NHBC Standards 2006

4.6 Vibratory ground improvement techniques


4.6 - S1Sitework standard
4.6 - S2Site supervision
4.6 - S3-S4Verification of completed treatment

4.6 - S1
All sitework shall:
(a) meet the Technical Requirements
(b) take account of the design
(c) follow established good practice and workmanship

Sitework that complies with the design and the guidance below will be acceptable for vibratory ground improvement.


4.6 - S2
The Builder shall ensure that the Engineer visits the site and provides competent supervision throughout the ground treatment process

The Engineer should provide competent full time site supervision throughout the period of the ground treatment process.

Some aspects of sitework may be the responsibility of the Engineer or his representative, or of the Specialist Contractor, rather than of the Builder.

Items to be taken into account include:

(a) location, depth and alignment of columns

Supervision should be provided to ensure that:

  • the minimum required depth of the stone columns is achieved, and they are correctly located. The Builder should provide sufficient profiles to enable locations to be checked
  • the stone columns are located either centrally under the foundations they are to support or in the predetermined staggered arrangement, at a maximum of 2 metres centre to centre and at the intersection of adjacent reinforced concrete strips
Stone columns located centrally in foundations or in predetermined staggered arrangement

  • missing stone columns are replaced
  • stone columns which are misaligned by more than 150mm in any direction are replaced
Stone columns that are misaligned by more than 150mm in any direction are replaced

  • a check on the location of all stone columns is made by the Engineer's representative prior to the specialist plant leaving the site.

(b) unforeseen circumstances

Allowance should be made for:

  • unforeseen changes in the site conditions, or trends which may affect site conditions. Changes should be recorded and reported to the Engineer immediately they become apparent
  • changes in the anticipated depth of the compaction point in excess of 25% should be recorded and reported to the Engineer and Specialist Contractor as soon as possible but no later than the end of the day on which they occur
  • variations of over 50% in the quantity of backfill used in compaction points of the same length. Variations should be recorded and reported to the Engineer and Specialist Contractor at the end of the day on which they occur
  • unforeseen obstructions requiring either local removal and backfilling prior to treatment, or realignment of, and additional columns, coupled with local amendment of foundation design
  • the effects of any of the above on the final efficiency of the treatment. These are to be fully considered by the Engineer and the Specialist Contractor. The Builder and NHBC are to be advised immediately about proposed remedial measures.
Unforseen circumstances


4.6 - S3
The Engineer shall require the Specialist Contractor to verify that the ground treatment is satisfactory

Items to be taken into account include:

(a) suitable testing

Tests should be carried out to establish the degree of ground improvement, its load-bearing characteristics and settlement potential.

The types of test that can be used are described in the following clauses. The Specialist Contractor should predict the results from his experience of work on the type of ground, prior to the test taking place. Prediction of the results and the degree of tolerance within those results is to be agreed with the Engineer prior to testing, and compared with the test results.

If for example a threefold improvement were predicted and only a twofold improvement achieved, this could mean that the ground was different to that indicated by the investigation, or that the treatment carried out differed from the specified treatment. In such a case, further investigation would be necessary.

Tests on ground containing clay soils may need to be delayed for a few days after the completion of treatment to allow excess pore pressures to dissipate.

The Engineer may choose any combination of the following tests:

  • 600mm diameter plate tests or dummy footing tests using long stiffened steel plates
  • mini zone test
  • in situ test
  • trial pits
  • zone test.


This test will not determine the design but will allow for an assessment to be made of the workmanship on the stone columns. Plate tests should be carried out on stone columns or treated ground at a frequency of at least one test per day per rig.


A mini zone test can be used as a limited substitute for zone tests. The test should be applied to at least two stone columns and the area of foundation which they support. The load may be applied through a rigid beam or stiffened plate using skips or other known loads arranged to give a uniform distribution of the load.

To be useful, mini zone tests should be continued for sufficient time for creep behaviour to be quantified and allowances for this time should be made in the overall project programme.

Mini zone test using skips


Where vibration will improve the ground itself, eg granular materials, then in-situ testing is appropriate. The improvement in density of deep fill (greater than 5m) should be checked in this manner. The improvement can be assessed when the test results are compared with the in-situ test results recorded during the pre-treatment investigation.


Trial pits can be excavated around trial stone columns to prove that they are fully formed and to the required depth and diameter. This is a destructive test and allowance should be made accordingly.


An isolated pad or strip footing is used, and up to 8 stone columns and the intervening ground can be tested. Loadings, which must simulate the dwelling loads, are held for 24 hours at pre-determined stages to examine creep behaviour.

Zone test

(b) written confirmation of completed treatment

On completion of the treatment the Engineer should:

  • from the results of the tests carried out satisfy himself that the treated ground has achieved the anticipated condition assumed in his design
  • once satisfied with the effectiveness of the treatment in relation to the design, advise the Builder and NHBC accordingly in writing
  • advise the Builder of any special precautions which should be taken for the positioning of services both beneath the dwelling and adjacent to it.

(c) record of the work

A comprehensive record of all works including information concerning the treatment, depth of fill, volume of stone used, on-site changes and all other relevant information, should be made available to NHBC.

4.6 - S4
The Builder shall ensure that treated ground is not disturbed by subsequent excavations

Ensure that the minimum clearance between excavations and foundations is not less than the depth of excavation minus the depth of the structural foundation.

Influence of drainage and other service trenches on the stability of the complete design