Analysis of jacking forces during pipe jacking in granular materials using particle methods

Trenchless technology is often used in congested urban areas or river crossings to install underground pipelines to minimize disturbance to surface traffic or other activities.Pipe jacking is a typical technique applied to jack pipe segments between two working shafts.However,the design of the jacking force is usually implemented using empirical methods.It should be emphasized that the jacking force will change for each site,depending on the magnitude of overcut,lubricants,work stoppages,geology and misalignment.A particle method is proposed to estimate the jacking force along the pipe.The microparameters are calibrated for sandy soils in Shenyang,so that the macroscale material behavior can be reproduced using the particle model.Hence,the normal force around the pipe circumference can be derived in the particle model,after which the interface friction coefficient is applied to evaluate the friction resistance mobilized at the soil-pipe interface.A modified Protodyakonov’s arch model can be used to assess the magnitude of earth pressure acting on the shield face.In the end,the combination of friction resistance and face pressure provides the jacking force.The efficacy of the proposed particle method is demonstrated by comparing calculated jacking forces with those measured in the field for three types of jacking machines in sandy soils under the Hun River,Shenyang.

Strengthening of Ultrafine Lamellar-Structured Martensite Steel via Tempering-Induced Nanoprecipitation

An ultrafine lamellar-structured martensite steel fabricated by heavy warm rolling(HWR)has shown an excellent combination of strength and ductility.By appending tempering at 400℃to HWR,we show that the comprehensive mechanical property of a lamellar-structured low-carbon martensite steel can be further improved to reach a yield strength of~1.8 GPa,an ultimate tensile strength of~2.0 GPa and a total elongation of~9.3%.This is achieved by tempering the HWR steel from 300 to 750℃,and the optimum tempering temperature is thus obtained.We find that the tempered ultrafine lamellar martensite contains high-density nanoprecipitates dispersed within the aligned martensite laths with reduced crystallographic variations.The ultrahigh strength of the steel is rationalized as mainly the result of grain boundary strengthening and precipitation strengthening,which contribute to yield stress by 610 MPa and 440 MPa,respectively.The good ductility is believed to be closely related to the capacity of the tempered grains to accommodate dense dislocations upon plastic deformation.The present thermomechanical processing provides a feasible routine for producing steels with ultrahigh-strength and good-ductility.

Granular Carbides-Assisted Ultrafine-Ferrite Fabrication in the Pearlitic Steel Without Severe Plastic Deformation and Annealing

The evolution of ferrite grain and cementite lamella during cold rolling in a granular carbide–pearlite steel has been investigated.Particular attention has been given to a quantitative characterization of changes in the ferrite grains.Electron backscattered diff raction and transmission electron microscopy observations show that the ultrafi ne ferrite(~388 nm)can be produced through low equivalent strain cold rolling without severe plastic deformation(SPD)and annealing.The average grain size of ferrite depends on the volume fraction,shape and distribution of granular carbides as well as interlamellar spacing of pearlite.A general explanation of granular carbides-assisted grain refi nement is that the embedded carbides between natural barrier will signifi cantly facilitate dislocation nucleation during cold rolling.Dislocation reaction occurs more drastically and quickly near these granular carbides.Such reactions promote the formation of high-angle grain boundaries.The formation of ultrafi ne ferrite grains and subgrains in steel after cold rolling toε=1.4 strain makes the strength and ductility increased simultaneously compared withε=0.6 cold-rolled steel.The results suggest a new material design strategy to obtain ultrafi ne-grained structure via the granular carbides assistance.