Novel assessment test for granular road foundation materials

Drivers for sustainability have made it necessary for the construction industry to adapt its traditional processes to become both more efficient and produce less waste. Performance based design and specification in the UK for motorways and trunk roads permits a very flexible approach to pavement design, material selection and performance related testing aimed at utilising materials to their maximum potential. However, it is clear that within the emerging philosophy of using materials that are ‘fit for purpose’ there are many technical challenges for design and specification. There is a need to develop suitable methods of evaluating materials prior to their being used on site. This project was born out of this requirement, with a particular emphasis on coarse granular materials due to their common role in capping construction and also their unique difficulty for measurement under laboratory conditions due to their large range of particle size. A novel assessment test for coarse capping materials for roads that can be used to indicate their likely short-term in situ performance, under controlled laboratory conditions before construction on site, has been developed during this research programme. Key findings relating to the behaviour of coarse capping materials, the use of stiffness measuring devices and variables that influence the measurement of composite stiffness are discussed in detail. The research highlights the necessity for adequate drainage and protection of foundation materials against increase in water content. When adopting a performance specification the timing of the pavement assessment is critical, both on site and in the laboratory. The performance measured on site should perhaps only be considered as a ‘snapshot’ relating to the stress state in the material at the time of testing

The assessment of coarse granular materials for performance based pavement foundation design

The need to use more recycled and marginal materials in pavement foundations is encouraging moves towards performance based specifications. Such an approach needs data on the fundamental material parameters of stiffness and resistance to permanent deformation (strength) to allow analytical design and achieve comparable compliance testing on site. Whilst a number of laboratory and field tests to measure such performance exist, all have limitations relative to either the particle size of materials that can be tested and/or correlation between the laboratory and field derived data. This paper presents the development of a large scale resiliently lined steel laboratory material box tests proposed for routine material assessment. The test utilises a Lightweight Deflectometer, similar to that proposed for field compliance testing to provide performance data from compacted large particle size granular materials. The system developed utilises a “soft” base condition to replicate typical subgrade stiffness and allows wetting and drying of materials to assess their environmental stability. The results show that such a test can simply provide suitable data for performance based design, but consideration needs to be given to the water content of materials, the time of testing after compaction, and the use of appropriate boundary conditions. These findings have both implications not only for the developed tests but for field compliance testing of pavement foundations

Lightweight deflectometers for quality assurance in road construction

The use of Lightweight Deflectometers (termed LWDs in Europe, and occasionally PFWDs in the USA) for construction quality control or material investigation for road construction has increased worldwide. In the UK the change in pavement foundation design to a ‘performance based approach’ has brought about the use of Lightweight Deflectometers for field assessment of stiffness modulus. This paper reviews the LWD as a field evaluation tool. It discusses in some detail the test variables that can influence and affect the field data quality, and presents brief summaries of recent fieldwork where an LWD has been used as a quality control tool. The paper concludes both on the LWD usefulness and also its limitations for a variety of earthwork and road assessment scenarios, and describes a field test protocol for its use on a variety of materials. The findings demonstrate the flexibility of the LWD but also show that its determination of ‘stiffness modulus’ may differ from that of the conventional Falling Weight Deflectometer (FWD) to a varying extent. The paper provides a useful reference document for LWD users, consultants, material specifiers, contractors and clients

Insitu assessment of stiffness modulus for highway foundations during construction

Several portable field devices that measure stiffness modulus are reviewed in detail in this paper including the German Dynamic Plate Test (also known as the Lightweight Drop Tester), the TRL foundation tester (UK), the Prima (Denmark) and the Humboldt Soil Stiffness Gauge (USA, also known as the GeoGauge). Laboratory and field data are presented which explain the many important influences on the measured data and demonstrate comparative performance with respect to the Falling Weight Deflectometer. These field data show significant scatter and site specific correlation. A strategy for compliance testing during construction, as part of a performancebased specification approach for the UK, is suggested. Conclusions are made regarding the devices’ relative merits and limitations, and considerations for their introduction into contractual use for routine assessment during construction

Geotechnical specifications for sustainable transport infrastructure [published as: Sustainable earthworks specifications for transport infrastructure]

The specification of the materials and methods used in earthworks and foundations for highways, railways and airfield runways can be approached in several ways. However, in part due to the sustainability agenda there is a need to use specifications that make best use of material properties, and a performance-based specification may be considered the best way to facilitate this. The advantages and disadvantages of the different specification approaches is described and discussed in this paper. The functional requirements of a performance-based specification for UK highway foundations are considered. The (recently researched) performance-based specification is explained, demonstrating the steps to its development, determination of the engineering requirements, suitable target values and the potential construction-related implications. It is shown that performance-based specifications offer the advantage of better incorporation of the principles that underpin sustainable construction but also require a fuller understanding of material behavior for their development and implementation. However, contractual issues and implications for construction need to be carefully considered to allow a full performance-based approach to be successfully adopted. It is considered in the UK that a staged implementation of a performance specification is necessary to permit the gaining of experience of both the process and the field measurement methods, some of which are relatively novel, and to reduce the risk of contractual disputes or potential failures and thus a negative reaction from the industry

Review of lightweight deflectometer for routine in situ assessment of pavement material stiffness

The use of a portable lightweight deflectometer (LWD) for construction quality control or material investigation for earthworks and road construction is increasing around the world. This paper reviews the LWD as a field evaluation tool, discusses the test variables and data quality and concludes both on its usefulness and also its limitations for a variety of earthwork and road assessment scenarios. The paper aims to provide a state of the art reference document for LWD users, consultants, material specifiers, contractors and clients. It reviews data from road foundations (subgrades, granular capping and sub-base) and fully constructed in-service (thinly surfaced) roads, to demonstrate the flexibility of the LWD but also show that its determination of ‘stiffness modulus’ may differ from that of the conventional Falling Weight Deflectometer (FWD) to a varying extent. A series of laboratory investigations are presented that demonstrate the sensitivity to uniformity of plate/surface contact, and also limitations in the interpretation of peak displacement from the device. In conclusion, a good understanding of the device workings and careful specification of the test variables are required to both analyse the data correctly and permit comparison between data sets. The device is concluded to be a useful and versatile field quality control and pavement investigation tool, if an understanding of the device issues is considered by data users

A review of geotechnical specifications for sustainable transport infrastructure

The specification of the materials and methods used in earthworks and foundations for highways, railways and airfield runways can be approached in several ways. However, due to the sustainability agenda there is a need to use specifications that make best use of material properties. The advantages and disadvantages of different specification approaches are described and discussed in this paper and a performance-based specification is considered the best way to facilitate sustainability. The functional requirements of a recently researched performance-based specification for UK highway foundations is explained, demonstrating the steps to its development, determination of the engineering requirements, suitable target values and the potential construction-related implications. It is shown that performance-based specifications offer the advantage of better incorporation of the principles that underpin sustainable construction but also require a fuller understanding of material behaviour for their development and implementation. However, contractual issues and implications for construction need to be carefully considered to allow a full performance-based approach to be successfully adopted. It is considered in the UK that a staged implementation of a performance specification is necessary to permit the gaining of experience of both the process and the field measurement methods, some of which are relatively novel, and to reduce the risk of contractual disputes or potential failures and thus a negative reaction from the industry

Laboratory assessment of coarse granular road foundation materials

A new UK ‘performance based specification’ for road foundations potentially allows any material that is ‘fit for purpose’ and encourages the wider use of marginal and recycled/secondary materials. The specification provides guidance with respect to a target stiffness, density achieved and maximum allowable rutting depth under construction traffic (to avoid damage to the subgrade during construction). Full-scale field trial sections are currently used and proposed for large schemes to assure as the suitability of proposed foundation materials. However, it is prudent to develop a routine, economical and laboratory-scale means to assess the performance and suitability of the foundation materials before any full-scale trials are undertaken. This paper presents the findings of recent research work at Loughborough University with regard to the development of a large-scale laboratory assessment test aimed specifically for granular materials. A series of test results on four granular materials are presented in detail, with an emphasis on their stiffness behavior, with some field data for comparison and preliminary validation of the laboratory method. The effects of a soft and rigid base condition, and wetting and drying of the material is shown to have a significant effect on the measured values of both stiffness and strength for the samples tested. However, there appears a reasonable relationship between the laboratory results for the soft base condition and the field data. Several recommendations for further work are made and practical observations and comments for the construction and measurement of these materials on site within a performance specification framework