Soft Marine Clay Research Articles

Use of recycled gypsum in the cement-based stabilization of very soft clays and its micro-mechanism

This paper presents an experimental study and micro-mechanism discussion on gypsum role in the mechanical improvements of cement-based stabilized clay (CBSC). A soft marine clay at two initial water contents (i.e. 50% and 70%) was treated by reconstituted cementitious binders with varying gypsum to clinker (G/C) ratios and added metakaolin to facilitate the formation of ettringite, followed by the measurements of final water contents, dry densities and strengths in accordance with ASTM standards as well as microstructure by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Results reveal that the gypsum fraction has a significant influence on the index and mechanical properties of the CBSC, and there exists a threshold of the G/C ratio, which is 10% and 15% for clays with 50% and 70% initial water contents, respectively. Beyond which adding excessive gypsum cannot improve the strength further, eliminating the beneficial role. At these thresholds of the G/C ratio, the unconfined compressive strength (UCS) values for clays with 50% and 70% initial water contents are 1.74 MPa and 1.53 MPa at 60 d of curing, respectively. Microstructure characterization shows that, besides the common cementation-induced strengthening, newly formed ettringite also acts as significant pore infills, and the associated remarkable volumetric expansion is responsible, and may be the primary factor, for the beneficial strength gain due to the added gypsum. Moreover, pore-filling ettringite also leads to the conversion of relatively large inter-aggregate to smaller intra-aggregate pores, thereby causing a more homogeneous matrix or solid skeleton with higher strength. Overall, added gypsum plays a vital beneficial role in the strength development of the CBSC, especially for very soft clays. • Gypsum fraction has a significant influence on the index and mechanical properties of the CBSC. • Optimal G/C ratio determined in CBSC was obviously greater than that of mortar or concrete. • Ettringite-induced filling and cementation are responsible for the improved mechanical performance of the CBSC. • The abnormal beneficial role of gypsum is closely associated with the increase in the density. • Design guideline for cementitious binders that are only applicable to soft clays was re-established.

Anisotropic and Noncoaxial Behavior of Soft Marine Clay under Stress Path Considering the Variation of Principal Stress Direction

The influence of the variation in the principal stress direction on the anisotropic and noncoaxial behavior was investigated through several hollow cylinder apparatus (HCA) tests that were performed on reconstituted soft clay specimens under a stress path with a fixed or continuous rotation of the major principal stress axis. In particular, the varying amplitude of the deviator stress (q) and intermediate principal stress coefficients (b) were focused upon. The results indicated that the shear stiffness and strength were strongly dependent on the fixed major principal stress angle. Furthermore, apparent noncoaxiality was observed between the orientations of the major principal strain increment and the major principal stress. Consequently, the noncoaxial behavior of the reconstituted soft clay was sufficiently and effectively described using rose and polar diagrams. Furthermore, the data fluctuation indicated that the noncoaxiality of soft clay was strongly dependent on both q and b during the pure principal stress rotation (PSR). The results obtained for undisturbed clay and sand are not expected to be applicable to reconstituted clay, because its behavior is in contrast to the former two. Regarding reconstituted soft clay, the degree of noncoaxiality was observed to increase during the second rotation (N = 2) compared with that during the first rotation (N = 1). However, this trend gradually weakened with an increase in q or b values. Moreover, regardless of whether N = 1 or 2, the degrees of noncoaxiality decreased with the increase in q for b = 0.5 whereas it increased with the increase in b values for N = 1. However, those for b = 1 were the smallest for N = 2. The results are expected to provide an experimental basis for the constitutive modeling of soft clay with fixed and continuous rotations of the major principal stress axes in ocean engineering.

Amending excavated soft marine clay with fine incineration bottom ash as a fill material for construction of transportation infrastructure

The fast development of transportation infrastructure has increased the consumption of construction materials, resulting in overexploitation and depletion of natural aggregates. Meanwhile, the increasing amount of solid wastes is another rising problem due to the growing economy and rapid urbanization. Instead of using natural aggregates as fill materials for transportation infrastructure, the solid wastes can be treated and reused as alternatives, which can reduce the reliance on natural resources and landfill. Therefore, in this study, the mixture of two solid wastes, i.e., excavated soft marine clay (MC) and fine incineration bottom ash (IBA) of municipal solid waste, was studied as a fill material for construction of transportation infrastructure. Mixtures of IBA and MC (IBA-MC) with different IBA/MC mass ratios (0/10, 1/9, 2/8, 3/7, 4/6, 5/5, 6/4, and 10/0) were prepared. The Atterberg limits, compactability, unconfined compressive strength (UCS), leaching of heavy metals, mineralogy, and microstructure of IBA-MC were studied. The results showed that, as the content of IBA/MC ratio increased, the water content, liquid limit, and plastic limit of IBA-MC decreased, changing it from plastic paste to semi-solid/solid granular material. The compactability and UCS of IBA-MC were improved when the IBA/MC ratio increased from 1/9 to 5/5. For the IBA-MC with IBA/MC ratio of 1/9 to 5/5, the leaching of heavy metals satisfied the criteria for construction material. In summary, IBA improved the compactability and strength of MC, while MC reduced the leaching of heavy metals from IBA, and IBA-MC with proper ratio could be compacted as a fill material for construction of transportation infrastructure.

Engineering properties of marine soft clay stabilized by alkali residue and steel slag: an experimental study and ANN model

Engineering properties of marine soft clay stabilized by alkali residue and steel slag: an experimental study and ANN model

Effects of frequencies on the cyclic behavior of marine soft clay under different consolidated stress paths

To experimentally investigate the effect of frequency on the cyclic behavior of marine soft clay under different consolidated stress paths, a series of cyclic tests on remolded Wenzhou marine clay were conducted under undrained conditions covering various frequencies and consolidated stress paths using a triaxial apparatus. The development of pore water pressure, stress–strain relationship, and resilient modulus was analyzed. Several useful conclusions were reached from this study. First, the trends of pore water pressure development can be divided into three stages: linearly increasing, slowly increasing, and stable. Second, the frequency has a more pronounced influence on the number of cycles of the steady value of pore water pressure than the counterparts of values of the ratio of the radial to vertical consolidation stresses, K. Then, a unified linear equation was established based on the consideration of different K values on a logarithmic scale. Finally, the shapes of the stress–resilient strain hysteretic loops and the range encompassed by the stress–resilient strain hysteretic loops are slight as the value of K decreases, which indicates that the K value has a slight effect on the shift in the type of elastic behavior of clay.

Numerical study on the suction force of jack-up mat foundation on marine clay seabed

A deep layer of soft clay is widely present on the seabed in the offshore area of China. The traditional jack-up spudcan faces the problem of difficult insertion and extraction in clay seabed. Jack-up mat foundation is more suitable in soft clay than spudcan for its lower requirements on bearing capacity. Due to the poor permeability of clay, the uplifting process of the mat foundation needs to overcome a large soil suction force. In this study, three-dimensional numerical model is established to investigate the suction force of jack-up mat foundation on marine clay seabed. The elastoplastic constitutive model, which can describe the structural and over-consolidation of soft marine clay seabed, was embedded in commercial software ABAQUS through UMAT subroutine. The cohesive contact model was adopted to stimulate the interaction between mat foundation and the seabed soil. Field test was conducted to verify the accuracy of the numerical model. Then, the effects of degree of over-consolidation, degree of structural, depth of clay, interface stiffness and eccentric uplift loading on suction force are studied in detail. Numerical results show that the effects of degree of over-consolidation and structural of soil have significant influence on the suction force. The suction force first increases rapidly and then decrease gradually as the thickness of the upper dimensionless clay layer increases. With the increase of the uplift eccentricity, the suction force reduces continuously.

Liquid limit of marine soft clay encountering desalination percolation process

The diffuse double layer (DDL) theory indicated that the liquid limit (LL) of clays decreases based on pore-water salinity during salinization tests conducted in a laboratory. However, LL also decreased based on pore-water salinity when authors conducted geological investigations in the regression depositional zone in Lianyungang city, contradicting the DDL theory. This contradictory finding is inferred due to the coupled effect of salinity leaching and erosion of the colloid during a desalination percolation process. To confirm this hypothesis, 3 model tests of distilled water percolation and seven accelerated percolation tests were designed in this study, and the soil/water characteristics after desalination percolation were determined. The results reveal that the pore-water salinity leaches are combined with the colloidal erosion during the desalination percolation process, leading to the gradual decrease in LL based on the reduction of salinity and suggesting that the dominant effect is the colloidal erosion. At the initial stage of the desalination process, the colloids easily erode from the clay matrix due to the weakened salinity inhibition on the colloid release, leading to the seepage channel expansion, and hence the increase in colloid dimension and concentration in leachates throughout percolation. Based on the further decrease in pore-water salinity, the colloids in the clay matrix continuously release and inversely clog the inner channels of the soils, decreasing the colloid dimension and concentration in leachates. When the percolation process is sufficiently long, all salinity would be leached out from the clay matrix, and only the erosion of the colloid would be identified.

Damping of a soft marine clay subjected to the combination of monotonic and cyclic shear stresses

In relation to the dynamic behavior of soils, the damping (D) and the dynamic shear modulus (G) has always acquired special relevance, since its evaluation has allowed the application of simplified models of dynamic soil behavior. They depend on many factors, being necessary to include the combination of monotonic (τo) and cyclical shear stress (τc) that are inevitably present under any structure. As part of an extensive testing programme, the objective of the present paper, besides the identification and classification tests, was to perform 139 simple cyclic shear tests with different levels of static stresses (τo) before the cyclic phase. This paper refers to the influence that the combination of monotonic (τo) and cyclic shear (τc) stresses has on the damping of a soft cohesive soil susceptible to sudden failure. Laboratory tests were carried out on undisturbed samples from the Port of Barcelona, in Spain. The samples were obtained at depths between 29 m and 52 m, that correspond to an effective vertical stress of (σ′ov) between 277 kPa and 413 kPa, respectively. In general, the results indicate that: a) the combination of τo and τc controls the damping ratio (D), b) in the combined monotonic and cyclic case, it is possible to establish empirical functions of the type D = D(N, γc), D = D(N, G/σ′ov) and D = D(N, u/σ′ov), for different (τo/σ′ov) ratios and c) it is possible propose a chart to evaluate the damping ratio as a function of N, G/σ′ov and u/σ′ov, for different (τo/σ′ov) ratios.

Undrained Deformation Characteristics of Soft Marine Clay Subjected to One-Way Cyclic Loading with Variable Confining Pressure

To gain a better understanding of the undrained deformation characteristic of saturated marine clay soil subjected to vehicle cyclic traffic load, a sophisticated dynamic triaxial was used to conduct a variety of undrained one-way compression cyclic experiments with variable confining pressure (VCP) as well as constant confining pressure (CCP). The results indicate that, compared to CCP test results, VCP is helpful to raise the axial resilient modulus (Mr) and restrain the permanent plastic strain ( ε a p ) development of the specimens. By normalization analysis of the measured data of Mr and ε a p , the virtually unique correlation between normalized average resilient modulus, normalized permanent axial strain after 1,000 loading cycles, and normalized mean normal stress is established, respectively, regardless of the values of CSR. Additionally, the VCP influence on ε a p is quantified and fitted by a power law function, which can be used for subsoil deformation prediction and provides new insights into the mechanics of strain accumulation under undrained cyclic loading conditions.

A study on soil interface interaction

Interfaces between soil-concrete, soil-steel, soil-root, soil-wood, soil-polymer frequently stumble upon various geotechnical engineering applications. Several studies agree that the material properties interfacing with the soil, the properties and the performance of the soil at the interface makes it even more multifaceted and interesting. Highly plastic, over consolidated clays, crushable calcareous sands, soft marine clays, clays with organic content etc., show unusual complications related to cast in situ and driven pile shaft capacity calculations, functioning of various infrastructural projects that comes in contact with soil. Several testing methodologies have been developed to examine the interface behaviour under different conditions, it substantiates determination of the interface shear strength parameters for design. Such studies involve real time understanding of the frictional anisotropy which is possible through large scale direct shear tests, tilting frames, simple shear tests and radial shear experiments for finding the interface between soil and material. In addition, microstructural investigations are also to be piloted to realize the shearing mechanism of the soil material interfaces. Yet standard testing conditions for evaluating the interface angle simulating the real time environment is not yet available. A systematic review of the study on appropriate mechanisms available for investigating the Soil Material Interface is done, which will aim to suggest improvements by critically identifying their shortcomings.

Undrained shear and stiffness degradation of intact marine clay under monotonic and cyclic loading

Optimal whole-life design for marine structures requires an accurate assessment of how their underlying geo-materials respond to various static and cyclic loads. This study experimentally investigates the undrained shear behavior and stiffness characteristics of intact, soft marine clay that outcrops an offshore wind farm in southeast China. A series of monotonic and cyclic triaxial tests were conducted on the highly compressible marine clay, and the test results were interpreted within the critical state framework. The elastic shear modulus measured using bender elements in triaxial tests was compared with the secant modulus that gradually degraded with the strain during monotonic and cyclic loading. An interesting finding was that the elastic stiffness measured at various loading stages increased exponentially with the mean effective stress; a power-law increasing trend was also observed in the secant modulus normalized by the void ratio at various strain levels. It was also demonstrated that the intact sample under cyclic loading is stiffer than that under monotonic loading at the same strain level. Furthermore, a close correlation between the nonlinear undrained shear stiffness and the elastic stiffness measured during monotonic or cyclic loading was observed, which is of vital importance when evaluating the stiffness degradation behavior of natural soils over strain ranges relevant to major engineering works.

Undrained anisotropic behaviour of K 0-consolidated marine clay under cyclic stresses

To investigate the effect of major principal stress direction angles on the cyclic response of soft marine clays, a series of directional shear tests with different major principal stress direction angles on undisturbed Wenzhou marine clay were conducted in the undrained condition by using a hollow cylinder testing apparatus. The dynamic responses of the marine clays were analysed, along with the shear stress-strain relationship, octahedral strain and excess pore water pressure. Results indicate that the specimens exhibit remarkable anisotropic deformation under various major principal stress direction angles, and a larger angle causes a smaller octahedral strain of specimen under the same cyclic stress level. With the increase in the major principal stress direction angle, the excess pore water pressure decreases. At β > 45°, the increment shear stress starts dominating, which causes the sign of generated pore water pressure to change from negative to positive.

Preloading system with partially penetrating vertical drains in marine soft clays in Egypt: 2D and 3D comparative FE study

A large-scale trial embankment provided with partially penetrating prefabricated vertical drains (PVD) was installed in consolidating marine clay deposits at East-Port said industrial zone project in Egypt. The trial embankment was constructed with a bottom area of 150x150m and a height of 5.5m to evaluate the efficiency of the improvement system and verify the design parameters. An intensive instrumentation system was built-up including shallow settlement indicators, vibrating wire piezometers, extensometer, and inclinometers. Two and three-dimensional finite element analyses (FE) were performed to study the effect of the preloading system provided with partially PVD in consolidating marine clay deposits. FE models were validated and showed good agreement with the field observations in terms of vertical displacement of embankment center, and lateral soil deformation beneath the embankment toe.

Freeze-thaw behavior of calcium carbide residue-plant ash stabilized marine soft clay

The freeze-thaw behavior of solidified soil has been studied extensively in previous researches. However, few studies have been carried out on freeze-thaw performance of calcium carbide residue-plant ash stabilized marine soft clay. This research is conducted to evaluate the influence of calcium carbide residue and plant ash on mechanical properties and freeze-thaw performance of solidified marine soft clay. Unconfined compressive strength and scanning electron microscope tests are both carried out in this study. The experimental result reveals that the compressive strength of solidified soil has an improvement with the increase in calcium carbide residue and plant ash contents. However, the freeze-thaw cycles have a negative influence on the strength of solidified soil and the compressive strength gradually decreases with the increasing numbers of freeze-thaw cycles. To better understand the influence, the durability index of solidified soil (DI) is defined and it indicates that the values of DI also gradually decreases with the increase in the numbers of freeze-thaw cycles. The freeze-thaw strength (qu(F-T)) of sample has an improvement with increasing unfrozen strength (qu(unfrozen)) and the final expression equation between the generalized strength (qu(F-T)/qu(unfrozen)) and the numbers of freeze-thaw cycles (n) under different CCR and plant ash contents is established. The stress-strain behaviors of solidified soil present the brittleness and strain softening performance. The failure strains are mostly 1.0–3.5% for different samples. To reveal the microstructure of solidified soil, scanning electron microscope test is conducted and it reveals that the gelling chemical products and dense structure are mainly responsible for the improvement of compressive strength of solidified soil with calcium carbide residue and plant ash. Finally, the stabilization mechanism of solidified soil with plant ash and CCR and influence mechanism of freeze-thaw cycles are analyzed and discussed.

Study on Engineering Properties of Deep Marine Soft Clay in Guangdong-Hong Kong-Macao Greater Bay Area

A large number of Marine soft clay are distributed in the Pearl River Delta region of China. The soft clay features mainly on high water content, high sensitivity, high compressibility, low density, low strength and low perviousness, which dramatically restricts local project construction. These features often cause engineering problems such as subgrade settlement, pavement cracking, house cracking, tilting and insufficient bearing capacity of the foundation. Therefore, analyzing the soft clay engineering properties in this area is of great significance for guiding engineering design and construction. Based on the statistic analysis of a large amount of in-situ exploration and experimental data in the western central urban area of Zhuhai, this paper focuses on the distribution and engineering properties of the deep Marine soft clay. The results suggests that the thickness of soft clay is at a range of 10m~ 55.1m. The average water content of soft clay is as high as 68.05%, the plasticity index is 28.16, the fast shear cohesion is only 4.93 kPa, and the internal friction angle is 3.26°. Statistical analysis of layered soft clay shows that the parameters of soft clay vary greatly in depth. The preconsolidation pressure and unconfined compressive strength increases with depth. The water content, liquid limit, plastic limit and plasticity index increase first and then decrease with depth, and the average values reach the maximum values at the depth of 10m ~ 25m. However, the direct shear strength index is on the contrary, and shows a track of decreasing first and then increasing. The minimum values of cohesion and internal friction angle come out at the depth of 10m-25m, with the values of 4.83kPa and 3.10° respectively. The results of in-situ test can better reflect the true mechanical properties of soft clay.

Improved Vacuum Preloading Method Combined with Sand Sandwich Structure for Consolidation of Dredged Clay-Slurry Fill and Original Marine Soft Clay

An improved vacuum preloading method combined with a sand sandwich structure (SSS) was developed to facilitate the consolidation of dredged clay-slurry fill and original soft marine clay for land reclamation. In this method, a novel sandwich structure is formed as a sand layer introduced on to original soft marine clay prior to the placement of reclaimed, dredged clay-slurry. This assists in transmitting the vacuum pressure to both the reclaimed, dredged clay-slurry and the original soft marine clay, through prefabricated vertical drains, with little vacuum loss, and the sand layer thereby improving the efficiency of the vacuum preloading. Field pilot tests were carried out on improved ground both with and without using a SSS in order to demonstrate the capacity of the improved vacuum preloading method combined with the SSS. Field measurements were performed to evaluate the ground settlement, degree of vacuum loss, effective consolidation stress, shear strength, and bearing capacity. The test data showed that better consolidation results were achieved using the improved vacuum preloading method combined with the SSS.

A fluid mechanics approach to evaluating marine soft clay strength by a ball full-flow penetrometer

Ball full-flow penetrometer is widely used for testing the ultimate bearing pressure q of marine soil to evaluate the undrained shear strength su by the proposed bearing capacity factor N, and thus N is one of the primary concerns of marine geotechnical engineers. However, plastic constitutive model solutions of N resolved by using classical rigid-plastic materials described by the Tresca yield criterion are not suitable for evaluating the low-strength marine soft clays with viscous fluid characteristics. In this paper, the validated computational fluid dynamics (CFD) approach is used to simulate the interaction between the ball with smooth and rough surfaces and marine soft clays described by non-Newtonian fluids with shear thinning characteristics and rate effect. Based on this, the effects of the rate effect parameter λ, density and strength of the marine soft clay on N are systematically analyzed. Furthermore, during the ball's deep penetration, the equations of N considering different λ at rough and smooth ball surface conditions are established. Finally, the stress state of the marine soft clays around the ball and the mechanism of the rate effect leading to the variation of N are elucidated by the pressure distribution contours on the ball surface, the shear rate contours of the marine soft clay on the ball surface, and the velocity field contours of the marine soft clay around the ball, which provides a basis for the undrained shear strength su evaluation of low-strength marine soft clays.

Model Test Study on Bearing Capacity and Deformation Characteristics of Symmetric Pile–Bucket Foundation Subjected to Cyclic Horizontal Load

The pile–bucket foundation that features a bucket slipped onto a monopile is a new type of symmetric offshore foundation supporting the wind turbine. Its load bearing and deformation resistance capacity are unclear, especially when subjected to cyclic horizontal loadings. In this paper, a model test has been designed and carried out for investigating the cyclic behavior of the pile–bucket foundation embedded in soft marine clay. Cyclic horizontal loads are applied in a displacement-controlled manner with different amplitudes and frequencies. The influences of cyclic loading parameters, including the amplitude, the frequency and the cycle number, have been studied from the perspectives of stiffness-degradation and damping effect that are evaluated from the recorded horizontal force–displacement relationships at the loading point. In addition, the influences of cyclic horizontal loading on the bending moment distribution and on the p–y curve have been presented and discussed. The results show that significant reductions in the foundation stiffness and in the soil resistance may be observed during the first few cycles when the loading displacement is relatively high.

Investigation of Penetration Process of Three Full-flow Penetrometers in Marine Soft Clay

This study presents an experimental investigation of the penetration process in marine soft clay in the Shanghai coastal area using three types (non standard Ball (N), standard Ball(S) and T bar) of full-flow penetrometers. Three main factors (the penetration resistance, strength degradation, and strength factor) are evaluated and discussed in the comparative analysis of the penetration process. The results reveal that there is a model to predict the soil strength degradation in the range of low soil sensitivities (ST < 8). In particular, for the Ball(N) penetrometer, which is a non-standard penetrometer, the degradation process is more readily affected by the soil properties. For the Ball(S) and T bar penetrometers, the test results indicate a certain relationship between the soil sensitivity (ST) and the strength factors of undisturbed and remoulded soils with low sensitivity (ST < 8). however, this relationship is not observed with the Ball(N) penetrometer. These tests can enrich the application of conventional full-flow penetrometers in marine soft soils while facilitating the adaptation of non-standard penetrometers for in-situ tests in various soft soils.

Improving the Stability of High Embankments Founded on Soft Marine Clay by Utilizing Prefabricated Vertical Drains and Controlling the Pace of Construction

Improving the Stability of High Embankments Founded on Soft Marine Clay by Utilizing Prefabricated Vertical Drains and Controlling the Pace of Construction