Xuguang Chen

In Situ Observation and Model Test on Zonal Disintegration in Deep Tunnels

The mechanical behavior of deep rock masses differs from that of shallow rock masses. Deep rock masses are classified into alternating fractured and intact zones. This unique geological phenomenon is known as zonal disintegration. It is difficult to explain the formation mechanism of such phenomenon under the framework of traditional rock mechanics. The fractured shape and forming conditions of this occurrence also remain unclear. In our present study, we conducted an exploration in deep tunnels of the Dingji coal mine in China to investigate the forming mechanism of zonal disintegration. This phenomenon was observed using a borescope TV. A geomechanical model test was then constructed based on the results of the exploration. An analogical material referred to as barites–iron–sand cementation analogical and optical scale multi-point displacement meters were developed independently for the model test. The model test was then used to validate zonal disintegration and monitor the formation process. The strain and displacement laws of the surrounding rocks during zonal disintegration were obtained and found to be non-monotonic. The fractured shape of zonal disintegration was determined, and the radii of the fractured zones were found to fulfill the relationship of geometric progression. The experimental results were in accordance with the in situ exploration findings. The mechanism of the zonal disintegration was revealed by theoretical analysis based on fracture mechanics. The fractured zones are reportedly circular and concentric to the cavern. Each fracture zone ruptured at the elastic–plastic boundary of the surrounding rocks and then coalesced into the circular form. The geometric progression ratio was found to be related to the mechanical parameters and the ground stress of the surrounding rocks.

Model Test of Anchoring Effect on Zonal Disintegration in Deep Surrounding Rock Masses

The deep rock masses show a different mechanical behavior compared with the shallow rock masses. They are classified into alternating fractured and intact zones during the excavation, which is known as zonal disintegration. Such phenomenon is a great disaster and will induce the different excavation and anchoring methodology. In this study, a 3D geomechanics model test was conducted to research the anchoring effect of zonal disintegration. The model was constructed with anchoring in a half and nonanchoring in the other half, to compare with each other. The optical extensometer and optical sensor were adopted to measure the displacement and strain changing law in the model test. The displacement laws of the deep surrounding rocks were obtained and found to be nonmonotonic versus the distance to the periphery. Zonal disintegration occurs in the area without anchoring and did not occur in the model under anchoring condition. By contrasting the phenomenon, the anchor effect of restraining zonal disintegration was revealed. And the formation condition of zonal disintegration was decided. In the procedure of tunnel excavation, the anchor strain was found to be alternation in tension and compression. It indicates that anchor will show the nonmonotonic law during suppressing the zonal disintegration.

Analogical model test and theoretical analysis on zonal disintegration based on filed monitoring in deep tunnel

Field monitoring carried out in a deep tunnel of the Dingji coal mine in China confirmed the zonal disintegration phenomenon by using the borehole TV. Based on field monitoring, an analogical model test was conducted to research the fracture shape and forming conditions of the rock mass in the Dingji mine. To perform the model test, an analogical material and optical sensor were developed independently. Through the test, the occurrence of zonal disintegration was confirmed and the forming process was monitored. The fracture pattern of zonal disintegration was determined, and the radii of the fractured zones were found to fulfil the relationship of geometric progression. The displacement laws of surrounding rocks during zonal disintegration were obtained and found to be non-monotonic. The test results are in agreement with the field-monitoring results. Through a theory analysis based on fracture mechanics, the mechanism of zonal disintegration was revealed. The fracture zones occur as circles concentric to the cavern periphery, which is the “false face”. Each fracture zone ruptures at the elastic–plastic boundary of surrounding rocks and then coalesces into a circle. The geometric progression ratio was determined; it is related to the mechanical parameters and ground stress of the surrounding rocks and calculated as follows: And the mechanism of the non-monotonic displacement law is revealed; the continuous formation of the “false face” causes the geostress redistribution and crack opening.

Investigation of scour effects on lateral behaviors of suction caisson

ABSTRACT The use of suction caissons can reduce the development costs of offshore wind energy and has broad application prospects. However scour around marine foundations is inevitable, it gravely affects the stability of marine engineering. Therefore, there is an urgent need to study the weakening effects of scour on suction caisson. In this study, the variation trends of remaining soil parameters (the effective unit weight and the peak effective friction angle) after scour are examined with consideration of the dilatancy and stress history of sandy silt. It is found that the parameters of shallow soil change considerably after scour, and the larger the scour depth, the greater is the change in the parameters. However, the deep soil is less affected. On the basis of these findings, scour effects on the ultimate moment capacity of suction caisson are studied. The ultimate moment capacity is found to greatly reduce under scour, and its calculated value is larger than the actual value when the effects of dilatancy and stress history are ignored. To simplify calculation, it is feasible to replace the ultimate moment capacity when both dilatancy and stress history are considered with that when only dilatancy is considered.

Stability analysis of suction bucket foundations under wave cyclic loading and scouring

ABSTRACT Suction bucket foundation is a typical type for offshore turbines. Scour caused by wave and current can reduce the stability of foundation and then endanger the whole structure. This paper details a series of suction bucket model tests performed in sand under wave cyclic loading. The model tests investigate the effect of scour on stability of bucket foundation by artificially excavated scour hole around the foundation. It is revealed that the behavior of foundation bearing capacity can be divided into two stages: the initial cyclic stage and the final stage (showing either cyclic stability or cyclic failure). When the wave circulation is stable, the sand on the front and back sides of the foundation is suspected to be liquefied. With the increase in scour depth, the stability of foundation is gradually reduced, the behavior of foundation gradually changes from a state of cyclic stability to cyclic failure, and the number of waves that can be withstood is drastically reduced. Finally, the height of the center of rotation of the suction bucket was observed to descend with the increase in scour depth.

Scaled Physical Model Test on Underground Gas Storages Based on a Pneumatically Flexible Loading System

A geomechnical model test was conducted to study the stability of a salt cavern during the gas injection/recovery period. In order to simulate the internal pressure in the cavern and accurately exert the geostress, a numerical controlling pneumatically flexible loading model test system was developed. This loading system also realized the automatically loading digitalization control and visualization display. The model cavern was built using a wooden mold and was set within a latex balloon. This designation realized the injection and extraction of the gas, which simulated the variation of gas pressure in prototype engineering. During the model test, the precisely optical-based sensors were buried into the model to observe the deformation of the cavern. The deformation laws of the cavern were separately obtained at the gas stability, gas injection, and gas extraction stages. In addition, the optimal gas pressure and safe gas pressure were determined through the deformation law versus the gas pressure. Through analysis, the gas extraction and the rate of gas injection/extraction were identified to be the most important factors that threaten the stability of the salt cavern. The model test also demonstrated that the middle layer was the most at-risk section during the operation period compared to the other regions.