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talkingpoint

Are secant piles walls as watertight as diaphragm walls? Duncan Nicholson discusses his experience.

Over the last 10 years secant pile walls have captured a large share of the UK basement wall market from diaphragm walls.

The watertightness of diaphragm walls was discussed in a paper by Puller in 1994. However, very little has been reported on leakage through vertical joints in secant pile walls.

In my experience, most secant pile walls on typical London sites meet watertightness requirements without needing significant grouting to the joints between piles. At these sites the top of the London Clay was typically about 12m deep. A perched water table of less than 3m head was encountered at the base of the overlying Terrace Gravel.

However, I have encountered four sites with leaky secant pile walls close to the River Thames.

At these sites, more than 10m head of water was present in the Terrace Gravel. Am I alone in this experience?

The ICE Specification for piling and embedded retaining walls (1996) recommends the same water tightness requirements for both diaphragm walls and hard/hard secant pile walls, namely: 'visible running water leaks are found which would result in leakage per individual square metre in excess of that stated in the particular specification'.

This encourages project specifications to quantify permanent leakage rates. However there is little published data to define these rates. Instead, it is normal to specify watertightness of exposed vertical joints on both diaphragm and secant walls in terms of 'damp, no running water' or similar phrases.

On site, 'running water' can be difficult to assess. A useful test is to place your hand on the wet patch and observe if a bead of water flows down your arm to your elbow.

I believe the specified long-term water tightness of a diaphragm wall (assuming joints at 5m centres) can be related to a 'mass wall permeability' of less than 10 m/s. A hard/hard secant pile wall would have a mass permeability of less than 5x10 m/s. This applies to cut-off walls to 15m depth.

Once the perimeter cut-off wall has been installed, a pumping trial can be performed.

The mass wall permeability can be used to assess the flow into the basement box before starting excavation.

My experience is that diaphragm walls built with reasonable workmanship generally achieve the specified watertightness. Water bars in the diaphragm wall stop-ends generally improve this situation. However, on four UK secant pile wall projects high leakage rates have been encountered below the proposed basement formation level and through the exposed walls.

At these sites permeable Terrace Gravel was encountered to 15m depths with a water table at 4m depth. To form a hydraulic cutoff, the perimeter walls were embedded in the underlying clay. Pumping tests within the basement box showed a wall mass permeability of about 10 -7 m/s. Flow rates into two of the basements were about 150 to 300m 3per day. This was 20 times the design value.

Two of the basements were excavated to 12m and about 90% of the exposed secant pile wall joints had to be grouted to achieve the specified 'damp, no running water' watertightness. The secant pile overlaps at basement formation level were within the verticality tolerances.

Where a permanent under-slab drainage has been designed, the project team is faced with grouting outside the wall with uncertain reductions in leakage rate, or the client has to accept the need for continual pumping costs and disposal costs.

Discharging low strength trade effluent to a Thames Water sewer costs between 16p and 23p per m 3. Over a 50-year design life this may result in disposal costs of £0.5M.

Where a 'watertight' base slab has been adopted, the client and the design team face the normal contractual problems of splitting responsibility between the wall contractor and the base slab contractor at the wall to slab joint.

Puller reports similar problems for diaphragm walls and I am not aware of a way of isolating this responsibility. His paper highlights problems with:

vertical joints between panels

base slab to wall joints

capping beam to wall joints

I believe secant pile walls are inherently less watertight than diaphragm walls. They can be made watertight on the exposed faces within a basement by grouting the joints but the section of wall below formation in permeable soil is very difficult to grout. In these cases 'watertight' slabs should be used to avoid long term pumping costs.

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