Analysis And Control For Upward Movement Of Shield Tunnel During Construction

It has been verified by many practical engineering that segments just out fromshield tail often take place partial or whole upward movement in the process of shieldtunneling. The upward movement leads to staggering of segment joints, cracks,disrepair or even axes deflection and the case would be especially serious when theshield goes through the area with shallow overburden. This paper analyzed thereasons for upward movement of the segments and the corresponding calculatingmethods.The buoyancy was divided into static buoyancy and dynamic buoyancy in thepaper. The static buoyancy was generated by the segment rings immersing in theliquid, such as slurry, grouts or underground water. And the dynamic buoyancyoriginated from the pressure in the grouting process, which brought about thesegments upward movement, staggering, crack, or other forms of damage. Thediffusion process of back-filled grouting was analyzed and divided into four phases,including fill, penetration, compaction and hydrofracture. The penetration andcompaction were considered as the main factors that originated pressure andbuoyancy to the segments.On the basis of Maag's spherical surface diffusion formula and cylinder surfacediffusion formulas, and with the assumption of half-spherical and arc surfacediffusion models, the grouts diffusion radius and the pressure to shield tunnelsegments were studied by substituting the intrinsic void percent of soil with theequivalent void percent to consider the existing of construction gap. As a result, thecorresponding formula was acquired. It is shown that the grouts diffusion radius andthe pressure to segments are related to many factors, such as the grouting pressure,grouting duration, grouts viscosity, soil permibility, thickness of the construction gap,radius of the grouting pipe, etc. And the relationship between them was also discussedfor engineering practice.The partial upward movement controlling model was proposed in the paper, which was used to analyze the effects of segments, bolts and the overburden, whenone segment (or several segments) or one segment ring (or several segment rings)experienced buoyancy. Then, a single segment upward movement control formulaand a whole segment ring upward movement control formula were put forward,respectively. The former adapts to the condition that the buoyancy concentrates on thebackside of one segment or several segments. And the latter is fit for the conditionthat one whole segment ring experiences buoyancy.The longitudinal upward movement controlling model was proposed, which wasused to analyze the force and distortion of the segments and ground whenexperiencing buoyancy, by assuming the shield tunnel segments as a long beam. Thetransverse rigidity efficiency and inflection factors were considered in thelongitudinal equivalent continuous model, so the transverse rigidity and longitudinalrigidity could be unified. Engineering practice shows that the longitudinal rigidity hasa close relationship with the transverse rigidity in proportion to it. In addition, theupward movement characteristic of shield tunnel in construction was simulated withFEA method.Finally, some problems and further studies were discussed.