The Filed Reduced-scale Model Test And Stability Research On The Tunnel Anchorage In Soft Rock

Anchorage is one of the critical components in a suspension bridge because its stability is a determining safety element.There is a suspension bridge over the Yangtze River and its tunnel anchors are proposed to be installed in a mudstone layer characterized by soft rock,shallow burial,and reservoir water influence.The soft rock is known for its softening in the presence of water and appreciable creep,so under high pulling loads the bearing capacity and long-term stability of tunnel anchorage represent critical factors.But till now systematic investigation in this respect is rare.This paper presents a study that had as its investigation object the tunnel anchorage of the said bridge.The bearing capacity behavior and the long-term stability of soft rock tunnel anchorage have been investigated systematically and in depth by means of a variety of techniques,including a geological survey,rock physical and mechanical tests,in-situ scaled-down anchorage tests,and numerical simulations.The major work and findings of the study are as below:(1)The basic physical and mechanical properties of the test zone and anchorage zone rocks were found by laboratory and large-scale in-situ mechanical tests,which made it possible to obtain mechanical parameters of anchorage mudstone and to understand to what extent water affects rock mechanical properties.(2)Natural state(non-soaked)1:10 in-situ scaled-down anchorage tests were conducted under design load(1P),overload(>1P),and ultimate load levels,revealing the deformation behavior and spatial distribution pattern as well as the failure behavior and failure mode of soft rock tunnel anchorage with a shallow burial depth.Displacement vs.time curves corresponding to different load levels were constructed from the model anchorage creep test,and the variation of model anchorage creep as a function of load level was established by analysis.(3)Natural state(non-soaked)and soaked 1:30 in-situ scaled-down anchorage tests were carried out at design load(1P),overload,and ultimate load levels,revealing the pattern and the extent of the influence that water may have on the displacement and characteristic strength of soft rock tunnel anchorage.In natural and soaked states,the failure behavior and mode of the tunnel anchorage are essentially the same,but the presence of water has a relation with a greater displacement and an appreciable drop in characteristic strength.Displacement vs.time curves corresponding to different load levels were constructed from creep tests,and the variation of creep in soaked state as a function of load level was established by analysis.(4)A comparison of the test results of models having various scaling factors indicated that for a given water moisture the characteristic strength grows with a decreasing scaling factor C(i.e,larger in model size).Scaling factor and moisture condition make no meaningful difference to displacement behavior & its spatial distribution pattern,failure behavior,or failure mode.(5)With the results of in-situ scaled-down anchorage tests known,the model parameters were identified using uniform design and PSO-LSSVM geotechnical displacement reverse analysis techniques.Mudstone creep parameters were found with the help of viscoelastic displacement inversion theory.Similarly,mudstone elastoplasticity parameters were estimated by use of elastoplastic displacement inversion theory.(6)A 3D elastoplastic numerical simulation that had been performed on the real tunnel anchorage at design load(1P),overload,and ultimate load levels provided an insight into the stressing,displacement variation,and distribution patterns,the development and distribution pattern of the plastic zone,and the load capacity.At design load,the rock in the neighborhood of the anchor is on the order of mm and the maximum anchor displacement is 2.5mm.The neighborhood rock mass is largely in an elastic state,and the surrounding rock is in a satisfactory stressing state,suggesting the anchor is in a stable surrounding rock.The bearing capacity of the real tunnel anchor is 8P and 7P respectively if 187 m and 197 m reservoir levels are considered.From the in-situ scaled-down anchorage test results and the 3D elastoplastic simulation findings,a safety factor of 3.5 and a safety margin of 6 are recommended for the real tunnel anchorage having regard to the rising of the reservoir level.(7)Further,with inversion creep parameters the real tunnel anchorage was studied for its long-term stability performance by addressing the real tunnel anchorage in viscoelasticity numerical analysis.The creep stabilizes 6 months after the application of design load(1P)for a water level of 197 m or 187 m.In the case of 197 m,both the transient and long-term displacements of the tunnel anchorage are slightly higher than the case of 187 m.The surrounding rock is essentially in an elastic state except for a limited zone before and after the anchor,which is plastic.Therefore,after the water level rises to 197 m the tunnel anchorage still satisfies the long-term stability requirement.