Experimental Investigation on the Vertical Bearing Characteristics of Jacked Piles in Saturated Silt Foundations under Excavation

A model test system for vertical bearing characteristics of the jacked piles in saturated soil foundations under excavation has been introduced. The system device comprises a soil pressure loading system, a model pile loading system, a soil vacuum saturation system, a model box, a model pile, and a control and data acquisition system. The soil vacuum saturation system designed for the model box of this test device can ensure that the saturated soil in the model box can reach a higher degree of saturation. Loading and unloading were conducted on the soil sample in the model box through the soil pressure loading system to simulate the soil excavation so that the soil sample and that in the field have the same stress state and history. The soil consolidation pressure, pile jacking pressure, pile tip force, soil consolidation settlement, and pile displacement at the top were collected and monitored in real time through the control and data acquisition system. This device is used to conduct an experimental study on the bearing characteristics of the jacked piles in saturated silt foundations under excavation. The results indicate that the static load test increases the residual pressure on the tip of the jacked pile while also increasing soil stiffness at pile tip and ultimate tip resistance, thereby increasing the pile top stiffness and ultimate load-carrying capacity. However, when the jacked pile is left undisturbed for the same time, the static load test on the jacked pile does not affect the pile skin friction resistance. There is a better linear relationship between the pile skin friction resistance and the undrained shear strength of the soil under the corresponding stress path during the static load test of the normally consolidated soil and the jacked pile after overburden pressure unloading. There is a good linear relationship between the ultimate resistance and the undrained shear strength of the soil under the corresponding stress path in pile sinking, normally consolidated soil, and during the static load test on jacked pile after unloading.

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.

Vertical Bearing Capacity of Precast Pier Foundation Filled with Demolished Concrete Lumps

The application of recycled compound concrete made of demolished concrete lumps (DCLs) and fresh normal concrete in pier foundation can effectively improve the utilization efficiency of construction waste resources. In this study, two prefabricated pier foundations based on recycled compound concrete (dimension of Ø800 × 2500 mm and Ø1000 × 2500 mm) and two cast-in-place pier foundations based on ordinary concrete (dimension of Ø800 × 2500 mm and Ø1000 × 2500 mm) were tested. Special attention was devoted to the load-settlement curve characteristics of the precast pier foundation of compound concrete, the load transfer law of the pier-soil system, the soil pressure distribution at the bottom of the pier, and the failure mode. The results showed that the Q-S curve of precast concrete pier foundation made of recycled compound concrete is slow deformation at loading, which is consistent with that of cast-in-place concrete pier foundation. The load transfer theory of pier-soil system is established, and its accuracy is verified by experimental analysis. The precast foundation of recycled compound concrete is the same as the cast-in-place foundation of ordinary concrete. The failure form of prefabricated pier foundation made of recycled compound concrete was a local shear failure, while the failure form of ordinary concrete cast-in-place pier foundation was piercing-type shear failure. The feasibility of relevant theoretical methods for calculating the vertical ultimate bearing capacity is examined.

Calculation Method for the Critical Thickness of a Karst Cave Roof at the Bottom of a Socketed Pile

The thickness of a karst cave roof at the bottom of a socketed pile plays an important role in the vertical bearing capacity of the socketed pile in the karst region. In practice, its thickness is simply recommended to be not less than 3 times the diameter of the socketed pile, regardless of the geological conditions and the size of the cave itself. In this study, we present an approach for calculating the critical thickness-to-diameter ratio of a karst cave roof η (η = h/d, the ratio of karst cave roof thickness to pile diameter) based on the generalized Hoek–Brown criterion by virtue of the limit analysis method, which considers the pile tip load, hardness degree of the intact rock, and rock mass quality. The analysis results show that less load at the bottom of the pile, higher quality of rock mass, and more hard rock all lead to a smaller critical thickness-diameter ratio, whereas the critical thickness-to-diameter ratio is greater. The validity of the proposed method is verified through a physical model test.

Study on the vertical bearing performance of pile across cave and sensitivity of three parameters

A new method was used to study the performance of pile across cave. This paper investigated the vertical bearing characteristics of piles cross caves using centrifugal model tests and a theoretical model of sensitivity. Twelve pile scenarios were selected, the first was a conventional pile, 24 cm long and 2.5 cm in diameter, with no karst cave as a control. In the other eleven scenarios the piles passed through karst caves of four different heights, of four different spans, and three different numbers of caves. The results reveal that increasing the height, span, and number of caves all are negative for vertical ultimate bearing capacity of piles. The axial force and unit shaft resistance of piles are great different. According to the ratios of the tip and shaft resistance, caves change the type of piles. The sensitivity of vertical ultimate bearing capacity to these factors from high to low is height, number, and span of caves. Importantly, the bearing characteristics of piles decrease faster once the height of the prototype karst cave is higher than 9 m, but decreases slowly when the cave’s span is greater than 9 m × 9 m.

Analysis of Vertical Load Transfer Mechanism of Assembled Lattice Diaphragm Wall in Collapsible Loess Area

Based on analysis of the formation mechanism and characteristics of the negative friction in collapsible loess areas, this study investigates the load transfer law of a wall-soil system under a vertical load, establishes the vertical bearing model of a lattice diaphragm wall, and analyzes the vertical bearing capacity of an assembled latticed diaphragm wall (ALDW) in a loess area. The factors influencing the vertical bearing characteristics of the ALDW in a loess area are analyzed. The vertical bearing mechanism of the lattice diaphragm wall in the loess area is investigated. The failure modes of the ALDW in the loess area are mainly shear failure of the soil around the wall and failure of the wall-soil interface. In the generation and development of negative friction, there is always a point where the relative displacement of the wall-soil interface is zero at a certain depth below the ground; at this point, the wall and soil are relative to each other. The collapsibility of loess, settlement of the wall and surrounding soil, and rate and method of immersion are the factors affecting the lattice diaphragm wall. The conclusions of this study provide a reference for the design and construction of ALDWs in loess areas.

A study on the vertical bearing capacity according to the inclination angle of single batter piles and the relative density of sand under vertical loads

This study analyzed the increase in the compressive bearing capacity of single-sloped piles according to the relative density and inclination angle when the batter piles are subjected to a vertical load in non-adhesive sand. An experiment was conducted with inclination angles of 0, 10, 20, 30, and 40, and the relative density of sandy soil was divided into 31% (loose), 53% (medium), and 72% (dense) and compared with the results of the earlier research. As a result of the experiment, when the relative density of the ground was medium and dense, the bearing capacity was greater than that of the vertical pile (0°) at all angles. The tendency noted was that the bearing capacity of the pile increased from vertical to 20° and gradually decreased after 20°. The same tendency was also exhibited by loose sandy soil, but with less bearing capacity than the vertical pile (0°) except for 20°.

Analysis of the loading of the gear casing of an electric locomotive provided that the bolts fastening it to traction motor are loosened

Objective: To study the dependence of the numerical values of the bearing reactions occurring in the bolted fasteners of the gear casing (GC) on the conditions of its fastening to the traction motor. To show the resulting redistribution of the bearing reaction values in the case of the lack of load bearing in one or two bolted fasteners from the three existing design ones. To calculate the equivalent stresses that appear in the gear casing structure under various abnormal conditions of its fastening to the traction motor under static loading. Methods: The equations of statics were used for the spatial system of forces, including three vertical bearing reactions and the weight of the casing itself. To study the stress-strain state of the casing in the case of its static loading, the finite element method was used, implemented in the ANSYS Workbench software package. Results: The cause of damage to the GC during operation has been found associated with its design flaws. Calculation formulas have been obtained for determining the vertical bearing reactions of the GC. The need to take into account the GC loading in the case of redistribution of loads due to the loosening of its fastening bolts has been found. The need to improve the design of the gear casing has been indicated. Practical importance: The probable cause of damage to the GC during operation has been found. A method is proposed for assessing the GC loading in the case of redistribution of loads between its bearing points.