International - Special 16th ECSMGE

Recent safety studies indicate a significant risk of backward erosion piping as a failure mechanism for dikes. Traditional measures, such as berm s or sheet pile walls take a lot of space or are expensive, in particular in areas with a high infrastructural density. Consequently, there is an urgent need for alternative piping measures. An innovative solution is the vertical sand-retaining geotextile, which has been developed and tested in the field. An optimisation of this method is a coarse sand barrier: a trench filled with coarse sand, covered by a clay layer. The coarse sand provides more resistance to erosion than the inherent sand of the aquifer with its uncertain composition with respect to d70, so that initiated pipes cannot continue to develop. Laboratory and field scale tests suggest that this is an effective and economically feasible method.

The onshore wind industry continues to grow rapidly with the construction and development of many large scale wind farms using large megawatt wind turbines. This development requires a clear understanding of the geotechnical foundation behaviour in order to achieve a safe and economical foundation design. The foundation design however includes the selection of various types of foundations, design methodologies and mathematical models, ranging from basic to advanced. The geotechnical designer needs to make a choice based on numerous codes and standards, design requirements and the anticipated geotechnical soil conditions. In some cases, standards are conflicting, or just lacking specific design guidelines for wind turbines. Based on extensive design experience some design considerations are described in this article.

This landmark highway project in Turkey had many demanding site constraints. This led to innovative construction techniques being adopted by the Client, Designer and Contractor.

This paper presents the findings on the behaviour of organic soft soil (clay and peat) using a variety of state-of-the-art laboratory tests. Moreover it addresses the importance of critical evaluation of the each test and the parameter determination. The tests are performed within a research program on behalf of research foundation “STOWA” to verify the stability of a local flood defence of the waterboard “Hoogheemraadschap van Delfland”. Oedometric tests (OED) and constant rate of strain tests (CRS) have been carried out to determine the preconsolidation stress, after which anisotropically K₀-consolidated undrained triaxial tests (TA-ACU) and direct simple shear tests (DSS) have been executed with three different consolidation paths. All tests have been performed on undisturbed samples to obtain reliable soil parameters.

The interpretation of laboratory tests is made in terms of pre-consolidation stress (s’vy), undrained shear strength (Su) and effective friction angle (φ’). This paper focuses on determination of the strength parameters at large strains, the so called critical state or ultimate state parameters, further on called ultimate state parameters (“ult”). The ultimate parameters can be used for a effective stress approach with Mohr-Coulomb strength model using effective friction angle and no cohesion. The ultimate parameters can also be used for a critical state soil mechanics (CSSM) approach with undrained shear strength related to the effective stress and overconsolidationratio (OCR).

The undrained shear strength (Su) is normalised by the vertical consolidation stress (s’_vc) to derive the undrained shear strength ratio (S). By plotting S versus the OCR, both the normally consolidated value (S;nc) as the exponent (m) to describe the overconsolidated behaviour are derived. The OCR is defined as s’_vy divided by s’_vc. This paper presents additional criteria to define the ultimate state of Su.

To obtain the ultimate state effective friction angle (φ’), the interpretation of TA-ACU is made by plotting t against s’. For the DSS tests, Su is plotted against the ultimate vertical effective stress (σ’_ult), which represents the vertical effective stress acting on the sample at the moment of failure. This paper presents additional criteria to define the ultimate state of φ’.

Lastly a comparison is made between two consolidation procedures to derive the undrained shear strength at in-situ stress conditions. Two identical samples have been tested, one sample is consolidated to the in-situ stress level before undrained shearing. The other samples is first consolidated to the pre-consolidation stress, then consolidated to the in-situ stress before undrained shearing, the so called SHANSEP method.

In the early 1960’s Henri Vidal, introduced the Terre Armée (Reinforced Earth®) construction technique. Henri Vidal conceptualized this method and built the first full scale demonstration. For 50 years since, Terre Armée has set the standards for reinforced soil structures and has been involved in more than 50,000 projects all over the world, accumulating knowledge and experience in the field of engineered backfills.