During the construction works of a dam, for an important project on the A14 Motorway in Italy it was found that the ground comprised of: gravel, loose sand and silty clay (at the bottom of the piles, 25m long) and the sand was below the water level.
At the beginning the contractor did not want to use bentonite and it was agreed to use only the sleeved CHS steel pipe as formwork. Due to the fact that the layer of gravel was very loose and did not allow the sides of the holes to stand, it emerged the necessity of consolidation by grouting using a pipe system. Unfortunately the injections were not successful because the voids of the gravel were too large and the bentonite flowed away, plus the bentonite formed a mixture of concrete-sand within the layer of sand and silt and the CHS pipe remained stuck when it was driven into the ground.
This sort of mixture is easy to recognise because when digging the ground the mixture comes up to the hole and solidifies. This means the procedure should be stopped otherwise the pipe gets stuck into it and it is nearly impossible to be removed.
Conclusion
It is good thing to use bentonite when we are not sure the sides of the hole can stand. It is better not to inject into layers of too large gravel because we cannot predict if the procedure is successful; generally the ground is never tested after the consolidation, but it should be done.
It is preferred to adopt the combined use of bentonite and steel sleeved pipe to avoid sticking, in this case, do not drive the pipe more than 1m into the cohesive strata.
Augusto Nasini, CEng MICE, ICE Member in Italy