Analysis of deep excavations using the mobilized strength design(MSD) method

Abstract: The population in Sweden and around the world is increasing. When population increases, cities become more densely populated and a demand for investments in housing and infrastructure is created. The investments needed are usually large in size and the projects resulting from the investments are often of a complex nature. A major factor responsible for creating the complexity of the projects is the lack of space due to the dense population. The lack of space creates a situation where a very common feature of these types of projects is the use of earth retaining systems. The design of retaining systems in Europe is performed today based on Eurocode. Eurocode is a newly introduced standard for the design of structures and is developed in order to make it easier to work cross borders by using the same principle of design in all countries. For the design of retaining walls in Sweden, Eurocode uses the old standard as the basis of the design procedure consisting of two separate calculations, ultimate limit state and serviceability limit state. Since soil does not consist of two separate mechanisms consisting of failure and serviceability, this approach to solving engineering problems fails to address the real behavior of soils. To handle this problem Bolton et. al. (1990a, 1990b, 2004, 2006, 2008, 2010) developed the theory of “mobilized strength design” where a single calculation procedure incorporates both the calculation of deformations and the safety against failure. The calculation uses conservation of energy and the degree of mobilized shear strength to study deformations in and around the retaining system and the safety against failure in mobilizing the maximum shear strength of the soil. The aim of this thesis was to introduce the theory of mobilized strength design to geotechnical engineers in Sweden working both in academia and in industry. Another aim of the thesis was to develop a tool that could be used to perform calculations of earth retaining systems based on this theory. The development of a working tool has resulted in a Matlab code which can in a simple way be used to calculate both deformations in the retaining system and the safety against failure by using the degree of mobilized shear strength presented in the theory. The Matlab code can handle ground layering with different shear strengths and weights of the soil. A comparison instrument in a Mathcad calculation sheet have been developed to produce results based on the original theory where the feature of soil layering is not incorporated into the calculation procedure. The thesis shows that the Matlab code developed performs well but is not yet sensitive enough to produce the same results as the Mathcad calculation sheet and needs to be further developed to make it more robust in order to handle all different excavation scenarios. v The theory of mobilized strength design has been introduced to geotechnical engineers in Sweden and the thesis studies the theory and shows the calculation procedure and how the different input values and calculations affect the analysis. The thesis also shows some areas in which the theory and the code can be modified and where further research can be performed in order to make them fully applicable to Swedish conditions. As an example the use of rock dowels drilled into the bedrock and attached to the retaining structure is a common feature for deep excavations in Sweden. Further research can be pursued on how to incorporate the energy stored in the rock dowels into the calculation procedure.