Ebook: From Research to Applied Geotechnics

This book constitutes the imprint of the Plenary Conferences delivered by distinguished geotechnical engineers of renowned international quality in the XVI Pan-American Conference on Soil Mechanics and Geotechnical Engineering (XVI PCSMGE), which was held from November 17 to 20, 2019 in Cancun, Mexico.

Experience and youth are combined in this special volume that includes the 9th Arthur Casagrande Lecture, the Plenary Conference of ISSMGE President, three Bright Spark Lectures, and thirteen manuscripts of the Invited Lecturers of practically all the technical sessions of the XVI PCSMGE 2019, among them Analytical and physical modelling in Geotechnics, Unsaturated soils, Soft soils, Foundations and retaining structures, Excavations and tunnels, Offshore Geotechnics, Transportation in Geotechnics, Natural hazards, Preservation of historic sites, Rock mechanics, Education and Energy Geotechnics.

This special volume covers both the research and the applied Geotechnics, including the recent developments in geotechnical engineering. Besides, it provides valuable references for engineering practitioners and graduate students and helps to identify new issues and to shape future directions for research. We are very pleased to disseminate all these contributions among the scientific community.

We would like to express our thanks to all the Invited Lecturers for their high quality and valuable contributions. In addition, we are especially grateful to Eduardo Martínez, Alejandra Liliana Espinosa and José Alfredo Promotor for their assistance with the preparation of this Invited Lectures Volume.

Norma Patricia López Acosta, Chairwoman, Editorial Committee

Alexandra Ossa López, Chairwoman, Technical Committee

Walter I. Paniagua Zavala, Chairman, Organizing Committee

Cancun, Mexico, November 17 to 20, 2019

Many large cities such as Tokyo, Bangkok, Río de Janeiro, Recife, Bogota and, of course, Mexico City, to name only a few, were built and are still being developed on soft soils. In many cases, these cities also experience regional subsidence induced by pumping of groundwater from deep local aquifers. Among the sources of uncertainty prevailing in the geotechnical characterization of these sites, soil properties spatial variability is one of the most difficult to deal with since the associated uncertainty cannot be eliminated only by improving laboratory and field-testing techniques. For an accurate evaluation of the subsoil conditions, spatial variations of the soil profile and mechanical properties together with the groundwater conditions must be assessed by performing a sufficient number of soil explorations, processing a generally large amount of data and developing either deterministic or probabilistic models of these variations. The techniques available to develop such models and some difficulties encountered to implement them are examined in this lecture. Some geotechnical analysis and design methods that take into account soft soils spatial variations are also reviewed together with constructions techniques aimed at mitigating consequences of soil variability.

The above considerations are illustrated with reference to Mexico City’s highly compressible volcanic lacustrine clays. Models of the spatial variability of these materials developed over the years for different projects using traditional and geostatistical techniques are presented. Some of the geotechnical analysis and construction methods used by geotechnical engineers to deal with soil spatial variability in this megacity once called by Terzaghi “the paradise of soil mechanics”, are also discussed.

The negative impact of climate change calls for additional sustainable and environmentally friendly techniques to be developed for the improvement of the engineering performance of civil infrastructure, such as landfill covers and slopes. Bioengineering using vegetation can be considered and promoted as a low-cost, aesthetically pleasant solution for greening landfill covers and improving shallow slope stabilisation. The mechanical effects of vegetation as soil reinforcement have been extensively studied, but the hydrological effects of vegetation on soil shear strength and water permeability are unclear. This study therefore presents an interdisciplinary research programme consisting of laboratory and field tests and centrifuge modelling. The programme explores the hydrological effects of plants on the performance of final landfill covers and slope stabilisation. Results show that suction induced by plants under a novel vegetated three-layer landfill cover is preserved better than that under a bare cover even after an extreme rainfall event with a return period of greater than 1000 years in Hong Kong. The laboratory tests and field trials demonstrate that the vegetated three-layer landfill cover system using recycled concrete can effectively minimise percolation at humid climate even without a geomembrane. Novel artificial root systems are developed for the centrifuge model tests. Heart-shaped roots have stronger pull-out resistance and higher preserved suction (hence higher soil shear strength) compared with tap- or plate-shaped roots. The heart-shaped root architecture is thus the most effective type in producing stabilisation effects on slopes.

In addition to the already common use of geotechnical centrifuges to represent, in a reduced-scale model, the state of stresses corresponding to a full-scale geotechnical structure, centrifuge techniques have increasingly been used for an additional purpose: accelerating flow process through geotechnical systems. This is particularly relevant in geotechnical problems involving low hydraulic conductivity scenarios, including flow through low-hydraulic conductivity shales, through unsaturated soils in general, and through unsaturated soils subjected to volumetric changes during infiltration (i.e. expansive clays). This paper provides an overview of recent analytical and experimental advances involving the use of centrifuge technology for the hydraulic and volumetric evaluation of expansive clays. In particular, a new centrifuge approach is presented for practical characterization of expansive clays aimed at implementation in conventional laboratories rather than research centers. The results indicate that, in spite of the significantly highly practical and expeditious characteristics of the new approach, the predicted swell-stress curve is the same as that obtained using time-consuming conventional experimental techniques.

Different phenomena influence the strength and volumetric behavior of unsaturated soils. Among the most important are suction hardening, hydraulic hysteresis and the influence of volumetric strains on the soil-water retention curves. Fully coupled hydro-mechanical models require including all three phenomena in their constitutive relationships. Among these phenomena, suction hardening is the most influencing as it determines the apparent preconsolidation stress, the position of the loading-collapse yield surface and the shift of both the isotropic consolidation and the critical state lines. In this paper, a simple fully coupled hydro-mechanical model is presented. Numerical simulations on both suction controlled and undrained triaxial tests show comparable results to more complex models.

Ground improvement techniques are commonly required for construction on soft clays. In general, the most traditional construction techniques use a combination of prefabricated vertical drains, temporary surcharge, reinforcement, stabilising berms or staged construction. In order to achieve shorter construction times, alternative techniques may be adopted. Examples of these include: lightweight fills, vacuum preloading, temporary surcharge, geosynthetic-reinforced pile-supported embankments, stone columns, geosynthetic-encased columns and cement injection techniques such as deep mixing. This keynote lecture will present the results of two recent case studies in which vibro stone columns and encased columns were used to strengthen a soft clayey foundation supporting embankment loading.

At zones of strong impedance contrast in which there is a significant change in stiffness between adjacent geomaterial layers, Winkler-based analysis methods predict abrupt changes in the internal pile reaction force effects for laterally-loaded foundation elements. In particular, the sudden de-amplification of moment when transitioning from a soft to stiff layer is accompanied by amplification of pile shear. From a design perspective, this is problematic when considering large lateral loads and moments acting on drilled shafts, because it can result in bulky transverse reinforcement designs that pose constructability challenges. This paper will review the challenges associated with the lateral performance of piles in zones with strong stiffness contrasts and present a large-scale experimental research program that investigates the lateral load transfer of rock-socketed deep foundation elements. The study seeks to better understand the ability of numerical and analytical methodologies in capturing the behavior at impedance interfaces, compare such with experimental observations, and derive lessons for the construction industry in how to optimize the design requirement using performance-based predictions for deep foundations embedded in stiff materials.

Constructive methods, tunnel lining, and soil treatment and conditioning depend fundamentally on ground and hydro-geological conditions. Regardless of specific difficulties, construction in homogeneous ground becomes, after adequate definition of the parameters above and the initial learning curve, a repetitive and uniformly controlled process, either through conventional or mechanical tunneling methods. Difficulties often arise when varying ground conditions are encountered along the tunnel alignment, especially if ground behavior presents significant contrasts in deformability, shear strength and permeability. In geological environments where soft ground overlays rock and tunnels have to be built crossing this interface, the above-mentioned contrasts normally occur at the same location. The most significant recent tunnel failures in Brazil occurred close to rock soil interface, showing the necessity of a review of current design and construction practice This paper intends to discuss main challenges associated to the rock-soil interface in the light of recent tunnel failures and present suggestions for robust design and construction methods.

The huge growth and intense development in the European offshore wind power sector over the last decade have created significant achievements within the wind turbine foundation technology. The state of the art focusing on geotechnical design aspects for Offshore Wind Turbine (OWT) foundations and important aspects for installation are presented in this paper. In place operational experience based on structural health monitoring campaigns and future trends are also discussed.

Carbonate sediments are prevalent in many major offshore oil and gas basins, as well as a growing number of regions assigned to offshore wind development. Identified as difficult from an engineering perspective, the failure to properly characterize and design for these sediments has adversely influenced several projects. This paper provides a brief geological perspective, and identifies broad trends and characteristics to be considered when defining the engineering properties of such materials. An overview of the challenges faced when founding offshore structures in such sediments is provided, drawing on experience gained over the last 30 years, and with an emphasis on current and emerging issues.

Quarry by-products (QB) are an industrial by-product of aggregate quarry processes. They are typically less than 1/4 on. (6 mm) in size and consist of coarse, medium, and fine sand particles, and a small clay/silt fraction. Quarry by-products are found abundantly all over the crushed rock extraction facilities in Illinois where they are produced during blasting, crushing, washing, and screening operations. Recent research conducted at the Illinois Center for Transportation (ICT) has evaluated the characteristics of QB materials collected from different quarries across the State of Illinois, and studied potential uses of QB in pavement applications. Because the Unconfined Compressive Strength (UCS) for QB materials was quite low, Portland cement and Class C fly ash chemical admixture stabilizers were used to improve the strength properties of QB materials which resulted in 10 to 30 times increases in laboratory determined UCS compared to virgin unstabilized QB samples. Such significant increases observed in the strength of stabilized QB materials have indicated suitability of QB for sustainable pavement applications. Full-scale test sections were constructed next with chemically stabilized QB base/subbase applications over a subgrade having a California Bearing Ratio (CBR) of 6% to represent medium volume flexible pavement applications. The test sections were evaluated for performance using Accelerated Pavement Testing (APT), which spanned over two years to include effects of harsh winter freeze. Field testing and forensic analysis techniques included Falling Weight Deflectometer (FWD) tests before and after trafficking, hot mix asphalt coring, Dynamic Cone Penetrometer (DCP) profiling of subsurface layers, and trenching to determine actual thicknesses and contribution of each pavement layer to the measured surface rutting. In general, results from APT and forensic analyses indicated satisfactory results and improved rutting performance.