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4.6.9 Acceptable methods of earthworks filling
Earthworks filling shall only be undertaken using methods acceptable to NHBC that are appropriate to the materials, site conditions and end-use. Issues to be taken into account include:
- conventional compaction
- non-standard compaction
- preloading and surcharging.
4.6.9.1 Conventional compaction
Placement and compaction of engineered fill should be undertaken in layers of controlled depth using appropriate plant. Acceptable methods are considered to be those based on the recommendations from the Specification for highways works (SHW) Table 6/4.
The type and size of the compaction plant should be based on the volume or depth of engineered fill to be placed, the classification of the fill to be compacted, and the specification requirements.
4.6.9.2 Non-standard compaction
This aspect covers all methods of compaction which are not covered by SHW Table 6/4. Details of non-standard plant and techniques should be submitted to NHBC in writing at least eight weeks before site works begin. Non-standard techniques include, but are not limited to:
- rapid impact compaction (RIC), using a rammer
- high energy impact compaction (HEIC), using polygon or triangular rollers
- dynamic compaction, using heavy drop weights dropped from a crane.
NHBC will require evidence of the effectiveness of non-standard plant or techniques, or demonstration of the suitability of proposals for the site conditions by trials as appropriate, particularly for large or complex projects. Site trials, including testing requirements, should be designed and agreed with NHBC prior to commencement. Trials must accurately reflect the proposed filling techniques and achieve specified performance requirements, and may require post-treatment investigation and load testing.
4.6.9.3 Preloading / surcharging
Placement of a surcharge may be an acceptable form of ground improvement but should not be used in place of the compaction techniques described previously. The design of the surcharge, including its magnitude and duration of surcharge requires careful analysis.
The effectiveness of surcharging should be demonstrated by monitoring or by trials. The requirements for monitoring and supervision are outlined in BS EN 1997-1 and BS 6031 and should be presented in the GDR. The surcharge load should be maintained until the achievement of the design requirements has been demonstrated. Refer to BRE FB 75 for further guidance.
Last updated: 2nd January 2025