Site Improvements

Vibratory ground improvement

This process of foundation construction may not suit every ground type and generate more risk than other types of ground improvement methods. Builders should engage experts and exercise caution if using vibro compaction.

Additional ground improvement techniques for hazardous ground comprise: driven piles, bored piles, ground beams and the use of suspended ground floors. All of these techniques must be designed and supervised by an independent engineer.

Ground Stability - Technique explained

This is a technique where stone columns are fitted to reduce settlement, improve the load bearing capacity and provide an adequate bearing strata for foundations. Where the method is to be used by builders we would strictly recommend that the following requirements should apply:

The builder engages an independent engineer. The engineer appointed must be in private practice, be qualified by examination in civil or structural engineering and possess professional indemnity insurance. The engineer will decide:

  • The suitability of dwelling design, dwelling layout and foundation design

  • The suitability of vibratory ground improvement technique – possibly in conjunction with a specialist contractor's engineer.

  • The extent and nature of ground hazards.

Written Confirmation and Agreement

Written confirmation should be obtained by the builder from the engineer and specialist contractor that the site is appropriate for the system chosen. The contractor and engineer should agree who is accountable for testing and procedures and decide on the categories and regularity of testing. Competent supervision should be provided by the engineer. The engineer should confirm to the builder when tests are carried out that they are satisfactory. Features such as design of foundations, drainage layout and design of suspended floors are the independent engineer’s responsibility.

Investigations and Completion

A specialist site investigation firm should be engaged by the independent engineer to carry out site investigations to evaluate site conditions and to explore for any harmful wastes or gas content. After installation of the stone columns, it is vital that other service trenches or drainage are not dug adjacent to the columns. Such action might remove adjacent support to the columns.

Confirmation of completion will be required by the certifying agent or Insurance underwriter both from the engineer and the vibratory ground improvement contractor that the foundations are acceptable to ensure the dwelling’s structural stability in view of the ground conditions present.


Minimum Floor Levels

If the highest possible level of the water table is within 250mm of the lowest floor in a dwelling or groundwater may pose a threat to the stability of the building, either the ground covered by the building should be gravity drained or some measure should be taken to safeguard the building.

Steps which could be taken include:

  • Raising the lowest floor level,

  • Incorporation of tanking in the floor and walls or,

  • In extreme cases, not building on the site.

Diagram A11: Distance from water table to lower floor level Diagram A23 - Ground protection within root protection area

It should be noted that all foundations should be designed by a qualified engineer and in accordance with the relevant codes of practice.