A comparison of devices for measuring stiffness in situ.

The variability between devices that measure the stiffness of a road foundation in situ and their accuracy are important considerations for the introduction of such field assessment methods into the construction monitoring process. The aim of this paper is to present the significant findings of recent research into the comparability of four such stiffness devices. Results have been obtained from commercial construction sites and large-scale field trials covering a wide range of material type and stiffness. In addition, controlled tests on a synthetic rubber were carried out to investigate repeatability, as well as a study to investigate the stress dependency of the computed stiffness values. The results show significant variations in the correlation coefficients, which were shown to be dependent upon material type and construction methods. Conclusions are drawn with regard to the efficacy and accuracy of the four stiffness measuring devices and proposals made concerning their suitability for the range of site construction conditions and materials

Performance parameters and target values for construction of UK road foundations

There is impetus in the UK to move away from empirical design of road foundations and method specification towards analytical design assured by end product testing during construction. Current research at Loughborough University, sponsored by the Highways Agency, is aimed at introducing such a performance based specification. This paper introduces the philosophy behind the research and explains the primary objectives. Selective results are presented from a full-scale field trial construction of a road foundation on a soft subgrade, which was characterised by a variety of field devices and subsequently trafficked. This paper focuses on insitu stiffness modulus (ER), measured by several devices for comparison. The setting of suitable target values for field ER is seen as a key factor and the influence of inherent variability in material response a concern. The discrepancy between devices and the problem of long-term stability are discussed and highlighted as a key focus for the remainder of the work

Stiffness behaviour of trial road foundations.

This paper reports the results of two full-scale field trials to determine the stiffness and density of granular materials receiving different levels of compaction energy and support. The research showed that there were greater improvements in density with initial input of compaction energy, and yet little change in density, but significant increases in stiffness, with the final applications of compaction energy. Density is thus an inappropriate indicator of performance (i.e. resilient elastic stiffness

Threshold stress and asymptotic stiffness of UK clays in the repeated load triaxial test.

Recent research at Loughborough University has produced a draft performance specification for pavement foundations. To assess the performance parameters of stiffness and resistance to permanent deformation of the subgrade, an extensive series of repeated load triaxial tests has been performed on a range of fine grained subgrades. Assessment of the data has shown that the deviator stress at which the cumulative permanent deformation starts to increase significantly (i.e. the threshold stress) is equal to half that at failure (and occurs at approximately 1% permanent strain). The resilient stiffness of the materials tends to low asymptotic values at higher deviator stress. Comparison between the elastic and plastic behaviour has shown that the deviator stress at ‘threshold’ is coincident with the stiffness asymptote. This paper presents a suggested simplified mechanistic design approach for pavement foundations based on these findings

Subgrade equilibrium water content and resilient modulus for UK clays

The main functional requirement of the subgrade is to resist excessive deformations during construction and in service. Deformation is controlled by ensuring adequate foundation resilient modulus. UK pavement design currently relies upon the determination of long-term CBR values for subgrades, which results in conservative pavement foundation design. To allow more realistic design an analytical approach is required, including measured values of resilient modulus at anticipated short- and long-term equilibrium subgrade water contents. Resilient modulus is determined from repeated load triaxial tests with on-sample strain measurement. Results from these tests on samples adjusted to model the subgrade behaviour at various design conditions are presented. These results are discussed, highlighting problems both with measuring small strains on undisturbed soils and in predicting and modelling accurately long-term soil behaviour. RÉSUMÉ: Le critère principal d’une sous-couche est de résister à des déformations excessives en construction puis en service. La conception des chaussées au Royaume-Uni est actuellement basée sur la détermination de l'indice portant Californien (CBR) à longterme des sous-couches. Pour une conception plus réaliste, il est nécessaire d'adopter une approche prenant en compte des valeurs de module de résilience mesurées à des degrés d'humidité de sous-couches à l'équilibre simulés à court et long-terme. Le module de résilience est déterminé à partir d'essais répétés en charge triaxiale avec mesure des contraintes sur l'échantillon. Dans cet article les résultats de ces tests sur des échantillons simulant le comportement de la sous-couche sous diverses conditions de conception sont présentés. Ces résultats sont discutés, en soulignant les problèmes résultant de la mesure de faibles contraintes sur des sols non-perturbés, et dus à la prédiction et à la modélisation du comportement des sols à long-terme

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

Performance based specification for road foundation materials

UK Pavement foundations are currently designed using a method specification whereby tightly specified materials are constructed using specific compaction methods and layer thickness. This process does not necessarily guarantee the performance of the materials, but it is assumed to be adequate based on past experience. However, it can be inefficient, leading to unnecessary restrictions when using stabilised, recycled or marginal materials and/or the inappropriate use of good quality aggregates. The UK Highways Agency (HA) funded a recently completed three-year research project to produce a draft performance-based specification for road foundations. The performance-based specification aims to enable more appropriate and efficient use of a wider range of materials, both natural and recycled. The performance parameters required of the materials were established as the stiffness and the resistance to permanent deformation, with both measured, ideally, in the laboratory for design purposes and during construction to ensure their performance on site. Pre-construction trials to demonstrate adequate material performance (both as individual layers and as a composite structure) are expected to feature prominently when the new approach is adopted. A further HA-funded project started in January 2000 to evaluate the implementation of this new specification. This paper outlines the philosophy of the draft performance-based specification produced, including what needs to be measured and how and when it should be measured. Its impact on the highways industry is then discussed

The response of flexible pipes buried in sand to static surface stress

A series of laboratory tests on thin-walled PVC-U (i.e. very flexible) pipes buried in sand is described. The tests were conducted in a glass-fronted test tank, the pipe being positioned up against the glass with its longitudinal axis perpendicular to the glass. This allowed direct observation of the sand-- pipe interactions. Photographs were taken through the glass allowing discrete measurement of pipe and soil displacements during pipe installation and subsequent surface loading. This paper discusses the influences on pipe response of installation method, cover depth and pipe stiffness as increasing static surface stress was applied. The results of the laboratory tests indicate very clearly the importance of well-controlled backfilling around flexible buried pipes to ensure their long-term performance. The stiffness of the pipe affects the way it behaves and hence its performance in resisting applied stresses. This is demonstrated by the observed changes in arching effects above pipes of different stiffness. The effect of increasing cover depth is demonstrated and confirms previous research findings regarding the influence of the ground surface on pipe performance. The results clearly demonstrate the valuable insight afforded by direct observation of the soil--pipe interaction during installation and the subsequent loading of flexible pipes