Test Study And Numerical Simulation Analysis Of Inflatable Anchors Mechanism

Inflatable anchor is a new type anchor, which was invented base on the soft soil's large deformation characteristics when under the external force. It comprises a rigid steel rod and an elastic rubber membrane that may be bored into place and then inflated with air or water to provide pullout resistance. The anchor system has features of inexpensive, reusable and easy to operate etc, which has broad prospective in soft soil anchoring engineering application. The study of the same new anchor has just started in foreign countries, but our country has not begun so far. Compared with general anchors, there is obvious difference on bearing mechanism and design theory, so it has important engineering significance and theory value to study more.First, from the model test beginning, based on the performance requirements of inflatable anchor, a set of inflatable anchor model and the corresponding test equipment model has been established, and a series of expansion tests and pullout tests that are embedded in both dry sand and saturated softy soil have been carried out. According to the deformed character of the inflatable anchor, displacement meter and grouting consolidation method were measured separately, the result shows the deformed body of the anchor inflated in the sand and soft soil is approximately a cylinder that the top is a bit bigger than the bottom, so the relationship among inflated pressure and volume calculated by the cavity expansion theory can be used to calculate the ultimate inflatable pressure. The pullout test shows the main factors that affect on inflatable anchor's bearing capacity are as follow: inflation pressure, expanded volume, soil density, embedded depth, length, thickness and roughness of the rubber membrane etc. To the sandy soil, the most significant factors are the inflation pressure and soil density, which grow exponentially into the relationship. The secondary factor is the rubber membrane length, which grows into a linear relationship. Roughness of the rubber membrane anchor has great influence on the increase of bearing capacity, while embedded depth of the anchor could not increase bearing capacity significantly. To the soft clay, when inflation pressure is smaller, pressure has bigger influence on the bearing capacity. When the inflation pressure reaches the limit expansion pressure, capacity expansion diameter becomes the main factor that controls the bearing capacity.Next, based on the reviewing and conclusion of the classical cavity expansion theory, applying Modified-cam-clay model for small deformation theory in the elastic region and for large deformation theory in the plastic zone, and simplifying effective post-yield stress path to a linear relationship, the formulas of the stress filed, displacement, pore pressure, and radius of the plastic region are deduced when the saturated clay is expanded under undrained condition. Based on this, the expansion mechanical characteristic is analyzed, and the theoretical formula of the surrounding soil's stress filed, displacement and the limit inflated pressure is derived under inflated expansion. And the law of the soil pressure and displacement is analyzed by classical calculation. Utilizing numerical software FLAC3D, an inflated model is established and the deformation of saturated soft clay and mechanical character are simulated based on modified-cam-clay model. The result is accord with theory analysis, which tested the exactness of theory analysis. Based on theoretical analysis and numerical simulation, the over-consolidation ratio, internal friction angle and shear modulus and other factors that affect on the limit expansion pressure of inflatable anchor were discussed, which showed that the soil stress history affects significantly at most on expansion pressure. With the increases over-consolidation ratio, the expansion of the limit soil pressure increases rapidly.Base on the model test, the mechanical characteristics in the uplift process and failure mode of inflatable anchor were analyzed, and the characteristics of load-displacement curves were studied particularly by dividing uplift process of inflatable anchor to four phases. The uplift bearing capacity formula was built. The result of the calculated value is well with the test result proved the usefulness of the formula. With the numerical simulation method, the law of the soil press before and after pullout was analyzed. The deformation character of the rubber membrane and the shear-displacement stress distribution of the anchor rod and load transfer mechanism also have been studied. The contrast of numerical simulation pullout profile and pullout tests profile states numerical analysis can reflect the pullout mechanic characteristics of inflation better. The dissipation of pore water pressure which affected on the ultimate bearing capacity was discussed.Finally, the practically use of the inflatable anchor was discussed. Although the inflatable anchor has larger bearing potential compared to similar anchor bolt, it has its flaws, such as its ultimate displacement is too large and rubber membrane's tension strength is limited, so there are two improved methods were designed:the method of strengthening steel and the consolidation of grouting method. The test result shows that:reinforcing steel inflatable anchor would transfer the load from rubber membrane to the wire, which increased the ultimate bearing capacity of inflatable anchor; while grouting consolidation type can effectively control its displacement and increase bearing capacity. Improved inflatable anchor would have good application prospects.