Steel-cased driven mini piles are often a very effective alternative to open bored mini piling in London Clay and other clay ground conditions where shrinkage and heave present actual or potential problems.
While many mini piling contractors have probably used the system in this situation over the years, from my experience it appears it is not generally considered as a candidate by engineers preparing specifications for piling works.
Steel-cased driven mini piles provide advantages in many situations and I believe these merits should be considered alongside traditional methods at the outset in the planning of a contract.
The system can also be a good hedge bet against problems that may be encountered later on site, when unstable or uncertain ground conditions are involved. There have been several instances where steelcased mini piling has provided a rapid alternative when open bored holes have collapsed or flooded and another approach has been needed.
Traditionally, open bored piles are used to counter clay shrinkage and heave. However, the use of these foundations is only really feasible if no groundwater is encountered during boring and the bored holes do not collapse or neck when the augers are extracted.
There are two commonly specified alternatives where open boring is not feasible. The first is hollow stem auger piles, where concrete is injected down the hollow stem of the auger as it is withdrawn. In mini piling, concrete is normally replaced by grout.
The second method is bored piles with temporary casing. Here the bores are drilled and cased to seal off perched groundwater inflow and concrete is placed as casing is withdrawn.
Hollow stem auger piles require a high degree of skill on the part of the site operatives and a high level of quality control. While integrity testing can be carried out a few days after pile installation, negative results would prove costly.
The problem with bored piles using temporary casing is the depth at which the casing can be installed in clay strata, which can often only be determined on site. The depth of installation depends on the size and power of the rig used to drill and case, which is often limited by site access conditions and headroom, particularly if working inside people's homes.
In the steel-cased driven mini pile system, pile holes are pre-bored - generally to 3m - using an auger drilling machine (the hole diameter being larger than the diameter of the casing) and a pile sleeve is inserted into the pre-bored holes for highly effective heave protection.
The sealed steel casing is inserted inside the sleeved hole and the casing is driven to a set using a drop hammer piling rig. It is quite common to achieve pile depths of 9m-10m, even driving into firm to stiff clay.
The system normally employs 150mm diameter and 220mm diameter piles and can be installed inside domestic properties. The gap between the heave sleeve and the casing can be filled or left open. In the latter case this provides full heave protection to the cased pile. The piles must be considered as columns over the sleeved length and designed accordingly.
The advantage of the steel-cased driven mini pile system is that a fully concreted pile is guaranteed.
If there is any doubt as to the set and pile working load, then the driven casing can be tested using dynamic pile testing techniques to predict the load carrying capacity.
This can be carried out before concreting, and on the same day as the pile is installed, thus reducing possible delays or costly remedial works.
Some engineers have reservations about driven piles because of possible vibrations resulting in damage to buildings close to piling works. This has not been a problem as pile driving starts 3m below ground level.