Giovanni Spagnoli

Undrained shear strength of clays as modified by pH variations

The undrained shear strength of clays is an important geotechnical parameter used during construction processes. Several laboratory tests were performed on kaolinite and smectite mixed with pore fluids with different pH values. Vane shear tests were carried out and it was found that the undrained shear resistance for clays increased considerably if the pore fluid had a high or a low pH. A possible explanation could be the dissolution of Al3+ which acts as a coagulant, increasing the internal shear resistance. Geochemical computations, Al measurements and ζ-potential experiments were performed to confirm this theory. The research suggests varying the pH may make a useful contribution to soil improvement techniques.RésuméLa résistance au cisaillement non drainé des argiles est un paramètre géotechnique important utilisé en phase de construction d’ouvrages. Plusieurs essais de laboratoire ont été mis en œuvre sur des kaolinites et des smectites préparées avec des fluides de différents pH. Des essais de cisaillement au scissomètre ont été réalisés et l’on a trouvé que la résistance au cisaillement non drainé des argiles augmente considérablement si le fluide interstitiel a un pH faible ou fort. Pour un pH faible, une explication possible pourrait venir de la dissolution des Al3+ qui agissent comme des coagulants, augmentant la résistance au cisaillement. Des calculs géochimiques, des mesures de teneur en Al et de potentiel ζ ont été réalisés pour confirmer cette théorie. Les résultats de cette recherche suggèrent que le fait de faire varier le pH de l’eau interstitielle peut contribuer utilement à l’amélioration de la résistance des sols.

Electro-osmotic flow in clays and its potential for reducing clogging in mechanical tunnel driving

Clogging during mechanical tunnel driving is not only a serious technical issue, but also an economic one. The costs of a tunnel excavation can easily rise and disputes between the awarding authorities and the executive companies may occur. Although the literature is full of cases describing the clogging in clayey soils and despite countermeasures being available, clogging still occurs. This study proposes an alternative method to diminish adhesion of clays on TBMs. Electro-osmotic flow experiments, spectral induced polarisation tests and Zeta-potential simulations were performed on kaolinite and smectite, mixed with several pore fluids under one critical consistency index. The results showed that the electrical parameters were not only influenced by the clay mineralogy per se, but also by the pore fluid chemistry. To apply the laboratory findings in in situ conditions, several theoretical considerations have been taken into account. Although further research is required, the study indicates electro-osmosis may be a new and revolutionary approach to deal with the clogging of TBMs.RésuméLe colmatage lors du creusement d’un tunnel est non seulement un grave problème technique, mais aussi une question économique. Ce travail de recherche propose une méthode alternative pour diminuer l’adhérence des sols argileux sur les tunneliers. Des expériences d’écoulement électro-osmotique, des essais de polarisation induite spectrale (SIP) et des simulations de potentiel zêta ont été effectués avec de la kaolinite et de la smectite, mélangées avec plusieurs fluides pour un indice de consistance critique. Les résultats ont montré que les paramètres électriques étaient non seulement influencés par la minéralogie de l’argile, mais aussi par la chimie du fluide interstitiel. Pour appliquer ces résultats de laboratoire aux conditions in situ, plusieurs considérations théoriques ont été prises en compte. Bien que des études supplémentaires soient nécessaires, on conclut que l’électro-osmose pourrait constituer une approche nouvelle et révolutionnaire pour traiter du problème du colmatage des tunneliers.

Assessment of bauxite refining residue (red mud) as a liner for waste disposal facilities

Red mud (RM), a waste arising from the bauxite refining, was studied to assess its suitability as a compacted liner material. The geotechnical and hydraulic conductivity (K) characteristics of the RM, as well as the effect of desiccation, were evaluated. RM has index properties satisfying the requirements to achieve low K. The compaction–permeability studies showed that RM may be compacted to a dense homogeneous mass using a moderate compactive effort, while lowest K (⩽1 × 10−7 cm/s) was achieved for RM compacted slightly (0.4–4%) wet of optimum. Shear strength test revealed high values of friction angle and cohesion for compacted RM, and its tight structure was also evidenced by its low compressibility and maximum settlement under load. Desiccation adversely affected the permeability of compacted RM, but to a lesser extent than generally observed for clays. These results suggest that RM could be potentially utilised as a low-permeability barrier to be laid down under engineered landfill sites.

Preliminary Design of a Trench Cutter System for Deep-Sea Mining Applications Under Hyperbaric Conditions

Seafloor massive sulfides (SMSs) are formed as precipitation products from hot hydrothermal fluids as a result of mixing with cold seawater and are mostly found at depths between 1500 and 3600 m. These deposits are formed in tectonically active zones of oceans (midocean ridges and “back-arc” spreading systems) and are the result of the global heat transfer from the mantle in the oceanic crust. As these fluids mix with the cold surrounding seawater, metal sulfides in the water are precipitated on or in the nearby seabed. The appearance of the solid mineral occurs as cylindrical chimney structures: the so-called black and white smoker (caused mainly by the presence of iron, copper, zinc, and sulphur). Larger sulfide occurrences are mostly originated in several generations of hydrothermal cycles, and they form deposits that can range from several thousands to about million tons. SMS contains pyrite (iron), galena (lead), sphalerite (zinc), and chalcopyrite (copper). Deep-sea mining is concentrating now to exploit such deposits. So far, the only known commercial project is the one developed by Nautilus Minerals, which is based on a horizontal system. This paper describes a preliminary design of a novel cutting tool developed for a vertical mining approach. The vertical mining method is preferred when rough terrain is expected and the device is easy to relocate. Using an atmospheric and hyperbaric cutting model, the cutting energy for two selected SMS deposits has been estimated and validated with real onshore excavation sites performed with a cutting tool normally used for diaphragm wall installation (i.e., the trench cutter technology). Preliminary results suggest that the estimated cutting energy ESP is 2.9 times higher with respect to the measured one in atmospheric conditions. This could be because the model considers the worst case scenario, i.e., higher energy due to the maximum cutting forces assumed. This factor has been used to design a cutting tool which is able to work up 2000-m water depth to mine SMS deposits.

Strength of soil reinforced with fiber materials (Papyrus)

Construction of building and other civil engineering structures on weak or soft soil is highly risky because such soil is susceptible to differential settlements, poor shear strength, and high compressibility. Various soil improvement techniques have been used to enhance the engineering properties of soil. Soil reinforcement by fiber material is considered an effective ground improvement method because of its cost effectiveness, easy adaptability, and reproducibility. Hence, in the present investigation, papyrus fiber has been chosen as the reinforcement material, and it was randomly included into the soil at four different percentages of fiber content, i.e., 5, 10, 15, 25% by volume of raw soil. The main objective of this research is to focus on the strength behavior of soil reinforced with randomly included papyrus fiber. Direct shear, consolidation, and displacement tests were performed on papyrusreinforced specimens with various fiber contents. The results of these tests have clearly shown a significant improvement in the failure deviator stress and shear strength parameters (c and φ) of the studied soil with a percent addition of 10% (the preferred percent). Moreover, this addition ratio reduced the displacement of the soil under loading. It can be concluded that papyrus fiber can be considered an appropriate soil reinforcement material.

Latest Technological Developments in Offshore Deep Mixing for Piled Oil and Gas Platforms

This paper presents some recent technological developments in deep mixing for the offshore sector. Deep mixing methods comprise in-situ soil treatment technologies where binding materials are added and blended with the original soils in order to improve their mechanical properties. The MIxed Drilled Offshore Steel (MIDOS) pile is introduced in this paper, which takes advantage of such deep mixing technologies. The comparison between the API approach and CPT-based methods for the prediction of the pile capacity are provided to validate the capability of the MIDOS pile as a foundational element for oil&gas structures in different geological conditions. The theoretical calculations are intended for initial estimation of pile sizing only and are not intended as a detailed design method.Copyright

Laboratory investigations to assess the feasibility of employing a novel mixed-in-place offshore pile in calcareous deposits

ABSTRACT Calcareous sands are typical of warmer seas and are encountered in several high growth locations around the world. Despite having high frictional resistance with friction angles exceeding those of siliceous sands, the in situ behaviour is characterised by particle damage and extreme contraction at high confining stresses. This behaviour results in very low values of skin friction for driven piles in calcareous deposits, where the contraction dominates the pile response. The MIxed Drilled Offshore Steel (MIDOS) pile is a novel mixed-in-place technology which has many advantages over driven steel piles and conventional drilled-and-grouted (D&G) piles. The MIDOS is based on the deep-mixing technology normally used as an onshore ground improvement technique. The mechanical technology and in situ pile performance were successfully demonstrated during an in situ test in silica sand. A laboratory based study was undertaken to assess the MIDOS performance in calcareous sand. Geotechnical tests, grout tests and steel–grout pull out tests were performed to assess the sands and to model the behaviour at the interface of the steel reinforcement and the grout body. These preliminary results demonstrate that the frictional shaft resistance at the pile–soil interface is comparable for both silica and calcareous sands, and as there is no stress relief or contraction during the installation process, the geotechnical performance of the MIDOS pile is deemed comparable for both soil types tested.

Some considerations regarding the use of helical piles as foundation for offshore structures

Europe is currently the leader in offshore wind energy, with the first offshore wind farm being installed in Denmark in 1991. As of 2010, there are 39 offshore wind farms in European waters with a combined operating capacity of 2,396 MW. Currently, the most used foundation type for offshore wind farms is the monopile. However, the increasing water depth and turbine size will slowly decrease the utilization of this solution. Among several other solutions, an interesting one might be the use of helical piles. The aim of this paper is to briefly describe the current foundation situation in the offshore wind farm industry and theoretical considerations with regard to the uplift capacity of the helical piles and their possible application in the offshore industry.

Liquid limit of mixtures of smectite, kaolinite and quartz powder with water and NaCl solution

Abstract Clay mineralogy plays an important role in determining the mechanical properties of clayey materials. Several liquid limit and water uptake tests were performed on kaolinite-smectite mixtures for different quartz powder content and water and / or NaCl content as pore fluids. To interpret the results, the diffuse double layer theory is used. The experimental results show the effect of quartz powder in reducing the liquid limit of smectite with water and 1 M NaCl as fluid. A linear relationship is obtained from the tests. The quartz powder acts only to dilute the liquid limit values. To better understand the behavior of the clays regarding the liquid limit, predicted values for the liquid limit of all the mixtures with water and NaCl as pore fluid have been developed assuming the linear law to be valid. The prediction is very good for water, but it becomes less good with lower liquid limit values/higher quartz contents. The correlation with NaCl as fluid is not that good when the quartz content is high. A correlation of water uptake and liquid limit is also presented. The data fit is good. The overall results confirm that the effect of the pore fluid type is significant when the smectite content in the mixture is high.

Geotechnical and machinery properties influencing the offshore pile drillability

ABSTRACT Foundations of offshore oil and gas production platforms or wind parks are in the majority of cases piled-based. Piles are mainly driven to a certain design depth using hammers. However, there are many situations were driven piles cannot be properly installed. Plug formation or presence of rock layers and/or boulders preclude the correct installation. Since time is a key factor in offshore operations, offshore drilled piles are not very common. Their high level of uncertainty is mainly due to the complexity to assess a pile drillability analysis. For these reasons, assessing the drilling time needed to overcome the problems is very important. The following research compared first of all several well-known equations used for estimating the tunnel boring machine (TBM) excavation rate. Second, a novel approach has been used, which is based on the estimation of the specific energy. After having performed continuous automatic diagraphy tests in Turin subsoils (Italy), values of friction angles and compressive strength were obtained. Specific energy was calculated at the scale of the continuous automatic diagraphy test as a function of ξ, which is a coefficient depending on the geotechnical parameters assumed by Nishimatsu. It is therefore possible to estimate the drilling rate for different values of friction angle and cohesion.