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4.6.12 Verification of engineered fill
Verification testing of engineered fill shall be undertaken to confirm compliance with the earthworks specification and its fitness for purpose. Items to be taken into account include:
- fill classification
- end product testing
- performance testing
- settlement measurement
- testing frequency.
Validation and verification of the filling 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 on a combination of random and targeted locations through the full depth of the fill to ensure consistency across the site. Higher risk locations (for example deeper engineered fill, heavier/larger buildings or where engineered fill was placed in less suitable conditions) should have a robust data set to confirm that fill has achieved the required geotechnical compliance criteria.
The testing frequency should be set out in advance of the works in the Earthworks specification.
4.6.12.1 Fill classification#
Classification testing including particle size distribution (grading), plasticity (Atterberg limits), 4.5kg rammer compaction tests or moisture content tests, should be undertaken throughout the earthworks to confirm that the engineered fill materials remain within the acceptability limits set out in the earthworks specification.
The type and frequency of classification tests should be site and material specific and should be confirmed in the earthworks specification. Acceptable test methods and frequencies of classification tests are given in Tables 9 and 10 of BS 6031:2009.
4.6.12.2 End product testing#
Fill compaction is most commonly established by comparing the in-situ dry densities and moisture contents with the laboratory determined maximum dry density and optimum moisture content. NHBC will normally accept engineered fill verified to achieve the criteria given in Table 6. There may be variation to these requirements where differing material types or foundations are adopted, or where there is a risk of future inundation of the fill.
The in-situ densities of engineered fill may be measured using sand replacement tests (SRTs), nuclear density tests (NDTs) or electromagnetic density gauges (EDGs). Where NDTs or EDGs are used, the results should be regularly calibrated against SRTs. Where there is variation in the fill materials, the use of NDT results to assess compaction may not be reliable.
4.6.12.3 Performance testing#
Table 9 indicates suitable tests to assess earthworks performance. The type and frequency of testing should be appropriate for specific site conditions and the proposed end-use. It is recommended that a minimum of three tests of each type are undertaken, so that a trend can be observed from the results with the exception of preloading by surcharge.
Table 9: Types of Performance Testing
| Test /Parameter | Description | Application |
|---|---|---|
| Preloading by surcharge | An area usually greater than 50m2 is raised above the proposed development level with soils or fill to model the proposed development loadings | For large sites or areas of deep fill (over 5m deep) or where highly compressible soils exist at depth, usually in conjunction with other performance tests such as zone tests, plate tests or probing |
| Zone load | A rigid base (usually made up of concrete) with areas related to the width of the proposed foundations, typically between 4-10m(2) . Suited to modelling the loading and depth of influence of a building on raft foundation. The test duration to be sufficient to identify the magnitude and timescale for completion of primary settlement, the clear trend of any secondary settlement (creep) and to confirm that load-settlement behaviour is in accordance with calculated predictions | For large sites or areas of deep fill usually in conjunction with other performance tests such as plate tests or probing undertaken to identify suitable locations for the zone test. Care must be exercised during the zone tests, pads/bases are susceptible to ground vibrations/ movement resulting from machinery/plant |
| Mini zone load/pad | Smaller than a zone test – with an area less than 4m2 and designed to model a typical (or heaviest) part of a spread (beam grillage or reinforced strip) foundation. The test duration to be sufficient to identify the magnitude and timescale for completion of primary settlement, the clear trend of any creep and to confirm that load-settlement behaviour is in accordance with calculated predictions | Usually in conjunction with other performance tests such as plate tests or probing to identify suitable locations for the zone test. Care must be exercised during the mini zone load tests, pads/bases are susceptible to ground vibrations/movement resulting from machinery/ plant |
| Surface points settlement | A stable concrete or steel level monitoring point set at the surface or at shallow depth to determine the nature of any fill settlement without applied loading – usually over a prolonged period to model self-weight consolidation and creep settlement as required | Where self-weight consolidation or creep settlement may be significant – usually associated with deep cohesive fill |
| Settlement rods | Plates placed at the underside of engineered fill to measure settlement caused by the placement of fill | Usually in conjunction with other performance tests such as zone tests and mini zone load tests |
| Magnetic extensometers | Can be placed at varying depths within engineered fill or underlying made ground in deep fill sites to record settlement from targeted zones | Usually in conjunction with other performance tests such as zone tests and mini zone load tests |
| Plate test | Usually 600-900mm diameter plate and loaded incrementally to 1.5-3.0 times the highest proposed bearing pressure. The peak load is held for up to 1 hour or until the rate of settlement does not exceed an agreed limit. The use of dual cycle plate load tests can provide a significant improvement on the assessment and interpretation of the data. Wherever possible, dual cycle plate testing should be employed | Inexpensive but may not load the fill as deeply as a foundation, limited to approximately 1.5x the plate diameter. Most suitable during fill placement |
| Probing | A range of techniques are available | Static cone testing can provide data on the surface to depth of soil, including soil stiffness and other important soil properties. Shallow techniques (including hand-held) may be limited in data quality |
| Permeability | Infiltration testing in boreholes or trial pits at the depth of interest | Where low permeability is required to limit groundwater (or ground gas) movement |
| Inundation | Similar to permeability testing but with added settlement monitoring | To assess the susceptibility of fill to settlement when it becomes saturated (from surface water, service trenches or rising groundwater) |
| Hand vane | Undrained shear strength of cohesive fill. Quick and easy but of limited reliability and accuracy | For cohesive fill. Any gravel within the soil matrix that comes into contact with the cruciform pattern of the 4no. vanes can significantly alter the results. It should only be used in conjunction with other tests as a confirmatory tool |
Proposals for testing should be agreed in advance of the works and should be set out in the earthworks specification with supporting test plan(s). Test locations should be selected to allow a satisfactory spatial and lateral spread of results.
The pass criteria for the performance tests should be detailed in the earthworks specification and Geotechnical Design Report/Geotechnical Design Statement and should be appropriate for the proposed end use and foundation type. Load-settlement zone load tests should be a minimum of 1.2 times the anticipated design load of the foundations and with the loaded area of a comparable scale.
4.6.12.4 Settlement measurement#
Settlement measurement of load tests or surface points should be undertaken by a suitably qualified land surveyor to a suitable accuracy, no greater than to the millimetre and less where site conditions are considered appropriate or are likely to reduce uncertainty and the required duration of monitoring. It is critical that over the long term, survey points are not damaged, disturbed, or affected by other activities. Additionally, the data should be related to stable datum outside the influence of any earthworks or Made Ground or existing unstable ground. A minimum of two datum points should be established to provide redundancy in case one is lost or damaged.
Extensometers in boreholes can be effective at determining the depth at which settlements are occurring where preloading or surcharge mounds are positioned upon underlying ground and self-weight consolidation is being assessed.
Plates with extension rods also provide means of measurement of settlement at sub formation and the underlying ground.
Settlement data should be plotted at both natural time and logarithm time to aid assessment of the data.
4.6.12.5 Testing frequency#
The frequency of testing for classification, compliance and performance purposes should be determined according to site conditions, fill materials and their consistency, filling techniques and the proposed end use as stated in the earthworks specification. Testing frequencies should be specified in accordance with industry guidance such as BRE 427 and also see additional guidance within Clause 4.6.1.
Where a Method Specification is utilised, in situ testing is required but typically would be based on the total volume of engineered
fill to confirm that the degree of compaction achieved by the adopted method on site is as expected. When End Product specification is utilised, in situ testing should be based on a minimum number and grid with each and every layer tested, eg 1 per 25m x 25m grid per layer and a minimum of 3 per layer per day. Where a performance specification is adopted, it will be necessary to undertake performance testing during fill placement, as well as upon completion, to ensure compliance with the earthworks specification.
The testing frequency based on Table 10 is generally considered as an acceptable minimum requirement.
Table 10: End Product compliance testing of placed engineered fill*
| Fill volume (m3) | Minimum no. of tests required |
|---|---|
| >100,000 | 2no. per 1,000m3 |
| 10,000 to 100,000 | 3no. per 1,000m3 |
| <10,000 | 5no. per 1,000m3 |
Notes
*Variation to the above may be necessary where differing material types or foundations are to be used.
Test types and frequencies should be reviewed according to the results obtained and site conditions. Where there is greater variation in site conditions or results than anticipated, an increased frequency of testing will be required. Conversely and where agreed with NHBC, a decrease in test frequency may be acceptable where consistency in test results is being achieved.
Last updated: 2nd January 2024