Research On Mechanical Behaviors And Peak Displacement Prediction Of Steel Casing Composite Piles Under Lateral Impact Loading

Bridges are threatened by various collisions,and the pile foundations may also undergo deformation beyond limits or even failure,thus endangering the bridge’s safety.Moreover,owing to the construction method of drilling cast-in-place piles,many piles of Chinese bridges are in the form of steel casing composite piles(SCC piles).The steel casing changes some mechanical properties of the original reinforced concrete(RC)pile under the interaction.Current research rarely takes the steel casing and RC pile as a composite structure into the collision analysis,which results in the unclarified mechanical properties of SCC pile under the lateral impact loading.However,once the SCC pile suffers a lateral impact,the failure modes and location of the pile shaft may be more difficult to identify due to the effects of steel casing,thus laying a structural safety hazard;there is also the possibility of causing large pile head displacement,resulting in the pile not satisfying service requirements anymore.Therefore,the mechanical characteristics of SCC monopiles under lateral impact loading are investigated in-depth through theoretical analysis,lateral impact and static loading tests,and finite element(FE)simulations.The prediction method for the peak pile head displacement during the collision and anti-impact design are also addressed.The main contents and conclusions of this study are as follows.(1)Tests on three sets of six SCC piles,FE simulations,and parametric analysis are carried out to reveal the effects mechanism of steel casing on the mechanical behavior of RC pile under lateral loading.The results show that the normal contact force between the steel casing and RC shaft is the direct reason causing the failure modes to transform from bending to bending-shear when the length of steel casing is short.Completely covering the equivalent plastic zone of the original RC pile by the steel casing is necessary to effectively and significantly improve the performance of the SCC pile.Moreover,the optimum lengthto-diameter ratio for a steel casing is proposed,and the related formula is established,which is the basis for classifying SCC piles.(2)Pendulum impact tests on 21 SCC piles and static loading tests on five corresponding pile specimens are conducted to clarify the impact behaviors of the SCC pile.The test results show that in the presence of the upper mass(during the service stage),piles with the optimum casing length-to-diameter ratio failed in the cap-casing joint owing to greater inertia.In the absence of the upper mass(during the construction stage and the structure over the pile is unbuilt),piles satisfying the optimum steel casing length-todiameter ratio suffer flexural failure within the steel casing–RC segment,while the piles not satisfying it develop more brittle damage in the uncased RC segment nearing the casing bottom,which may change from bending to bending-shear failure with increasing impact velocity.In addition,the damage location,deformation patterns,and the relationship between the absorbed energy and pile head displacement are similar for an identical pile under lateral impact and static loading.(3)The FE model of the SCC pile subjected to lateral impact loading is established to solid the base for the theoretical method.The mechanical behaviors of the SCC pile under lateral impact and static loading are also compared.The results show that when the pile reaches its peak displacement during the collision,the impact energy absorbed by the structure mainly transfers to the deformation energy of the pile and soil.At this time,the energy dissipation ratio,internal force distribution,and deformation characteristics of the pile are also the same as those under an equivalent static load that results in the same displacement.But the internal force of the critical section is higher under the impact loading due to the material strain rate effects.Therefore,by considering the strain rate effects with the static analysis results of the pile,the relationship between the deformation energy and peak pile head displacement under impact loading can be reasonably derived.In addition,the deformed shape of SCC piles has two categories: single and double plastic hinges.(4)Based on the deformation and energy absorption characteristics of the SCC pile under static loads and incorporating material strain rate effects in the cross-section analysis,the maximum absorbed impact energy-pile head peak displacement relationship is derived.Then,an energy-based method for predicting the peak pile head displacement of the SCC pile under lateral impact is proposed.The method allows efficient prediction of peak pile head displacement under various collision scenarios by only running a structural static loading analysis and several sectional moment-curvature analyses.The proposed prediction method has been validated against experimental and numerical simulation results with a deviation within 15% and low dispersion.(5)Based on the predicted peak displacement under the lateral impact,a ratio index of the pile head displacement is proposed and employed to classify the level of impact-induced damage and anti-impact performance of the SCC pile.The yield pile head displacement under the lateral static loading is taken as the dimensionless divisor,which effectively avoids the influences of pile size on the classification values.Then,an anti-impact design for the SCC pile is proposed based on the ratio index.By combining the peak displacement prediction method and ratio index under the lateral impact loading,the proposed design method can provide a reference for the collision analysis of the SCC pile.