Vibratory ground improvement techniques

4.5.7Acceptable methods

Vibratory ground improvement techniques shall only be conducted using methods that are appropriate to the ground conditions and acceptable to NHBC.

There are two main vibratory methods commonly used in the UK. These are known as the ‘dry bottom feed’ and ‘dry top feed’ methods. A third method, infrequently used in the UK and known as the ‘wet bottom feed’, is also acceptable to NHBC.

Dry bottom feed method

The dry bottom feed method is adopted in weaker soils or situations where there is a high water table and the bore hole is liable to collapse between vibrator insertions.

The vibrator penetrates by its mass, air flush and vibration. At design depth, the stone is introduced via a hopper into a pipe fixed to the size of a vibrator. The stone, generally of 40mm size, exits the pipe at the tip of the vibrator and discharges in to the bottom of the bore hole. The stone is then compacted into the surrounding soil by repeated withdrawal and insertion of the vibrator.

Dry top feed method

The dry top feed method is only used on cohesive soils where the bore hole can remain open. The vibrator penetrates the weak soil, or fill, by its mass, air flush and vibration to form a bore hole.

Once refusal or design depth is reached, the vibrator is removed and stone fill introduced into the bore hole, the ‘charge’ is generally 500-800mm deep. The vibrator is re-inserted and ‘packs’ the stone into the surrounding strata. Successive charges of stone are added and compacted, bringing the column up to working level. The stone grading is generally 40-75mm.