海底犁式挖沟机犁体动力学数值仿真分析

有效模拟海底犁式挖沟机犁体土壤挖掘过程及分析犁体受力情况对整机结构优化及提高操控性具有重要意义.本文分析了线性Drucker-Prager塑性屈服准则的土壤弹塑性本构方程,结合一阶线性水波对海底土壤的动压力方程,建立犁体、海底土壤及海流的动力学方程,采用ABAQUS无网格粒子法进行仿真模型的数值求解.通过陆地试验结果验证基于无网格粒子法的犁体挖掘土壤仿真模型的可行性.针对海底作业情况仿真,结果表明水波对犁体牵引力产生正弦影响.同时揭示犁体作业时土壤的应力变化过程,随着土工参数值的增加,犁体牵引力呈非线性增加,其中弹性模量影响较大,不排水剪切强度及摩擦角影响近似.

海上风机大直径单桩动态打桩过程数值仿真分析

在海上风电场建设中,基础施工是项目建设中最重要的关键环节之一,因此研究超大直径单桩动态打桩过程中桩-土-水相互之间的耦合效应有非常重要的意义。分析了海上风机大直径单桩的动态打桩过程,以江苏启东H3海上风力发电场建设过程中的沉桩为工程背景,使用商用有限元软件LS-DYNA进行建模,基于多物质ALE流固耦合法,通过定义多物质材料和流固耦合关键字建立了流体(海水和土壤)和固体(桩体)之间相互作用的耦合机制。对动态打桩过程中土体的应力、应变、体积分数以及沉桩阻力的变化规律进行了仿真分析,发现在桩端附近的土体有明显的应力集中区,应力分量的等值线具有类似于“应力泡”的层状结构,这表明桩端土体具有类似球形的扩张形式。随着贯入深度的增加,桩端处的压应力也在逐渐增大,但各应力集中区的影响范围和等值线的轮廓形状变化不大,说明动态打桩过程对土壤的影响范围是有限的。每次击打过后,由于应力波的反射作用和土体受压缩出现的塑性屈服,桩体会先向下移动一段距离再向上回弹一段距离,从而导致沉桩阻力先增大后减小,直到下一锤击又开始。为海上风机大直径单桩的动态打桩过程提供了数值仿真建模参考和实用的分析方法。

Slope analysis based on local strength reduction method and variable-modulus elasto-plastic model

Employing an ideal elasto-plastic model,the typically used strength reduction method reduced the strength of all soil elements of a slope.Therefore,this method was called the global strength reduction method(GSRM).However,the deformation field obtained by GSRM could not reflect the real deformation of a slope when the slope became unstable.For most slopes,failure occurs once the strength of some regional soil is sufficiently weakened; thus,the local strength reduction method(LSRM)was proposed to analyze slope stability.In contrast with GSRM,LSRM only reduces the strength of local soil,while the strength of other soil remains unchanged.Therefore,deformation by LSRM is more reasonable than that by GSRM.In addition,the accuracy of the slope's deformation depends on the constitutive model to a large degree,and the variable-modulus elasto-plastic model was thus adopted.This constitutive model was an improvement of the Duncan–Chang model,which modified soil's deformation modulus according to stress level,and it thus better reflected the plastic feature of soil.Most importantly,the parameters of the variable-modulus elasto-plastic model could be determined through in-situ tests,and parameters determination by plate loading test and pressuremeter test were introduced.Therefore,it is easy to put this model into practice.Finally,LSRM and the variable-modulus elasto-plastic model were used to analyze Egongdai ancient landslide.Safety factor,deformation field,and optimal reinforcement measures for Egongdai ancient landslide were obtained based on the proposed method.

A nonlinear multi-field coupled model for soils

A nonlinear multi-field coupled model for multi-constituent three-phase soils is derived by using the hybrid mixture theory. The balance equations with three levels (constituents, phases and the whole mixture soil) are set up under the assumption that soil is composed of multi-constituent elastic-plastic solid skeleton (which is different from the linearization method) and viscous liquid and ideal gas. With reasonable constitutive assumptions in such restrictive conditions as the principles of determinism, equipresence, material frame-indifference and the compatible principle in continuum mechanics, a theoretical framework of constitutive relations modeling three-phase soil in both non-equilibrium and equilibrium states is established, thus the closed field equations are formed. In the theoretical framework, the concept of effective generalized thermodynamic forces is introduced, and the nonlinear coupling constitutive relations between generalized dissipation forces and generalized flows within the system at nonequilibrium state are also presented. On such a basis, four special coupling relations, i.e., solid thermal elastic-plastic constitutive relation, liquid visco-elastic-plastic constitutive relation, the generalized Fourier’s law, and the generalized Darcy’s law are put forward. The generalized or nonlinear results mentioned above can degenerate into the linear coupling results given by Bennethum and Singh. Based on a specific dissipation function, the concrete form of generalized Darcy’s law is deduced, which may degenerate into the traditional form of Darcy’s law by neglecting the influence of skeleton deformation and temperature. Without considering temperature and other coupling effects, the nonlinear coupled model in this paper can degenerate into a soil elastic-plastic constitutive model.

Organic carbon isotope and molecular fossil records of vegetation evolution in central Loess Plateau since 450 kyr

Significant uncertainties remain regarding the temporal evolution of natural vegetation during the Quaternary, and drivers of past vegetation change, on the Chinese Loess Plateau （CLP）. This study presents analyses of total organic carbon isotopic composition （TOC） and n-alkane ratios （C31/C27） from the Lingtai loess-palaeosol sequence on the central CLP over the last 450 kyr. The results demonstrate that the vegetation in this region comprised a mix of C3 and C4 plants of herb and woody growth-form. C3 plants dominated for most of the last 450 kyr, but this did not lead to extensive forest. C3 woody plants were more abundant in MIS9 （S3 period） and MIS5 （S 1 period） during warm and humid climate conditions. Herbs increased in the region since 130 kyr, possibly as a result of increased aridity. On the orbital timescales, there was a reduction of C3 herbal plants in MIS 11 （$4） than in M1S 12 （L5）, and in Holocene than in the last glacial period. Our isotope and n-alkane proxy records are in agreement with Artemisia pollen changes in the region, which is/was the dominant species in this area and varying due to different heat and water conditions between glacial and interglacial periods. Though the climate in MIS1 （SO） was similar to that in MIS11 （S4）, a significant increase in woody plants during the Holocene suggests the impact of human activities and ecological effects of changes in fire activity.