4 Foundations

4.6 Engineered fill

This chapter gives guidance on meeting the Technical Requirements and recommendations for using engineered fill to support residential developments, external works, and infrastructures.

Introduction#

On construction sites, filling or earthworks is often required for various purposes, such as preventing flooding, enhancing drainage, planning roads, or establishing a suitable ground profile. Engineered fill may also be used to replace existing made ground overlying competent strata at shallow to moderate depths to form the new development platform or be used below raft foundations on sites with shrinkable clays and trees in lieu of deep trench fill foundations.

This chapter details the information that NHBC requires to ensure that engineered fill is correctly placed to support low rise residential building structures, external works, and infrastructure while avoiding potential risks of excessive settlement or ground failure.

Scope

This chapter gives guidance for engineered fill to be placed for the support of the proposed development.

The interaction between engineered fill, underlying ground conditions and the development are important and site-specific. The primary objectives of this chapter are to ensure that:

the ground investigation is appropriate to the proposed development, site conditions and nature of the filling
geotechnical ground model(s) are developed taking into account the proposed development and underlying ground and groundwater conditions
the engineered fill is placed to a suitable earthworks specification
appropriate geotechnical laboratory acceptability testing and in situ, compliance testing is undertaken and documented, and verification is provided to NHBC
engineered fills do not settle excessively or have the potential to cause excessive differential settlement between properties founded upon the fill and external areas
engineered fill and the underlying ground supporting building foundations shall limit building settlements to less than 25mm and minimise angular distortion or tilt to 1:400
the design and detailing of foundations, infrastructure and external works suit the placed fill and underlying ground conditions taking account of the overall ground model and any geohazards in the ground beneath or nearby.

The geotechnical and environmental suitability of historic fill or fill placed without full reporting should not be relied upon without appropriate site investigation, characterisation, compliance testing and assessment being undertaken like other made ground deposits would be assessed. This may need to include long-term monitoring or loading trials to determine performance.

Assessment of historic fill within land reclamation schemes and marginal sites is generally outside the scope of this chapter. For sites with uncertainty about the site’s suitability for future residential development, please consult NHBC or consider using NHBC Land Quality Service for a bespoke consultation. You can find details about this service at:

www.nhbc.co.uk/builders/products-and-services/ consultancy/land-quality-service, or
Email: lqs@nhbc.co.uk

The following are not considered in this chapter and will require more specific guidance from other sources:

backfilling to retaining walls, buried features such as culverts, services or access chambers
capping and sub-bases to roads, drives, parking, and hardstanding areas
filling beneath adoptable highways
soil stabilisation or soil mixing
piling mats or other specific forms of working surface for construction activities
fill containing hazardous substances or required to provide mitigation against the presence of contamination, or to be placed on contaminated sites
use of reinforcing geogrids.

The builder is responsible for ensuring that any earthworks filling proposals do not breach planning requirements or waste and environmental regulations.

The following Figure 1 is to assist users in navigating through this chapter and it outlines how a proposed or existing earthworks fit into the applicability and use of this standard.

Definitions for this chapter#

Acceptability testing	Geotechnical and chemical suitability testing of the source material(s) intended to be used as engineered fill
Beam grillage	A reinforced concrete foundation designed to support load-bearing walls or structure and to span a loss of support and reinforced with traditionally cut and bent rebars and closed-loop links
Buried batter	An inclined buried interface of natural soil and made ground or between made ground and engineered fill and potentially liable to cause differential settlement at the ground surface
Cohesive fill	Containing clay of natural origin, comprising greater than 15% fines passing a 0.063mm sieve. Suitable for use as engineered fill but requires greater moisture control than granular material. Behaves in a plastic manner and can be deformed and remoulded by hand. Suitability is typically dependent on moisture content. Only clays with a plasticity index of less than 40% are acceptable as fill, where they are required to support building foundations. This includes Class 2 (general fill) and Class 7 (selected fill) in accordance with the Specification for Highway works (Series 600 ‘Earthworks’)
Compliance testing	Geotechnical suitability testing of the engineered fill as it is being placed in layers on site
Differential settlement	Settlement of one part of a building or structure relative to another. While the total settlement of a structure may interfere with functions such as service connections, it is the differential settlement that causes structural damage
Earthworks	Permanent change in ground profile and/or level, which may be due to the placement of fill (filling), the raising of the ground level (embankment) or the removal of material (cutting)
Earthworks specification	An earthworks specification describes the design requirements of proposed earthworks and contain how the earthworks should be undertaken and what acceptance criteria should be used to demonstrate its suitability
End-use	The nature of the proposed final development located over the fill, including the buildings and their foundations, external works and infrastructure features and associated activities in their normal use
Engineered fill (referred to as ‘fill’)	Fill that is selected, placed and compacted to an appropriate specification so that it will exhibit the required engineering behaviour to support structural loadings for buildings, external works (drives, paths, external walls) and infrastructure (roads, drains and services) without excessive settlement or risk of shear failure. Selected (structural) fill and General fill are examples of Engineered fill
Formation level	The prepared surface on which the engineered fill or the superstructure is built
General fill	Suitable for the support of access roads, drainage, driveways, and services buried within it or supported on it. Elements supported on general fill are usually less sensitive to differential ground movements than shallow foundations for residential buildings. Some degree of variability can be accepted in the completed fill, and fewer tests may be acceptable than for structural fill, however it will still require a suitable regime of compliance and verification testing
Geotechnical Design Report	A report that is used to develop a suitable and functional earthworks design for a proposed end use, which includes the full details on the interpretation of design data and justification for the design including relevant layout drawings. They are produced for Geo Category 2 and 3 projects where engineered fill is being used to support building foundations
Geotechnical Design Statement	Statement A brief and abridged version of a Geotechnical Design Report for Geo Category 1 projects where engineered fill is not being used to support building foundations
Granular fill	Free draining material predominantly comprising sands and/or gravels. Good for use as engineered fill. Having less than 15% fines (less than 0.063mm sieve size). Behaves in a non-plastic manner. This includes Class 1 (general fill) and Class 6 (selected fill) in accordance with the Specification for Highway works (Series 600 ‘Earthworks’)
Ground improvement	Techniques used to stabilise or improve the strength of soils in situ. This includes dynamic compaction, rapid or high energy impact compaction, surcharging, and installation of vertical drains or vibro stone columns. These techniques may be applied to historic uncontrolled fill, loose native soils upon which engineered fills are to be placed or following placement of fill
Highwall	The steep edge of quarry or opencast workings. Where the quarry is backfilled, it usually forms a zone of contrasting material behaviour where ground movements could be excessive and affected by significant differential settlement
Historic fill	Comprises ‘Made Ground’ and is not suitable for supporting building foundations, access roads, drainage, driveways and buried services without full investigation and assessment which should be provided in a Ground Investigation Report
Immediate settlement	Settlement which takes place during the application of a load as a result of elastic deformation without change in the water content
Inundation settlement/ collapse compression	Occurs in poorly compacted Made Ground or fill placed dry of optimum moisture content and with high air voids. The failure mechanism is typically attributed to the subsequent rising of groundwater or surface water percolation, where the voids are filled with water and cause delamination. Burst pipes and leaking drains present a risk. It can occur in both cohesive and granular soils, with greater risk in cohesive soils
Landscape fill	Unsuitable to support shallow foundations for buildings, ground floor slabs for buildings, other critical elements sensitive to differential ground movements, or any external works such as pavements, services or garden walls. Loading may be limited to construction equipment. There may be no geotechnical testing requirements for landscape fill other than for workability reasons. Landscape fill may be specified for garden areas alongside earthwork slopes and bunds in some developments. They are sometimes referred to as Bulk fill
Made ground	Material that has been placed but to an unknown or unrecorded filling specification. Its geotechnical and geo-environmental properties need to be established by appropriate ground investigation and testing, for it to be reclassified and its suitability for reuse established
Material classification	Assignment of materials into groups and classes for earthwork purposes. The groups and classes are established by testing of the geotechnical properties of a material to determine its suitability for use as fill. The properties may include moisture content, grading, plasticity, compaction and recompacted CBR and shear strength etc. Testing continues during earthworks placement to ensure material properties remain consistent and within defined limits to achieve the required compaction or fill performance
Performance testing	Load-settlement testing of the completed engineered fill at its surface and ahead of building foundations
Primary settlement or consolidation	Occurs after immediate settlement of material on loading. It occurs as pore water pressures dissipate and usually comprises the most significant element of the total settlement of a cohesive fill. Completion of primary settlement is most easily identified by graphical analysis of settlement data showing a ‘levelling off’ behaviour as secondary settlement then commences
Raft foundation (Semi-rigid raft)	A foundation consisting of continuous slab elements and down stand beams designed in accordance with Chapter 4.4 to spread superstructure loads over the building footprint and provide a capacity to span a defined loss of support
Reinforced strip	A reinforced concrete foundation designed to support load-bearing walls or structure and to span a loss of support and typically reinforced with two layers of mesh reinforcement
Secondary settlement or consolidation	consolidation Occurs after Primary settlement (ie, creep) and may continue for many years including during the life of the development depending on the soil types
Selected and Structural fill	Suitable for the support of shallow foundations for buildings and other critical elements sensitive to differential ground movements or as backfill underneath ground-bearing slabs against earth-retaining structures. The completed fill is required to have a high degree of uniformity, high stiffness and low settlement potential and requires high degree of supervision and control and a high frequency of selection classification, compliance and verification testing
Self-weight settlement	Settlement induced in a fill due to its own mass. The magnitude of self-weight settlement is a function of both the stiffness of the fill, and its overall thickness. The contribution of self-weight settlement can be significant in cohesive soils
Soil Modification	Mechanical and/or chemical treatment of soils to create a suitable moisture content of an engineered fill to achieve the specified requirements of minimum relative compaction and maximum air voids. Processes include (non-exhaustively); destoning, formation of windrows with mechanical mixing or turning to promote air drying, addition of low-dosage (no greater than 2% by dry weight) binders such as lime and/or cement in order to moisture condition, lower plasticity and/or control silt fraction to promote optimal compactibility
Soil Stabilisation	Mechanical and / or chemical treatment of soils to create substantial gain in strength to produce structural soil layers contributing significantly towards the performance or stability of earthworks. This would include (non-exhaustively) soil mixing with high dosage chemical admixtures or grouts, hydraulically bound materials and reinforced soils overlying weaker strata. It is currently outside the scope of this Chapter and not accepted for the support of foundations
Specification for earthworks	Are acceptable methods by which satisfactory compaction of fill materials can be achieved. The three main types of ‘Specification for earthworks’ used in the UK are Method, End Product and Performance
Total settlement	The combined magnitude of immediate, primary and secondary settlements

Clauses for 4.6 Engineered fill

Last updated: 2nd January 2024