Modeling Test and Numerical Simulation of Vertical Bearing Performance for Rigid-Flexible Composite Pouch Piles with Expanded Bottom (RFCPPEB)

Rigid-flexible composite pouch piles with expanded bottom (RFCPPEB) are generally considered as new symmetrical piles in practical engineering, but their bearing characteristics and design method are still not completely understood. The objective of this study is to investigate the vertical bearing performance and the optimal design scheme of RFCPPEB. Hence, laboratory modeling tests for this symmetric structure and an ABAQUS three-dimensional (3D) numerical simulation analysis were used to study the vertical bearing characteristics on bottom-expanded piles and rigid-flexible composite piles with expanded bottom. The vertical bearing capacity, shaft resistance, pile tip resistance distribution rule, and load sharing ratio of RFCPPEB were analyzed and verified using different bottom expansion dimensions and cemented soil thicknesses. The results revealed that the optimal bottom expansion ratio of rigid bottom-expanded piles was 1.8 when the ratio of pile body to bottom-expanded pile head was 9:1. When the bottom expansion ratio (D/d) was increased, the bearing capacity of bottom-expanded piles was significantly increased at D/d = 1.4 and D/d = 1.8 compared to that of D/d = 1.0, reaching 1.67 and 2.29 times, respectively, while for D/d = 1.6 and D/d = 2.0, the ultimate bearing capacity remained unchanged. Besides, shaft resistance played an important role in the bearing process of the rigid bottom-expanded piles and RFCPPEB. When the shaft resistance was increased, the ultimate bearing capacity of the pile foundation was significantly improved. The shaft resistance of RFCPPEB was increased with increasing cemented soil thickness. The increases in the shaft resistance and thickness of the cemented soil showed a nonlinear growth, and the maximum shaft resistance was approximately 75 cm from the pile top. When the diameter of the expanded head was 1.8 times the diameter of the pipe pile and slightly larger than the thickness of the cemented soil (0.5 times the diameter of the pipe pile), the optimal amount of concrete 425.5 kN/m3 required for per unit volume around piles was obtained, with the RFCPPEB ultimate bearing capacity of 7.5 kN. For RFCPPEB, the soil pressure at the pile tip was directly proportional to the pile top load under small load and was decreased in the form of a half quadric curve under large load. It reached the most reasonable position where the slope of the quadric curve was the largest when the thickness of the cemented soil was larger than 0.5 times the diameter of the pipe pile.

Field Tests on Bearing Characteristics of Large-Diameter Combined Tip-and-Side Post Grouted Drilled Shafts

This paper presents a field study on the axial behavior of four large-diameter drilled shafts embedded in coarse sand. The grouting and loading test procedures were reported. The bearing capacity of shafts (TS1 and TS2) and grouted drilled shafts (TS3 and TS4) were herein determined by the bi-directional static test and top-down load test, respectively. The enhancement mechanism of bearing characteristics of the grouted shafts was discussed in detail. The test results indicate that the bearing characteristics and load transfer mechanisms of the test shafts were significantly affected by the quantity of pressurized cement slurry and the mechanical properties of the soil surrounding the shafts. Furthermore, the tip resistance of shaft can be mobilized more rapidly and fully after grouting, the side and tip resistance are mobilized in a more synchronized and coordinated manner due to the pre-mobilization of the grouted cement. Additionally, the standard penetration test (SPT) prediction model was introduced to calculate and predict the SPT blow counts of soil after grouting. The results show that the post grouting has a more obvious improvement on the strength of cohesionless soil.

Estimating an empirical equation connecting subgrade reaction modulus with cone penetration test for sandy soils

In geotechnical engineering, the coefficient of subgrade reaction is regarded as one of the most important parameters used for describing the interaction of soil and structure as well as describing some soil characteristics, subgrade reaction coefficient can be calculated theoretically using many different formulas, laboratory via specific well-known tests, and in site through field plate loading test. On the other hand, the cone penetration test is one of the most frequently used field tests to investigate the soil. The lately carried out researches showed a good relation between the subgrade coefficient and the tip resistance collected from the CPT, but the results obtained from the proposed method are still doubtable. In this paper, fifteen plate load tests and thirty CPTs, already collected for private site investigation project, have been used for finding the best fit equation connecting the subgrade reaction coefficient Ks with the tip resistance qc. The finds of the established equation have been compared extensively with those of other well-known related equations. The results show the ability of the concluded equation to get Ks results in the acceptable range of sandy soils. However, the depth and shape effect on the suggested formula need further investigations since all the plate load tests in this project have been carried out on the soil surface with a 45 cm diameter circular plate.

Static Load Test and Numerical Analysis of Influencing Factors of the Ultimate Bearing Capacity of PHC Pipe Piles in Multilayer Soil

Prestressed high-strength concrete (PHC) pipe piles have been widely used in engineering fields in recent years; however, the influencing factors of their ultimate bearing capacity (UBC) in multilayer soil need to be further studied. In this paper, a static load test (SLT) and numerical analysis are performed to obtain the load transfer and key UBC factors of pipe piles. The results show that the UBC of the test pile is mainly provided by the pile shaft resistance (PSR), but the pile tip resistance (PTR) cannot be ignored. Many factors can change the UBC of pipe piles, but their effects are different. The UBC of the pipe pile is linearly related to the friction coefficient and the outer-to-inner diameter ratio. Changes in the pile length make the UBC increase sharply. Low temperatures can produce freezing stress at the pile–soil interface. The effect of changing the Young modulus of pile tip soil is relatively small.

Stratigraphic profiling, slip surface detection, and assessment of remolding in sensitive clay landslides using the CPT

A multi-year cone penetration testing program was initiated at a landslide subject to episodic retrogression in Mud Creek, Ottawa, to assess whether a hand-operated mobile CPT could yield new insights into the current degree of remoulding under progressive failure in metastable areas of a landslide where conventional tracked rigs are unable to gain access. The mobile CPT rig permitted tests to be performed through the entire thickness of the Champlain Sea deposit at a penetration rate of 0.5 cm/s, with similar results to tests performed at the standard 2 cm/s. Measurements of pore pressure varied considerably with cone size, with the magnitude of pore pressure response decreasing with cone size. The elevation of the slip surface was identified in the tip resistance as the point of transition between the remolded soil above the slip surface and the intact soil below the slip surface, whereas a further 0.5 m of penetration was required to elevate pore pressures to values indicative of the intact soil behaviour. In-situ measurements of shear strength of corresponding layers between the intact and remolded profiles to be compared indicating that the soil above the slip surface had remolded to 50% of its fully remolded strength.

Development and operation of an automatic air pluviation system for physical modelling applications

This paper presents an air pluviation system, developed to facilitate 1-g physical model tests in granular soils. The deposition process is fully automated and requires minimal input from the operator, thereby significantly reducing the time required to deposit large volume of granular material, improving the uniformity of the prepared specimens, and the reliability of test results. The components comprising the pluviation system have been calibrated to produce loose-to-very dense sand beds, of relative density that ranges between Dr=7% and Dr>100% of the maximum density achieved with the procedures described in the pertinent standards. The testing chamber where sand is deposited is instrumented with an array of pressure sensors, and the rig is equipped with a miniature Cone Penetration Testing (mini-CPT) device. Measurements from the earth pressure sensors and cone tip resistance profiles are used to evaluate how friction at the sand-chamber interfaces affects the distribution of geostatic stresses inside the chamber, the uniformity of sand beds, and boundary effects during deposition and during mini-CPT testing. The air pluviation system allows preparing layered sand profiles by adjusting the deposition parameters on the fly, and this feature is demonstrated via the analysis of mini-CPT tests performed in layered sand beds.

Field Study on Deformation and Stress Characteristics of Large Open Caisson during Excavation in Deep Marine Soft Clay

Oujiang River North Estuary Bridge in Wenzhou is the world’s first double-deck suspension bridge under construction with three-tower and four-span. It is the first time to build large open caisson foundation in the deep marine soft clay in estuary with strong tide, extending the application scope of caisson. To study the deformation and stress characteristics of large open caisson during excavation and ensure the safety of anchorage excavation, a large number of sensors are arranged in the caisson. By analyzing the change of tip resistance, lateral soil pressure, and posture parameters during caisson excavation, the stress characteristics and deformation of caisson are described. The result shows the following. (1) Because of the thixotropy of soft clay, the reaction force of partition wall in deep soft soil area of caisson is similar to that of blade foot, and the reaction force of blade foot can be effectively reduced through the layering construction of caisson. (2) The height of caisson construction and the sand-bearing stratum will obviously affect the plane torsion angle of caisson. When the caisson enters the sand-bearing stratum, the lateral soil pressure increases significantly, which leads to the increase of the plane torsion angle. (3) The inclination and central deviation of caisson are sensitive to the caisson construction and stratum property. It can be found that the lateral soil pressure, plane torsion angle, inclination, and central deviation of caisson are sensitive to stratum property, and inhomogeneity of stratum easily leads to inclination of caisson. Based on the field monitoring data, the stress characteristics and geometric posture of caisson during sinking are studied, which provide technical guidance for scheme design and subsidence prediction analysis of caisson in deep marine soft clay. It can provide a good opportunity to study the behaviors of large caisson foundation constructed in deep marine soft clay and has great significance and reference value for construction optimization of anchorage structure.

A Framework to Predict the Load-Settlement Behavior of Shallow Foundations in a Range of Soils from Silty Clays to Sands using CPT Records

Using a set of cone penetration test (CPT) records,the current paper develops a general framework based on regression analyses to model the load-settlement (q-s) behavior of shallow foundations resting on a variety of soils ranging from silty clays to sands.A three-parameter hyperbolic function is employed to rigorously examine the obtainedq-s curves, and to determine the model parameters.Also, the results of someCPT soundings, including the corrected cone tip resistance (qt) and the skin friction (Rf),are adopted to predict the results of plate load tests(PLT). The findingscorroborate the high accuracy of the proposed model, thereasonable performance of the hyperbolic function and the use of the Volterra series to predict the q-s curves.Moreover, the obtained curves from the newly developed model arecompared to those from other methods in the literature which cross-confirms the efficacyof the current model. Asensitivity analysis isalso conductedand the exclusive effects of all the contributing parameters are assessed among which Rfis shown to be the most influential. Ultimately, simple solutionsare adoptedto determine variouskey geotechnical parameters, like the ultimate bearing capacity (qult), the allowable bearing capacity (qa) andthe modulus of subgrade reaction (ks).

Verification of strength resistance of sandy soil using small scale penetrometer tests

This study focuses on utilizing cone penetrometer models to determine strength (resistance) of sandy soil and also assessment how the relative density and the angle of friction effects on the measured cone penetration resistance in sandy soil. Simple empirical equations are used also to determine the cone penetration resistance components such as the sleeve resistance and the tip resistance. Simple comparison is performed between the measured and calculated soil strength and well agreement is noticed between them.