Effects of anti-pull ties on the bearing behaviors of shallow tunnel-type anchorages in soft rock

Tunnel-type anchorages(TTAs)installed in human gathering areas are characterized by a shallow burial depth,and in many instances,they utilize soft rock as the bearing stratum.However,the stability control measures and the principle of shallow TTAs in soft rock have not been fully studied.Hence,a structure suitable for improving the stability of shallow TTAs in soft rock strata,named the anti-pull tie(APT),was added to the floor of the back face.Physical tests and numerical models were established to study the influence of the APT on the load transfer of TTAs,the mechanical response of the surrounding rock,the stress distribution of the interface,and the failure model.The mechanical characteristics of APTs were also studied.The results show that the ultimate bearing capacity of TTAs with an APT is increased by approximately 11.8%,as compared to the TTAs without an APT.Also,the bearing capacity of TTAs increases approximately linearly with increasing height,width,length,and quantity of APTs,and decreases approximately linearly with increasing distance from the back face and slope angle of the tie slope.The normal squeezing between the tie slope and the surrounding rock increases the shear resistance of the interface and expands the range of the surrounding rock participating in bearing sharing.Both tension and compression zones exist in the APT during loading.The tension zone extends from the tie toe to the tie bottom along the tie slope.The range of the tie body tension zone constantly expands to the deep part of the APT with an increasing load.The peak tensile stress value is located at the tie toe.The distribution of compressive stress in the tie body is the largest at the tie top,followed by the tie slope,and then the tie bottom.

Bearing behavior and failure mechanism of squeezed branch piles

The current practice for the design of squeezed branch piles is mainly based on the calculated bearing capacity of circular piles. Insufficient considerations of the load-transfer mechanism, branch effect and failure mechanism, as well as overreliance on pile load tests, have led to conservative designs and limited application. This study performs full-scale field load tests on instrumented squeezed branch piles and shows that the shaft force curves have obvious drop steps at the branch position, indicating that the branches can effectively share the pile top load. The effects of branch position, spacing, number and diameter on the pile bearing capacity are analyzed numerically. The numerical results indicate that the squeezed branch piles have two types of failure mechanisms, i.e. individual branch failure mechanism and cylindrical failure mechanism. Further research should focus on the development of the calculation method to determine the bearing capacities of squeezed branch piles considering these two failure mechanisms.

Bearing Behavior of Cast-in-Place Expansive Concrete Pile in Coral Sand Under Vertical Loading

The low side friction of piles in coral sand results in the low bearing capacity of foundations.In this paper,expansive concrete pile is utilized to improve the bearing capacity of pile foundations in coral sand.Both model tests and numerical simulation are performed to reveal the bearing mechanism of expansive concrete pile in coral sand.Results showed that the lateral earth pressure near pile increases obviously and the side friction of piles is improved,after adding expansion agent to the concrete.The horizontal linear expansion is 1.11%and the bearing capacity increased 41%for the pile,when 25%expansion agent is added.Results in finite element numerical simulation also show that ultimate bearing capacity increases with the increase of the linear expansion ratio.Besides,the area for obvious increase in side friction is below the surface of soil about three times the pile diameter,and the expansion leads to a high side friction sharing of the pile.Therefore,the cast-in-place expansive concrete pile is effective in improving the bearing capacity of piles in coral sand.

Influence of bolt dynamic installation on topography characteristics and mechanical behaviors of CFRP interference-fit bolted joints

As the controlled research of Dynamic Installation(DI)and Static Installation(SI),a new interference installation method was developed based on electromagnetic loading to enhance the mechanical properties of composite structures.Four different interference-fit sizes were considered,ranging from a net fit to 2.0%.The experiments were conducted to evaluate the installation resistance and the mechanical behavior of the joint under external loads.Meanwhile,an FFA model to model the stress distribution and damage behavior of the bolt-hole contact interface was established.The load-displacement curve and damage modes of experiments were used to verify the FEA results.The results show that the installation resistance during DI process was remarkably lower than that of SI process corresponding to all interference-fit sizes,and the stress amplitudes induced by interference were larger and widely distributed.The damage of the hole wall was positively correlated with interference fit size,but DI can significantly reduce the damage compared to SI.In performance tests,DI enhanced the static bearing capacity and extended longer fatigue life of the joints than SI.DI methods can be an effective way to achieve highly reliable interference joints in composite structures.

Effect of local openings on bearing behavior and failure mechanism of shield tunnel segments

Local failures(loss of concrete or reinforcement)can severely compromise the bearing capacity of shield segments,damaging the tunnel structures.To investigate the effects of local openings on the bearing behavior and failure mechanism,four full-scale bending tests were conducted on specimens with different opening positions and diameters;monitoring of load,displacement,and concrete strain was performed during loading.The test results reveal that both the opening position and diameter significantly influence the bearing characteristics of the segment.The failure process includes four sequential stages distinguished by three critical loads,namely the cracking,failure,and ultimate loads.Subsequently,the numerical model of the local failure segment was established using the elastoplastic damage constitutive relation of the concrete and verified by inversing the full-scale test results.Based on the numerical model,parametric analyses were performed to comprehensively investigate the influences of the opening position,concrete loss,and reinforcement loss on the bending capacity.Furthermore,an analytical model was proposed,indicating that the opening position is the primary factor decreasing the bearing capacity,followed by the opening diameter and reinforcement loss.The results of this study can provide a theoretical basis for the safety assessment and remedial design of subway shield tunnels under extreme breakthrough conditions.

Experimental Study on Force Behavior of Steel Rein forced Concrete Transfer Beam Structure with Basement of Large Space

Based on comparative test of two transfer story models, in one of which the transfer beam and basement column is constructed of steel reinforced concrete, and the other is constructed of ordinary reinforced concrete, its force behavior, ductility and failure mechanism under vertical and horizontal loads are studied. The results show that loading bearing and seismic behavior of transfer story structure with steel reinforced concrete beam and basement column is good. The relative design suggestion is put forward.

State-of-the-art on resistance of bearing-type bolted connections in high strength steel

With the recent development of material science,high strength steel(HSS)has become a practical solution for landmark buildings and major projects.The current codes for design of bearing-type bolted connections of steel constructions were established based on the research of conventional steels.Since the mechanical properties of HSS are different from those of conventional steels,more works should be done to develop the appropriate approach for the design of bearing-type bolted connections in HSS.A review of the research carried out on bearing-type bolted connections fabricated from conventional steel and HSS is presented.The up-to-date tests conducted at Tongji University on four connection types fabricated from three grades of HSS with nominal yield strengths of 550,690,and 890 MPa are presented.The previous research on failure modes,bearing resistance and the design with consideration of bolt hole elongation are summarized.It is found that the behavior of bolted connections in HSS have no drastic difference compared to that of conventional steel connections.Although the ductility is reduced,plastic deformation capacity of HSS is sufficient to ensure the load redistribution between different bolts with normal construction tolerances.It is also found that behavior of each bolt of multi-bolt connections arranged in perpendicular to load direction is almost identical to that of a single-bolt connection with the same end distance.For connections with bolts arranged in parallel to load direction,the deformation capacity of the whole connection depends on the minimum value between the end distance and the spacing distances in load direction.The comparison with existing design codes shows that Eurocode3 and Chinese GB50017-2017 are conservative for the design of bolted connections in HSS while AISC 360-16 may overestimate the bearing resistance of bolted connections.

The behavior of a rectangular closed diaphragm wall when used as a bridge foundation

The rectangular closed diaphragm(RCD)wall is a new type of bridge foundation.Compared to barrette foundation,measuring the performance of RCD walls is relatively complicated because of their incorporation of a soil core.Using the FLAC3D software,this paper investigates the deformation properties,soil resistance and skin friction of a laterally loaded RCD wall as well as the settlement,axial force and load-sharing ratio of a vertically loaded RCD wall.Special attention is given to the analysis of factors that influence the performance of the soil core.It was found that under lateral loading,the RCD wall behaves as an end-bearing friction wall during the entire loading process.The relative displacement between the wall body and the soil core primarily occurs below the rotation point,and the horizontal displacement of the soil core is greater than that of the wall body.Under vertical loading,the degree of inner skin friction around the bottom of the soil core and the proportion of the loading supported by the soil core increase with increased cross-section size.The wall depth is directly proportional to the loading supported by the outer skin friction and the tip resistance of the wall body and is inversely proportional to the loading borne by the soil core.

Field testing of geosynthetic-reinforced and column-supported earth platforms constructed on soft soil

This paper is focused on the behavior of geosynthetic-reinforced and column-supported （GRCS） earth platforms in soft soil. By analyzing the data of a 15-month long field monitoring project, the bearing behavior and effectiveness of GRCS earth platforms are discussed in detail. It can be found that the soil arching is generated when the filling reaches a certain height. The measured pressure acting on the soil in the center of four piles was smaller than that acting on the soil between two piles. The elongation and the tension of the geogrid located in the soil between piles are both larger than the corresponding values on the pile top. The skin friction of piles is relatively small in the soil layer with low strength and the load transfer of the axial force in those layers is significant; meanwhile, the opposite situation occurs in the soil layer with high strength. The pore water pressure at shallow locations increases slightly with the filling height and is greatly affected by the increasing filling load. The layered settlement is directly proportional to the filling height, and the corresponding amount is relevant to the locations and the properties of specific soil layers. Additionally, the lateral displacement of the embankment increases with greater loading and decreases with increased depth. These suggest that the use of GRCS system can reduce lateral displacements and enhance the stability of an embankment significantly.