Abstract Controlled low-strength material (CLSM) is widely applied in many backfill engineering applications because of its excellent workability and sustainability. However, for CLSM using native soil as fine aggregate and industrial by-products as a binder, the flow-ability and self-leveling performance will deteriorate, and the setting time will be slower, which is unsuitable for construction applications. The addition of additives is regarded as an effective technique for improving the performance of CLSM. Although the effects of nano-SiO2 on the performance of concrete have been the subject of many studies, little research has been done on its effects on CLSM. The present work investigated the effects of adding nano-SiO2 as an additive. The flow-ability, hydraulic penetrometer, and unconfined compressive strength (UCS) tests are performed with the increasing nano-SiO2 content. Lastly, the mercury intrusion porosimetry (MIP) test is carried out to identify the effects of nano-SiO2 on the morphology of pores. For controlled low-strength materials using native silt soil, significant improvements in setting time and strength are observed with the increase of nano-SiO2. A decrease in the average pore diameter and an increase in the percentage of pore diameter below 20 nm are also observed in the specimen with nano-SiO2.

The evaluation of the trench stability under the action of ocean waves is an important issue in the construction of an immersed tunnel. In this study, a two-dimensional coupling model of a wave-seabed-immersed tunnel is proposed for the dynamic responses of a trench under wave action in the immersing process of tunnel elements. The porous seabed is characterized by Biot consolidation equations. The k − ε model and RANS equation are adopted to achieve the flow field simulation, and the level set method (LSM) is used to capture the free surface between the water and air. The proposed numerical model is verified using the experimental data and analytical results. Then, the transient liquefaction and shear failure in the vicinity of the trench are discussed at two different conditions, namely, after the foundation groove is excavated and after the tunnel element is placed. The pore pressure amplitude on the weather side slope is demonstrated to be significantly smaller than that on the lee side slope. Also, the distribution of the surrounding flow field and pressure field change dramatically after the tunnel element is settled, leading to the significant changes of seabed stability.

A theory of pseudo-Scholte wave propagating in a saturated porous medium loaded on its interface by a viscous compressible liquid is described. The porous medium is simulated by the Biot theory with high-frequency correction, and the overlying liquid is simulated by the linearized Navier-Stokes equation. An analytical expression for the complex dispersion equation of pseudo-Scholte wave through boundary conditions is established. Then the Riemann sheets related to body waves are discussed and the real and imaginary parts of the complex dispersion equation are separated and solved numerically. The resulting phase velocity, attenuation, as well as displacement and pressure fields are analyzed and comparisons are drawn with the non-viscous model. Finally, a set of parametric analyses is carried out to describe the effects of the phase velocity ratios of the S-wave in the porous medium to Ls-mode in overlying liquid on phase velocity and attenuation of the pseudo-Scholte waves.

Summary The paper focuses on the propagation of low-frequency pseudo-Rayleigh and pseudo-Scholte waves at the liquid/soft porous sediment interface with an underlying hard porous sediment half-space. The overlying liquid is assumed to be ideal compressible medium and the porous sediments are modelled by Biot theory. Based on the boundary conditions, the closed-form dispersion equations of far-field interface waves are deduced using 2-D Helmholtz decomposition theorem and Fourier transform. The velocity and attenuation of pseudo-Rayleigh and pseudo-Scholte waves are determined by Newton iteration in a reasonable rooting interval. The analytical expressions of the displacement field and liquid pressure distribution caused by interface waves are also derived. Then, the dispersion equations for four degenerate systems are derived as special cases by assuming the thickness of the liquid layer or the sandwiched porous soft sediment layer to be zero or infinite. Lastly, numerical examples are used to verify the degeneracy of the system and to analyse the propagation characteristics of pseudo-Rayleigh and pseudo-Scholte waves. They show the dependences of the velocity and displacement field on dimensionless modulus and dimensionless wavelength. When the dimensionless wavelength is small or very large, the phase velocity and displacement field calculated by the present system is the same as the special cases, thus proving the validating of the new system.

The content and distribution pattern of the gas hydrate are the key factors affecting the free surface reflection of the gas hydrate-bearing sediment. In this study, a theoretical model of wave reflection in gas hydrate-bearing sediment is discussed for the incidence of P1-wave and S1-wave to analyze the influences of the gas hydrate. The gas hydrate-bearing sediment is modeled by a Biot-type three-phase theory and assumed to be composed of sediment grains with connected pore occupied by the mixture of fluid and hydrate. The incoming wave is split into three reflected compressional waves and two reflected shear waves at the free surface of a gas hydrate-bearing sediment half-space. The analytical solutions for various reflected waves at open-pore and sealed-pore boundaries are obtained in terms of displacement potentials. Then, two-phase type models based on Biot theory are proposed by assuming the gas hydrate and sediment frame are weakly coupled or essentially consolidated to verify the rationality of the theoretical model. A third two-phase type model is given by assuming there is no gas hydrate in the pore. At last, the influences of the content and distribution pattern of gas hydrate, the frequency of the incident wave, and the permeability condition on the horizontal displacements and displacement ratios are analyzed respectively through numerical calculation. It is revealed that the solutions based on the Biot-type three-phase theory are very consistent with the solutions based on Biot theory if the gas hydrate and sediment frame are weakly coupled or essentially consolidated. Furthermore, the model proposed in this study can be used to analyze the influence of the distribution pattern of the hydrate.

The propagation of interface waves at the interface between a fluid-saturated porous medium and a fluid has been extensively investigated in the last three decades due to its various and wide applications in several fields including earthquake engineering and materials testing. Although the sea floor is usually covered with porous marine sediment, the previous interface wave theories are rarely used for submarine acoustic problems for the following reasons. 1) In addition to hard porous media, unconsolidated soft porous media exist widely in the seabed, which are seldom considered in previous studies. 2) The depth of seawater is limited, and in many cases it cannot be regarded as a half-space. 3) The fluid-saturated porous medium model cannot describe the effect of a small number of bubbles caused by decomposition of organic matter in the sediment. Hence, the present paper focuses on the low-frequency pseudo-Scholte waves at the interface between an overlying fluid layer of finite thickness and a quasi-saturated porous half-space. The overlying fluid is assumed to be ideal compressible water and the quasi-saturated porous media are assumed to be sandstone and unconsolidated sediment and modeled by Biot theory. A fluid equivalent model is used to analyze the effects of the bubbles in the pores. Based on the boundary conditions, the closed-form dispersion equations of far-field interface waves are derived by using classical potential function method. The velocity and attenuation of pseudo-Scholte wave are determined by Newton iteration in a reasonable rooting interval. The analytical expressions of the displacement field and fluid pressure distribution caused by pseudo-Scholte waves are also derived. Then, based on the derived theoretical formulation, the numerical examples of calculations are presented. Our calculation results show that the stiffness of porous medium significantly affects the mode, phase velocity, displacement and fluid pressure distribution of interface waves; the phase velocity of the pseudo-Scholte wave in the finite-thickness fluid/fluid-saturated porous half-space is related to the ratio of the wavelength to the thickness of the fluid layer; the phase velocity of the shear wave is insensitive to a small number of bubbles dissolved in the pores, but the existence of bubbles has a significant influence on the phase velocity of the compressional wave and the pseudo-Scholte wave. Furthermore, the existence of bubbles can significantly affect the distribution of the pore pressure.

To study the effect of particle shape on shear modulus of sand, four sand samples with different particle shapes and grain sizes are prepared, including Fujian standard sand and artificial quartz sand. Image Pro Plus and Matlab program can extract the geometric parameters of particles from images of four samples obtained by scanning electron microscope. Two geometric parameters, radius angularity and form index, are improved and studied. Through statistical data of sand particle, the results show that radius angularity and form index describe different aspects of particle shape. As such, shape factor of sand is proposed based on radius angularity and form index to generalize particle shape. Then, a series of model tests in Ko condition is conducted to obtain the shear modulus of samples under different vertical stresses by bender element deployed in the sand. The tests show that shape factor is closely related with shear modulus of sand. Finally, based on Hertz-Mindlin contact model, shape factor is introduced to derive the formula of sand's shear modulus, which agrees well with the measured one.

Based on Leclaire's extension of the Biot model for saturated porous elastic medium, the propagation characteristics of the body waves in a three-phase porous elastic medium with two types of solid phase components are studied. Firstly, based on the Helmholtz decomposition of the displacement vector, the characteristic equations of the body waves in saturated frozen soil are established. Then the effects of the volume fraction of each phase, the shape of the particles and the contact between the particles on the inertial parameters, viscous parameters and elastic parameters are studied. The model of saturated frozen soil is degraded. The characteristic equations and propagation characteristics of body waves are analyzed assuming only liquid water or ice in pores. Finally, a numerical calculation is carried out to explore the relationship among phase velocity, attenuation coefficient of waves and parameters of soil consolidation, saturation, porosity, particle contact, wave frequency etc. The calculation results show that, different from saturated soil, there are 5 types of body waves in saturated frozen soil, i.e. 3 compressional waves and 2 shear waves. All body waves present dispersion and attenuation. The dispersion and attenuation of P1 wave and S1 wave are much smaller than those of P2, P3 and S2 waves. Body wave propagations are significantly affected by cementation parameters, saturation and porosity, but are slightly affected by contact parameters.

The propagation of Raleigh waves at free boundaries of solid-quasi-saturated porous media and gas-quasi-saturated porous media is compared in this paper. Based on the equivalence theory, a small amount of solid particles embedded in pore-water of solid-quasi-saturated porous media, or small amount of gas embedded in pore-water of gas-quasi-saturated porous media, together with the pore-water, are equivalent to a new pore-fluid. The dispersion relation modeled by Biot theory is derived, to study the influence of the pore-filler and saturation on the phase velocity of body and Rayleigh waves. Numerical analysis is carried out on pore-space gas hydrate-bearing sediments and slight gas-bearing sediments. It is found that the types of pore-filler have great influence on the phase velocity. The presence of gas hydrate in the pore slightly increases the velocity of P1-wave and S-wave, but has little effect on the Rayleigh wave. But even a small amount of gas in the pore can significantly reduce the velocity of P1-wave. The velocity of S-wave is almost independent of saturation in gas-quasi-saturated porous media, and under the influence of P1-wave, the velocity of Rayleigh wave decreases rapidly and then tends to stabilize with the decreases of saturation. The effect of the type and saturation of the filler is influenced by the dry-frame stiffness.

Since the shear waves involved in in-situ and laboratory measurement methods vary significantly in terms of the frequency range, it is necessary to consider the effects of frequency on the shear wave velocity. In this study, sand particles are assumed to be spherical solid particles with an equal radius and identical material properties, and sand skeletons are regarded as granular aggregations generated through the random packing of sand particles. It is also assumed that the sand particles only undergo elastic deformation during shear wave propagation. Based on a spherical particle model, a formula is obtained for calculating the shear wave velocity in sand, with the shear wave frequency as an extra influencing parameter. The quantitative calculations demonstrate that the shear wave velocity decreases with an increase of sand porosity, and accelerates with increases of vertical effective stress and elastic modulus of the sand particles. It is also indicated that both the particle density and Poisson’s ratio of the sand particles have negligible effects on the shear wave propagation. The frequency dispersion characteristics of shear wave propagating in sand are also discussed. Moreover, the critical frequency is defined and its analytical expression is derived. The calculation results obtained using the proposed equations agree well with the in-situ measurement results and bender element test data.

Excavation beneath the original basement can effectively utilize the underground space, which is one of the popular ways to improve the parking problem in the dense urban areas. Excavation beneath the original basement will change the behavior of original pile. If the excavation depth exceeds a certain value, it may lead to buckling instability of the original piles due to the reduction of ground confining pressure around piles. In this paper, a theoretical analysis is performed to estimate the bucking critical load and the effective length of a single pile. At first, the scheme of floor-addition of basement is briefly introduced with the case of Zhejiang Hotel Extension Project. Secondly, the total potential energy of the pile-soil system under the condition of excavation beneath the original basement is set up based on the Winkler elastic beam theory. Finally, the expressions of critical load and effective length of single pile are deduced by using the minimum potential energy principle. Based on the proposed theory, the influence factors of critical load, including half-wave number n and excavation depth, are analyzed. It is shown that the buckling critical load of pile shaft converges with an increase in half-wave number n; by increasing the excavation depth, the buckling critical load decreases rapidly. The proposed theory may provide guideline to estimate the supported pile behavior of excavation beneath original basement under existing buildings.