4.6 Vibratory ground improvement techniques
|4.6 - S1||Sitework standards|
|4.6 - S2||Site supervision|
|4.6 - S3-S4||Verification of completed treatment|
(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.
The Engineer should provide competent site supervision at critical stages (e.g:
- inspection of setting out and materials
- column installations during early stage of the work
- where installation data differ from design assumptions
- if changes in treatment layout are required)
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
- missing stone columns are replaced
- stone columns which 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.
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 appropriate combination of the following tests with the agreement of NHBC:
- 600mm diameter plate tests
- dummy footing/mini zone test
- zone test
- in situ test
- trial pits
600MM DIAMETER PLATE TESTS
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.
DUMMY FOOTING TEST/MINI ZONE TEST
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.
An isolated pad or strip footing is used, and up to 8 stone columns and the intervening ground can be tested. Loadings, which should simulate the dwelling loads, are held for 24 hours at pre-determined stages to examine creep behaviour.
Where vibration will improve the ground itself, eg granular materials, then in-situ testing is appropriate.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.
(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.
Ensure that the minimum clearance between excavations and foundations is not less than the depth of excavation minus the depth of the structural foundation.
Particular attention is needed for excavation below the water table.