4.6 Engineered fill

4.6.8 Compatibility of fill with foundations, infrastructure and external works

Placed engineered fill shall support foundations, infrastructure and external works as required without excessive settlement. Items to be taken into account include:

building and foundation types
engineered fill performance and Foundation Options
building settlements and relative movements
external works and infrastucture.

This section provides guidance on building foundations that will generally be acceptable to NHBC. Where more rigorous structural or geotechnical analysis is carried out alternative solutions may be acceptable, but these must be agreed with NHBC in advance of development.

4.6.8.1 Building and foundation types#

Engineered fill covered within this chapter should be capable of supporting foundation loads from buildings of three storeys or equivalent weight.

Where compliance testing of engineered fill demonstrates a high level of consistency and quality in combination with a high level of supervision and control, a suitably structurally designed reinforced strip foundation may be acceptable. This can be so in conjunction with ground hazards and risks associated with excessive settlements being suitably addressed. A suitably structurally designed beam grillage will also be acceptable.

Where engineered fill has been placed to a suitable earthworks specification with a satisfactory verification testing, a semi-rigid raft foundation in accordance with Chapter 4.4 will be acceptable to support buildings.

Reference should also be made to the requirements of Chapters 4.2 ‘Building near trees’ and Chapter 4.3 ‘Strip and trench fill foundations’ as appropriate. Cohesive fill material should be taken as having a similar volume change potential (and requirement for minimum foundation depths) as natural clays of the same plasticity index.

4.6.8.2 Engineered fill performance and Foundation Options#

Residential developments founded on engineered fill are more sensitive than other structures or infrastructure founded on the same material and requires foundation solutions that will mitigate the potential for both aesthetic and structural damage over the design life of the building. Any foundation solution adopted should be sufficiently robust to accommodate the predicted future settlements and prevent damage to building superstructures. To provide the required stiffness, rigidity and robustness against unwanted movements, all foundations on engineered fills should be reinforced. Unreinforced strip footing would not be acceptable to NHBC under any circumstances.

Table 8 indicates the minimum requirements necessary for a shallow foundation to be used on engineered fill and considers different foundation types that may be used based on the achieved compaction of the engineered fill only. The table should only be used where no other residual geotechnical risks are present and assumes that the engineered fill is placed on competent natural ground.

The foundation specified should consider the holistic geotechnical design (eg bearing capacity and settlement) and also allow for any retained geotechnical risks outside or below the engineered fill. Table 8 should not be used to justify a less robust foundation where unresolved geotechnical risks remain.

Where the requirements of Table 8 have not been significantly achieved, for example poor execution or poor site supervision or inadequate testing or reporting, or where the verification of the earthworks falls outside the minimum requirements given below, an alternative foundation solution outside this chapter will need to be considered – ie, solutions from Chapter 4.4 ‘Raft, pile, pier and beam foundations’ or Chapter 4.5 ‘Vibratory ground improvement techniques’. Where piles are considered, the potential for negative skin friction on the piles due to creep settlement of the placed engineered fill and/or the consolidation of any underlying soils should be assessed.

Table 8: Engineered fill Performance and Acceptable Foundation Options

Proposed outcome	Reinforced strip	Beam grillage	Semi-raft
Minimum relative compaction (% maximum dry density)¹	>95	Majority of results >95. However, results show some variability and non-compliances (see Figure 7) No results <90	Majority of results >95. However, results show some variability and non-compliances (see Figure 7) No results <90
Maximum air voids (%)	<5	Majority of results <5 However, results show some variability and non-compliances (see Figure 7) No results >10	Majority of results <5 However, results show some variability and non-compliances (see Figure 7) No results >10
Supervision	A very high degree of independent supervision. Full time independent resident geotechnical engineer dedicated to inspection, supervision and testing of the fill	A high degree of independent supervision. Full-time inspection of the filling by an independent resident geotechnical engineer	Good/normal supervision. Part-time inspection of the filling by an independent geotechnical engineer
Contractor	Experienced/specialist earthworks contractor	Experienced earthworks contractor	Suitably experienced groundworker or earthworks contractor
Engineered Fill	Fill generally of consistent depth and properties throughout. Fill behaviour should have been proven by precedent or trials	Fill generally of consistent or steadily varying depth. Little variation in fill properties	Effects of variations in fill depth and properties should be assessed with predicted settlements established
Foundation performance testing	Zone load tests and/or mini zone load tests and Plate load tests across fill depth and at surface²	Mini zone load tests and Plate load tests across fill depth and at surface²	Plate load tests across fill depth and at surface²
Verification	Earthworks Verification Report produced by a third-party organisation	Earthworks Verification Report produced by an experienced earthworks contractor	Earthworks Verification Report produced by an experienced earthworks contractor/groundworker
Foundation design	Design by a suitably qualified Engineer to relevant British Standards. Designed for moment to span 2m as simply supported and 1.0m as a cantilever³	Design by a suitably qualified Engineer to relevant British Standards. Designed to span 3m as simply supported and 1.5m as a cantilever	Design by a suitably qualified Engineer to Chapter 4.4 ‘Raft, pile, pier and beam foundations’

Notes

1. Modified Proctor hammer (4.5kg).
2. Minimum 600mm diameter.
3. Foundations to be typically reinforced with mesh reinforcement at top & bottom.

In circumstances where there is a potential increased or significant geotechnical residual risk of long-term creep settlements, for example as a result of the type or thickness of placed engineered fill or underlying thickness of made ground beneath the engineered fill, a stiff reinforced concrete raft or stiff beam grillage foundation may be required.

4.6.8.3 Building settlement and relative movements#

The total settlement between any part of the building foundation and the surrounding ground or external area, should not normally exceed 25mm over the 60-year design life.

Where engineered fill or the underlying ground, is undergoing steady uniform settlement a total foundation settlement of over 25mm may be acceptable, provided the differential settlement is compliant with the guidance below and the foundation solution is sufficiently robust.

The tilt or distortion (differential settlement) of any part of the foundation should not exceed 1 in 400. The potential differential settlement of any foundation should be assessed where:

the depth of fill significantly varies,
there is a tapered layer of compressible material,
a quarry highwall or buried batter is present.

Where predicted total or differential settlements of any part of the building foundation exceed the above criteria further assessment will be required to determine suitable alternative foundation solutions. The presence of highwalls, in particular will require rigorous assessment including their location, geometry, and observed load-settlement behaviour of the engineered fill following remediation, if they are to be considered by NHBC to be suitable to build residential development upon.

The effects of differential settlement between external areas and buildings should be assessed in particular where these are piled or supported on vibro columns. Additional means of support or measures to mitigate ground movement may be required where excessive settlements could be realised, ie, the use of flexible drainage and/or surcharging.

The following Figures 3 and 4 show examples of engineered fill over rolling landscape and over buried features where part removal of the feature was necessary to mitigate excessive tilts.

4.6.8.4 External works and infrastructure#

It will generally be acceptable to support low height retaining walls and boundary walls on mass concrete or reinforced foundations placed on engineered fill. Walls should be provided with joints and/or reinforcement as considered appropriate to provide flexibility and structural integrity while accommodating ground movements particularly at locations where depths of engineered fill and settlements may vary. Geotechnical and structural designs are required where retaining walls are in excess of 0.6m in height.

Wall foundations, hardstandings and roads should be designed according to appropriate guidance and achieved geotechnical properties of the engineered fill.

Where external works and infrastructure pass from engineered fill to the natural ground or where the depth of engineered fill varies significantly, the potential for differential ground movements should be assessed and catered for. Additional measures should be provided as necessary eg the use of geogrid or futureproofing drains by potentially laying to steeper falls and the use of flexible jointing.

For areas not within the influence of building foundations (ie, hardstanding, gardens, infrastructure or external works areas), a maximum total long term settlement of over 25mm would generally be acceptable subject to serviceability design limits. See Figure 2 above for an approximate delineation of these areas.

Last updated: 2nd January 2024