1.  Engineered fill in context

These notes are written around land development and industrial development projects. These types of projects are distinct from other projects which have substantial earthworks components such as (non-exhaustively) civil engineering projects like roads construction, rail construction and flood defence construction. While there is substantive technical overlap with projects of this type, the context is markedly different and thus consideration of the earthworks would require a different synthesis. As such, these notes should not be used outside of land development and industrial development context.

In broad terms, engineered fill is any soil placed in such as fashion as to achieve specified engineering criteria. Typically, this involves placing the soil in thin layers and applying some form of compactive effort in order to increase the as-placed density of the soil. Nominally, the engineering criteria are adopted in service of a geotechnical or structural model such that inherent design assumptions are realised. These assumptions may involve strength, stiffness, permeability, chemical stability or combinations of such parameters. This is communicated to the earthworks contractor via the earthworks specification and realised in the field via an inspection & test plan (ITP) distilled from the specification.

Specifications can be standardised for particular applications – see for example the Specification for Highway Works [1] which was developed based on multi-decadal experience by the Transport Road Research Laboratory & the Highways Agency and is designed to adequately specify large volumes of fills for road construction. In land development and industrial applications, specifications are often bespoke and can be somewhat nebulous in how they are generated and organised as amalgams of standard specifications with particular requirements that can often be a source of conflicting information and confusion in application. Notwithstanding, it is generally the case in land development projects that specifications for earthworks are some form of end-product requirement for fill with some form of performance requirements. Respectively, these criteria are usually based around achieving a minimum relative compaction with maximum air voids in the fill and some limiting settlement of the fill overall.

1.1.        Definitions of engineered fill

There is a certain amount of colloquialism in defining engineered fills but broadly there are three categories (these definitions follow the NHBC’s new definitions):

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 or against earth-retaining structures. The completed fill is required to have a high degree of uniformity, high stiffness and low settlement potential and requires a high degree of supervision and control and a high frequency of selection classification, compliance and verification testing.

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.

Bulk / 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 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.

Other fill distinctions that are worth noting include (again to NHBC guidance [2]):

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’)

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’)[Note: see also Section 3.1.2]

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.

2.  Broader concepts

2.1.        What do you want to achieve?

Earthworks for land development and industrial development is a relatively straightforward enterprise but can frequently become muddled where a centralised well-defined scope is not provided – this leads to ill-defined technical problems and thus technical solutions that are either not fit for purpose or wholly over-designed. In recent years the general trend has been commodification of design where, in land development projects, the developer (or their appointed consulting engineer) will approach specialist subcontractors such as piling and ground improvement contractors, in order to establish a working scheme which is often subject to re-arrangement and multiple low-intensity phasing. Frequently schemes will then be procured through the groundworker or alternatively, the groundworker will be approached to provide a complete development plan based on bespoke (and often loose and informal) design & construct contractual arrangements.

Industrial schemes are generally procured differently. It is quite typical that a Principal Contractor will be engaged by the developer on an all-risk (usually rigorous and onerous) contractual basis to provide a functional industrial scheme based heavily around an architect’s scheme and a basic civil/structural arrangement from a consulting engineer. The scheme is typically rigidly fixed and is usually either a single phase or small number of high-intensity phases. The Principal Contractor will usually engage and manage subcontractors on bespoke (equally or more onerous) design-and-construct contractual arrangements to build the scheme.

The point here is that the procurement route is contextually important and adequate definition of the project earthworks requirements is the best way to simplify the scheme because it ensures the developer is purchasing the correct solution for the problem at hand. All too often, passive consulting engineers allow the procurement route to define the scope of works or draw a line at the ground defining anything below as a ”contractor-designed element” instead of leading the technical development of the project. There’s nothing technically wrong with this except that the structural requirements are often frustratingly ill-defined. For example, a structural engineer will often define the requirements of a large-area, ground-bearing floor plate in terms of bearing capacity and settlement instead of the stiffness assumption that (presumably) informed the structural model that formed the basis of the design of the structural design. This often leads to large scale ground improvement underneath fill that is both unnecessary & costly and doesn’t necessarily provide what’s actually needed.

So, in sum, it is crucial that there is enough practical experience and technical expertise in the design team to adequately manage the scope of the earthworks. Contract trading is a poor approach (e.g. if you go to a piling contractor they will try to sell you piles) but, alas, much of the industry is geared this way.

2.2.        Settlement is the key

While examination of methods of assessing settlement is out with the scope of these notes, it should be understood universally that the most important aspect of placing fills is assessing the associated settlements induced in the formation by the fill once placed and of the fill itself – one of the primary tenets of structural fill which distinguishes it from general fill is its very low susceptibility to self-weight compression which can be quite substantial for deep fills. This can be done qualitatively – for example express calculation of settlement would not be required where thin fills are placed on stiff ground. However, the majority of land development sites need some form of settlement estimate to demonstrate that the fill will not settle excessively and/or that the relative movement of the fill and structure is not excessive. Some common misinterpretations of settlement giving rise to woefully high remedial costs are as follows:

1. General fill placed on marginal formations with piled structures – here the fill settles relative to the structure giving rise to an unacceptable serviceability condition and downdrag loading on the pile.
2. Poorly compacted general fill around a “podium” of structural fill giving rise to excessive or collapse compressions of the general fill and gross unacceptable variation in settlement at the interface of the two fill types.
3. Upfilling onto compressible formations without adequate or properly completed settlement modelling or monitoring giving rise to commencement of structures before an acceptable rate of settlement is achieved. Commenced structures are then subject to excessive settlements post-construction. Delays to construction programmes because settlement rates are too high to commence.

2.3.        Bearing capacity and stiffness

It is not uncommon for structural engineers to specify bearing capacity requirements for structural fill. For example, it may be a requirement for houses on structural fill in a land development scheme to have a requirement for 100kN/m² bearing capacity with no more than 25mm over the design life of the structure. Where the fill is deep (i.e. where the fill is greater than, say, twice the foundation width) this can be a trivial requirement as structural fill is highly compacted – settlement of the fill is far more critical to examine. Structural fills will often achieve undrained shear strengths of well over 100kN/m² or effective stress parameters of f’ = 34°+ which means the ultimate bearing capacity will be very high. For thin fills where the underlying formation soils will be engaged by the foundation, layered bearing capacity calculations would be required.

Stiffness, on the other hand, can be more difficult to achieve. More specifically a consistent stiffness across the site. This is a consideration in, for example, industrial applications where large-area floor plates on fill may require a high and consistent stiffness to facilitate a lean structural design of a ground-bearing slab. In such a scenario, the most useful criteria to specify in the earthworks is a minimum subgrade reaction modulus along with an acceptable working range.

2.4.        The whole picture – thinking about ground investigations

It is beyond the scope of these notes to discuss ground investigation scoping and procurement in detail. However, in broad terms intrusive ground investigation strategies for earthworks for land development projects should yield the following basic results:

Solid geology: information sufficient to facilitate contour mapping of the rockhead, description of weathering, assessment of mining risk and, where shallow, excavatability assessments

Spatial variability of overburden and groundwater: information sufficient to allow the mapping of discrete soil horizons spatially distinguished by composition, consistency, strength and density. Facilitates contour mapping of groundwater levels and measurements of seasonal variation

Formation compressibility: where an area of the site is expected to be filled, the ground investigation should characterise the compressibility of this area of the site. Investigations should focus on the consolidation characteristics at appropriate stress levels and it is quite important that the expected fill depth is at least roughly known in advance of specifying the testing for such parameters such that the stresses can be properly specified

Compaction characteristics of cut materials: in cut areas of the site sufficient sampling must be undertaken for laboratory testing such that the compaction and stiffness characteristics of the site can be assessed. It is critical that sampling is sufficient to be representative of soil variability. This latterly informs the material reusability assessment

Obviously, ground investigations can have a much broader scope particularly in respect of environmental and geoenvironmental considerations which fall out with the scope of these notes. However, anthropogenic soils (historic fill / made ground) are considered further in Section 3.1.1 in geotechnical terms as generally anthropogenic soils are poorly characterised in this sense.

3.  How is engineered fill produced?

In very simple terms engineered fills for land development are produced through application of compactive effort, possibly with manipulation of the native moisture content of the material, to achieve a defined density of soil, the soil being a complex three-phase system of solid soil particles, air and water. A detailed treatise on compaction theory is outside the scope of these notes. Compaction is not well understood by general practitioners in civil / structural consulting and so this simplification is adequate. More advanced considerations of unsaturated soils are not considered as this is a substantial topic of research and engineering application in and of itself. In respect of producing engineered fill, as a first consideration, the suitability of soils for compaction is considered.

3.1.        Unsuitable soils

The NHBC have compiled a useful list of soil materials unsuitable to produce engineered fill from, summarised as follows:

1. Topsoils, soil containing topsoil or soils with organic matter content greater than 6% by volume
2. Degradeable, reactive or otherwise non-durable materials
3. Expansive materials such as slags or gypsum (subject to testing of relative risk of expansion)
4. Materials capable of releasing contaminants or hazardous gases
5. Material with a calorific value greater than 7MJ/kg placed within the top 1m of the finished development (subject to other good practices to mitigate combustion)
6. Materials containing harmful substances which pose risks to human health or the environment
7. Material where the total potential sulphate is greater than 0.25%
8. Structureless chalk (grades Dm and Dc)
9. Other materials (categorised as materials that are “generally not used as engineered fill on residential sites”) – examples might include coal or lignite, slates or material that is inherently flaky, calcareous soils or materials which are subject to change under compaction (owing to, for example, particle breakage)

Such materials should ideally be identified during the ground investigation stage and spatially quantified during the interpretative stage thus being adequately accounted for in the geotechnical design report / statement.

3.1.1.         A closer look at made ground

Anthropogenic soils (or colloquially made ground) is a general category of material which is of non-natural origin. Current codes of practice do not adequately provide a means to characterise these soils geotechnically and moreover, the overwhelming ground investigation effort in respect of made ground is geared towards geoenvironmental categorisation. Recognising this, the AGS [3] produced a guidance note (very rarely used by ground investigation practitioners but in itself and excellent contribution) to facilitate adequate description of made ground.

Given that nominally the balance of land development projects has shifted from greenfield to brownfield environs, it could hardly be more important to adequately describe made ground in order to understand what can be done with it geotechnically. The following table summarises a useful classification:

 

 

Class 1	Class 2	Class 3	Class 4	Class 5
Granular material	Cohesive material	Organic material	Non-cohesive fine-grained anthropogenic materla	Other identifiable material
Sand- and gravel-sized particles of flint, sandstone, brick, concrete, etc.	Silt and clay	Non-natural organic matter, decayed vegetation, decomposed waste of non-natural origin	Amorphous materials such as chemical precipitates, crystalline material, chemical salts, pastes, sludges, powders such as Galligu or foul limes	Plastics, timbers, sawdusts, metals
Predominantly mineral soil constituents. Proportions of inclusions should be termed in approximate percentage by volume as: •    Rare (<5%) •    Occasional / scattered (5-20%) •    Numerous / frequent (20-40%) Abundant (>40%)		Important to note that peats and relic topsoils would not be identified as anthropogenic soils)	Predominantly non-mineral soil constituents. Here, the site history as prepared in the desk study portion of the ground investigation, will be important in understanding the nature of the constituent materials. Screening, soil washing and other cleaning techniques can be considered.
Provided geoenvironmental factors are amenable, these soils can be geotechnically competent or improved to be so		Generally incompetent materials and engineering options will typically involve by-passing these soils (maybe driven piles) or excavate-and-replacement

Ashes, clinkers, slags, cinders and charcoals are considered separately, needing special consideration owing to potential for expansion and nascent contamination (metallic compounds, sulphurous compounds and PAHs).

3.1.2.         Can all mineral soils be compacted to an engineering specification?

In short, no. Critical to the production of engineered fills is the facility of the soil to achieve a low air voids content. As shown in Figure 3.1 below, uniformly-graded (sometimes referred to as evenly-graded) soils can struggle to achieve low air voids content as the constituent particle sizes are too much the same (the analogy here would be to imagine applying compactive effort to a volume of single-size ball bearings – no amount of compactive effort will reduce the space between the balls as there is not facility to move the balls closer together or for smaller balls to take-up the void space).

Where sandy natural material is expected or where selected granular fills are to be produced or imported, nominally the uniformity coefficient (a measure of how uniformly graded the material is by comparing proportions of particle sizes extracted from the grading curve) is defined to some minimum value in order to assure the material is not too uniform (this would not be the case, for example, for drainage materials where high uniformity can be beneficial in promoting higher permeability and rapid seepage of water). This is shown intuitively in Figure 3.2 where higher fines content promotes higher uniformity coefficients thus improving compactability (note how flat the curve with only 1% fines is). Soils such as this are not common but, for example, some of the marine sands encountered in the northeast of Scotland around Inverness can be very uniform and difficult to compact.

Other soils, such as calcareous soils and filler materials such as pulverised fuel ash (PFA) are unsuitable to produce material to achieve density-driven, low air voids specifications. This is owing, respectively, to particle breakage (resulting in uniformity of particle size) and nascent uniformity of particle size. Figure 3.3 shows typical grading curves for PFA. Observe the flatness of the curves and the resulting difficulty in achieving lower air voids.

Figure 3.1. Relation between dry density and moisture content for various soils and aggregates (2.5kg rammer). Reproduced from Novak & Gilbert [4]

Figure 3.2. Compaction tests results for silty fine to medium sand showing the change in behaviour with increasing silt content from uniformly graded to well-graded fill. Reproduced from Novak & Gilbert [4]

Figure 3.3. Variation in dry density with moisture content for PFA from a range of sources. Reproduced from Novak & Gilbert [4]

3.2.        What is the effect of being wet or dry of optimum?

Indicative optimum moisture contents for normal soil types used to produced fills are given as follows:

Fill type	Rough range of optimum moisture content in modified proctor test (%)
Sand	6-10
Sand-silt mixtures	8-12
Silts	11-15
Clays	13-21

The effects of being either wet or dry of optimum moisture content are summarised as follows (reproduced from Lambe & Whitman [5]):

Fill engineering property	Comparison
Structure –   Particle arrangement –   Water deficiency   –   Permanence	Dry side more random Dry side lower porewater pressure; more susceptible to swell Dry side structure more susceptible to change / collapse / instability
Permeability –   Magnitude –   Permanence	Dry side more permeable Dry side permeability susceptible to reduction
Compressibility –   Magnitude     –   Rate	Wet side more compressible in low stress range; dry side more compressible in high stress range Dry side consolidates more rapidly
Strength –   Undrained remoulded –   Drained remoulded –   Undrained on saturation   –   Drained on saturation –   Porewater pressure at failure –   Stress-strain modulus –   Sensitivity	Dry side higher Dry side somewhat higher Dry side somewhat higher if confined Dry side slightly higher Wet side higher Dry side much higher Dry more sensitive

While there are factors that would imply being dry of optimum is preferrable (higher stiffness, strength), materials compacted dry of optimum are often rendered into a metastable condition meaning that the body of fill is susceptible to substantial volume change, particularly with changes in moisture content. The most onerous result is collapse compression, wherein a small change in moisture content results in gross compression of the body of fill because the dry material will necessarily yield a high air voids content thus facilitating volume loss on inundation.

Materials compacted slightly wet of optimum are more amenable to achieving lower air voids while achieving adequate relative compaction. Materials that are compacted excessively wet of optimum will fail to achieve adequate relative compaction which is the main challenge associated with compacting materials wet of optimum moisture content.

3.2.1.         The 5-point compaction test

Idealised compaction would be determined wholly by the minimum plausible specific volume of the soil. This is most readily examined in the laboratory by measuring dry density. In order to produce a useful means to visualise the relationship between moisture content and dry density including determination of optimum moisture content, at least five points are required to develop the curve. This curve will then provide a means to establish working moisture content limits for a required relative compaction and minimum air voids content. This is illustrated in Figure 3.4.

In developing the curve, compactive effort is standardised at two levels (light and heavy compaction to British Standards, referred to often as standard and modified Proctor tests respectively in American parlance). Light compaction is undertaken with a 2.5kg rammer dropped from 300mm at 27 blows per layer in three layers – this imparts 595kJ/m³ of energy. Heavy compaction is undertaken with a 4.5kg rammer dropped from 300mm at 62 blows per layer in three layers – this imparts 2,675kJ/m³ of energy. It is the purview of the designer then to establish what the appropriate compaction effort is for the earthworks application under consideration. Figure 3.5 illustrates this. For example, area QRTU would be suitable for foundation for buildings whereas area STVW would be acceptable for mass / bulk fills. It should be noted that for sites with high volumes of fill, multiple curves will be need to reflect the spatial and constituent variability of the fill materials proposed for use.

Figure 3.4. Determination of acceptable limits of moisture content. Reproduced from BSI, 2009 [6]

Figure 3.5. Selection of compactive effort for engineered fill. Reproduced from Novak & Gilbert [4]

With reference to Figure 3.6, the following curves are identified:

1. Idealised compaction
2. Modified (4.5kg) laboratory compaction
3. Standard (2.5kg) laboratory compaction
4. Laboratory static compaction at low imparted energy
5. Field compaction – 6 passes of rubber-tired roller
6. Field compaction – 6-passes of sheepsfoot roller

It is useful to observe here that normal field compaction effort will usually result in compaction effort well in excess of the 2.5kg compaction curve (noting that the above data is relatively dated). Advances in compaction efficiencies in newer plant mean that modern well-controlled earthworks practice will usually yield compactive efforts commensurate with the 4.5kg rammer curve almost by default.

Figure 3.6. Comparison of field and laboratory compaction. Reproduced from Lambe & Whitman [5]

The following is a simplified summary of appropriate compaction plant in respect of soil type:

Broad soil description	Compactability	Appropriate plant
Sand and sand-gravel mixtures	Good; structural fill possible;	Vibratory drum roller
Sand or sand-gravel mixtures with silt	Good; structural fill possible;	Vibratory drum roller
Sand or sand-gravel mixtures with clay	Good to fair; structural fill possible;	Vibratory rubber tyre; pneumatic tyre rollers;
Low plasticity silts	Fair to poor; structural fill possible with competent / specialist earthworks contractor;	Sheepsfoot rollers to breakdown clods; pneumatic tyre to finish; any excess water will make compaction untenable;
High plasticity silts	Poor; structural fill likely not possible;	Sheepsfoot rollers to breakdown clods; pneumatic tyre to finish; any excess water will make compaction untenable; modification likely required;
Low plasticity clays	Fair to poor; structural fill possible with competent / specialist earthworks contractor;	Sheepsfoot rollers to breakdown clods; pneumatic tyre to finish; any excess water will make compaction untenable;
High plasticity clays	Poor; structural fill likely not possible;	Sheepsfoot rollers to breakdown clods; pneumatic tyre to finish; any excess water will make compaction untenable; modification likely required;
Organic soils	Not compactable	–

 

3.3.        Soil modification and soil stabilisation: proper definitions

Soil stabilisation is a commonly used term which typically encompasses activities of mixing dry binders into thin soil layers prior to compaction using specialised plant. The most common dry binders are lime and cement. It is a common misunderstanding that the objective here is to improve the strength of the soil substantially to effect better fills. This can be in the case of stabilisation but not so in modification and the distinction can be made as follows:

Soil stabilisation is any process which substantially and purposefully improve the strength of the soil in order to produce a structural soil layer i.e., a layer of strength integral to stability.

Soil modification is any process which effects changes in grading or moisture content to engender a condition of compactibility to some specified requirement.

More simply, soil stabilisation is the act of optimising soil for strength. Soil modification is the act of optimising the soil for compaction. Here, only the latter is considered in respect of producing engineered fills.

3.4.        What does lime do?

Detailed examination of the chemical processes of lime when introduced to soil is not within the scope of these notes, however, the effects can be categorised as follows:

• Dehydration of the soil
• Ion exchange and flocculation of the soil
• Pozzolanic reactions

Here a distinction must be made between quicklime and hydrated (slaked) lime. Quicklime, being mainly calcium oxide (CaO), is substantially more reactive than hydrated lime (Ca(OH)₂) – calcium hydroxide where controlled introduction of water has already been introduced in the production of the powder.

Quicklime will immediately take up 32% of its own mass of water in the surrounding soils on introduction and the exothermic reaction will further reduce moisture content within the soil. A good rule of thumb here is a 1% quicklime addition will produce a net 4% reduction in soil moisture content in well-controlled conditions. In contrast, 1% hydrated lime would produce a net 1% reduction in soil moisture content.

In the short-term, flocculation of the clay particles engenders a more friable and granular condition of the soil which facilitates compaction to a much greater degree. In geotechnical terms the flocculation results in a decrease in the plasticity index owing to an increase in the plastic limit of the soil without a substantive change in the liquid limit (at typical field dosages). This is illustrated in Figure 3.7.

It should be further noted that there will be substantive changes to the compaction curve with the maximum dry density reducing and a shift in the position (increase) of the optimum moisture content. This is illustrated in Figure 3.8. This effect compounds the drying effect in respect of moisture content control but it is important to assure that compaction curves are based on modified samples to ensure correct reference density and optimum moisture content and it can be a source of confusion if modified soil field densities are referenced against (higher) unmodified laboratory densities. In low-plasticity materials, this effect is suppressed and it is mainly the moisture content reduction that is beneficial.

Longer-term effects include carbonation and formation of weak cementitious compounds between soil particles but these effects are normally not relied upon for geotechnical capacity.

Figure 3.7. Classic chart of the effect of lime on the plasticity of London Clay [7]

Figure 3.8. Illustrative effect of lime on the compaction properties of fill materials [4]

3.5.        What does cement do?

Application of dry cement to soil results in complex chemical reactions, the detail of which are outside of the scope of these notes. However, in brief, the porewater of the soils induces rapid hydration of the cement which results immediately in hydrated calcium silicates, hydrated calcium aluminates and hydrated lime. These hydration products bind adjacent cement grains enclosing the soil particles – this is particularly effective at the silt particle size. In addition, the elevated pH of the porewater induced by the cement addition (this is caused by the disassociation of the hydrated lime) causes the dissolution of the soil silica and alumina which are natively acidic resulting in hydrous silica and alumina which subsequently react with liberated calcium ions to form further compounds (calcium silica hydrates and calcium alumina hydrates) which harden and bond the soil particles.

In addition, the reaction is exothermic and will result in moisture content reduction though more in line with hydrated lime effects than quicklime effects. While after mixing, the soil-cement mixture will cure and harden resulting in additional strength gain, these effects are considered secondary for engineered fills at low cement dosages. The primary benefit, particularly with silty soils, is the binding of difficult finer fractions to produce a soil which is more granular in composition and thus more readily compactable. Boulder clays in Scotland are a good example here as they can be very silty and extremely difficult to work at native moisture contents without modification.

3.6.        What other additives and processes are purposeful?

While there are other compounds which can be beneficial, implementation is difficult. This is mainly owing to how the industry has developed over many decades which has centred on the introduction of dry powders. In addition, standard specifications such as the Specification for Highway Works which are commonly invoked and the rigidity of Local Authorities and other regulating bodies, stifle innovation in the field.

GES has undertaken some laboratory work using polymers which are effective in controlling moisture content, but practical implementation on a site-wide scale is, to date, impractical as it involves very small and very precise additions to effect substantial moisture content adjustments.

4.  What to expect from NHBC’s new guidance?

The following section is a rough need-to-know summary of the NHBC’s new guidance for engineered fills.

4.1.        Basics

The document pertains to fill under low-rise residential building structures, external works and infrastructure. It does not pertain to fill under adoptable roads and road construction make-up (this would remain under the purview of the Local Authority Roads Department), fill for reinforced earth structures (Principal Designer), fills for construction platforms (Principal Designer) or stabilised soil (note see section 4.2 in respect of the technical definition of stabilised soils as distinct from modified soils).

The general thrust of the guidance is to assure limited settlements (no more than 25mm) with limited associated angular distortions (no more than 1:400) for structures built on fill. The guidance is also a vehicle for the NHBC’s Land Quality Service particularly where historic fills are on site.

4.1.1.         Technical reporting structure

In respect of technical approvals, the following are the technical reports required to satisfy NHBC:

1. Ground Investigation Report appropriate for the proposed development and properly reported
2. A Geotechnical Design Report (GDR) or Geotechnical Design Statement (GDS) depending on the proposed development. The GDR/GDS should define the geotechnical category of the development as:

1. Simple wherein a design statement is satisfactory
2. Conventional wherein the GDR is satisfactory

• Complex which falls outside the scope of the guidance and design must be by a specialist geotechnical deisgner

1. Materials Classification (Acceptability) Assessment
2. Earthworks Specification (A combination of the GIR, GDR / GDS and Earthworks Specification is required to be submitted to NHBC with at least 8-weeks lead-in for approval)
3. Earthworks Verification Report (on completion of the earthworks or at agreed interim stages)

4.2.        Fill designations

Fills are delineated as follows:

• Engineered fill is taken to be material selected, placed and compacted to a specified engineering behaviour. This would include “structural fills” and “general fills”
• General fill is fill that is selected, placed and compacted to be suitable for support of roads, drainage, driveways and buried services
• Structural fill is fill that is selected, placed and compacted to be suitable for support of shallow foundations and other structural elements
• Granular fill is taken to be free-draining, non-plastic, low-fines materials
• Historic fill is generally what is called made ground
• Landscape fill, occasionally referred to as “bulk fill” is non-structural fill for external works usually such as garden areas and the like
• Modified soil is soil optimised for compaction, typically for structural fill, use via moisture content adjustment. This can include low-dosage additions of lime and/or cement
• Stabilised soil is soil optimised for strength via adjustments to the physical character of the soil through formation of cementitious by-products. This typically involves the use of high-dosage cements, grouts or other chemical admixtures and is generally not acceptable to the NHBC and falls outside the scope of the guidance

4.3.        Formation soils

The guidance contains a list of unacceptable formation soil onto which proposed fills cannot be placed. These include:

1. Topsoil and organic matter
2. Frozen soils
3. Contaminated soils
4. Soils with an undrained shear strength of less than 40kN/m² (noting that ground improvement techniques may be employed to engender acceptability)

It should be noted that it is acceptable to fill onto compressible soils where the compressible soils are allowed to settle adequately and where adequate compaction can be achieved. It is important to note that relative movement must be assessed which means that settlement analyses should form part of the GDR.

4.4.        Precluded material for use as fill

Table 3 of the guidance is comprehensive and delineates materials by suitable, hazardous, requiring NHBC acceptance and unsuitable. Particular points to note:

1. Maximum permissible organic content is 6%
2. Maximum material calorific value is 7MJ/kg
3. Maximum permissible total potential sulphates is 0.25%

4.5.        Acceptable fill treatments outside of compaction

• Air-drying
• Addition of low dosage lime and/or cement
• Water addition

4.6.        What are NHBC looking for from their fills?

In general, the guidance is advocating for performance specification based on end-product requirements which can be summarised as follows:

1. Structural fill will have end-product requirements of 95% relative compaction with no more than 5% air voids under compactive effort equivalent to a 4.5kg rammer augmented by load test or settlement monitoring
2. General fill will have end-product requirements of 95% relative compaction with no more than 5% air voids under compactive effort equivalent to a 2.5kg rammer augmented by settlement monitoring where fill depth is greater than 5m
3. General fill will have end-product requirements of 90% relative compaction with no more than 10% air voids under compactive effort equivalent to a 2.5kg rammer augmented by settlement monitoring where fill depth is less than 5m OR an agreed method specification (the guidance more-or-less advocates the Specification for Highway Works here)

4.7.        What does this mean for foundations?

The guidance allows three options depending on compliance level and level of supervision. The idea would be that failure to achieve full compliance would give the option to move to the next most applicable option.

1. Reinforced strip: full end-product compliance for earthworks undertaken by specialist contractor with very high-level of independent supervision and independent assessment for validation testing and verification reporting
2. Beam grillage: partial end-product compliance for earthworks undertaken by earthworks contractor with high-level of supervision and contractor self-certification for validation testing and verification reporting
3. Semi-raft: partial end-product compliance for earthworks undertaken by groundworker with normal-level of supervision and contractor self-certification for validation testing and verification reporting

To be clear, where a reinforced strip foundation is proposed, it has been clarified with NHBC that the requirement is only to deal with the flexural requirements associated with the 2m simple support and 1m cantilever, i.e. a layer of mesh top and bottom – not to deal with shear forces which would require a deeper beam and fixed cage consistent with the beam grillage option.

4.8.        Some important points for earthworks contractors

• Electromagnetic density gauges are allowable. Suggest increased SRT measurements where used
• The simplest performance criteria is compliant fill with surface point settlement monitoring (see Table 9 of the guidance)
• Frequency of testing is now codified as a minimum number of tests per volume of fill and so this is now a level playing field for all contractors, as follows:

Fill volume (m3)	Frequency
> 100,000	2 per 1,000
10,000 to 100,000	3 per 1,000
< 10,000	5 per 1,000

 

5.  Implementation and cost

Implementation of earthworks schemes can be very simple but for larger scale land development projects, earthworks can be a complex enterprise. All schemes will have their own idiosyncrasies but invariably a primary driver will be cost and more specifically trade-offs between scheme options. Figure 5.1 below shows indicative costs (from earthworks contractor at 2023 prices) of bulk, modified and imported fills as well as muck-away prices. The following scenarios might be some choices the Engineer would need to consider:

1. Low bulk fill cost versus augmented foundation requirements and settlement challenges
2. Conversely, high fill modification costs versus nominal foundation requirements
3. Certainty of imported fill versus very high cost and prospective muck-away costs (which are increasingly more onerous and are often not properly considered)
4. Compounding effect of modifying soil to reduce /eliminate muck-away in lieu of imported fills

Figure 5.1. Indicative earthworks cost versus volume

 

6.  References

[1]	National Highways, “Manual of Contract Documents for Highway Works (MCHW) Volume 1,” Department for Transport, 2023.
[2]	National House Building Council, “NHBC Standards,” National House Building Council, 2024.
[3]	(AGS), Association of Geotechnical & Geoenvironmental Specialists, “Description of anthropgenic materials – A practitioner’s guide,” AGS, Bromley, Kent BR1 1LT, 2018.
[4]	P. Novak, P. Gilbert and N. Trenter, Earthworks: A guide, London: ICE Publishing, 2015.
[5]	T. W. Lambe and R. V. Whitman, Soil Mechanics, New York: John Wiley & Sons, 1969.
[6]	British Standards Institute, “Code of practice for earthworks,” British Standards Institute, London, 2009.
[7]	P. T. Sherwood, “Views of the Road Research Laboratory on soil stabilisation in the United Kingdom,” Cement, Lime and Gravel Limited, New York, 1967.