A New Technology for Increasing the Load-Carrying Capacities of Offshore Foundations in Soft Clays

There are many underground construction projects, including tunnelling and deep excavations, in Singapore. However, the underground construction is challenging when encountering Singapore soft marine clay due to its poor engineering properties, such as low shear strength and stiffness. Cement stabilisation through deep mixing or jet grouting has been commonly used to treat the soft marine clay for underground construction in Singapore, which can significantly increase the shear strength and elastic modulus, as well as decrease the permeability of soft clay. However, the process of manufacturing traditional Portland cement (PC) leads to negative environmental impacts, e.g. high CO2 emissions and energy consumptions. In addition, the PC is not very effective for stabilisation of marine clay with high water content. Hence, this study proposes to use a novel sustainable binder consisting of two industry by-products, namely carbide slag (CS) and ground granulated blast furnace slag (GGBS), to stabilise Singapore soft marine clay for underground construction. By replacing PC with industry by-products, the associated environmental impacts are significantly reduced. Specimens of soft clay stabilised by both CS-GGBS and PC with two binder contents were prepared in laboratory and then tested at different curing ages. The key engineering properties related to underground construction, including unconfined compressive strength, elastic modulus and permeability of the stabilised clays with different binders were compared. The results indicate that the CS-GGBS-stabilised soft clay can achieve much higher unconfined compressive strength (up to ~300%) and elastic modulus (up to ~600%), and lower permeability (one order of magnitude) than the corresponding PC-stabilised soft clay, which are beneficial to the soft stabilisation for underground construction.

There are many underground construction projects, including tunnelling and deep excavations, in Singapore. However, the underground construction is challenging when encountering Singapore soft marine clay due to its poor engineering properties, such as low shear strength and stiffness. Cement stabilisation through deep mixing or jet grouting has been commonly used to treat the soft marine clay for underground construction in Singapore, which can significantly increase the shear strength and elastic modulus, as well as decrease the permeability of soft clay. However, the process of manufacturing traditional Portland cement (PC) leads to negative environmental impacts, e.g. high CO2 emissions and energy consumptions. In addition, the PC is not very effective for stabilisation of marine clay with high water content. Hence, this study proposes to use a novel sustainable binder consisting of two industry by-products, namely carbide slag (CS) and ground granulated blast furnace slag (GGBS), to stabilise Singapore soft marine clay for underground construction. By replacing PC with industry by-products, the associated environmental impacts are significantly reduced. Specimens of soft clay stabilised by both CS-GGBS and PC with two binder contents were prepared in laboratory and then tested at different curing ages. The key engineering properties related to underground construction, including unconfined compressive strength, elastic modulus and permeability of the stabilised clays with different binders were compared. The results indicate that the CS-GGBS-stabilised soft clay can achieve much higher unconfined compressive strength (up to ~300%) and elastic modulus (up to ~600%), and lower permeability (one order of magnitude) than the corresponding PC-stabilised soft clay, which are beneficial to the soft stabilisation for underground construction.

India has a vast extend of coastal area and has got large deposits of soft marine clays extending to a depth of 15–60 m. Construction on marine clays is a big challenge because of its high compressibility, low shear strength and excessive settlement. Ground Improvement techniques such as sand drain, gravel drains, PVD, stone columns etc. are widely used to improve the engineering properties of sub soil. It was found from field studies that in case of very soft clay, if sand or coarse aggregate alone is provided, diameter of drain gets reduced and continuity of drain get broken, due to necking action of very soft clay. In order to reduce these effects and to increases strength and continuity characteristics of drainage media a hybrid mix comprising of aggregate, sand and synthetic fibre is developed through experimental model studies. The major limitation of displacement vertical drains is the remoulding of soil surrounding each drain caused by installation which reduces permeability and efficiency of drain. The problems of smear zone can be reduced by multiple driving of vertical drains in the same spot which will enhance efficiency of drains by forming new flow paths, as there is increase in diameter and drainage media get penetrated into smear zone. In this paper efficiency comparison of various vertical drains such as sand drain (single and double driving), PVD, and hybrid mix drain (single and double driving) is studied by assessing the behavior in laboratory on large scale model bed tanks. The efficiency is evaluated by comparing settlement with time for different loadings for each types of vertical drain. From the results it is noted that double driven hybrid mix shows much better performance compared with other vertical drains and can be effectively used for very soft clays.

Marine clays are soft clays with low shear strength and high compressibility. These soils are prone to seawater intrusion, due to which their engineering properties may be altered. Therefore, the effect of salinity on engineering properties of marine clay has to be investigated prior to any engineering activity in those areas. In this paper, an attempt has been made to study the effect of salinity on various geotechnical properties of Cochin marine clay. As sodium chloride is the most abundant mineral salt in seawater, the effect of 0%, 0.5%, 1%, 2%, 3% and 4% sodium chloride on marine clay for different curing periods were studied. The results of the study show that liquid limit decreases whereas the strength and compressibility characteristics improve as the concentration of salinity increases. Leaching, however, reverses these improvements, amplifying compressibility by 30% and decreasing pre-consolidation pressure by almost 50%. ITherefore, prior to design of any foundation structures on coastal areas prone to salt water intrusion, the behaviour of clay with variation in salinity should be investigated.

Large deposits of marine clays are encountered all along the Indian coastal belt. The marine clays are soft clays characterized by low shear strength and high compressibility. In Cochin, most of the areas consist of soft marine clay deposits. The presence of weak marine clays demands the use of expensive deep foundations in these areas. Also the settlement of foundations is a major problem associated with the structures constructed over these areas. This paper presents the results of laboratory studies to determine the improvement of engineering properties of Cochin marine clay by introduction of compacted lime column and lime fly ash column techniques with and without preloading. The test results indicate that the compacted lime column and lime fly ash column improve the physical, chemical and engineering characteristics of soft clay very effectively. From the studies, it is noted that the columnar intrusion with preloading can substantially improve the soil properties and can lead to economy in design of foundations and retaining systems during deep excavations.

Soft soils pose abundant engineering issues due to its low bearing capacity and shear strength. Comprehensive study on soft soil’s physical properties such as shear strength and ability to store water (porosity) could help in devising the optimum ground improvements and foundations techniques. Therefore, physical properties of soft marine clay in Nibong Tebal were thoroughly studied using 2-Dimensional Resistivity Imaging (2-DRI) method in conjunction with porosity measurements, standard penetration test values (SPT-n) and particle size distribution (PSD) analysis. The 2-DRI profile depicts three lithologies, which are unsaturated topsoil, saturated soft clayey soil and saturated sandy soil in the area. The soft soil extends up to 32 m in thickness where it overlies the sandy layer and could be correlated back to lithology profile from borehole record. Additionally, soil samples were collected at three locations along the survey line for porosity measurements via saturation porosimetry method. The samples demonstrate that the clay layer has a very large porosity range and signifies that the soil will compress tremendously under load. On the other hand, SPT-N values of the soft clay is also very low; thus, could be classed as very soft to soft cohesive soil with very low shear strength as compared to a higher range SPT-n values of the sandy layer. The PSD result also compliments the 2-DRI, porosity and SPT results to show distinct differences between topsoil and the soft clay layer in terms of the presence of fine grains. These results further indicate that the thick upper layer is not capable of bearing immense loads such as high-rise infrastructures due to the soil’s high porosity and low shear strength. Hence, the area must undergo ground remediations prior to any infrastructure developments on the land.

Electrokinetic Stabilisation (EKS) technique is a combination process of electroosmosis and chemical grouting. This study involves the investigation on the EKS technique performances in stabilising soft clay soils. Stabilising agents will assist the EKS technique by inducing it to the soil under direct current and its movements which is governed by the principle of electrokinetic (EK). The objective of this research is to study the effectiveness of EKS technique in increasing the strength of soft clays. Two reactors were set up by using 1.0 M of calcium chloride (CaCl2), sodium silicate (Na2SiO3) as the electrolyte and stainless steel plates as the electrodes. EKS technique was being performed for 21 days period of time with a constant voltage gradient (50 V/m). This technique was carried out in two phases where the difference between them is a combination of the stabilising agent. The two combinations of stabilising agents in phase 1 and phase 2 were CaCl2 – distilled water (DW) and CaCl2 – Na2SiO3, respectively. The difference was, one was using distilled water while the other was using Na2SiO3. Results of the strength, liquid limit (LL), plastic limit (PL), plasticity index (PI), pH and ion concentration test towards untreated and treated soil were presented. Showing the strength of treated soil for both phases was increasing near the cathode section with 27.83 kPa and 27.67 kPa. LL and PI for treated soil showed the highest value which occurred near the cathode, while PL seems consistant with the values from untreated soil. The Calcium (Ca+) and sodium (Na+) concentrations in soil were increasing compared to the untreated soil, hence it has proven that the application of stabilisers in EK treatment is more effective in increasing the strength and the stability of soils.

Structures construct on soft clays are often affected by stability and settlement problems due to high compressibility, low shear strength and low permeability of soft clay which will lead to bearing capacity failure and excessive settlement. The soft clay samples had the dimensions of 50 mm diameter and 100 mm in height. The bottom ash column had two (2) different area replacement ratios which were 4% and 9% (10 mm and 15 mm diameters respectively) with the bottom ash column penetration ratio of 0.3, 0.7 and 1.0. The results of the unconfined compression test show that there is an improvement in shear strength of soft kaolin clay when reinforced with bottom ash column. For samples with area replacement ratio 4%, the results show the increment of 25%, 37.5% and 50% at height penetrating ratio of 0.3, 0.7 and 1.0, respectively in shear strength. Meanwhile, for samples of 9% area replacement ratio, the shear strength of the soft kaolin clay increased about 14.29, 28.57 and 57.14%, respectively. It can be concluded that by reinforcing the soft clay using singular bottom ash column, the shear strength of the soils increases and become more significant as the area replacement ratios and the column penetration ratios increase.

A series of subsurface investigation including in-situ and laboratory tests has been carefully planned and executed for a proposed residential and commercial development over soft marine clay at Tg Tokong, Penang Island. This paper presents the approach taken in determining and developing fundamental geotechnical engineering design parameters of the soft clay. The subsurface investigation was carried out in two phases to suit the overall development implementation plan namely subsurface investigation done near-shore with about 70 nos of boreholes, 50 nos of cone penetration tests (CPT) and 100 nos of Mackintosh probes and those that were carried out off-shore with 72 Nos of boreholes. The samples extracted from the field work were subjected to grain size analyses, Atterberg limits, oedometer test and consolidated undrained compression. Subsequently, correlations were established between physical soil properties with undrained shear strength from field vane and laboratory tests. The subsurface investigation has suggested that the soft marine clay at the northeast of Penang island comprises of a very soft upper marine clay layer overlying a stiffer lower marine clay. An intermediate stiff clay is sandwiched between these two marine clay layers. The soft clay was also subjected to x-ray diffraction to observe the minerology make up. The primary clay mineral was kaolinite/chlorite followed by smectite. The clay is anticipated to pose minimum effect onto the swelling and compression behaviour.

Electrokinetic (EK) treatment of soil/sludge contaminated with metallic and organic pollutants has proven its success concerning efficiency and cost. The study carried out in the laboratory aimed at the feasibility of this technology for the simultaneous elimination of aluminium and sulphates from sludge obtained from the clarification of industrial dairy wastewater by coagulation-flocculation with aluminium sulphate. Work in this area is scarce due to the presence of bacteria in the sludge. The EK reactor comprises three compartments: the anode, the cathode, and the central part, which contains the sludge to be treated. A constant voltage of 15 V is applied between two platinum wire electrodes immersed in each electrolytic compartment. The EK treatment employed gave satisfaction despite the bacterial front in the 3/5 of reactor sludge fractions. Indeed, the removal rates are approximately 72% for aluminium and 92% for sulphates with current values below 5 mA, and relatively low energy consumption and cost (648 kWh/ton and 25.9 $/ton of sludge, respectively). For EK transport, the absence of hydraulic convection exerted on the sludge would favour the displacement of aluminium and sulphates by electromigration. Electroosmosis also contributes to this displacement. Thereby, analysis of sludge after this EK treatment predicts a possible valorisation but after liming. The sludge is rich in nutrients with low heavy metals concentrations. Although the study is more interesting for aluminium because of its toxicity, the behaviour of sulphates suggests the possibility of using EK to decontaminate soils or sludge from anions like nitrates, chlorides, etc.

Very soft and soft clays are to be consolidated to enable them to support embankments supporting highways/railways, heavy duty pavements at container yards and to support runways. Such clays are predominant along the sea coast with thickness of layers extending to large depths. Among different ground improvement options, strengthening of soft clay by vertical drains with preloading is the best option and is preferred by engineers (Sarika S, Deepankar C, Salunkhe JS (2015) Ground improvement of marine clay for highway construction at Mumbai, India. IFCEE ASCE 2015; Kumar Pichumani N, Aminul I (2021) Reclamation and ground improvement of soft marine clay for development of offshore terminal 4, JNPT, Navi Mumbai. Indian Geotech J-021-00521-y). The ground treatment time with vertical drains depends on consolidation characteristics of soft clay for proposed drain with given spacing. Prefabricated vertical drains (PVDs)/geocomposite drains are widely used as they are efficient over other forms of drains. Often, designers consider the value of radial coefficient of consolidation based on the value of vertical coefficient of consolidation. Earlier studies on soft marine clays have reported the ratio of radial to vertical coefficients of consolidation to vary from 1.5 to 4 (Chu et al. in J Geotech Geoenvironmental Eng 2002:724–732, 2002, Marchetti et al. 2004). In the present paper, the influence of radial coefficient of consolidation on the treatment time for soft clay at one of the stacking yards of Visakhapatnam port trust is evaluated from 3-dimensional consolidation. The design of ground improvement system with PVD and preload is done for different values of radial coefficient of consolidation to highlight its influence in ground improvement program in soft clay. The study revealed that the overall degree of consolidation (U) is not much affected for cr/cv values of 2.5–4.0 at closer spacing of drains (0.8 m) as the variation in values of U is about 5%. Further, in triangular pattern arrangement of drains, the variation is relatively smaller compared to square pattern arrangement of drains.KeywordsSoft clayGround improvementPreloadingVertical drainRadial consolidationCoefficient of consolidation

This dissertation is the first Geotechnical Engineering doctoral thesis in Griffith University, and a detail study of the soft clay as encountered in Southeast Queensland is carried out. In the study process, due to the insufficient laboratory equipments and access to Geotechnical softwares, the dissertation has to be presented in a practical format. In addition, laboratory tests were conducted to investigate the application of chemical (cement and lime) treatments. Three case histories (Sunshine Motorway, Port of Brisbane Motorway, and Gold Coast Highway) are presented in this thesis. The main focuses are on the following aspects: i) soil parameters needed in engineering design from laboratory tests and field measurement, ii) the behaviour of constructed embankment on soft ground with and without ground improvement, iii) the performance of ground improvement techniques. The methods which have been employed to achieve the main aims were conventional methods. Laboratory test data and field measurement data, which were utilised for back-analyses and prediction of constructed embankment of soft ground behaviours, were obtained from QDMR. Thick layer of soft sensitive marine clay were found in the studied areas with up to 13m depth. The performance behaviour of constructed embankment on Southeast Queensland soft clay deposit has been evaluated based on the interpretation of test data, the theoretical analyses and conventional methods for settlement, lateral displacement, and excess pore pressure dissipation. Detail study of the estuarine soft clay as encountered in the Sunshine Motorway is carried out. 33 borehole data were examined to delineate the soft clay profile, which is about 10.5m thickness and varies substantially along the longitudinal section of the motorway. The laboratory value of the coefficient of volume decrease ranged from 1 to 5 MN/m2 and the laboratory values of the coefficient of consolidation are in the range of 0.25 to 0.5 m2/year. The Port of Brisbane Motorway embankments are installed with vertical drains and consist of three sections. Embankment A had drains at 3 meter spacing, and Embankments B and C had drains at 1.5 meter spacings. The maximum settlement obtained after 226 days of monitoring is shown. It can be seen that vertical drain treatment significantly increased final settlement. This increase varied from 70 to 80%. It can be concluded from the settlement results that vertical drains would have increased rate of consolidation. A trial embankment was constructed along the Gold Coast Highway. This embankment was divided into three sections, one section contained no ground improvement, and the other two sections had stone columns at 2m spacing and 3m spacing. For embankment with 3m spacing, the maximum settlement was 490 mm. For embankment with 2m spacing, the maximum settlement was 386 mm. For embankment without stone column, the maximum settlement was 522 mm. Based on the laboratory tests, for cement treated samples with 5 percent to 15 percent cement content, the maximum unconfined compressive strength increased from 132 kPa to 370 kPa for 7 days curing period; these values for 28days curing increased from 170 kPa to 405 kPa. For lime treated samples with lime contents from 2 percent to 15 percent, the maximum unconfined compressive strength increases from 47 kPa to 199 kPa (for 7 days curing period). Results indicated that, 2 percent lime has little effect on peak unconfined compressive strength. This thesis summarises some ground improvement techniques used in Southeast Queensland, and demonstrated the applicable of chemical stabilisation. Overall it was concluded that the addition of cement and lime has favourable effects on the strength characteristics of Southeast Queensland soft clays.

Soft clays in coastal areas have low shear strength and high compressibility. Thus construction activities for infrastructure developments in these deposits often pose geotechnical problems due to large time dependent settlements and lateral movements. Ground improvement techniques are adopted to reduce the water content of the soft clays by preloading techniques with vertical drains. Depending on the magnitude of the surcharge used substantial immediate settlement with lateral movements can takes place during preloading. This in turn causes stability problems in the loaded areas. The use of vacuum assisted preloading has now become a popular method in Australia where substantial loads need to be carried out to meet a desired rate of settlement and mitigate undrained failure. To assist the vacuum propagation to significant depths, vertical drains are used in conjunction. At the Port of Brisbane, Australia, vacuum assisted surcharge preloading and conventional surcharge preloading schemes were used to reduce the time required for consolidation and long term settlement in soft Holocene clays. The design of the combined vacuum and surcharge fill system and construction of the embankment are described in this paper. A comparison made on the performance of a combined vacuum surcharge loading system with a standard surcharge fill highlights the clear benefits of vacuum consolidation. Field monitoring data on surface and sub-surface settlements, pore pressures and lateral movements on test embankments performed during construction are presented. An analytical solution for radial consolidation that considers both time-dependent surcharge loading and vacuum pressure to predict the settlement and associated excess pore pressures in soft clay deposits is also proposed.

Soft clays are problematic soils as they present high compressibility and low shear strength. There are several methods for improving in situ conditions of soft clays. Based on the geotechnical problem's geometry and characteristics, the in situ conditions may require reinforcement to restrain instability and construction settlements. Granular columns reinforced by geosynthetic material are widely used to reduce settlements of embankments on soft clays. They also accelerate the consolidation rate by reducing the drainage path's length and increasing the foundation soil's bearing capacity. In this study, the performance of encased and layered granular columns in soft clay is investigated and discussed. The numerical results show the significance of geosynthetic stiffness and the column length on the embankment settlements. Furthermore, the results show that granular columns may play an important role in dissipating the excess pore water pressures and accelerating the consolidation settlements of embankments on soft clays.

This paper describes a bench-scale study dealing with the removal of heavy metals by electrokinetic (EK) remediation from sediment of the Great Backa Canal (Vojvodina, Republic of Serbia), with an emphasis on the dependence of removal efficacies on the physicochemical states of the heavy metals and sediment chemistry. Sediment samples were spiked with the following heavy metals (mg kg− 1): Zn 4400, Ni 900, Cu 1140 and Cd 57. In addition to determining the pseudo-total metal content in the contaminated sediment before and after EK treatment, BCR sequential extraction was also performed to examine the distribution of the contaminants in the sediment. Conventional EK remediation (EXP I) was ineffective in removing the heavy metals investigated, so two enhanced processes were developed. In both these processes, the mass of treated sediment was reduced to avoid the presence of inactive electric field areas in the sediment and increase current density. The first enhanced experiment (EXP II) used acetic acid (HAc) solution (pH 2.9) as an anolyte. Combined with the smaller sediment mass, this resulted in an increase in overall removal efficacies (9% for Zn, 15% for Ni, 10% for Cu and 15% for Cd). The second enhanced experiment (EXP III), as well as using HAc solution as an anolyte, made use of a cation exchange membrane in the cathodic chamber to minimize pH changes in the region adjacent to the cathode, which negatively influenced the removal of some heavy metals. However, no improvement in removal efficacy was achieved in EXP III. Since the redox potential of the sediment drops during the EK process, metals removal is limited by the formation of their sulfides. In conclusion, the removal of heavy metals by EK remediation is governed by a complex interplay of the complexation, precipitation and reduction processes, and the difficulties encountered in their optimization can explain the unsatisfactory effectiveness achieved by the described remediation procedure. Improved understanding of the behavior of metal ions during EK treatment can be useful in predicting and enhancing the efficacy of the process.