Sand Foundation Research Articles

Experimental simulation study on dynamic response of offshore wind power pile foundation under complex marine loadings

Offshore wind power structures are subjected to complex dynamic loads in marine environment during their service. At present, the real sea conditions can be simulated to the maximum extent by the mixed test technology of boundary layer wind tunnels, wave pools and shaking tables. However, due to the high cost of test and the lack of a complete and mature combined simulation laboratory which can be operated for wind, wave and seismic loads, it is difficult to carry out such joint tests. It is advantageous to use self-developed mechanical devices to simulate marine environmental loads within the controllable errors and which is in simple operation. A series of dynamic model tests were carried out on saturated sand foundation in order to explore the feasibility of studying dynamic response of offshore wind power pile foundations through simulating marine environmental loads by simplified methods. Based on Stokes second-order wave theory and harmonic superposition method, the time history of wind, wave and current load is derived and programmed, then the wind, wave and current load on offshore fan model structure is simulated by using self-developed complex dynamic loading system. Finally, the dynamic response of single pile foundation in saturated sand under complex load is analyzed according to the test results. The test results show that the load displacement curve of single pile foundation in saturated sand is of work hardening type. The maximum pile bending moment at the peak of dynamic load increases with the increase of loading times, but the cumulative effect of bending moment caused by the change of load amplitude should be considered. The dynamic load in marine environment will cause the increase of pore pressure around the pile, but the increase of pore pressure on loading side is not obvious in comparison to that on the opposite side. The displacement change of pile body is basically in accordance with the fluctuation trend of test load and increases with load. The cumulative rate of displacement of single pile decreases with the increase of number of cycles under the long-term cyclic wind wave load. In this study, the method combining theoretical calculation of wind, wave and current load with model test of complex loading system can consider the characteristics of dynamic loads of marine environment more accurately, compared with the empirically determined cyclic load waveforms in conventional tests, the obtained response law is more in accord with actual sea conditions.

Axial Bearing Mechanism of Post-Grouted Piles in Calcareous Sand

Post-grouted piles, as a foundation form for large-span and large-scale structures on calcareous sand, are expected to provide a high bearing capacity, but research on the response of post-grouted piles subjected to axial load in calcareous sand is still in the exploratory stage. In this paper, a model test is constructed for static pressure piles in calcareous sand under axial loading. The response of axial compressive piles, with and without post-grouting, in calcareous sand were investigated, and the test results were compared with those of axial compressive piles, with and without post-grouting, in siliceous sand. The influence of post-side-grouting on the response of a single pile subjected to axial compressive load in calcareous sand and its bearing mechanism were further analyzed. The results show that the change in shaft resistance, caused by the lateral extrusion of calcareous sand, is less than the negative effect caused by particle breakage during pile driving, so single piles without post-grouting in calcareous sand exhibit weaker axial bearing behavior than that in siliceous sand. A single pile with post-side-grouting in calcareous sand can provide a higher bearing capacity by increasing the shaft resistance and tip resistance compared with a single pile without post-side-grouting, and the increased ratio of the bearing capacity of piles, after grouting in calcareous sand, is better than that of piles in siliceous sand. Post-side-grouting can not only strengthen the surrounding soil by the solidification effect of injected cement grout, but it can also have a strengthening effect on the tip resistance. In addition, ideal-geometry grouting has more obvious advantages in improving the bearing behavior of pile foundations than annular point grouting, and higher stability in improving the bearing properties of pile foundations is evident for ideal-geometry grouting. Therefore, it is suggested that a directional grouting device should be adopted in actual projects in the future to form a more stable pile-soil interaction system and to expand the application prospect of pile foundations in calcareous sand.

Research on improvement of Terzaghi’s bearing capacity equation for coral sand foundation

The mechanical characteristics of coral reef sandy soil is significantly different from the sand and clay soils. Currently, Terzaghi's bearing capacity equation is still commonly applied to evaluate the bearing capacity of coral reef, while the calculation result is not consistent with the actual condition. Based on a port project in east Africa, plate loading test of the coral reef sandy soil has been conducted and the P-S curves are obtained, and the bearing capacity values obtained from the plate loading test results are compared with the calculated results based on the Terzaghi’s bearing capacity equation. According to multiple linear regression and the optimal regression equation selected by the significance test, the modified Terzaghi foundation equation of bearing capacity in coral reef sandy soil area is proposed, and the modified Terzaghi foundation equation has important reference value for the calculating of the bearing capacity of the coral reef sandy soil foundations.

Marmaray (Bosphorus Crossing)

It is possible to construct deeper and longer immersed tunnels when the connection to land, ventilation, emergency evacuation systems, connection details between elements and similar key features are assessed and designed appropriately. Being the deepest and one of the most challenging immersed tunnels constructed to date, the Bosphorus Crossing included various innovative techniques.The Bosphorus Crossing project is located in Istanbul, Turkey, connecting the European and Asian sides of the city. The works included a double-track railway immersed tunnel with a length of 1387 m at a maximum depth of approximately 60 m below sea level. The tunnel consists of a total of 11 tunnel elements 15.3 m wide and 8.6 m high. Elements were constructed as reinforced concrete (RC) with an external steel waterproof membrane and with a transition to sandwich section for the element joint areas. Element no.11 also had a full-sandwich section in the vicinity of the access shaft connection. Design life of the permanent structures is 100 years. Elements were prefabricated in two drydocks approximately 40 km from the immersion site. Due to the expected severe seismic shaking, a sand foundation was unsuitable. A gravel foundation was placed low and roughly levelled using a remotely operated blade. Under-base grouting was injected from inside the tunnel elements to fill all gaps between the gravel foundation and the bottom slab.The tunnel is in a high seismic zone of Magnitude 7.5, which was considered in the design. Towards the Asian end of the tunnel, ground improvement using compaction grouting was used to minimize the risk of soil liquefaction. A total of 1.2 million m3 of seabed was excavated, of which 118,000 m3 was contaminated soil that had to be dumped in a confined disposal facility created on land nearer the Black Sea. The adjacent bored tunnels were constructed using tunnel boring machines (TBMs) that were driven at depth through pre-mixed soil into receiving chambers at the two ends of the immersed tunnel.

Shake table testing of liquefaction mitigation efficiency on pile foundations in sand stabilised with colloidal silica

Shake table testing of liquefaction mitigation efficiency on pile foundations in sand stabilised with colloidal silica

Research on sinking and leveling control technology of tripod bucket foundation in sand

High capacity wind-turbines are increasingly installed offshore and further away from the coast, where strong foundations are required, and their installation is challenging. The so-called multi-bucket foundation is a promising candidate to be used for such scenarios. However, the sinking control technology, which is commonly used to ease the installation, is not very mature. In this paper, a series of theoretical studies and tests are carried out to investigate the sinking process of a wide-shallow tripod bucket foundation (WTBF). First, the mechanism of seepage-failure of a WTBF at different depths is investigated using both empirical formulas and experiments. In addition, the optimal method to calculate/predict seepage-failure of a WTBF in the sand and the value range of the kfc coefficient are determined. Then, an experiment to determine the maximum adjustable initial tilt-angle (based on seepage failure control) is conducted. This enables the improvement/determination of the control index and control methods for seepage failure as well as the tilt-angle during the sinking process. Finally, an automatic control system for the sinking and leveling of the multi-bucket foundation, which uses a dual-index control method (seepage failure, leveling angle) is developed. The feasibility of this approach is verified through experiments.

Seawater based MICP cements two/one-phase cemented sand blocks

The Sporosarcina pasteurii strains can grow and multiply in seawater medium and have certain urease activity. The two-phase and one-phase cementation methods can be used for the construction of ocean islands and reefs far away from the mainland in seawater seawater based MICP (Microbial-Induced Carbonate Precipitation). The compressive strength, carbonates content and porosity of sand blocks cemented by seawater based MICP with two-phase and one-phase methods are measured and compared. The results show that the strength and carbonates content of sand blocks with two-phase biocementation are greater than that of one-phase. The SEM images show that the principle of seawater based MICP cementing loose sand blocks is that carbonates particles grow between the sand particles, and them act as a bridge to bind the loose sand particles into a whole. The seawater based UPB of different pH can induce calcium carbonate deposition at different time. The seawater based UPB of different pH can be selected according to the actual project to reinforce the island reef sand foundation and prepare building materials under one-phase biocementation.

Performance evaluation of PM4Sand model for simulation of the liquefaction remedial measures for embankment

Non-linear seismic deformation analyses of an earth embankment founded on liquefiable soil were performed using the FLAC 2D and PM4Sand as the constitutive model to simulate the seismic behaviour of loose sand foundation. PM4Sand model was calibrated to the cyclic behaviour of Nevada 120 sand. The untreated embankment-foundation system was subjected to three sinusoidal shaking events. Though the computed deformations are in reasonable agreement with measured results, the excess pore pressures (EPP) in the central foundation region were found to be overpredicted. The temporal variation of sand permeability was considered for the simulation of untreated sand deposit supporting the embankment. The low effective stresses computed to be regained following the dissipation of EPP for the case with variable sand permeability implies that the numerical model is not capable of accounting the shear-induced dilation in the foundation soil underlying the embankment. So, the class-C1 simulations were performed to evaluate the numerical model by adjusting the dilation parameters in the model. Following this, the three different liquefaction mitigation measures such as dense sand columns within the foundation soil, gravel berms along the side slopes of the embankment, and vertical sheet piles below the embankment toes were simulated and comparisons with experimental data are reported.

Experimental Study on the Treatment Effect of Vibroflotation Gravel Piles for Saturated Sand Foundations in Coastal Areas

Experimental Study on the Treatment Effect of Vibroflotation Gravel Piles for Saturated Sand Foundations in Coastal Areas

Investigating the hydro-mechanical properties of calcareous sand foundations using distributed fiber optic sensing

In this study, a model test was carried out to investigate the hydro-mechanical properties of calcareous sand, and distributed fiber optic sensing technologies were utilized to monitor the entire process. The test procedure had three stages: one-way water supply, one-way drainage, and two-way drainage. The actively heated fiber optic (AHFO) method was used to monitor the soil moisture profiles, and soil deformation was captured using the high-accuracy optical frequency domain reflectometry technique. The Pearson correlation function was used to describe the relationship between soil compression and moisture content. The results indicated that the AHFO method allowed soil moisture measurement in saturated and unsaturated zones with a bias of 0.027 m3/m3. Water infiltration in calcareous sand can lead to uneven settlement, toppling, and horizontal deformation, with most settlements occurring during the watering stage. The ground settlement was mainly caused by particle sliding and rotation during the watering and drainage processes. The results show that settlement occurred earlier when the soil was closer to the water supply chamber. Moreover, tension cracks can easily appear between the calcareous sand and fine-grained soil interlayers. The Pearson correlation coefficients were larger than 0.8, indicating that the soil layer was compressed owing to particle sliding and rotation, and the moisture contents had a linear correlation with compressive strains.

Frictional behavior of granular materials exposed to dynamic normal load

Understanding the frictional characteristics of granular materials (e.g., fault gouge and sand foundations) exposed to dynamic normal load are important for investigations of engineering geology disasters and safety assessment. Direct shear tests on granular materials under dynamic normal load were performed in the laboratory using a dynamic shear box device. Experimental results showed that the shear strength and normal displacement varied with variations of normal load, identified by a phase shift and normal stress loading peak. The peak shear stress decreased with increasing normal load frequency and shear velocity, in particular, the peak shear stress changed nonlinearly with the increase of normal load amplitude, characterized by a critical point. The influence of normal load amplitude on the shear strength and normal displacement was analyzed in detail. Furthermore, the results showed that dynamic normal load can both enhance and reduce the shear strength on granular material, and the frictional behaviors were velocity-amplitude-frequency dependent, which can be well interpreted by the Rate and State Friction constitutive law. These findings provide guidance in the design of geotechnical engineering projects when dynamic normal loads are encountered (e.g., blasting, earthquakes, earth tides and reservoir effects).

Experiments and modelling of horizontal loading tests on small-scale foundations in sand

The accurate prediction of settlements of shallow foundations under operational conditions is still an open issue for both scientists and engineers, in particular in the case of complex loading conditions (e.g. combined vertical and horizontal loads). The complexity of the soil mechanical response and the difficulty in managing a multiscale problem (ranging from the local scale of the soil representative elementary volume to the macro-scale of the structure) makes a rigorous modelling of such problems particularly demanding for standard numerical tools such as three-dimensional finite-element codes. In this perspective, the well-known macroelement approach may represent a valid simplified approach, capable of overcoming these limitations and allowing a comprehensive description of both serviceability limit state and ultimate limit state of such systems, particularly useful even for practitioners. In the paper, based on a small-scale 1g experimental campaign on a strip foundation, the effect of embedment is investigated over horizontal loading paths at a constant vertical load. In particular, relatively low vertical stress values are considered for the foundations, as it often happens in real structures under usual working conditions. A simple macroelement plastic model, based on a strain hardening law with non-associated flow rule is then proposed and validated over the available experimental data.

Effects of embedment depth on the pullout capacity of bucket foundations in sand

A series of 1 g experimental tests was conducted to investigate the effects of embedment depth on the pullout capacity of bucket foundations in sand. Polycarbonate model buckets with a constant diameter of 150 mm and length-to-diameter (L/D) ratios of 0.5, 1.0 and 1.5 were employed varying loading rates from 0.002 N/s to 4.1 N/s. The slow loading rate of 0.002 N/s induced drained conditions characterised with substantially small suction force and high skirt resistance. The fast loading rate of 4.1 N/s induced nearly undrained conditions characterised by large suction force and almost full uplift of the soil plug. The pullout capacity increased almost linearly with the L/D ratio. A 50% increase in embedment depth resulted in an average increase in pullout capacity of 40%. The pullout capacity and suction force inside the bucket rapidly increased with the loading rate in the partially drained condition, and converged to nearly constant values in the undrained condition. In the nearly undrained condition, the suction force at the tip was approximately half of that under the lid. The simplified approach reasonably predicted the capacities of buckets with various L/D ratios.

Load sharing ratios and interaction effects for piled raft foundations in sands

In piled raft foundations, the load shared between the piles and the raft is affected by a complex soil–structure interaction. However, the number of piles and their location can be optimised, thereby making piled rafts more effective and economical. In this study, the influence of three key design parameters (the number of piles, pile length and pile spacing) on pile–pile–soil and raft–pile–soil interactions was investigated; each of these parameters can lead to improvements in the performance and efficient load sharing of piled rafts. The results of small-scale model tests in sand showed that an increase in pile number or length proportionally decreased the load taken by the raft. A change in the pile spacing or compaction state of the sand bed also affected the raft's contribution, although to a lesser extent. Load sharing between the piles and the raft was found to be not only sensitive to the foundation size but was also dependent upon its settlement. Under favourable pile–pile–soil, raft–pile–soil and pile–raft–soil interaction effects, the combined load capacity of the piled raft increased. These interaction effects were non-linear and sensitive to settlement, which resulted in non-linear sharing of the load between the piles and the raft. The effect of pile inclusion and the number of piles on the shared load was also investigated using a simple hyperbolic model while considering the above interaction effects.

Effect of Pile's Number on the Behavior of Piled Raft Foundation

This paper introduces an experimental research on the behavior of the piled raft foundation (PRF) in sand of two states (loose and medium sand). A small model has been tested in a soil container and the vertical load was applied to the foundation through a pneumatic jack. The settlement of the foundation was measured using a displacement transducer; three sensors were attached to the pile heads to measure the axial load borne in a group by each pile by the Arduino data logger. The laboratory experiments were carried out on models of (1 pile), (1x2), (1x3), (2x3), (3 x3), (3 piles triangular), (4 piles diamond), (5 piles), and (9 pile circle), as well as to tests on an unpiled raft. Test variables were pile length, number of piles, and sand density. It is noticed that when the piles increased to nine piles in the group, the bearing capacity increased by 40%. The effect starts to increase when one pile is just placed under the raft as the bearing capacity of the piled raft increased by 3% and 7% when the pile length is 15D and, 20D, respectively. The reduction in the settlement is also observed to be smaller and no economic advantage is achieved with more increase in the number of piles. If the number of piles reaches (6) piles, the influence of the piled raft on settlement reduction disappears.

Comparison of the free vibration analysis of a fluid-conveying hybrid pipe resting on different two-parameter elastic soils

This study focuses on the dynamic behavior of a cantilevered fluid conveying hybrid pipe (FCHP) consisting of two different materials (aluminum and steel), and resting on a two-parameter elastic soil. To analyze the fluid-structure interaction in a hybrid pipeline conveying fluid, the pipe was modeled as an Euler-Bernoulli beam, and the equation of motion that describes the behavior of the dynamics of the Euler-Bernoulli beam was obtained. The critical velocities and complex frequencies of the system were obtained by utilizing the differential transformation method (DTM). Although studies on pipes supported on two-parameter elastic soil and FCHPs consisting of two different materials have been conducted separately by several researchers, a detailed study investigating the behavior of these two systems together has not been published so far. The stability of such a system is affected by various parameters. Therefore, in this study, the effects of the two-parameter elastic soil, the length ratio of the hybrid pipe materials, and the soil types and properties on the dynamic instability of a FCHP were investigated in detail. The results of this study showed that the critical flow velocity increased with the existence of a two-parameter elastic soil; the critical flow velocity of the FCHP increased with increasing foundation and shear modulus values; although the length ratio affected the instability of the cantilevered FCHP with and without elastic foundation, the FCHP resting on stiff clay foundation was more affected by this parameter in comparison to the FCHP resting on dense sand foundation having higher foundation and shear modulus values; and the critical flow velocity curves contained S and Z shaped segments associated with the instability–restabilization–instability sequence of the system.

Numerical modelling of the wave interaction with revetment breakwater built on reclaimed coral reef islands in the South China Sea—Experimental verification

The South China Sea (SCS) is an important channel, which plays a significant role in global economic trade and in the maintenance of world energy security. A series of artificial lands have been successfully built on the top of natural coral reefs in the SCS by the way of reclamation in recent years. In order to prevent those artificial lands from wave scouring and impacting, a great number of revetments and breakwaters have been constructed along the margin of these artificial lands. The revetment breakwaters have great significance and practical value to ensure the stability of these reclaimed lands, and to guarantee their normal long-term service performance. In this study, taking the reclamation project in the SCS as the engineering background, a computation model for the interaction between ocean waves, revetment breakwater and its calcareous coral sand foundation is established by taking the CFD solver OlaFlow as the computation platform which was developed based on the open source library OpenFOAM. Then this established computation model is verified by some laboratory testing data of wave profile and wave impact which have been measured in several wave flume physical model tests. The comparison between the testing data and the computational results indicates that the computation model established adopting OlaFlow can reliably simulate the wave generation, propagation, the dissipation of wave energy as well as the complicated interaction between ocean wave, the revetment breakwater and its calcareous coral sand foundation. This verification work will be a solid basis for the subsequent investigation of the interaction between severe ocean waves and the revetment breakwaters in large-scale, as well as the quantitative evaluation of the stability of the revetment breakwater build on reclaimed coral sand foundation in the SCS.

Load-settlement response of piled raft foundations in sand

ABSTRACT The load-settlement behaviour of piled raft foundations (PRFs) resting on sand subjected to vertical uniformly distributed load is studied using three-dimensional elasto-plastic finite element (FE) analyses. The unified Clay and Sand Model (CASM), based on the critical state soil mechanics, is used as the soil constitutive model. In the present study, 38 rectangular, strip, and circular raft connected to different pile configurations resting on four different types of sands in their loose, medium, and dense states are considered. The accuracy of the finite element analysis is confirmed through verification and validation with other numerical and experimental studies. A detailed parametric study is conducted, which includes soil elastic and critical state parameters as well as different dimensions of PRFs. A guideline chart is provided, which will be helpful for a design practitioner to choose the suitable PRF configurations considering serviceability limit states of safety and economical design aspect. This flow chart is based on (i) combined effects of various parameters on PRF load settlement behaviour, (ii) load sharing between the raft and the pile groups, and (iii) a preliminary cost analysis.

Development and preliminary application of a plate loading model test system considering stress state

To explore the effect of stress state on the bearing characteristics of foundation soil, this study proposed a self-developed plate loading test system. The system can be used to simulate the actual stress state of foundation soil within a given stratum, achieve uniform loading or gradient loading of lateral stress in simulating the k0 consolidation state, and simulate overburden stress around the foundation under the two loading modes. This study conducted plate loading tests on siliceous sand foundation under different loading modes using this system. It was found that (Ι) the bearing capacity and deformation modulus of foundation soil measured by the system under flexible loading were greater than those measured under rigid loading. Also, foundation settlement was smaller under flexible loading. (ΙΙ) Vertical stress in foundation soil was relatively uniform under both loading modes and was more uniform under flexible loading.

Resonance vibration approach in soil densification: laboratory experiences and numerical simulation

This paper describes the improvement effect and mechanism of strengthening a liquefied sand foundation using the cross-vibration wing resonance method, through an indoor model test and numerical simulation. The results obtained from the model test showed that a vertical drainage tube was formed during vibration compaction, and finally a crater with a depth of 40 mm and a radius of 150 mm was formed with sloping sides. The sand layer obtained a good improvement effect after resonance vibration, especially in the middle-lower sand deposit. The variation in excess pore water pressure showed different behavior in three stages of the vibration process, and the value after treatment was less than before with a decrease of 18.81%. The vibration energy in the horizontal direction gradually decreased to zero, however the absorption of vibration energy of the soil presented obvious nonuniformity along the depth direction. The results of the numerical simulation were similar to the model test results, including the scope and variation of pore water pressure, and the ground settlement after treatment.