Study On Macro And Meso Mechanism Of Construction-induced Interfaces And Real-time Compaction Quality Assessment Of Roller Compacted Concrete Dam

In the construction of roller compacted concrete(RCC)dams,concrete with large area is compacted by vibratory rollers in a layer-by-layer manner.Such a unique construction technique determines that the compaction quality and interlayer bonding strength are key factors influencing the safe operation of RCC dam.This leads to two significant scientific problems for ensuring the construction quality of RCC dams:(1)how to reveal the layer forming mechanism of RCC dams from the aspect of construction control,and(2)how to decide appropriate construction parameters according to the varying conditions of construction sites.Existing research mainly adopted small-scale rolling tests or vibration compaction molding tests of specimen to investigate the mechanism of interlayers of RCC dams.These testing methods have several aspects of problems,which include(1)the scale effect of roller drum;(2)the obvious difference between the plate vibration and actual rolling vibration;and(3)the failure to reveal meso mechanism of the embedding and interface bonding behavior of RCC interlayer aggregate under a precise simulation of actual construction condition.Although existing technology in monitoring dynamic rolling parameters is able to ensure the real control of rolling process,it fails to monitor the RCC compaction and interlayer bonding quality directly.In addition,despite there have been studies using certain compaction indexes to monitor and assess compaction quality in real time,few of them has investigated the applicability of these compaction indexes in RCC dam material.To address the limitations of existing research,it is necessary to(1)study the macro and meso mechanism of layer forming during RCC construction with reliable indoor simulation measures,(2)propose a monitoring index and its control criterion that is able to directly characterize the comprehensive effect of multiple compaction parameters on interlayer bonding quality under varying jobsite construction conditions,e.g.VC(vibrating-compacted)values of concrete placement,and(3)further investigate the key technology of real-time assessment of RCC compaction quality,so as to develop a new theory and method of RCC compaction quality control.The achievement of the above-mentioned research objectives has both theoretical significance and engineering practical values for ensuring construction quality of RCC dams,which can provide strong technical support for lean construction and long-term safe operation of RCC dams.According to the control requirements for RCC compaction and interlayer bonding quality,this dissertation comprehensively analyzed the influence mechanism of compaction process on RCC layer forming by combining macro simulation compaction tests and meso discrete element simulations.Via simulating the actual construction conditions,the effect of compaction parameters and the quality control criteria of RCC interlayer bonding were studied.Real-time monitoring indicators of RCC compaction quality were analyzed and improved,based on which a real-time comprehensive assessment method of RCC compaction quality was proposed.The studies provide a new way for the lean control of RCC dam construction quality under changing construction conditions.Main research results of this dissertation are as follows:(1)A multi-parameter adjustable laboratory device for simulating RCC compaction was developed.This device is not only able to simulate the construction process of roller compaction,but also able to adjust different compaction parameters(roller static load,excitation force,roller velocity,compaction passes,etc.)in multiple levels.It solved the problem that the plate vibration mode of vibration compaction forming test fails to reflect the actual rolling process and the shortage of roller scale-reduced effect for the compaction test using small-scaled roller.In addition,it is more time-saving and cost-effective to perform lab-scale simulation with the device than actual field compaction test.The developed device provides an economical,convenient,and effective platform to study the influence of different compaction parameters on the compaction quality of RCC dam materials and then optimize the compaction parameters.The device underpins the subsequent meso mechanism analysis of RCC layer surfaces and real-time evaluation of compaction quality.(2)A discrete element modeling method was proposed to simulate the layered construction process of RCC.The method includes(a)the particle model generation of RCC aggregates according to the actual irregular shapes and gradation,(b)the modeling simulation procedure of the entire process of RCC layered construction,and(c)the determination of meso contact parameters and equivalent load of roller drums.A PFC case study was implemented based on an actual simulated compaction test.By the case study analysis,a RCC double-layer compaction model was established,which is consistent with the actual dam material characteristics(mix proportion,aggregate gradation,porosity,etc.)and the entire layered construction process.The model lays the foundation for subsequent meso mechanism analysis of layer forming during RCC construction.(3)The meso mechanism of layer forming during RCC construction was analyzed based on discrete element simulation.The concept of embedded value of interlayer aggregate and its calculation method were proposed.The generation of PFC model for RCC core samples containing interlayer contact faces,and a meso simulation method for direct shear test was proposed.On this basis,the influence of different compaction parameters was analyzed on the embedded value of RCC interlayer aggregates,the compaction characteristics of upper layer dam material,the stress variation of interlayer dam material and the interlayer bonding quality.The above analysis revealed the mechanism of layer binding performance,which can not only enrich the technical methods on investigating mechanisms of layer forming during RCC construction,but also be used as an effective supplement for the physical test.In addition,it provides theoretical support for subsequent research on the control criteria for RCC interlayer bonding quality while adopting the improved unit compaction work indicator,which reflects the comprehensive effects of compaction parameters.(4)Based on the meso mechanism analysis of layer forming during RCC construction and considering the characteristics of RCC materials,the improved unit volume compaction work and improved unit area compaction work indicators considering VC values were proposed.Simulation compaction tests of RCC materials with different VC values were conducted with the developed multi-parameter adjustable device.Quantitative relationship was analyzed between the improved unit compaction work indicators and the RCC compaction quality and interlayer bonding quality.Evaluation models of RCC compaction and layer bonding quality were established based on unit compaction work.A method for determining real-time control criteria of compaction parameters was proposed to address the variation of VC values in construction sites.The research findings in this chapter can provide a new way for the lean control of RCC dam construction quality under changing construction conditions.(5)Multiple improved frequency-domain acceleration indexes and a stiffness indicator considering the lag phase angle were proposed.Correlation between the proposed indexes and compaction passes was analyzed.Relationship models between the proposed indexes and the compaction density of dam material were established,of which the relative error in predicting compaction density was statistical analyzed.Results show that the proposed indexes were able to monitor and evaluate the compaction quality of RCC dam materials with high precision and good stability,which demonstrates their potential to improve the real-time characterization accuracy of RCC compaction quality,and to provide a theoretical basis for real-time comprehensive assessment of RCC compaction quality.(6)Based on XGBoost algorithm,a real-time comprehensive assessment model of RCC compaction quality was established,which takes into account compaction parameters,frequency-domain amplitude and time-domain variables.A real-time procedure for RCC compaction quality assessment was proposed,of which the prediction accuracy was analyzed.The compaction quality at arbitrary position of the entire compaction area was evaluated by inverse distance space interpolation method,which can provide a new way for improving the accuracy of RCC compaction quality assessment.