traditional form of waterproofing and Premcrete, as a manufacturer, have decided to exclude this product from their range, and replaced it with a more advanced combined self healing and fully bonded membranes as cited above
• Liquid Applied Membranes ie: Epoflex MMA , Hydroguard PUA or Hydroprufe LG. These vary from Hand applied (brush or roller) self install products, to specialist hot and cold spray applied applications.
• Cementitious Slurry Membranes ie: Hydroseal FX or Hydrorend
• Cementitious crystallisation powders/slurries ie; Hydroseal PX
• Loose Laid Membranes and DPMs such as Hydroprufe DPM. When incorprated within a wider waterproofing design, Premcrete can extend the warranty to cover the whole substructure protection.

Type B Integral Protection;

• Water resistant concrete admixture or additive ie Hydrocrete, and ancillary water bars such as Hydrostop BR and Cemflex VB
• Concrete designed to less than 0.2mm crack width which as waterproof concrete in its own right, but typically won't come with a warranty from a supplier or manufacturer.
• Crack Induced concrete
• Sheet steel piles with welded clutches are recognised as a form of waterproofing, but aren't typically included in a warranty from a waterproofing supplier/manufacturer.

Type C Water Management & Other Drainage methods;

There are two ways of introducing drainage.

The First, is to drain and manage the internal ingress of water into the structure, commonly known as Type C Waterproofing, with the use of the following;

• Cavity Drainage membrane systems, including perimeter Drainage channels, and Sump Pumps ie: Hydroflow HP

The Second is where external water needs to be removed, either temporarily or permanently. These include;

• Site drains (To remove surface water during construction)
• Site dewatering (To remove Groundwater during construction)
• Land Drains to permanently remove occasional groundwater.
• Surface drains and surface drainage membranes ie: Hydroflow HM to permanently remove surface water

BS 8102:2022 - Groundwater

Whilst it's essential to comprehend the risk associated with the water table before preparing a detailed waterproofing design or issuing a CSSW report, it's essential to recognize that a waterproofing designer operating in compliance with BS 8102:2022 is expected to assume a water table to the full height of the basement (even within the Capillary zone, if this applies) partcularly if the soil survey is either lacking or inconclusive

Even when the site investigation indicates dry conditions, the risk of some waterlogging in the future should be assumed. Groundwater requires time to drain away in the most permeable of soils, and this can result in limited pressure periodically coming to bear against the structure.

BS 8102:2022 - Defects

BS 8102:2022 acknoeldges that whilst the ideal waterproofing solution would be defect-free; it should however, be taken into account that defects might occur in the waterproofing, which then, if subjected to water pressure, could result in the required internal Grade of peformance not being achieved.

These defects could be manifested as follows:

• Defects owing to design
• Defects owing to poor workmanship
• Inappropriate use of the materials being used and defects owing to the specific properties of the materials being used
• Defects caused by follow-on trades and site operations.

The guidance states that construction methods, installation personnel, materials and protection thereof, used to realize the design should be such that the defects in all of the above are avoided. The potential for defects should be recognized and catered for in the design.

Contingency planning for dealing with any localized defects or system failure that arise should be included as part of the overall waterproofing design for the structure.

In either case, the issue of repairability should be taken into account and the form and feasibility of remedial measures after completion of the project with finishes in place assessed.

So if this is the case, we are expected to consider the Consequences of Defects.

BS 8102:2022 - Consequences of Defects

When designing to compensate for defects, here are some pratical examples of how we can approach this with the Types of Waterproofing known to us;

Type A & B Waterproofing

• Ensure all areas remain accessible for remedial work.
• Remove the risk of groundwater pressure

Type C Waterproofing

• As long as the drainage has been adequately designed and installed, all that is required is to ensure that drain runs and sumps/pumps remain accessible for maintenance.

BS 8102:2022 - Design Philosophy

The design philosophy embedded in BS8102:2022 is based on a comprehensive approach to managing water ingress in below-ground structures. The guidance is rooted in the precautionary principle, emphasizing that at any point during a structure's lifespan, groundwater presence is a possibility and therefore should be anticipated in the design phase. This implies that the design should not only prevent water ingress but also be resilient to it when it occurs.

Furthermore, it is recognized within the standard that no system is infallible; defects can and do occur. As such, the design should take into account the potential for flaws within the waterproofing system. This includes an understanding of the types of defects that might occur, their possible causes, and the likely ingress points for water.

Crucially, the standard advises that the consequences of any such defects must be thoroughly evaluated. The design process should consider the impact of water ingress due to these defects, particularly in relation to the intended use of the structure. Where the consequences of defects could be severe, such as flooding which might lead to structural damage or loss of use of the space, the design must include strategies for remediation.

BS8102:2022 guides that remedial measures should not only be effective but also feasible. This means that the design should allow for the repair or upgrade of the waterproofing system without excessive cost or disruption. It suggests that an approach for regular maintenance and inspection may be necessary to ensure ongoing integrity of the waterproofing measures.

In essence, BS8102:2022 promotes a design philosophy that is preventative, realistic, and adaptive. It advocates for waterproofing solutions that are robust, inspectable, maintainable, and repairable, ensuring that the below-ground spaces remain usable and protected throughout their designed life expectancy.

To summarise....

For a basement waterproofing design to comply with BS 8102, the following points must be considered:

• It must be assumed that groundwater will be present at some time of the structures life
• It must be assumed that defects will exist within the system
• The consequence of those defects must be considered
• If the consequence of those defects is unacceptable (such as flooding) then remedial work to the installed system must be feasible

BS 8102:2022 - Risk Assessment

A risk assessment should be carried out to consider the long term water pressures, the effects of water penetration (i.e. is it acceptable or not?), and the use of external drainage.

The risk assessment table in BS8102:2022, known as Table 1, is a fundamental component of the standard that provides a framework for assessing the potential risks associated with Water Table and the likelihood of water ingress in below-ground structures, and then providing guidance as to which Types of waterproofing are acceptable in those scenarios.

BS 8102:2022 - Drainage

Reference is made within the text of BS 8102, (and also shown on diagrams) to the use of subsoil drainage. It acknowledges that, wherever possible, hydrostatic pressure should be removed or reduced, by the appropriate installation of land drains (ideally Maintainable) and/or drainage membranes.

The reference to the use of subsoil drainage reflects a proactive approach to water ingress management in below-ground structures. Here are some elaborations to consider on the key points mentioned:

1. Hydrostatic Pressure Management: The standard stresses the importance of reducing or eliminating hydrostatic pressure against the structure. Hydrostatic pressure is the force exerted by water in the soil surrounding the structure. If not managed, it can lead to water ingress through walls, floors, and joints.
2. Installation of Land Drains: The implementation of land drains is recommended as a method to divert water away from the structure. Properly installed land drains can channel water to a suitable drainage point, thus reducing the amount of water that comes into contact with the below-ground structure.
3. Maintainable Drainage Solutions: BS 8102:2022 advocates for the use of maintainable drainage systems. This implies that the drainage solutions should not only be effective at the point of installation but also throughout the life of the structure. Maintainable systems allow for inspections, cleanings, and repairs, ensuring long-term effectiveness.
4. Drainage Membranes: The use of drainage membranes is another solution mentioned in the standard. These membranes are designed to provide a path for water to travel along, effectively bypassing the structure and thus preventing water from exerting pressure on the structure itself.
5. Integrated Waterproofing and Drainage Design: The standard underscores the need for an integrated design approach that combines waterproofing and drainage. This means that the waterproofing measures and drainage systems should be designed to work together to provide comprehensive protection against water ingress.
6. Site-Specific Solutions: It is also implied that subsoil drainage solutions need to be tailored to the specific conditions of the site. Factors such as soil type, local water table levels, seasonal variations in water levels, and the overall design of the structure must be taken into account.
7. Compliance with Regulations and Standards: Any subsoil drainage system should comply with local building regulations and standards, ensuring that it is not only effective but also legally acceptable.

In summary, BS 8102 acknowledges that managing water at the source and diverting it away from the structure through well-designed, maintainable subsoil drainage is a fundamental aspect of protecting below-ground structures from water ingress.

BS 8102:2022 - Condensation

BS 8102:2022 emphasizes the importance of appropriate ventilation, either through natural or mechanical means, to mitigate the risk of condensation in below-ground structures.

It underscores the necessity of a ventilation strategy that aligns with the intended use of the space to maintain air quality and prevent moisture accumulation.

Conclusion

Well thats a wrap on my second article during my CSSW training stint, I must say that although, that was rather tedious (and probably too lengthy), it certainly helps to put 'pen to paper', so to speak!

Lets hope my notes provide any interested readers with at least one further insight on the subject of keeping basements and below ground structures dry!!

Your feedback is always appreciated (I would hate to know how many spelling and grammar mistakes are lurking above!), and meanwhile if you need help on a specific project, please do get in touch [email protected] or check out the websire at www.premcrete.com. I'm getting to like these early engagement challenges!

Over and out.