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Very Soft Organic Clay Applied for Road Embankment

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In this thesis strategies for using very softorganic clay as a fill material for road embankment construction are compared, and an optimisation scheme is presented.

 This thesis concerns optimisation of the use of clay in embankments. Also recommendations are presented based on a rational method for conditioning and emplacing very soft organic clay in a safeand economic way. 

Very soft organic clay poses serious problems for embankment construction in deltaic areas. It is well known that this clay has very poor engineering properties; therefore it cannot be used as embankment fill material without improvement.

 Dewatering by evaporative drying, dewatering by horizontal sand drainage (clay-sand layers in a sandwich construction) and pre-loading (surcharging) are considered to be the simplest, most practical and most cost-effective methods for the improvement of the properties explored in this study. Computer programs for simulation of evaporative drying, for simulation of consolidation, and for slope stability calculation were used to analyse clay behaviour.

 Field and laboratory testing programs for a trial embankment were set up in order to study the relations of parameters, to examine suitability and workability of the clay, and to verify the improvement methods. An optimisation technique was applied as a rational method for using very soft organic clay for road embankments leading to an economic construction. Optimisation models were developed for this particular problem. These models consist of an equipment selection model and a multi-objective optimisation model for clay-sand layered embankment constructions. 

The equipment selection model provides an optimum selection of the size and number of the earthmoving equipment fleet, which yields minimum equipment costs. A Genetic Algorithm (GA) was applied as optimisation technique to find the optimum solution. The results indicate that the equipment selection model and GA can efficiently be used to find the optimum, which is minimum equipment costs. The obtained equipment costs were used as parameters in the development of cost models that are part of the multi-objective optimisation of clay-sand layered embankments.

 A special computer program was developed to integrate the embankment design to the GA optimisation process that is applied to find economic solutions in the multiobjective optimisation model. 

The calculation in the design part is performed using Artificial Neural Network (ANN) models. These models were developed from the results of the simulations in order to cope with complexity of the problem and to reduce computation time of the GA optimisation process. 

The ANN models for the calculation of the amount and duration of settlement, and of the stability were developed from the consolidation simulation program and the slope stability calculation program respectively. The model validation shows that with the aid of ANN models results are obtained that are almost identical to the results calculated by simulation programs. 

The computer program has performed the optimisation process by using ε-constraints, or the so-called trade-off approach i.e. a situation in which the parameters can be changed as long as they contribute to a global optimum. 

The Pareto curve that illustrates this approach can be constructed from the set of optimum solutions. A proper decision for a clay-sand layered embankment construction can be obtained by the assistance of the trade-off curve. A sensitivity analysis was performed to study the sensitivity of the solution for design parameters, which include those related to embankment dimension, material properties, and material costs.