NHBC Standards 2006

4.6 Vibratory ground improvement techniques


This Chapter gives guidance on meeting the Technical Requirements and recommendations for vibratory ground improvement techniques.


4.6 - D1Design standards
4.6 - D2-D3Statutory requirements and other standards
4.6 - D4Hazardous ground
4.6 - D5Notification
4.6 - D6Site investigation
4.6 - D7Suitability of ground conditions
4.6 - D8Confirmation of suitability of proposed treatment
4.6 - D9Compatibility of layout and design for the treated ground

4.6 - D1
Design shall meet the Technical Requirements

Design that follows the guidance below will be acceptable for foundations on ground improved by vibratory techniques.


4.6 - D2
Design shall comply with statutory requirements

Design should be in accordance with relevant Building Regulations and other statutory requirements.

4.6 - D3
Design shall follow relevant Standards and Codes of Practice

Relevant British Standards and Codes of Practice include:

BS 5930Code of practice for site investigations
BS 8004Code of practice for foundations
BS 8110Structural use of concrete
BS 10175Investigation of potentially contaminated sites - Code of practice.


4.6 - D4
The design of foundations shall be undertaken by an Engineer and take account of the characteristics of the site, its ground and any hazards

The foundation design should be carried out by an Engineer in accordance with Technical Requirement R5 - see Chapter 1.1.

In this Chapter, the term "Engineer" means an engineer who is independent of the specialist contractor responsible for the vibratory ground improvement techniques.

Details of ground hazards to be taken into consideration are given in Chapters:
4.1 'Land quality - managing ground conditions'
4.2 'Building near trees'


4.6 - D5
NHBC shall be notified before work starts on site

NHBC Rules state: "If a Home is to be constructed on a Hazardous Site you must before making Application for Inspection notify the NHBC in writing of the particular hazards which arise. You must do this at least 8 weeks before work begins on the site."


4.6 - D6
The Engineer shall commission a site investigation and advise the interested parties

The site investigation should take account of:

BS 5930Code of practice for site investigations
BS 10175Investigation of potentially contaminated sites - Code of practice.
Chapter 4.1 'Land quality - managing ground conditions'

The site investigation should at least determine:

  • the depths and properties of the natural materials under the site, including the presence of caves, workings, or natural phenomena such as rocks or soils which dissolve or erode when exposed to the passage of water. The Engineer should establish the scope of, and supervise, the site investigation, taking account of the findings of the desk study. Data for comparison with post treatment properties should be established
  • the extent and nature of any areas of filled ground on the site, including:
    • the proportions and distribution of constituent materials
    • the state of compaction of the fill material throughout its depth
    • the grading and particle size distribution of fill materials
    • the potential for gas generation from fill materials
    • the potential for spontaneous combustion of fill and/or natural deposits
  • the presence and extent of any existing or redundant services and drains, and what information is available regarding the extent and nature of the backfill to the excavations
  • the presence, level and nature of any ground water, and if it is likely to rise and cause heave or collapse by saturation
  • whether the site has been previously occupied by any structure, and whether these structures have left any potential underground obstructions or hardspots, eg basement walls, floor slabs etc
  • whether there are any contaminated substances or gases present or suspected.

The Specialist Contractor should be involved in the investigation.

The results of the investigation should be sent to NHBC.


4.6 - D7
The ground shall be suitable for vibratory ground improvement

The Engineer should assess the ground and be satisfied that it is suitable for treatment. Vibratory ground improvement techniques suitable for various ground conditions are detailed in Appendix 4.6-A.

Items to be taken into account include:

ground conditions acceptable for treatment

Conditions acceptable for treatment are only those within zones A and B of the chart.

Conditions acceptable for treatment

Note: an enlarged chart appears in Appendix 4.6-A.

ground conditions not acceptable for treatment

The following ground conditions are NOT acceptable for treatment:

  • soft clays with an undrained shear strength less than 30kN/m2. The wet process may be applicable to clays with a lower undrained shear strength but such sites should be examined individually. Under no circumstances can the treatment of clays with an undrained shear strength of 15kN/m2 or less be accepted
  • ground with peat layers close to foundation level or the base of the stone column, or where intermediate layers of peat are thicker than 200mm either as a single layer or the sum of the thicknesses of individual layers throughout the length of the stone column
  • voided filled ground, eg old water tanks, pottery, glass bottles, concrete rubble or brick fill of unsuitable grading
  • chalk fill or clay fills subject to:
    • collapse settlement
    • collapse caused by saturation
    • rising or fluctuating water levels
  • filled ground still settling or expected to settle:
    • under its own weight
    • where there is a high organic content
    • where decay is continuing
Filled ground

  • fill, containing degradable material where organic material forms more than 15% of fill by volume
  • highly contaminated ground, eg toxic waste, or where inflammable, explosive or toxic gas generation will take place (stone columns may act as vertical vents)
Stone column vent

  • clays with a plasticity index greater than 40%
  • highly sensitive soils liable to collapse or remoulding
detrimental factors

Factors to be considered include the following:

  • where partial depth treatment of filled ground is proposed, the Engineer should be satisfied as to the anticipated performance of both the treated and untreated zones. The Specialist Contractor should take responsibility for the treated zone and the decision as to the depth of treatment
  • the minimum depth of soil treated should allow for the interaction of adjacent foundations
Interaction of adjacent foundations

  • stone columns may form vertical drains allowing the passage of water to a moisture susceptible strata, or provide seepage paths for gases
Stone column soakaway

  • obstructions and variations in the density of fill and natural ground (hard spots)
  • alterations to the oversite level before or after treatment or disturbance of ground by excavations after treatment
  • the location of changes in the profile of the natural underlying ground eg edges of pits or quarries, slopes, or manmade obstructions such as soakaways or drainage runs
  • long term lowering of water table causing settlement of existing adjacent buildings
Dry process lowering of water table

  • short term rise in local water table due to large volumes of water used in wet process during construction causing settlement or heave of existing adjacent buildings
Wet process raising of water table

  • surface water sewers should be used for rainwater disposal where possible, but where soakaways are necessary, these should be positioned so that their construction and operation is not detrimental to the treated ground
  • soils with a modified Plasticity Index of 10% or greater should have foundations designed to accommodate volume changes, and the depth of concrete foundation should be in accordance with Chapter 4.2 'Building near trees'.
Building near trees


4.6 - D8
The builder shall obtain written confirmation from the Engineer and Specialist Contractor that the site is suitable for the proposed ground improvement system

Confirmation that the site is suitable for the proposed system should be made available to NHBC.

The Engineer and Specialist Contractor should agree the following in writing before work commences on site:

  • a detailed schedule of work
  • a programme of work
  • what tests are to be carried out on completion of the work
  • responsibility for procedures and tests.

For details of tests see Sitework clause S3.

The following should also be taken into account:

  • the layout and depth of the stone columns and the accuracy to be achieved (see Sitework clause S2)
  • what factors of safety have been incorporated into the design to allow for unforeseen contingencies
  • the criteria for non acceptance of the vibrating poker work
  • what calculations and case histories are required to justify the ground improvement proposals together with the layout of the stone columns and details of the equipment and process to be used on site.

These written agreements should be made available to NHBC before work commences on site.


4.6 - D9
Design shall ensure that site layout and dwelling design are compatible with the treated ground

Items to be taken into account include:

limitations of the treated ground

The Engineer should:

  • undertake discussion with the Specialist Contractor to confirm the feasibility of proposals
  • determine the loads to be imposed by the buildings and assess against the results of the site investigation
Ground bearing capacity

  • consider limitations of the configuration of the dwellings:
    • T-block vulnerable at junction
    • vulnerability of long blocks
  • avoid siting buildings in locations where major changes in ground conditions can be expected
Major change in ground conditions

  • advise and discuss design criteria with NHBC at the design stage.
limitations of ground support

The Engineer should:

  • establish the likely limits of ground movement
  • allow for ground movement in the design, including where appropriate:
    • position and spacing of movement joints
    • flexibility of masonry mortars
    • masonry reinforcement.
Brick reinforcement and movement joints in walls

drainage and service trenches

The Engineer should consider the influence of drainage and other service trenches on the stability of the complete design (see Sitework clause S4).

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

suitable foundation types

The following criteria should be incorporated in the foundation design to ensure the compatibility and overall stability of the foundations and superstructure:

  • only two types of foundations are suitable, both of which should comply with the minimum criteria for areas of reinforcement as defined in BS 8110. They are:
    • reinforced concrete strip foundation

      reinforced concrete strip foundation

    • reinforced concrete raft or semi-raft foundation positioned on a uniformly compacted bed of hardcore
Raft or semi-raft foundation

  • the depth of foundations to be a minimum of 600mm below the surface of the treated ground, and founded on firm material of adequate bearing capacity
  • where the treated ground is of a granular nature, a reinforced concrete strip foundation will normally be acceptable provided that the full depth of all fill material is treated
  • if the treated ground is of a cohesive nature, a suitably designed raft, semi-raft or reinforced concrete strip foundation will normally be acceptable. The reinforced concrete foundation should be designed to span between the centres of adjacent stone columns
  • if partial depth treatment of filled ground is proposed then a suitably designed reinforced concrete raft or semi-raft foundation should be used
  • if during excavations for foundations in treated ground it is found that excessive depths of concrete are required, then precautions should be taken to ensure overall stability of the foundations, and the Engineer should be satisfied that construction of the foundation will not be detrimental to the treated ground.
use of suspended ground floors

Suspended ground floors should be provided for all dwellings where vibratory ground improvement has been carried out.

notice to NHBC

Notice of the proposed development should be forwarded to NHBC.

Inform NHBC of the appointment of the Specialist Contractor and of the anticipated commencement date for treatment.