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4.6.10 Site work
The developer or builder shall ensure suitable supervision, sampling and testing are performed throughout the entire duration of the earthworks. Filling shall be undertaken by a competent Contractor using materials, personnel and plant appropriate for the nature of the site conditions and proposed end-use. Issues to be taken into account include:
- supervision and records
- site preparation
- material handling
- material acceptability and compliance testing
- actions in respect of non-compliant fill
- protection of fill.
4.6.10.1 Supervision and records#
The developer or builder must arrange for a suitable site supervision and recording of the works appropriate to the scale and complexity of the project and proposed end-use of the fill. The recording of the site works should be in accordance with the requirements of the earthworks specification. Guidance on the level of information that is required to demonstrate the quality of the works is available in BS EN 16907-5.
4.6.10.2 Site Preparation#
The site should be prepared in accordance with the earthworks specification with all vegetation, topsoil, fly tipped, and unsuitable materials removed prior to filling. If suitable, topsoil should be placed in segregated stockpiles for reuse onsite or elsewhere. Where excess topsoil is present, this may be removed under the Material Management Plan (MMP).
The formation should be prepared by proof rolling with any hard or soft spots identified and remediated in accordance with the earthworks specification. Temporary drainage may be required to keep the formation dry.
Relic structures, for example old foundations and hard standings, should be removed from below engineered fill areas in their entirety wherever practicable. As a minimum, relic structures and obstructions should be removed to at least 2m below any proposed foundation. All structures or uncharacteristic elements (including boulders) greater than 1.0m in diameter/length left in the ground at formation should be surveyed and their positions recorded.
When recording the depth of engineered fill, it is essential (eg for use in subsequent pile designs) that the formation should be surveyed prior to filling on a maximum 10m grid arrangement.
Site constraints and features to be preserved (for example monitoring wells, public rights of way, watercourses and trees to be retained) must be clearly identified and all necessary protection and safety measures must be implemented. Alternatively, their removal or any reinstatement measures should be specified as appropriate.
Where the sub formation is sloped, the sides should be stepped at a maximum height of 600mm to allow the engineered fill to be benched into the existing natural ground or achieved otherwise as stated in the earthworks specification. Figure 5 shows what may be required on a sloping site close to the development platform where filling is required.
4.6.10.3 Material handling#
In addition to the required classification testing, site-derived materials should be routinely inspected during excavation to ensure consistency and correlation with the expected ground model.
Imported fill should be subject to periodic inspection from the donor site prior to arrival at the site and regular inspections once delivered. Classification testing should be undertaken as required prior to placement, particularly where the material is not provided with independent certification and where visual assessment suggests the material classification varies.
Engineered filling should not be undertaken in inclement weather conditions. Where the engineered fill is cohesive, placement and filling should be paused during extended periods of rain. For sensitive materials like chalk, care should be taken not to over-compact the fill further than specified.
Engineered fill must be placed and compacted in accordance with the specification, within a defined moisture content range close to the optimum moisture content as derived from relevant laboratory testing.
For cohesive fill materials, the moisture content is critical in achieving the desired dry density. Engineered fill that is compacted too dry of the optimum moisture content (OMC) risks being vulnerable to collapse settlement or heave when wetted. Engineered fill that is compacted too wet of optimum moisture content risks being too weak to provide adequate support without excessive settlement.
Engineered fill should proceed in near horizontal layers (to assist in shedding surface waters) with each layer fully compacted before commencing the following layer. The maximum layer thickness for method compaction should be based on the type of fill material, the classification of the fill material, placement and the method and plant employed.
For end product compaction, an uncompacted layer thickness of up to 250mm subject to site-specific requirements would be acceptable. The maximum particle size should not be greater than 1/3 of the compacted layer thickness or 75mm, whichever is lower.
Differing engineered fill material types should not be placed in the same layer, they should be placed at predetermined layers and defined depths appropriate to their properties and the proposed end-use.
Filling activities should be planned and carried out so that:
- vibrations do not disturb existing or nearby properties, excavations, slopes, buildings, services or infrastructure
- filling and earthwork machinery does not overload retaining features, slopes, buried services or infrastructure
- ground movement including around the area of filling does not harm adjacent buildings, services or infrastructure
- temporary slopes (filled or in cut) are safe and stable
- under compaction and over-compaction are avoided
- the rate of placement of fill does not result in excess pore water pressure leading to failure of the underlying ground.
4.6.10.4 Material acceptability and compliance testing#
Classification and compliance testing should be in accordance with BS EN 16907-5 and BS 6031:2009. Laboratory testing should be undertaken by a UKAS and/or MCERTS accredited specialist. All testing should be to a current British or EN Standards eg BS 1377, using appropriately calibrated equipment. Testing should be undertaken throughout the duration of the fill placement in accordance with the guidance in clause 4.6.12.
4.6.10.5 Actions in respect of non-compliant fill#
Where testing indicates compacted engineered fill does not fully comply with the requirements of the earthworks specification, appropriate action should be agreed as soon as possible with NHBC. Possible actions include:
- drying of engineered fill material prior to further placement of subsequent layers
- excavation and replacement of the engineered fill represented by the test failure
- additional compaction (if the failure is shallow or at the surface) and retesting
- if the non-compliance is regarded as a marginal outlier and represents part of a normal spread of test data, it may potentially be deemed acceptable without further action. It should be recorded in the Earthworks Verification Report
- undertaking appropriate performance testing.
The possible extent of non-compliant engineered fill needs to be carefully assessed and whether the non-compliance may indicate unsuitable placement and compaction techniques or poor site conditions or excessively variable fill or unsuitable material. Where there is a divergence from the earthworks specification the Designer should be notified, and advice sought.
Remedial action needs to be carefully undertaken and recorded avoiding disturbance to previously placed compliant engineered fill.
4.6.10.6 Protection of fill#
Compacted engineered fill should be protected from deterioration due to trafficking, poor weather and pooling surface water. Suitable measures include laying the surface to falls to assist drainage or use of a durable granular sealing layer or sacrificial layer.
Last updated: 2nd January 2024