This survey is a collaboration between the Association of Geotechnical and Geoenvironmental Specialists (AGS), British Drilling Association (BDA) and Federation of Piling Specialists (FPS). It builds upon the AGS/BDA 2017 survey ‘Spotlight on the industry’ which identified that poor procurement of ground investigation was amongst the top three concerns of the responders.

The purpose of the survey is to identify the level of understanding of, and detail the concerns with, the current procurement processes for UK ground investigation services. The responses will be used by Joint Industry Working Groups under the Procurement of Ground Investigation Steering Group to develop new or amend existing procurement guidance.

We welcome responses from across the industry and wider participation from all stakeholders will bring greater insight. Your response will be treated in confidence and the results of the survey will be shared with the geotechnical community.

The survey can be completed here and will take around 5-10 minutes to complete. The survey will close on Tuesday 31st December 2019.

Full Name: Dr Claire Stone

Job Title: Quality Manager

Company: i2 Analytical Ltd

Having decided at an early age that I wanted to work in analytical sciences in the environmental field, I was lucky enough to undertake an Analytical and Environmental Chemistry degree. Having completed my degree, I then studied for a PhD in Analytical Chemistry and through this work and post-doctoral work specialised in metals analysis, quality control/assurance and other inorganic analytical techniques. Having joined i2 in 2006, I first worked I was first employed as a method development chemist, I then went on to run the inorganic analysis departments, before becoming Quality Manager in 2009.

What or who inspired you to join the geotechnical industry?

It’s hard to say – I’ve wanted to be involved in this type of work for so long I don’t think I can honestly remember who or what first inspired me to do it!

What does a typical day entail?

As well as managing my Quality team to ensure all our laboratories are maintaining our accreditation and extending our scope of testing to ISO 17025 and MCERTS, I also work closely with our Technical teams to develop novel and innovative solutions both in terms of analytical methods and techniques. Being a senior chemist, I also speak to our customers about any challenging sites or problems they have and look to work with them to produce a cost effective analysis proposition.

Are there any projects which you’re particularly proud to have been a part of?

The “asbestos dustiness” method development and accreditation of this particular technique is the project I’m most proud of. This is an innovative solution to provide additional lines of evidence for customers who may face challenges when dealing with asbestos contaminated sites.

What are the most challenging aspects of your role?

Ensuring the highest quality standards are met whilst ensuring that the work carried out by the whole of i2 is commercially fit for purpose.

What AGS Working Group(s) are you a Member of and what are your current focuses?

I’m a member of both the Laboratories Working Group and the Contaminated Land Working Group and my current focus is on bringing more environmental chemistry input to the Laboratories group and ensuring that labs are well represented on the Contaminated Land group. Personally I’m looking at the challenges of deviating samples in respect to both geotechnical and geo-environmental analysis – an analysis is only as good as the sample provided!

What do you enjoy most about being an AGS Member?

The AGS events are always enjoyable and I was lucky enough to speak at one of them – having a presentation really well received by such a diverse audience has certainly made these events my favourite aspect of being an AGS member.

What do you find beneficial about being an AGS Member?

Being involved with a wide range of disciplines working in the geotechnical sector means that both personally and professionally strong relationships can develop and through collaboration more opportunities and challenges present themselves.

Why do you feel the AGS is important to the industry?

The AGS provides a focal group to work through a variety of challenges within the sector and I think the fact that it’s run by the members for the members gives it a great strength.

What changes would you like to see implemented in the geotechnical industry?

I would like to see more practical cross discipline experience in the people involved in the industry. I think that by appreciating and understanding the impact of others work and roles the whole industry would benefit.

Now more than twenty years ago in his review in rethinking construction, Egan said that the construction industry at its best is excellent, but at the time of his review there was concern that it was underachieving and needed to deliver more inherent value. His conclusions also highlighted that the industry in general needs to educate and help its clients to differentiate between best value and price. The same conclusions apply to the Ground Engineering industry, in fact more so since unlike other products of construction built out of the ground, almost all our work is usually concealed from sight, in the ground. For us as an industry then, there really is an imperative to be able to provide evidence and compelling description of our value. Without doubt we are aware of this and often internally discuss this, but our mission is to take this external and present the evidence in terms and language that is understood by our clients and multi-disciplinary partners who hopefully become our advocates.

So, what is value? Clearly value is not necessarily the lowest cost or the quickest solution. A straw poll of leaders in major infrastructure yielded responses along the lines of value being ‘…the most effective way to achieve an outcome with legacy being important…’ This description highlights need for definition of outcome to measure ultimate success, optioneering to assess the most efficient or effective approaches to get there, and an eye on the timeline and downstream benefits. It’s arguably the journey to the outcome and downstream legacy benefits where we need to work most to ensure that counterparts and clients understand the value of our work.

Certainly, in the public sector, delivery of an outcome is rightly increasingly emphasising more than just simply cost with a move away from just transactional business to integrated, collaborative outcomes-focused delivery. Social value is often mentioned in tender criteria for ground engineering work, but a blended approach considering all of the Capitals is the direction of travel in major project procurement, these being human, manufactured, financial, natural, social and intellectual. Our industry regularly makes substantial contributions in all the Capitals areas, and especially in the circular economy, but would benefit from work, perhaps most appropriately initiated by the AGS, to measure and document outcomes more explicitly against these criteria in an array of common project tasks, building a body of compelling case study evidence. This would need to be accessible in the widest sense, kept current and be in those common areas that the target audience can easily relate to in language they appreciate. Doing so would probably place us at the forefront of the construction industry ahead of our colleagues in other related disciplines but we need it more given our benefits to a project are usually less obvious as noted above, unlike architecture, structural or civil engineering.

A decade ago the Waste Resources Action Programme (WRAP) initiated a study with similar intent, advocating for the use of geosystems in civil engineering applications. The primary objective was reduction of wastage but this was to be achieved by greater knowledge and use of engineered geosystems (geosynthetics) as actually selected alternative options to conventional ground engineering construction approaches. In doing so this raised the profile of the geosynthetics industry principally through the presentation of case studies quantifying cost, time and environmental benefits by evaluation against more conventional construction approaches. The guidance was presented in an accessible way with information that clients and the developer market could relate to by including non-technical information and quantification in directly relevant terms, these being mainly financial savings.

Quantifying the benefits, the information was compelling mainly from identifying, especially in time and cost terms, the advantages of re-using site-won spoil which would otherwise have been sent to landfill and substituted for imported higher specification aggregates as well as high carbon steel and concrete. Case studies included back analysis of actual construction of environmental, financial and carbon cost of works including a grade separated highway interchange, a noise/environmental bund, retaining walls and fill platforms as compared to the delivery of the initial design. The study did present challenges in compiling evidence in that initial design information was sometimes not developed in detail and required reasonable assumptions in quantities to derive environmental and financial costs. However, the outcomes were nevertheless clear in terms of potential benefits.

Refreshed and expanded upon, this approach could be used as a template for building information on the benefits that the ground engineering industry brings, by utilising case studies underpinning common themes with clients. Current conversations within our industry are commonly too internally-focused, and availability of this type of information is invaluable to allow us to take our regular conversations externally and talk in those terms clients, counterparts, developers and, for that matter, the general public understand. Generally, the industry has a wealth of experience in the benefits of various ground engineering tasks which are almost waiting to be documented.

Case studies could include several case studies valuing a focused and appropriate site investigation versus the usual acknowledgement by decision-makers on the ‘need for boreholes’ without understanding the specific direct downstream benefits these provide in risk mitigation and options for geotechnical design and the opportunities for more sustainable or innovative solutions. They may also include common work in optimising retaining walls through further analysis and the direct opportunity to slim down or shallow the wall through more analysis. Ground improvement is essentially entirely directed to optimisation of shallow foundations and surely would be more commonly used or requested if information on applicability and advantages was better described in a non-technical and quantified way. Earthworks and re-use of materials is an area where there is perhaps most to gain through this approach.

In summary then, surely with the tools and knowledge we now have and routinely use, the time is ripe for us to take the initiative and move one step further in talking about value with our clients, regularly including specific value statements in our work, describing short term investment for longer term gain, development of non-technical guidance documents to demystify the industry, surveying our clients to let them know we’re serious in focusing on our customers and solicit areas for work to improve our offering, creating a collective compendium of quantified case studies identifying the value that the ground engineering industry contributes to society including the value ground engineering can bring to enhancing sustainable solutions. Some of this the AGS has certainly initiated but is this not a wider role for the AGS to initiate through a working party or similar?

Remembering the sage-like but obvious conclusion from Egan, we as specialists need to educate and help our clients differentiate between value and lowest price.

Article provided by Patrick Cox, Director Major Projects at AECOM

Introduction

The use of the Dynamic Cone Penetrometer (DCP) also commonly known in the UK as the TRL Penetrometer or TRL Probe has been under discussion recently in the AGS Safety and Geotechnical Working Groups due to concerns raised by members regarding the significant danger of injury from damage to underground utilities, manual handling injury and the quality of the geotechnical data output.

The Dynamic Cone Penetrometer (DCP) is an instrument designed for the rapid in-situ measurement of the structural properties of existing road pavements constructed of unbound materials. The robust and simple design means that the DCP is quick and easy to use, portable, low cost and suitable for use in locations where access may be difficult. It is commonly used in the UK to determine a California Bearing Ratio (CBR) profile for pavement design.

This article discusses the history of the test, the safety concerns and geotechnical design limitations of the DCP in modern practice and provides alternative methods which must be considered by the Designer.

Historical Background

The earliest technical references to the Dynamic Cone Penetrometer (DCP) suggest that it was developed in 1959 by Professor George F. Sowers, Professor of Civil Engineering, Georgia institute of Technology, Atlanta, USA¹. The original DCP used a 15 lb weight dropping over 20 ins and used a 40^o cone. This DCP was originally developed for field exploration and for verifying individual footing foundations during construction. The DCP was further developed in South Africa for the evaluation of in-situ pavement strength or stiffness of newly constructed roads in the 1960s. Dr. D. J. van Vuuren designed this version of the DCP with a 30° cone².

The Transvaal Roads Department in South Africa began using the DCP to investigate road pavement in 1973³. Kleyn reported the relative results obtained using a 30° cone and a 60° cone. In 1982⁴, Kleyn described another DCP design, which used a 60° cone tip, 8 kg (17.6 lb) hammer, and 575 mm (22.6 in) free fall. This design was then gradually adopted by countries around the globe including the USA and by Transport Research Laboratory (TRL) in the UK. In 2004, the ASTM D6951-03 Standard Test Method for Use of the Dynamic Cone Penetrometer in Shallow Pavement Applications described using a DCP with this latest design⁵.

Typical DCP/TRL Probe

DCP Procedure

The 8 kg free fall hammer is manually lifted and dropped through a height of 575mm. The distance of penetration of the cone tip is then recorded and the cycle repeated. Continuous measurements can be made down to a depth of approximately 850mm or when extension rods are fitted to a maximum recommended depth of 2 metres.

DCP testing consists of using the DCP’s free-falling hammer to strike the cone, causing the cone to penetrate the base or subgrade soil, and then measuring the penetration per blow, also called the penetration rate (PR), in mm/blow. This measurement denotes the stiffness of the tested material, with a smaller PR number indicating a stiffer material. In other words, the PR is a measurement of the penetrability of the subgrade soil.

Technical Output from the DCP

The most common use of the Dynamic Cone Penetrometer (DCP) is to provide a quick and simple field test method for evaluating the in-situ stiffness of base and subgrade layers for roads and highways, and DCP testing has been used in many countries and US States for subgrade evaluation and QA/QC procedures. The greatest advantage offered by the DCP is its ability to penetrate underlying layers and accurately locate zones of weakness within the pavement system.

Correlations have been established between measurements with the DCP and conventional in-situ CBR so that results can be interpreted and compared with CBR specifications for pavement design. TRL Report TRL587⁵ is the most common correlation method used in the UK and is widely specified. A typical test takes only a few minutes and therefore the instrument provides a very efficient method for obtaining information which would normally require the digging of test pits.

Technical Limitations

This instrument is typically used to assess material properties down to a depth of 1000 mm below the surface. The penetration depth can be increased using drive rod extensions. However, if drive rod extensions are used, care should be taken when using correlations to estimate other parameters, since these correlations are only appropriate for specific DCP configurations. The mass and inertia of the device will change and skin friction along drive rod extensions will occur.

Correlations to CBR in homogenous, fine grained soil types are potentially the best use of the tool, or for finding boundaries of known (engineered or natural) layers where there is significant difference in resistance i.e. soft clay over dense gravel. Heterogenous soil types, e.g. Made Ground, with a recommended CBR from DCP may represent a false materials characterisation presented in a report to a civils designer not fully understanding the technique’s limitations.

Health and Safety Concerns

The nature of the test means it is manually operated from ground level without view of the materials and ground conditions being penetrated. The standard equipment comprises in basic terms a metal cone which is connected to metal rods which are driven into the ground using a metal weight and slide mechanism. Incidences of personal injury from manually driving or pushing metal spikes or road pins into buried services have been well documented and must be considered a significant risk. The equipment is not insulated and the procedure requires the operative to hold the equipment^6,7 and includes the potential to force-drive through obstructions which might include unobserved buried services.

Although not a DCP test but effectively a similar process, AGS in 2017 (SP Energy Networks) reported an incident where an operative drove a steel road pin into a HV electrical cable and more recently a Safety Alert was released by Geoffrey Osborne in 2019 relating to a similar incident. Driving of road pins on Network Rail requires a permit as it is deemed a significant risk and many contractors have banned the use of road pins or the driving of other metal rods (i.e. earth spikes) into the ground.

With regard to manual handling, the equipment is bulky and unstable unless held firmly at what may be head height for some operators including the 8 kg hammer at the top. Generally, it requires two persons to perform the operation, but is often carried out by single operators to minimise cost. Although some DCP systems have mechanical jacks to extract the rods the typical use in the UK is by back hammering or manual pulling if there is resistance during extraction of the rods and cone. These practices could lead to not only damage of the equipment but the risk of musculoskeletal injuries must be considered.

The device was designed before modern standards in health protection, or the practicality of manual handling in a more regulated industry was considered. It is the role of the Designer in CDM to reduce or preferably eliminate risks. Designers specifying a DCP as part of a compliant ground investigation to obtain data for pavement design are simply failing in their CDM duty where this hazard is identified because a CBR value can be obtained in a safer way which can reduce or even eliminate the risk.

Alternative methods for CBR

The common practice to determine CBR values before DCPs were through collecting a bulk sample from hand excavated pits and carrying out laboratory testing. It is acknowledged that excavating a trial pit to obtain a sample is not without risk, but that is a recognised industry methodology and would be expected to have mitigation to avoid underground services strikes.

In situ CBR values can be obtained from surface or the base of a hand excavated trial pit using In-situ CBR equipment or Plate Load Test (PLT) equipment in accordance with BS 1377 procedures. The limitation of these tests is that they only provide a single value and not a profile and results can be detrimentally affected by coarse materials in mixed soils or near surface desiccation in fine soils.

If a CBR by depth profile is essential for design then the use of equipment such as the Lightweight Deflectometer (LWD) could be considered. The LWD is a device that estimates the in-situ modulus of a material using the impulse load produced by the impact of a falling weight. LWDs are particularly useful for estimating the moduli of asphalt, aggregate base, granular subbase and subgrade pavement layers. LWDs consist of a mass (often 10 kg), an accelerometer or geophone, and a data collection unit and are designed to be light enough to be moved and operated by one person.

Conclusions

The use of DCP to provide QA/QC data for newly constructed highways and earthworks where the location of buried services is known is considered to be a useful, quick, low cost and relatively low risk method. However, when DCPs are specified along routes for new highways or more generally to determine CBR values for pavement design the risk becomes significantly greater from injury through damage to buried services and the method has technical and quality concerns.

The method has been extended for purposes beyond its original design which often do not take into account the modern environment. The construction CBR value provided through laboratory testing which also provides soil type and compacted density, or through an in-situ surface test such as In-situ CBR/PLT is recommended as being faithful to the original correlation with Californian rock gravel performance under relevant loading conditions.

The use of LWD equipment could reduce both the risk of injury from damage to buried services and manual handling even further and this method will provide a CBR depth profile which is now the most common reason in ground investigations for specifying a DCP.

The justification for the Designer to specify DCP must be carefully considered and not driven by lowest cost and a robust risk assessment must be carried out. Ground investigations, especially those on brownfield sites, using the DCP may not be considering the technical limitations of the test and may not fully take into account the safety risks. Therefore, Designers under CDM may be failing in their duty to eliminate or reduce risk and through specifying DCPs are putting persons at harm.

The AGS is currently considering the safe use of DCPs and alternative methods in order to reduce or eliminate the safety risks to members. It would welcome opinions and thoughts from its members and also technical data which could support the promotion of alternative methods.

References

¹ George F. Sowers and Charles S. Hedges: 1966 : Dynamic Cone for Shallow In-Situ Penetration Testing – Vane Shear and Cone Penetration Resistance Testing of In-Situ Soils, ASTM STP 399, Am. Soc. Testing Mats., p. 29.

² van Vuuren, D. J. : 1969 : Rapid Determination of CBR with the Portable Dynamic Cone Penetrometer, The Rhodesian Engineer.

³ Kleyn, E. G. : 1975 : The Use of the Dynamic Cone Penetrometer (DCP). Transvaal Roads Department, South Africa.

⁴ Kleyn, E. G., Maree, J. H., and Savage, P. F. : 1982 : The Application of a Portable Pavement Dynamic Cone Penetrometer to Determine in situ Bearing Properties of Road Pavement Layers and Subgrades in South Africa. The European Symposium on Penetration Testing, Amsterdam, Netherlands.

⁵ ASTM D6951 / D6951M – 18 : 2018 : Standard Test Method for Use of the Dynamic Cone Penetrometer in Shallow Pavement Applications.

⁶ Zohrabi, M and Scott, P.L : 2003 : TRL Report TRL587, The correlation between the CBR value and penetrability of pavement construction material. Transport Research Laboratory

⁷ Done, S.; Samuel, P. : 2004 :User manual UK DCP 2.2. Measurement of road pavement strength by Dynamic Cone Penetrometer.

⁸ Jones C R and J Rolt (1991). Operating instructions for the TRL dynamic cone penetrometer (2nd edition). Information Note. Crowthorne: Transport Research Laboratory.

⁹ BS 1377 : 1990 : Methods of test for soils for civil engineering purposes. British Standards Institute

Article contributed by James Harrison, previously Delta Simons and Julian Lovell, Equipe Group

NHBC is the UK’s leading warranty provider for new homes in the UK with a market share of around 80%. We deal with over 4,000 sites per year from 9,000 house builders on our Register with around 165,000 new homes currently registered annually.

 

Builders on the NHBC Register are required to build new homes in accordance with NHBC’s Standards to be acceptable for Buildmark warranty cover. The Standards specify the Technical Requirements of NHBC along with the performance standards to be achieved in the design and construction of new homes. Guidance is also provided on how the performance standards may be met.

Part 4 of the NHBC Standards covers foundations including requirements for; land quality, building near trees, strip and trench fill foundations, raft, pile, pier and beam foundations, and vibratory ground improvement techniques.

Consultants and/or specialist contractors preparing remediation strategies or substructure designs for NHBC registered house builders should be aware of the requirements of NHBC’s Standards. This is to ensure that the designs prepared will satisfy NHBC’s Technical Requirements and performance standards in order to be acceptable for Buildmark warranty cover.

Additionally landowners, land developers, development agencies and third parties remediating brownfield land for sale to house builders for residential development should also be aware of NHBC’s Standards and requirements. This will help house builders to avoid potential difficulties when they register the site for Buildmark warranty if the remediation strategy adopted, and the verification of any works undertaken, prior to acquisition of the land does not satisfy NHBC’s requirements. NHBC can offer support to non-NHBC registered companies remediating land for subsequent sale for residential development through a Land Quality Endorsement (LQE) service (www.nhbc.co.uk/lqe)

 

The NHBC Standards are available online at:

http://www.nhbc.co.uk/Builders/ProductsandServices/Standardsplus2019/#7

In Part 4 of the NHBC Standards, Chapter 4.1: Land Quality – managing ground conditions provides a framework for managing geotechnical and contamination risks with the objective of ensuring that:

• All sites are properly assessed and investigated for potential geotechnical and contamination hazards.
• Foundations and substructure designs are suitable for the ground conditions.
• Sites are properly remediated where necessary or appropriate, and design precautions are taken.
• Appropriate documentation and verification are provided to NHBC.

On potentially hazardous sites, NHBC seeks to adopt a pro-active risk management regime on developments registered for Buildmark warranty with the aims of:

• Reducing the potential for defective or damaged buildings.
• Ensuring risks to human health are addressed.
• Mitigating the likelihood of claims against Buildmark and significant claims costs for both house builders and NHBC.
• Avoiding reputational damage for all stakeholders.

With the increased use of brownfield and marginal land for residential development, sites frequently have significant geotechnical and environmental issues that need to be satisfactorily addressed in order to meet the requirements of NHBC.

It is essential that a holistic approach to the overall remediation strategy is adopted for all brownfield and marginal sites. This is to ensure that both the geotechnical and environmental issues are considered in tandem and are aligned, so that the aims and objectives of each individual strategy are not unduly compromised, and the overall strategy delivers the desired performance over the 60-year design life required for new homes.

When undertaking technical assessments for new residential developments on brownfield or marginal sites with complex geotechnical and environmental issues, typical areas of concern and focus on proposals and designs submitted to NHBC for Buildmark warranty cover include:

a) Geotechnical

• Inadequate or insufficient geotechnical site investigation and testing.
• The presence of soft and compressible alluvial soils prone to large and potentially long-term settlements.
• Deep un-engineered or partially engineered fills within landfills and quarries.
• Landforming on sites with deep Made Ground or compressible soils with significant cut and/or upfilling resulting in heave or significant settlements.
• Clarity of objectives for geotechnical remediation strategy including proposed foundation solutions.
• The appropriateness of proposed ground treatment(s) to achieve the required bearing capacity and settlement characteristics for the development platform.
• Suitability of engineered fill specifications particularly when proposed for the direct support of spread foundations, including; compaction method, testing regime, testing frequency, treatment of failures and verification.
• The potential for collapse compression/inundation settlement on deep fill sites due to rising groundwater or infiltration including consideration of building drainage solutions on the site.
• The robustness of the verification reporting on the implementation of the earthworks strategy.
• Planned development over, or in close proximity, to high walls.
• Long term settlements secondary and/or creep settlements over the 60-year design life of the new homes. Typically, NHBC will require any assessment to demonstrate that angular distortions/tilts will be no worse than 1 in 400 over the design life.
• Potential short and long-term differential movements between rigid foundations solutions (e.g. piles) and external areas over the design life of the new homes.
• Robust analysis and assessment of short and long-term settlements due to; building loads, raised ground levels, consolidation of cohesive soils and long-term creep in deep backfills and/or Made Ground. This includes the adoption of appropriate geotechnical parameters in settlement analyses/estimates.
• Theoretical settlement estimates/assessments validated by load testing and/or settlement monitoring to demonstrate the actual load/settlement characteristics of the remediated development platform.
• Due allowance for negative skin friction in pile designs on sites with deep fills and Made Ground or significant upfilling.
• Co-ordination of geotechnical and environmental remediation strategies.
• Co-ordination of post-remediation infrastructure/development works to ensure the geotechnical remediation works undertaken are not compromised or adversely affected (e.g. further changes to ground levels after remediation)

b) Environmental concerns

• The adequacy of desk studies/Phase 1 assessments.
• Robustness and completeness of conceptual site models.
• Inadequate or insufficient geo-environmental ground investigation, testing and assessment following Phase 1 investigations.
• The appropriateness of the contaminant testing suite and the frequency of testing and distribution on the site.
• The use of appropriate assessment criteria for contaminants in soils and controlled waters.
• The interpretation and assessment of environmental data obtained from ground investigations and appropriate updating of the conceptual site model.
• Appropriate characterisation of ground gas regimes and any protection measures that may need to be adopted.
• The robustness of the verification reporting following implementation of the environmental remediation strategy.
• Co-ordination of geotechnical and environmental remediation strategies.
• Co-ordination of post-remediation infrastructure/development works to ensure the geo-environmental remediation works undertaken are not compromised.

Often when things go wrong, the investigation of claims against Buildmark identify how these could have been avoided as the following examples demonstrate:

Example 1 – Excessive ground movement

In this example, the new homes were constructed on a site with soft compressible soils, including peat, to depths of 9m below the original ground levels. The properties were on piled foundations with a cantilevered path around the perimeter of the building to support services and safeguard the threshold at entrances to the buildings. However, during development, a piling mat was installed, and ground levels were also subsequently increased by up to 1m to suit final development levels. No measures were implemented to mitigate settlement of the underlying soft ground due to the increase in ground levels. After 12 months, 200mm to 300mm of settlement was recorded around the properties resulting in excessive differential movement occurring between the rigid piled substructure and the external ground. This incurred significant remedial costs on the affected properties. Claims on these properties could have been avoided by ensuring that the strategy for the site not only considered the foundation solution for the buildings but also the effects of raising ground levels on the underlying soft compressible soils.

Example 2 – Excessive ground movement

This example is another situation where potential for post development ground movements was not considered. In this case, the site was underlain by soft and very soft alluvial clays to depths of 15m. Piled substructures were adopted due to the poor ground conditions, but no consideration was given to post development settlements that may occur. A piling mat was provided to facilitate the piling works and some upfilling of the site was undertaken. This resulted in consolidation of the soft alluvial soils with between 75mm to 100mm of differential movement between the substructure and external that compromised access to the building and incoming services. Once again, claims on these properties could have been avoided by ensuring that the strategy for the site not only considered the foundation solution for the buildings but also the effects of raising ground levels on the underlying soft compressible soils.

Example 3 – Contamination in soils

On this rural development of 5 plots with extremely large rear gardens, fibrous boarding was identified in the gardens following occupation by the homeowners. Subsequent investigations revealed the presence of Asbestos Containing Material (ACM) in the topsoil and subsoil, typically within 500mm of ground level but locally up to 1m in depth. The volume and concentration of ACM in the soil was determined to represent a significant risk to human health and substantial remediation was required in the gardens. Investigations identified; inadequate site investigations were undertaken prior to development along with lax control during the decommissioning and demolition of buildings previously occupying the site. The claims on these properties could have been avoided with an adequate site investigation and conceptual site model for the site, together with appropriate controls during the demolition of the original buildings.

In the examples given above, the claims against Buildmark warranty resulted in significant claims costs and disruption to the lives of the affected homeowners. These could have been avoided had the ground conditions been fully appreciated and appropriately managed, during both the development of the scheme design and subsequent construction.

In summary, it is essential that all brownfield and marginal sites with complex ground conditions are appropriately investigated and assessed to identify the potential geotechnical and environmental risks. This should include consideration of; the works required to create the development platform and any additional risk this may create, the types of building proposed and the intended foundation solutions etc.

For residential sites registered for NHBC Buildmark warranty, development designs and proposals will be assessed against the requirements of NHBC’s Standards and expected to satisfy Chapter 4.1: Land Quality – managing ground conditions.  All designers providing services to NHBC registered house builders and/or land owners or land  developers remediating brownfield sites for residential end-use should ensure they are familiar with NHBC’s requirements to; avoid development proposals being determined as unsuitable for Buildmark warranty cover, to assist in reducing risks and to help prevent claims for the benefit of all stakeholders, particularly home owners.

If you’d like to learn more about our LQE service please visit www.nhbc.co.uk/lqe or contact us on lqe@nhbc.co.uk

Article contributed by Adam Gombocz, Senior Geotechnical Engineer and John Jones, NHBC Engineering Manager