What to expect from NHBC’s new guidance on engineered fills

We sat on the industrial liaison panel for the NHBC’s new guidance for engineered fills. Here’s a quick need-to-know summary. 

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 below on fill designations 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.

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:
    • Simple wherein a design statement is satisfactory
    • 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)

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

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.

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%

Acceptable fill treatments outside of compaction

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

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)

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.

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

Related Tags

Related Blog

What to expect from NHBC’s new guidance on engineered fills

We sat on the industrial liaison panel for the NHBC’s new guidance for engineered fills. Here’s a quick need-to-know summary. 

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 below on fill designations 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.

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:
    • Simple wherein a design statement is satisfactory
    • 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)

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

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.

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%

Acceptable fill treatments outside of compaction

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

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)

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.

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

Tags

Related Blog

What to expect from NHBC’s new guidance on engineered fills

We sat on the industrial liaison panel for the NHBC’s new guidance for engineered fills. Here’s a quick need-to-know summary. 

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 below on fill designations 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.

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:
    • Simple wherein a design statement is satisfactory
    • 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)

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

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.

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%

Acceptable fill treatments outside of compaction

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

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)

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.

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

Related Tags

Related Blog