Vibro Stone Column Foundations – Recognised Good Practice
In order to manage risk LABC Warranty is required to assess the structural design and installation of vibro stone columns and their associated foundations (referred to as vibro foundations below) as part of the audit process on behalf of the Underwriter. The following guidance outlines recognised good practice with regard to vibro foundation design and installation that is acceptable to LABC Warranty.
The design and installation of vibro foundations should be in accordance with the LABC Warranty Technical Manual and recognised publications from British Standards, Eurocodes, CIRIA, BRE and ICE, in particular BS8004, EC7, ICE Specification for Ground Treatment and Notes for Guidance and BR391 Specifying Vibro Stone Columns.
Site Investigation Report
A site specific geotechnical site investigation should take place and be in accordance with BS5930/EC7 and extend into adequate strata beneath the filled or poor strata above. The investigation should include enough geotechnical testing to enable accurate geotechnical design of the vibro stone columns in accordance with proven design methods.
Drawings and Calculations
Foundation drawings and calculations should be prepared by the Structural Engineer indicating the required bearing capacity and settlement characteristics for the purposes of design of vibro stone columns. In general foundations should be designed for maximum settlements of 25mm or differential settlement of 1 in 500 unless more stringent measures are required by the structural engineer.
Strip foundations should be designed to span between vibro stone columns and must incorporate top and bottom reinforcement.
Irrespective of vibro foundations, Chapter 5.2 needs to be fully complied with, with respect to trees/cohesive soils.
The vibro designer/contractor should be appropriately qualified to carry out the works.
The vibro designer should prepare a specification for the works including:
- Written confirmation of the suitability of the ground for vibro treatment.
- Confirmation that the available site investigation is sufficient for the purposes of vibro design and installation.
Design calculations for vibro stone columns based on recognised methods and based on the parameters from the Site Investigation Report and indicating the proposed depth of vibro. Vibro treatment must extend through the full depth of filled or poor ground.
- If there are high groundwater levels, soft clays or peats present, then these need to be specifically addressed in the design.
- Vibro treatment is not acceptable in voided ground, clay fill, chalk fill, settling ground, highly contaminated ground, ground with a high volume change potential or across sudden changes in ground.
- Drawings indicating the vibro stone column layouts and the associated bearing capacity and settlement characteristics. Columns should be at more than 2m centres.
- Proposed type, rate and criteria for testing.
Plate Tests should be carried out:
- With 600mm diameter plates loaded to 3 times working load or 11 tonnes minimum.
- At a minimum rate of 1 per 100 vibro stone columns or 1 per rig per day, or a minimum of 2 tests, whichever is greater.
- Note that plate tests do not provide a direct indication of the anticipated settlement of the completed structure.
Penetration Tests should be carried out:
- At a rate of 1 per 20-50 stone columns or 1 test for not more than 500m2, with a minimum of 1 test for each structural unit.
Dummy Footing Tests should be carried out:
- With 1500 x 600mm plates, loaded to 1.5 times working load for a minimum period of 13 hours.
- At a minimum rate of 1 per 100 stone columns or 1 per 10 houses, whichever is greater.
Penetration tests wont be required if dummy footing tests are carried out at the above rate.
Testing should be carried out across the full site and cover all the various ground conditions.
The vibro designer should provide a validation package including:
- Copies of all installation logs (directly referenced to the drawings). Logs should include date, column number, depth, diameter, weight of stone and surface level.
- Copies of all tests (directly referenced to the drawings).
- Interpretation of tests.
- Confirmation that the as-built vibro installation has achieved the required bearing capacity and settlement characteristics.
Alternative vibro types and design methods
If there are queries with regard to anything not covered above, please contact LABC Warranty prior to commencement of construction. If it is intended that the vibro design and/or installation is to deviate from the above guidance, then please contact LABC Warranty for agreement prior to commencement of vibro.
The following documentation (in accordance with the above guidelines) should be submitted to LABC Warranty for assessment. Items 1-3(c) should be submitted prior to commencement of vibro on site and items 3(d)-(f) should be submitted as soon as they become available, preferably prior to construction continuing over the vibro stone columns.
- Geotechnical site investigation report to adequate depth and detail.
- Foundation drawings and design calculations. Strip footings should be designed for the specified bearing pressures and be designed to span between vibro stone columns.
- Written confirmation from the vibro designer that the ground conditions are suitable for vibro treatment, including endorsement of the site investigation report.
- Vibro column layouts.
- Vibro design calculation.
- Vibro installation logs (with vibro column numbers referenced to the vibro layout drawing).
- Copies of all testing carried out (locations referenced to the drawings) and interpretation of test results.
- Written confirmation from the vibro designer that the as-built installation has achieved the required bearing capacity and settlement characteristics.
Please Note: Every care was taken to ensure the information in this article was correct at the time of publication on 28.3.2018. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.