Water jetting is a technique that aids pile installation and is generally associated with silent and vibration free piling methods.
Dense granular materials and all clays above firm are difficult to penetrate by purely jacking the pile. Other techniques are employed to reduce the driving force required, the primarily methods being pre-augering and water jetting.
High pressure water is delivered to the pile toe via a 50mm diameter jetting lance which is temporarily secured into the pan of the pile by a disposable clip and a securing weld. A nozzle is fixed to the end of the lance just above the end of the pile. Once the pile is ready to drive water is delivered by a purpose build pump through the lance to the toe of the pile. In coarse granular soils the soil is disturbed locally at the pile toe by the water and reduces the driving resistance thus aiding pile penetration. Jetting in fine to medium granular soils generates a local increase in pore water pressure which reduces particle friction. In clays and silts the water acts as a lubricant.
The flow is monitored and regulated by the operator. Water flow is maintained throughout the driving cycle to prevent damage to the nozzle. A minimum supply of 75 gallons per minute is usually required, which equates to a standard hydrant supply. This is discharged into a skip from which the jetting unit draws it.
Under normal circumstances water arising from the jetting process dissipates into the surrounding strata, but when the ground is all clay it will tend to return to the surface, and will need to be disposed of.
When water supply is a problem bowsers can be used to provide the water. Water can be reused if it can be captured after use, which is possible if the installation is in pure clays and a lead trench is dug to contain the water as it returns to the surface. Stilling tanks will have to be set up to remove the fines in the water as they will block the pump filters and cause damage.
In granular soils the water jetting disturbs the soil but it does reconstitute itself once jetting ceases. This is demonstrated by reaction being generated on the piles once driven.
Water jetting can be used close to buildings but monitoring will be required as ground settlement could occur due to the loss of fines from the soil. Care must be taken when adjacent to poorly constructed basements which could be subject to water penetration. In certain circumstances water jetting can induce heave of the soil due to pressure build up when jetting into granular soil with a impermeable cap above, but we have the expertise to analyse the soils information and advise you of the associated risks.