Soakaways and Basements, not Always a Good Combination.

Soakaways are a means by which surface water (roof/rainwater) can be discharged into the ground.  These have a range of benefits, in that they deal with surface water where there is no access to mains drains, they place a lesser demand on those systems if they are present, and they can provide a saving on your utility bills versus discharge into a mains system. They are commonly employed within new-build developments and we often see them.

The design of soakaways is a science in itself and should be undertaken by a suitably qualified Civil Engineer. It is not the design of soakaways that I want to comment upon, but their use in association with basement waterproofing systems, and implications upon the waterproofing where soakaways do not function as intended.

There is a good page of information on soakaway design/consideration here (Wilsham Civil Engineers); they also comment on soak-away failure where these are either incorrectly designed OR even in some cases where they are correctly designed. The recommendation is that informed risk based design decisions should be made by experienced designers – good advice!

Because I’m often asked to deal with problem basements, you can imagine that I’m seeing those issues where reliance has been placed on soakaways, but they have not performed, causing issues as a result.

For example, this was a case where an external adhesive bitumen sheet membrane had been employed to protect a reinforced masonry retaining wall/reinforced concrete raft structure, with land drains included with a view to de-watering the ground so that water pressure would be prevented/minimised.

Groundwater penetration at the junction of the added escape well and the existing structure, after an external tanking repair had been implemented.
Groundwater penetration at the junction of the added escape well and the existing structure, after an external tanking repair had been attempted.
Water collecting within the screed at low level.
Water collecting within the screed at low level.

They had to retrofit an escape-well at the request of Building Control, meaning that the existing tanking had to be cut and spliced to, which didn’t help; however the site was dense clay and the land drains linked into a soak-away.

The basement space suffered water penetration in association with heavy and prolonged rainfall, and the consensus we reached was that the soak-away was reaching capacity, with this causing water to surcharge up the land drains to pressure upon and penetrate past the external tanking system.  This because it was only an intermittent issue with very heavy rainfall.

The other item of interest, which is typical in these scenarios is that rainwater collected by the substantial roof area, discharged into gullies draining into the soak-away.  In essence the design was such that when the soak-away did not function, the roof acts as a mechanism to collect water, which was then channeled directly to the ground around the basement, this being exactly what we don’t want, and especially undesirable with a tanking system which is unlikely to be perfect / free of defects and therefore reliant on effective land drainage.

The main part of the solution was to to install internal cavity drainage waterproofing, but we also installed a single mains powered pump in the soak-away (accessible concrete ring chamber) which would activate just below the land drain inlet level and pump to daylight  Therefore if the soak-away did start to fill, at least there was a degree of mitigation – note that the internal cavity drainage provided the primary protection.

This next example is a bit of a silly design but not necessarily unheard of.

The properties were basement apartments, in a clay site, where the form of waterproofing was internal cavity drainage.  This method is entirely reliant on the continuous removal of water, in that water must be removed via a drainage channel system, so that it does not build-up/stand and pressure on the cavity drainage membranes installed above the channels – if water does not pressure it will not ever come past them, so reliable drainage equates to a dry space.

In this case the internal drainage channel system discharged to external soakaways in clay.

The implication of this design is that there was a direct route for water outside of the basement, into the basement.  Whether the soakaways functioned or not, an open penetration through a basement structure is never a good thing.  Residents were getting out of bed in a morning and into several inches of groundwater, imagine that!  Obviously not good:

General area post strip out.
General area post strip out.
Large holes in the ground retaining structure allow free penetration of water, meaning that the basement acts like a ‘sump’.
Spot the drainage channel running out through block on edge underpinning, just to left of soil stack.

When I worked for Basement Systems over in Connecticut USA briefly post-university, one of the sayings over there was that ‘stupidity in construction is digging a hole in the ground and expecting it not to fill with water’. Obviously there is a bit more to it than that, however whoever designed the system in question, probably would have benefited from such advice!

Sites do not necessarily need to be clay either, there is an example in this post of issues with a tanked basement in a limestone site, and while I don’t categorically know if the land drains in that case relied on soak-aways, they certainly assumed incorrectly that the ground would accept water and prevent pressure in the process.  Most people/deigners would assume imestone to be very well drained because of the fractured structure of this rock, but as per BS8102 you should always assume pressure.

There are examples where you know a soakaway isn’t going to cause an issue, such as sloping sites where the top of the soak-away is below the invert level of your land drainage, but apart from this and in summary, where habitable space is concerned and the consequences of failure are typically dire, I wouldn’t make use of any system which is totally reliant on drainage, where that drainage subsequently links to soakaways.

Having said that, I wouldn’t design systems for habitable space that are 100% reliant on land drains irrespective of where they drain to.  Call me paranoid but this is after 10+ years of dealing with tanking systems where pressure came to bear (therefore ineffective land drainage) with the result being penetration.

There are better ways !


3 thoughts on “Soakaways and Basements, not Always a Good Combination.”

  1. James. An excellent paper on the subject. My thought has always been ignore any internal gully/drain and put a sump pump in. Design out any potential problems at the design stage.

    Liked by 1 person

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