Doctoral Theses

The present doctoral thesis could be of interest in any field where rock fracture plays a fundamental role, such as civil engineering, mining engineering or energy engineering.

This research tries to provide answers to the general demand for tools and methodologies for the evaluation of rock fracture in noched components, setting aside the conservative application of classic fracture mechanics that has been traditionally used to deal with these sorts of issues. In particular, the applicability of the Theory of Critical Distances (TDC) and the Strain Energy Density (SED) criterion are studied in the scope of rock mechanics, trying to highlight the advantages and limitations of both methodologies on rock type materials.

Thus, this work is focused on the behaviour of the fracture of six types of rocks of varied lithology, under the presence of “U” shaped defects with different radii, different loading conditions (mode I and mixed mode I+II) and different temperature conditions up to 250ºC, a common temperature range in geothermic applications, for example.

Over the last years there has been a great development of offshore wind energy due to the increasing demand on renewable energies instead of the traditional ones and the need of new locations.

In those sites, like Cantabrian Sea, where continental platforms are narrow, floating platforms are needed. One of the most used mooring systems is the one formed by tension lines and plate anchors.
The aim of this thesis is to analyze the behaviour of plate anchors embedded in the seabed, focusing on the ultimate capacity of them.

The study is developed from two perspectives: experimental and numerical. The former consisted in small scale tests with application of vertical loads on plate anchors embedded in sandy soil, while the latter consisted in simulations of this scenario both in 2D and 3D analyses. Santander Bay sand has been chosen as the soil for both cases. Circular and square anchors have been tested at several depths in order to analyze the influence of the geometry of the plate anchor.

Finally numerical and experimental results have been compared. The comparison also included other results proposed in the bibliography.

With the aim of characterizing in a simple and economic way the waste disposed in a landfill, a field campaign, using pressuremeter and cone penetration test, was undertaken to determine the shear strength and the stiffness of the studied landfills.

In the last years, Mechanical and Biological Treatments (MBT) have been introduced in waste management. The main purposes of these treatments are to decrease the activity of the organic matter within the waste mass and to reduce the weight and volume of the landfilled waste, but it is not clear how these treatments affect the shear strength of the residues. The materials generated in the Meruelo’s landfill treatment facilities have been studied in the laboratory using large scale Direct Shear (300 x 300 m) and large scale Triaxial (Ø100 m) tests. As the materials generated are disposed together, several mixtures with variable proportions of each of them have also been studied.

Finally, the accident occurred in a real non-treated MSW sanitary landfill has been studied. The possible causes of the accident along with the shear strength of the involved wastes have been determined.

Piles are usually classified on the basis of their behaviour, construction procedure, materials used and cross-sectional shape.

According to the construction methodology, piles can be classified as in situ piles and driven piles. In situ drilled and grouted piles have been extensively used in carbonated soils, where only very low shaft capacities can be developed. There is a sound understanding of the performance of this pile type, thanks to the extensive experimental research conducted so far, the existence of numerous field tests; both at large (1:1) and reduced scales; as well as laboratory tests carried out at reduced scale.

Due to the high installation costs associated to the construction of drilled and grouted piles, alternatively grouted driven piles with circular hollow sections were developed. For this particular pile type, there is a limited number of field tests; both at large and reduced scales; and a relatively large number of laboratory tests at reduce scale. Nevertheless, due to the uncertainty associated to grout cover thickness, the difficulties method for measuring this cover thickness and estimating grout shrinkage, grouted driven piles with circular hollow sections have never been used for practical applications.

In order to improve the driving process, both for steel and concrete piles, several alternatives have been considered and studied so far. Shaft grouting has been used in these piles with the aim of either filling in the gap between the pile and the surrounding soil; generated during the driving process; or filling in lateral membranes attached to the pile surface. So far, for practical applications, only the first system has been used for driven steel piles with open cross-sections.

The work presented in this thesis focuses on a new type of pile; the shaft grouted driven concrete pile (SGDCP). This new pile type has been protected under an international patent.

This driven concrete pile has a square cross section with a hollow tube along its longitudinal axis, which is connected to a network of secondary lateral tubes. Each lateral tube, located in perpendicular direction to the pile axis, directly connects the main axial hollow tube with the pile surface, and is fitted with one-way valve. Following the driving process, the pile surround is pressure grouted to increase its shaft capacity, achieving a greater skin friction than in a conventional driven concrete pile.

As a consequence, several advantages have been identified: the total pile length in a deep foundation can be reduced, cutting down on construction period and reducing foundation cost.

The first practical application of this new pile type is also presented herein, explaining the geotechnical problem, showing the pile design, describing the on-site works and finally presenting and discussing the results. With the aim of assessing the increase in pile shaft capacity as a result of the pressure grouting, dynamic load tests were conducted before and after the injection process. These tests showed how the shaft capacity was significantly increased due to the pressure grouting, with shaft capacities between two and six times greater than the ones registered before the injection process.

Stone columns are a soft soil improvement method usually employed in embankments and structures foundations under soft soils. The study carried out in this PhD Thesis is focused on the analysis of:

– The influence of the density of the gravel in the column and
– The encasement of stone columns with geotextile.

With the aim of study the influence of the density of the gravel, laboratory small scale tests have been carried out with two different densities and several areas replacement ratios. These tests have been developed in a Rowe-Barden cell where the behavior of a unit cell is analyzed. The cell employed is instrumented in order to measure total and pore pressures as well as displacements. The results obtained were analyzed from the point of soil-column stress ratio, settlement reduction and dissipation of pore pressures. In order to complement the study, numerical analyses simulating the laboratory tests have been executed employing several constitutive models. Their results have been compared with the experimental ones. Finally, experimental results have been compared with the ones obtained from the most relevant analytical solutions. The study of encased stone columns has been carried out in a similar way as the mentioned previously for the analysis of the density. Small scale test similar to those presented before but this time using two different geotextiles. This time, as well as before, experimental results have been compared with the ones from the numerical simulations and the ones obtained from the most important analytical solutions.

The study of friction of the geosynthetics used for landfills both for basal-liner and capping systems is a very important issue. These lining systems typically contain a large number of material interfaces (geosynthetics/geosynthetics or geosynthetics/soil), many of which have low shear strengths. This introduces potential failure surfaces along the side slopes and base of the fill mass.
A research project about this subject has been undertaken at University of Cantabria. In this investigation, a methodology for direct shear tests between two geosynthetics and a soil and a geosynthetic has been developed, achieving the friction parameters and analysing interaction mechanisms of different contacts.

Later on at Technical University Bergakademie Freiberg was developed a shear strength model of the geomembrane/geotextile interface. On the one hand a shear model has been developed, on the other this model was introduced in numerical modelling code for advanced geotechnical analysis, FLAC3D. There is an excellent agreement between laboratory results, shear model and numerical model.

In situ tests are one of the most usual methods of subsurface and soil investigation, specially related to soil identification, strength and deformation parameters. Taking into account only the in situ tests, the penetration ones are surely the most usual.

Penetration tests are not deeply well known. There are many widespread correlations with the Standard Penetration Tests (SPT), sometimes they are not so accurate, but they are considered useful to estimate some parameters related to soil identification, strength, compressibility and displacements.

The main problem is that there are so many different kinds of penetration tests, each one with its own characteristics. It is very difficult to correlate the results from different tests among them. The actual equations only work approximately. The main aim in this thesis has been to improve those equations.

The penetration in these tests is intimately related to the potential energy of the hammer (nominal energy). This energy is due to the mass of the hammer and the free fall of the hammer after being released from a certain height. Then the hammer strikes on the anvil that lies on upper part of the drive rods.

So, it is necessary to understand correctly and deeply how this energy is transferred to the drive rods and how it is transmitted through the rods to the cone to realize how the real behavior in these tests is.

The penetration length really depends on the energy, not on that nominal energy, but on a portion of that energy that is effectively transferred to the rods (ENTHRU) and, in order to be more precise, the energy that reaches the cone (ENTHRUcone). The ENTHRU is measured by means of monitoring the upper part of the drive rods (close to the anvil). To calculate the ENTHRUcone it is necessary to correct the ENTHRU in three ways. First it is needed to take into account the energy loss in the energy transmission through the rods, and the energy loss due to the skin friction of the rods with the soil around them. It is also necessary to add the energy due to the rod weight that is penetrating into the soil.

The main hypothesis assumed in this thesis is based on the fact that the ENTHRUcone has to be greater than a certain value or minimum energy (energy threshold) to be able to cause penetration.

A large amount of tests were carried out in Arija (Burgos). These tests were performed on a very homogeneous and thick siliceous sandy soil.

The measured data were analyzed and the final results confirmed a correlation between the penetration length and the ENTHRUcone, and it can be stated with certainty there is an energy threshold.

To conclude, two main equations are presented in this thesis, one approximate and another one more accurate, to correlate the results between two different dynamic penetration tests. The correlations depend on the ENTHRUcone (or ENTHRU), the energy threshold, the nominal cone base area and the total penetration length in the two penetration tests whose results are related.

The analysis of the radial consolidation process around the column and the column-soil interaction is based on the study of a unit cell approach, which consists of a central column of gravel and the surrounding soil. In this thesis, a unit cell in small scale has been reproduced with the aim of analyzing load transfer between soil and column, settlement reduction and radial consolidation process that happen when a rigid vertical load is applied on surface.

Tests with two different geometries are carried out.

From the results, some conclusions related to consolidation process, stress concentration factor and settlement reduction have been obtained for each testing geometry.

The influence of the replacement area has been studied comparing the results of both geometries.

Finally, the results are presented and interpreted using some existing analytical solutions related to consolidation process and stone columns deformation.

Stone columns, either by the vibro-replacement or vibro-displacement methods, are one of the most common improvement techniques for foundation of embankments or structures on soft soils. The main effects usually considered with respect to the untreated ground conditions are: improvement of bearing capacity, reduction of total and differential settlements, acceleration of consolidation, improvement of the stability of embankments and natural slopes, and reduction of liquefaction potential.

In this thesis, a new closed-form solution that includes the radial and vertical interaction between soil and column has been developed. The solution gives all the stresses and displacements at any time by means of a simple spreadsheet.

The instrumentation of two different field sites where the ground was improved with stone columns is shown and analysed.