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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 A11: Distance from water table to lower floor level

Advice on construction and Rerouting

Where there is an existing drain(s) running under the proposed location of the building, the drain(s) should be appropriately re-routed around the building. Expert advice should be sought from an engineer who is qualified by examination, in private practice and is in possession of professional indemnity insurance if there is any doubt about table levels or possible flooding.

Retaining Walls

Below are two typical blockwork retaining wall details.

Please note the retaining walls below are not designed to accommodate load from roads, parking areas or buildings, this is for around the site boundary.

Retaining walls should be designed by a qualified engineer who is qualified by examination, in private practice and possesses professional indemnity insurance.

Diagram A12: Typical blockwork retaining wall details

Diagram A12: Typical blockwork retaining wall details


Root Presence

The presence of roots can cause movement of the foundation which causes structural damage. It is important to determine existing root growth and to check excavations carefully for roots and fibrous material.

Diagram A13 - Visible roots around trench

Photo- Roots found during construction

Assessing future root growth

Site conditions can be possible indicators for future root growth on sites, for example:

  • Roots will break through hard rock.
  • Roots are likely to grow along way to meet water / wet soil.
  • Roots will grow on the windward side where the tree is located in an exposed windy area.
  • Roots will stop at the water table.

Diagram A14 - How tree roots really grow

Diagram A15 - Root growth towards moisture

Water Table Location

Roots will not extend down below the level of the water table as detailed in the diagram below

Diagram A18 - Root growth with respect to the water table

For further details see:

  • Building Research Establishment (BRE) Digest 240 & 241: Low-rise buildings on shrinkable clay soils, Parts 1 & 2.

  • BS 5837: 2012 Trees in relation to design, demolition and construction – recommendations.

  • BRE Digest 298: Low-rise building foundations – influence of trees in clay soils.

Tree and Root Removal

Expansion of ground with removal of trees

Generally the retention of trees is considered a worthwhile objective as long as they are not a threat to houses or services on site. The possibility of ground expansion after the removal of trees should be noted also.

Root removal

In some cases it may be possible to remove up to a quarter of root growth; however, it must be considered that new roots may grow to compensate for roots that are removed and what direction these may take. In the event that cutting of roots must occur, specialist advice should be sought.

Diagram A20 - Example of where roots may be removed

Solving Tree Problems

Problems associated with trees and possible solutions

The roots can have an impact on a building and cause structural cracks. To prevent this happening - it is possible to build a barrier. After you cut back root growth a 150mm minimum concrete wall could be constructed to stop further growth.

In addition if you think pipe work could be penetrated by roots it may be worthwhile encasing the pipes in concrete.

Diagram A21 - Tree problems and possible solutions

Foundations near Trees

Trees themselves are nice to look at and enhance any garden; however, tree roots can present hidden problems for your building.

The closer a tree is to a new building the deeper the foundations must be taken down. The minimum depth for any foundation is 900mm; however this depth could be greatly increased, depending on how close the tree is to the new extension.

The basis of design is to ensure that the foundations are taken down to such a depth, that they will not be affected by tree roots.

The trees themselves do not necessarily have to be large varieties, such as Oak or Beech, even small trees, such as Plum or Apple, could have an effect on foundations.

Table A2 - Depth of Foundation with respect to tree types

The table above outlines the depth of the foundations required.Alternatively, measure tree height and foundation proximity (including trees outside plot boundary). Determine soil’s volume change potential (if impossible, assume it’s high); and the minimum acceptable depth of foundation to comply with BS 5837: Code of Practice.

NOTE: Foundations greater than 2.5m in depth must be designed by a suitably qualified engineer.

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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.