Vibratory ground improvement techniques

4.5.6Compatibility of the ground, design and treatment

Vibratory ground improvement techniques shall be compatible with the treated ground, site layout and the home design. Issues to be taken into account include:

  1. limitations of the treated ground
  2. limitations of ground support
  3. suitable foundation types
  4. use of suspended ground floors
  5. notice to NHBC.

Limitations of the treated ground

The engineer should:

  • avoid siting homes in locations where major changes in ground conditions can be expected
  • consider limitations of the configuration of the homes including the vulnerability at junctions and of long blocks
  • determine the loads to be imposed by the homes, and assess these against the results of the site investigation
  • discuss the feasibility of proposals with the specialist contractor
  • confirm the required load and settlement performance of the treated ground
  • 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 and account for this in the design, including:

  • the position and spacing of movement joints
  • the flexibility of masonry mortars
  • masonry reinforcement

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 EN 1992-1-1. They are:
  • reinforced concrete strip foundation
  • reinforced concrete raft or semi-raft foundation positioned on a uniformly compacted bed of hardcore
  • For both types of foundation, top and bottom reinforcement should be provided.
  • 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 unless a more rigorous structural analysis is carried out to show that an alternative detail is acceptable.
  • 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 homes where vibratory ground improvement has been conducted, unless the engineer can substantiate an alternative solution that is acceptable to NHBC.

Notice to NHBC

Where vibratory ground improvement is proposed, NHBC should be informed of:

  • proposed development
  • appointment of the specialist contractor
  • proposed start date of treatment.