Fact: Water is Heavy.... (Tanking Structures & Loading).

Floating structure - swimming pool.

Floating structure - swimming pool.

So I am reasonably sure that you are well aware of this, but do you understand the implications in relation to basements, cellars and waterproofing/tanking systems?

From what we see, many do not appreciate the forces which may be involved, including in some cases those that should certainly know better.  Lets look at an example.

We were asked to inspect a stone built cottage with a cellar beneath which suffered periodic issues of flooding in association with prolonged and heavy rainfall.

The homeowner advised that when sufficiently heavy rainfall occurred, a depression at the end of the garden filled with water to the degree that his children could use an inflatable dinghy and row around.  At the same time, the cellar would fill with water to a depth of approx. 400mm, this giving indication of the degree of saturation in the surrounding ground.

Flooding prevented any real use of the space and so a contractor was sought and found who would remedy the issue.  A specification was put forward and agreed, to install a multi-coat cementitious tanking render to the walls and floor.

This form of waterproofing functions by seeking to totally block water out of the internal space, by providing a physical barrier to water.  Like with any 'tank', holes or defects within it will allow passage of water, and so such systems must be perfect or free of any such defect through which water may pass.

The system was installed, the garden filled with water once again, and so did the tanked cellar.

At this point the homeowner called the installer, who returned along with the tanking manufacturer.  A solution was devised and subsequently installed.

The garden filled with water once again, and so did the tanked cellar.

At this point I understand that the installing contractor was no longer trading, and we were contacted with a view to providing advice and an appropriate solution.

Upon attending the property, I asked a various questions including: Was any investigation undertaken to identify the floor construction, before specifying cementitious tanking?  Answer: No I don't think so.

(Forgive the Jargon) Obvious case of the structure not being strong enough to resist the loads imposed upon it by hydrostatic pressure, resulting in floatation of the floor structure and differential movement & cracking at the wall floor junction, resulting in water penetration....

In simple terms, water filled the ground around the tanked 'box' to the degree that the box tries to 'float'.  In a structure of this type/age, where a minimal depth floor construction likely exists, spanning in-between the retaining walls; the floor represents the weakest element, and when water pressures upward upon it, the floor may move, typically causing cracking at the wall/floor junction, and failure in the tanking.

When I inspected, this is what I could see:

Flexible tanking applied at wall/floor junction.

A flexible tanking product had been applied to the wall floor junction in areas, localised patch repairs had been undertaken to cracking in the floor at the door threshold, and a sump pump was included in one corner of the cellar space.

Evidently, the problem of differential movement and cracking had been identified, with a flexible product being included to address this, albeit unsuccessfully.  The inclusion of a sump pump in a solid chamber (so would not de-water beneath the structure) had no tangible benefit as only accepted water once penetrating past the tanking.

The problem was addressed by installing a maintainable drainage channel system at the wall floor junction, in a concrete chase, linked to a triple pump sump pump system in a solid liner.  This design still makes use of the existing tanking barriers to the walls and floor, however relieves the pressure by collecting groundwater, this solving the problem.

But, my point is that one should consider the nature of the structure and the relationship between it and any proposed waterproofing system.  In this case, they clearly did not consider this appropriately.

Lets look at some other cases where the forces of hydrostatic pressure have come in to play:

This one (supplied by Triton http://www.triton-chemicals.co.uk/) shows an adhesive bitumen sheet membrane tanking system applied to the internal face of a retaining wall, with a block wall constructed in front of it.  This could not be tied back as this would obviously puncture the tanking, and when water pressured, causing the bitumen membrane to bow inwards, it pushed over the inner leaf:

Inner block wall pushed over by hydrostatic pressure.

This is a photo from a project which suffered the same issue, showing water causing the tanking to bow in:

Bitumen sheet membrane 'bows' in with hydrostatic pressure.

Bitumen sheet membrane 'bows' in with hydrostatic pressure.

This is the same project with a before/after shot, note that the inner block leaf was taken down where exposed visible because of health & safety concerns:

Bitumen tanking pushes over inner leaf (left), after installation of Trace system (right).

Finally, the extent of flooding present within this basement caused the insulation and screed (containing under-floor heating) over to float!

Screed laid on insulation floats as a result of flooding.

In closing, consider the nature of the structure, or call Trace and we will consider this and all other appropriate factors on your behalf.

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