Chao Li

Downhole dynamic compaction (DDC) improves soft ground by delivering high-energy impacts inside boreholes, where repeated hammer blows compact soil–rock mixture backfills and exert dynamic loading on surrounding soil. However, conventional penetration records cannot reveal the internal stress evolution, deformation, and energy transfer mechanisms that govern pile formation. This study established a three-dimensional coupled continuum–discrete numerical model, in which the native ground was simulated by continuum zones, and the in-hole soil–rock mixture backfill, together with the hammer, was simulated by discrete particles. The model was validated against a field test with three staged backfilling operations and nine field blows, reproducing the measured penetration attenuation with a maximum single-blow deviation of 6 cm and a cumulative relative error of 5.8%. The results indicated that the first blow after each filling mainly caused backfill rearrangement and borehole bottom compression, while later filling and tamping increased lateral deformation around the upper backfill, with limited additional movement at depth. Vertical compression predominated beneath the borehole bottom, whereas lateral compression predominated at the off-axis monitoring point. The displacement and plastic-work distributions supported a conceptual four-zone interpretation of variable-section pile formation, including upper contact expansion, intermediate radial compression, lower backfill expansion and shallow disturbance.

Variable cross-section cement–soil pipe piles are an innovative soft ground improvement technology. They are tubular, special-shaped cement–soil mixing piles characterized by a tapered profile along the pile shaft (larger diameter at the top and smaller at the bottom) and an internal soil core. They offer advantages including reduced material consumption, lower engineering cost, and shorter construction duration. However, the systematic theoretical understanding of their bearing performance remains insufficient. In this study, the bearing mechanism and influencing parameters of variable cross-section pipe piles were systematically investigated via full-scale field tests, numerical simulations, and laboratory model tests. An exponential decay constitutive model considering the strain-softening behavior of cement–soil was developed and implemented through secondary development in the ABAQUS platform for parametric analysis. Laboratory model tests were further conducted to advance the understanding of the bearing mechanism of variable cross-section pipe piles. The results show that the ultimate bearing capacity of the proposed variable cross-section cement–soil pipe pile is approximately 189% higher than that of the conventional ones. The expanded outer diameter and expanded height are the dominant factors affecting the bearing capacity, while the inner diameter and pile length have a comparatively minimal influence: increasing the expanded outer diameter from 0.6 m to 1.2 m and the expanded height from 0 m to 5 m increased the ultimate bearing capacity from 445 kN to 868 kN and 936 kN, respectively. The effective pile length is determined to be 6 m, and the recommended minimum wall thickness of the pipe pile is 1/4 of the inner diameter. Laboratory tests further demonstrated an abrupt change in axial force at the variable section. The findings provide reliable theoretical support for the engineering design and field application of cement–soil variable cross-section pipe piles.

In recent years, bidirectionally reinforced composite foundations have been widely used in highway, railway, and bridge engineering with notable results. The key mechanism is the soil-arching effect, which arises from the self-adjustment of the soil and directly affects the bearing capacity of the foundation. In this study, numerical simulation was employed to analyze the vertical stress in the subgrade soil and the transfer of particle contact forces from the macro and micro perspectives. The existence of the soil-arching effect was confirmed, and its variation under loading was revealed. To quantify the degree of the soil-arching effect, the stress transfer efficiency of the soil between piles was introduced. Subsequently, a bidimensional theoretical model was established based on the coordinated deformation among the embankment, the horizontally reinforced cushion, the vertical piles, and the soil. In this model, the combined effects of the embankment soil-arching, the reinforcement of cushion net, and the stress diffusion were incorporated. A method for the calculating of the pile–soil stress ratio of bidirectionally reinforced composite foundation was proposed, and the influence of various factors on this ratio was explored. The results indicate that the soil-arching effect can be divided into three stages according to the height of the subgrade fill: no-arch stage, transition stage, and soil-arching stage. Reducing pile spacing or increasing cushion thickness can improve the stress transfer efficiency. When the pile length is appropriate, the stress in the foundation soil at 0.55 times the pile depth was contoured, enhancing stability. The pile–soil stress ratio decreases with the increase in filling weight and pile spacing, increased first and then decreased with increasing internal friction angle of filling materials, and increased with the increasing height of embankment, the number of geogrid layers, and the cohesion of filling materials.

In order to accurately analyze the liquefaction and the reinforcement effect of saturated silty and sandy soils treated by dynamic compaction, a hydro-mechanically coupled explicit analysis method was proposed. The method, in combination with the cap model, was carried out using the Abaqus finite element software. Then, parametric analysis was carried out by means of the development and dissipation of excess pore water pressure, effective soil stress and the relative reinforcement degree. And the effects of the drop energy, tamper radius and soil permeability on the liquefaction zone and soil improvement of saturated soil were examined. The results demonstrated that the liquefaction zone and the effective reinforcement were determined by the drop energy rather than the permeability or tamper radius. A 2.5-times increase in drop energy can increase the maximum liquefaction depth by 1.1 m (4.6 m to 5.7 m) and the effective reinforcement depth (I_r ≥ 0.08) by 0.6 m (1.2 m to 1.8 m). It is recommended that the reinforcement effect should be improved by a lower drop energy with a low drop height and a heavy tamper in actual projects. It should also be noted that a smaller tamper radius was conducive to local soil improvement but also generated higher localized excess pore water pressures. Soil permeability critically controls liquefaction potential and excess pore water pressure dissipation. Low permeability soils experienced significant liquefaction depths and slower consolidation, whereas high permeability gravels (k = 10⁻² m/s) showed minimal liquefaction and great improvements in depth. To diminish the effect of the underground water, the gravel cushions should be used to drain pore water out before dynamic compaction.

With the rapid development of lightweight and environmentally friendly building materials, foamed concrete has been widely adopted as a novel material for lightweight filling and foundation applications. However, its bearing capacity under localized loading conditions requires further investigation. This study focuses on silt-based foamed concrete with a 30% silt content. A series of unconfined compression tests and triaxial shear tests were conducted to determine its key mechanical properties. Large-scale indoor model tests were then carried out to evaluate the effects of the wet density (600 kg/m³, 700 kg/m³, and 800 kg/m³) and the loading position on vertical bearing performance. The results show that silt-based foamed concrete exhibits a basin-shaped deformation pattern under vertical loading, similar to traditional foundations. Based on experimental data and shear strength parameters, a formula for the ultimate bearing capacity of silt-based foamed concrete was developed by extending Terzaghi’s bearing capacity theory. This provides a theoretical basis for its application in geotechnical engineering.

The cement–soil pipe pile is a novel blend of cement and soil, designed to enhance load-bearing capabilities while cutting down on the need for cement. Its tubular construction is key to its strength. To delve into how the pile’s cross-sectional size affects its load-bearing properties, we took into account the soil–cement’s strain-softening behavior. Laboratory tests examined the load-bearing properties of piles. We created an exponential decay Mohr–Coulomb model in ABAQUS for further development, performed field tests, and built a numerical model incorporating wall thickness, pile diameter, and length. The unit volume ultimate bearing capacity was used to evaluate pile performance, with a focus on a 600 mm diameter pile. The results show that wall thickness minimally affects load-bearing capacity, needing to be at least a quarter of the diameter. Larger diameters increase the ultimate bearing capacity, but the capacity per unit volume declines. The 600 mm diameter pile boasts the highest unit volume ultimate bearing capacity. The pile’s effective length is roughly 10 m. Beyond this, extending the pile length increases the single pile’s ultimate bearing capacity by less than 5%, but the unit volume capacity starts to dwindle.

In recent years, super-long and super-large diameter pipe piles have been gradually applied to the foundation in the Yellow River flood area. However, its bearing mechanism is not clear, especially the unclear bearing characteristics of the pile under the eccentric state, which limits its application and development. In this regard, this paper uses the method of combining field test and numerical simulation to analyze the bearing characteristics of super-long and super-large diameter pipe piles under different pile lengths, different pile diameters, different diameter-thickness ratios, and different offsets. Combined with the specific deviation form of the pipe pile, the calculation formula of the vertical ultimate bearing capacity of the super-long and super-long diameter pipe pile in the Yellow River flooding area under the influence of the construction effect is modified. The results show that when the length of the pipe pile changes, the vertical bearing capacity changes the most, and the vertical ultimate bearing capacity of the pipe pile increases linearly with the increase of the length of the pipe pile. When the wall thickness of the pipe pile increases, the vertical bearing capacity increases approximately linearly, but the reduction of the pile displacement decreases exponentially. The greater the deflection of the pipe pile, the smaller the vertical ultimate bearing capacity. When the deflection of the pipe pile is greater than 0.35°, the vertical ultimate bearing capacity decreases rapidly with the increase of the deflection. On the basis of the traditional formula, considering the deviation form of the pipe pile, the reduction coefficient of the bearing capacity correction formula of the super-long and super-large diameter pipe pile is proposed, and the correction formula is compared with the field example. It is proved that the formula can accurately calculate the bearing capacity of the super-long and super-large diameter pipe pile. The research results of this paper are of great significance to the application and promotion of super-long and super-large diameter pipe piles in the Yellow River flood area and the evaluation of vertical ultimate bearing capacity. At the same time, the research results of this paper can also provide a reference for the study of bearing characteristics of super-long and super-large diameter pipe piles in other foundations.

To resolve insufficient traditional bolt supports due to the complexity of geological conditions, the optimal design of an expanded head bolt was investigated by using theoretical calculations and experiments. The results show that the drawing capacity of an expanded head bolt is affected by the bearing capacity of front and rear ends, side bearing capacity, and side friction resistance. For a circular anchor bolt, stepped anchor bolt, and semi-ellipsoidal anchor bolt, with an increase in the front section’s radius, the lateral friction resistance of the inner anchor section is gradually shared by the bearing force of the front end of the inner anchor section; the bearing effect of the front end of the inner anchor section is enhanced; and the pulling performance of the anchor bolt is enhanced. Therefore, the pulling force of the circular anchor bolt is at the maximum, followed by the stepped anchor bolt, and the semi-ellipsoidal bolt is at the minimum. The increase in the rear section can provide greater lateral friction resistance and end-bearing force. Compared with cylindrical enlarged head anchors, the circular, stepped, and semi-elliptic enlarged head anchors have a smaller front section but a larger rear section, and the reduction in the front section’s bearing capacity is less than the increase in the side bearing capacity and rear-end bearing capacity; thus, the cylindrical bolt has the lowest pulling force. Compared with the front radius, the back radius has more influence on the drawing ability of the enlarged head anchor. The longer the inner anchorage section, the larger the distribution range in the compression zone that is formed in the soil body and the smaller the range in the tension zone that is formed in the rear. The increase in the length of the inner anchorage section is conducive to improving the reinforcement effect of the soil in front of the anchorage section in the bolt. Therefore, this parameter plays an important role in the redistribution of the soil in front of the force. The ultimate pull-out force of a circular table-shaped tensile bolt is the highest, followed by the stepped bolt, and the semi-elliptic bolt comes in third, with the cylindrical bolt exhibiting the lowest pull-out force; the circular table-shaped enlarged head anchor constitutes the best style design.

This paper presents a numerical investigation on the improvement mechanism of dynamic compaction (DC) in saturated soil of a weak layer with high levels of groundwater, using an improved fluid–solid coupling method with a Drucker–Prager–Cap soil model. The numerical model is verified by comparing with published test data at first, which show that the dual-phase coupling method can approximately reflect the development law of excess pore water pressure and the improvement effect of layered saturated soil foundation under DC. Then, based on the numerical model, the influences of the thickness and depth of the weak layer as well as tamping energy on the development and dissipation of excess pore water pressure, effective stress, and the relative degree of reinforcement during DC were investigated. The results showed that the embedment depth and thickness of the weak interlayer may greatly affect the effective reinforcement depth of DC. Meanwhile, the tamping energy and the groundwater table also play a great role in the improvement of the layered saturated soil under DC. The groundwater table should be lowered by dewatering or adding a drainage layer to achieve a better compaction effect during DC.