Ebook: Modern Geotechnical Design Codes of Practice

Geotechnical design is a product of local history, engineering practice, availability of construction materials and, of course, the geology of each site. All of these factors vary from region to region, which is why a standard recipe for developing geotechnical codes of practice does not exist. It is probably fair to say that the differences in geotechnical design codes worldwide are much larger than exist between steel or concrete design codes. Steel and concrete are quality controlled materials and the uncertainty in their engineering behaviour is very similar from region to region. Thus, concrete and steel design codes have been able to take advantage of worldwide research efforts in their calibration over the decades.

Modern geotechnical design codes are generally striving towards a similar harmonisation, both with their counterpart structural design codes and between regional geotechnical codes. However, harmonising geotechnical design codes is not an easy task. An excellent example of the challenges faced in harmonisation is presented by Eurocode 7. Although aiming to create common terms of reference, Eurocode 7 still required several design approaches to accommodate the needs of all member states, along with national annexes enabling each member state to define their own set of safety factors. Why was this diversity in design approaches necessary? And what do code developers have in mind when they make their choices in adopting a design approach or set of safety factors? Some answers to these questions will be given in this book.

The impetus for this publication started with an international workshop on Safety Concepts and Calibration of Partial Factors in European and North American Codes of Practice, which was held on November 30 to December 1, 2011 at Delft University of Technology, the Netherlands. The aim of the workshop was to exchange ex perience and transfer knowledge between code developers, practitioners, and researchers on code development, safety concepts and the calibration of partial factors in modern geotechnical codes of practice. The attendees, who were leading authorities from Europe and North America, provided interesting and valuable insights into the development of their own national codes. This workshop led to the idea of collecting contributions from geotechnical code developers worldwide into a single book, providing a resource that can be referred to as a guide in the years to come.

The papers collected in this book are organised into three sections: Code Implementation describes choices relating to safety concepts, target reliabilities, and design approaches; Code Application addresses their application to specific geotechnical problems; and Code Development includes papers discussing directions for future developments.

The editors would like to acknowledge the support of the following committees who have substantially contributed to and supported this publication: the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) Technical Committee for Safety and Serviceability in Geotechnical Design (TC 205), Technical Committee for Engineering Practice of Risk Assessment and Management (TC 304), and the Comité Européen de Normalisation (CEN) Technical Committee for Structural Eurocodes (TC250).

The paper describes the ways in which Eurocode 7 has been implemented by National Standards Bodies in Europe. It identifies key differences between the 33 countries involved in their choice of Design Approach for different foundation types and the introduction of different factors based on the design situation being considered and the consequences of failure. Finally, the paper gives details about the plans that are in place to develop the Eurocodes as a whole and Eurocode 7 in particular.

The harmonisation of anchor design within the limit state framework of the Eurocode presented a challenge due to the different design and testing practices in use in the various countries. An Evolution Group (EG1) was set up by TC250/SC7 tasked with developing a standard that conformed with the limit state philosophy and which could be used in the member countries. This paper presents the basis of a draft standard that has been prepared by EG1, which considers the ULS and SLS of the restrained structure and of the anchor itself and introduces the force FServ.k which considers the effect of prestress on the anchor force. The application of the recommendations is illustrated by a design example

In Poland, designing with the use of limit states and partial factors has been a common practice for over 30 years. However, there are many differences between Eurocode 7 and Polish practice and previous codes. The paper presents some of these differences. The state of Eurocode 7 implementation in Poland is presented in general. Eurocode gives opportunity to choose one of the three Design Approaches. The reasons of the choice of the Design Approaches in Poland are presented. The Polish National Annex to EC7, as well as the Polish EC7 guide, are discussed. Reliability issues and the National TC Geotechnics's structure reliability approach are also raised.

The paper investigates the philosophy and use of characteristic soil property values in Eurocode 7. Due to the spatial nature of soil variability, characteristic property values are shown to be problem-dependent and a function of two competing factors: the spatial averaging of properties along potential failure planes, which reduces the coefficient of variation of property values; and the tendency for failure to follow the path of least resistance, which causes an apparent reduction in the property mean. The Random Finite Element Method provides a self-consistent framework for quantifying and understanding this behaviour, and for deriving characteristic values satisfying the requirements of Eurocode 7. It is widely accepted that characteristic values may be over-conservative if they do not account for the spatial averaging of property values. Conversely, this paper argues that characteristic values based only on variance reduction techniques may be unconservative, if no account is taken of the apparent reduction of the mean along potential failure planes. Simpler probabilistic methods can be effective in guiding design through quantifying uncertainty. However, further research is needed to assess when such methods are applicable, and when they are significantly in error and require further attention.

For a long time the framework of geotechnical design in Germany was provided by the basic code DIN 1054 and various specific design and construction codes together with code-like recommendations for specific geotechnical structures. All these regulations were based on a global safety concept. This design philosophy was commonly supported by long-term experience and was understood to be not only safe but also economical. Under these circumstances the transition to the new design philosophy of Eurocode 7 and the Limit State Design was accompanied by a lot of discussions and adjustments until finally a broad acceptance was achieved. This contribution highlights the long tradition of geotechnical design in Germany based on the global safety concept. It shows the difficulties involved in the transition to the Eurocode 7 design principles and illustrates the current status of German Limit State Design in geotechnical engineering.

This paper deals with the implementation of Eurocode 7 in French practice by means of national additional standards. The architecture of French geotechnical design standards is presented. In particular, principles of safety management and of determination of characteristic value are explained. Then, for each geotechnical structure type, the values of the partial factors used are summarized and the methodology of their calibration is briefly discussed.

This paper examines how the different measures to achieve safe and reliable geotechnical designs are implemented in Eurocode 7. These measures include: use of the concept of geotechnical categories to take account of the complexity of geotechnical designs, procedures for managing the design and execution processes, requirements for the selection of characteristic parameter values, and recommended partial factor values associated with the three Design Approaches. How the consequences and reliability classes, set out in the head Eurocode, EN 1990, are implemented in Eurocode 7 for geotechnical designs is also examined. In additions, the paper presents the results of surveys to investigate how Eurocode 7 is being adopted in the different European countries and the some experiences in those countries with its implementation.

The performance of geotechnical structures is governed by the spatial average soil properties, such as the average shear strength over an appropriate length or area or volume of soil. This is considered in Eurocode by defining the characteristic value as being “a cautious estimate of the value affecting the occurrence of the limit state” and stating that the selection of this value should take account of, among other factors, “the extent of the zone of ground governing the behavior of the geotechnical structure at the limit state being considered”.

To quantify the characteristic values, 3 statistical properties are required: The arithmetic mean, the variance and the scale of fluctuation. The mean and the variance (or equivalently the standard deviation or the coefficient of variation) are known to most geotechnical engineers. The scale of fluctuation may be interpreted as the distance within which soil properties are largely correlated, whereas for greater distances the soil properties are largely uncorrelated or statistically independent. This degree of spatial correlation considerably influences the variance of the soil properties averaged over a certain geometrical zone.

This paper provides guidelines for the assessment of the characteristic soil properties for the use in geotechnical designs, taking into account explicitly soil variability, as well as the soil volume affected by the design.

Examples to illustrate the practical application of the proposed procedures for typical geotechnical routine work are provided.

The Eurocodes were officially adopted in Germany on 1st July 2012. Since then, application of the partial safety concept has been mandatory in all areas of structural engineering. The partial safety concept creates the impression that partial factors based on probability theory are applied to actions or effects of actions and resistances depending on the degree of uncertainty in each case. This is hardly possible in geotechnical engineering. European geotechnical engineers have therefore decided that the partial factors for the permanent and variable actions from the ground should be the same as those used in other areas of structural engineering for the sake of consistency in the field of construction. In Germany, the partial factors for the resistances from the ground have been selected so that the safety level is more or less the same as the tried-and-tested global safety level. In other words, the application of the partial safety concept results in approximately the same dimensions for foundations and geotechnical structures as those obtained with the global safety concept in the past. Thus, on closer inspection, the partial safety concept in its present form is, in geotechnical engineering at least, a modified global safety concept. Users are faced with the challenge of having to apply a concept which in some respects is new and more complex. However, this is reasonable when one considers the important contribution that has been made to placing European construction standards on an urgently needed common basis, thus promoting the unification of Europe.

The Eurocode system allows each nation to specify, for designs to be constructed on its territory, which of its three geotechnical “design approaches” are to be used and what values are to be given to partial factors. These are published in the National Annex to Eurocode 7 (EC7). This paper reviews the choices made by British engineers working under the auspices of BSI to produce the UK National Annex. The UK has chosen to use Design Approach 1, judging that it has the potential to provide the best balance between safety and economy over a wide range of design types, while allowing broad compatibility with previous designs. It also facilitates use of finite element analysis, a feature not shared with Design Approach 2. At this stage in development, values for the partial factors have been selected on the basis of calibration against past experience; probabilistic calculations have not been used to a significant extent in this process. Particular difficulties have been encountered in providing for pile design to EC7, for which the code gives separate requirements for design based on load testing and design based on calculation using ground properties. In practice in the UK, these are often used in combination, and it was found necessary to vary the factors offered for calculations by EC7 as a function of the amount and type of testing undertaken.

In the Netherlands, due to the large infrastructure and waterway projects from the 1980's onwards, much experience was developed using probabilistic design. Due to the vast extent of e.g. the Eastern Scheldt Storm Surge Barrier, these structures could not have been designed economically using conventional engineering. The probabilistic approach is reflected in the building and geotechnical standards which were introduced in the early 1990's. These standards were based on a target probability of failure or reliability index β. Probabilistic analyses (Monte Carlo) were undertaken to determine the partial load and material factors. These analyses were also performed for geotechnical structures, leading to a large set of material factors for soil properties. The old method of Ultimate Limit State analysis consisting of overall resistance factors was then replaced by partial factors. Consequently, when Eurocode 7 was established around 2000, Design Approach 3 adopting partial load and material factors was preferred in the Netherlands as its philosophy is similar to the 1991 Dutch building codes.

The use of the limit state design (LSD) is not common practice among dam engineers. However, it has been applied in the design of structures since the middle of the last century, and its application in the geotechnical area has increased over the last two decades, mainly as a result of both the European pre-norm and European codes concerning geotechnical design. While it is accepted that, due to the hazard involved in dam operation, extreme caution must be exercised in the process of dam conception and safety assessment, there are no significant reasons why dam safety and serviceability cannot be analyzed using the LSD approach. This case study presents an example of the application of the LSD of the foundation of a large concrete gravity dam, in static conditions.

A number of issues concerning the verification of ULS using numerical analysis are described by means of three simple benchmark problems. There are pros and cons associated with the two limit state factoring approaches: material factoring approach (MFA) and load and resistance factoring approach (LRFA). Based on these simple examples, for the calculation of factored structural forces, LRFA provides a more consistent level of conservatism. MFA often also provides satisfactory levels of conservatism, but when there is no soil yield, the factoring can have a negligible effect, and where there is a lot of yield, the degree of conservatism can also be too low. For the verification of geotechnical ULS, MFA is more straightforward because it involves the factoring of input parameters and identifies the critical failure mechanism, as well as its margin of safety. LRFA requires particular geotechnical failure forms to be verified but forcing models into particular forms, even in simple cases, is difficult to achieve. Alternatively, outputs of mobilised resistance can be compared with independently calculated limiting values, but this was found to be unworkable for the passive resistance of an embedded retaining wall where the factored output of horizontal stress exceeded the factored passive limit, no matter how deep the wall embedment. Consequently, when using numerical analysis, it is recommended to employ both approaches LRFA and MFA, which together are equivalent to DA1 or DA2+DA3 in Eurocode 7, in order to benefit from the advantages of each.

The influence of ground water level on shallow foundation design is presented for the ultimate limit state of the bearing resistance, according to the formulation presented in annex D of EN 1997:2004. Probabilistic and deterministic methods were used and compared. Concerning probabilistic methods, the advanced first-order second-moment method (AFOSM) was applied and the results were validated by Monte Carlo simulations. For the deterministic calculation, the partial factors method recommended by the Eurocode and applied in most practical cases, was implemented. For the assumptions herein made the width B determined by the probabilistic method is always smaller than the one obtained deterministically.

The Load and Resistance Factor Design (LRFD) methods are being widely adapted for deep foundations design. The benefits of the LRFD method are two-fold: (1) that uncertainties related to the design of a deep foundation are rationally incorporated into the engineering process, and (2) that the foundation design is harmonized with the superstructure design. The key issue in the wide application of the LRFD method for foundation design is the specification of resistance factors that are appropriate for various geotechnical conditions and design criteria. In recent years, a number of agencies have been calibrating resistance factors using regional/local load test data and design methods. These calibrations have to be used carefully since the design methods are not always in consonance with the load test data interpretation methods. In addition and more significantly, little effort has been made to address the dichotomy of design at the strength and service limit states. This paper first describes the current LRFD practice and resistance factor calibration approach. A performance based design (PB-LRFD) methodology that permits concurrent calibration and design of deep foundations at both the strength and service limit states is then presented. In the PB-LRFD approach, project specific resistance factors are obtained based upon the available site assessment and load-test data. Depending upon the scope of the subsurface investigation and field load testing/verification program, the proposed methodology may be considered in three-levels. The applicability of the PB-LRFD methodology is illustrated through foundation designs that satisfy strength and service limit state criteria.

Computational Limit Analysis is a numerical tool that allows the user to rapidly and directly determine the ultimate limit state for a general problem geometry without the need to iterate or timestep to a solution. This paper summarises recent work placing Computational Limit Analysis within a rigorous theoretical framework that can be used in the context of modern limit state design codes such as Eurocode 7. It then illustrates how a Computational Limit Analysis approach may be carried out for a range of example foundation and retaining wall problems within the Eurocode 7 context.

A particular focus of the paper is to examine the key issue of action/resistance factoring as used in Eurocode 7 Design Approach 1 Combination 1 and Design Approach 2.

This paper proposes a simple design code format for assigning design values of effective friction angle of clean sands from in-situ tests incorporating all sources of geotechnical uncertainty, including model uncertainty. New transformation models are proposed using a high-quality database of undisturbed sands. The quantitative characterization of uncertainty is pursued using a Bayesian hybrid Markov Chain Monte Carlo simulation framework. Design factors are calibrated using the posterior distributions output by the Bayesian framework. The effects of the aleatory and epistemic components of geotechnical uncertainty are discussed and assessed quantitatively. The paper concludes by demonstrating how to apply the results practically in geotechnical design.

The first experience with limit state design approach was introduced in the Czech Republic already 45 years ago for shallow foundations. However just the experiences with the second version which was implemented in 1987 (25 years ago) will be discussed here in more detail. The paper describes both limit states Ultimate and Serviceability limit states.

Mainly the focus will be on:

- definition and distinction of the design into 3 geotechnical categories

- demands on the site investigation

- nominal values of soil properties for different geotechnical categories

- design approaches within limit states

- definition of material partial safety factors

- design calculations for both limit states

- evaluation of up to date experience with respect to probability of failure.

A complete, formal geotechnical design code of practice addressing all aspects of geotechnical engineering really does not exist in the USA at the national level. However, a geotechnical design code has been developed at the national level in response to the needs of the structural engineers as part of the American Association of Highway and Transportation Officials (AASHTO) specifications, primarily focusing on structure foundations, buried structures, and retaining walls. Since 1994, AASHTO began migrating from Allowable Stress Design (ASD) to Load and Resistance Factor Design (LRFD), the USA equivalent to Limit States Design (LSD). AASHTO fully adopted LRFD, at least in the transportation Sector, in 2007, and ceased updating their Allowable Stress Design and Load Factor Design specifications in 2002.

A key issue for the USA geotechnical community regarding geotechnical design code has been to strike a balance between prescriptive minimum design requirements and levels of safety (i.e., load and resistance factors) and the flexibility needed by geotechnical engineers to apply engineering judgment to address site specific issues and local experience.

Summarized is the historical development of geotechnical foundation design code in the USA for transportation applications. With regard to the geotechnical portions of the current AASHTO LRFD Bridge Design Specifications, the development and selection of load and resistance factors is discussed. Key considerations for geotechnical design specification development are identified. Finally, gaps and future development needs for USA geotechnical design codes of practice are presented.

This paper reports lessons learned from reliability theory-based LRFD calibration for internal limit states for reinforced soil walls subjected to soil self-weight under operational conditions. The example of the ultimate pullout limit state for steel strip reinforced soil walls is used to demonstrate key issues. A unique feature of the general approach is the use of bias values that include the influence of model error and other sources of variability in input parameters on nominal load and resistance values. The paper shows how bias values can be used to: a) develop analytical load and resistance models that are statistically more accurate and avoid unwanted dependencies; b) select load factors satisfying a target exceedance value, and; c) compute resistance factors meeting a target probability of failure (reliability index value). The general approach has application to rigorous LRFD calibration of a wide range of geotechnical soil-structure design problems.

Canada has two national codes of practice which include geotechnical design provisions: the National Building Code of Canada and the Canadian Highway Bridge Design Code.Both ofthese codeshave beenusing a load and resistance factor format for about two decades now, but are still in the process of adopting a reliability-based design framework. This paper describes the advancesplanned for these codes.