非饱和土剑桥模型的基本框架

分析了用有效应力原理和用两个应力变量建立非饱和土力学理论的区别与联系，借助于有效应力原理将饱和土的剑桥模型推广到非饱和土，并与现有的几种塑性模型作了对比分析。     

Monitored and simulated variations in matric suction during rainfall in a residual soil slope

This research combines field, laboratory and numerical investigations to estimate the development of a wetting front within a 1.2 m residual soil mantle on a steep forested slope during rainfall events. The field-monitored variations in matric suction due to rain-water infiltration during various events revealed that the maximum infiltration rate was much higher when the wetting front resided in the upper 20 cm of soil compared to the case when the wetting front advanced to depths > 20 cm. Laboratory investigations on soil hydraulic properties (i.e., soil water characteristic curve, and hydraulic conductivity) were useful to establish the parameters of a multilayer finite-element model for one-dimensional vertical infiltration. These parameters were subsequently calibrated by matching the predicted and field measured transient pore water pressure responses during actual rainstorms with irregular rainfall patterns. The calibrated simulation model was used to assess the migration of the wetting front under uniform rainfall with different intensities. Based on the numerical results, a hyperbolic equation was developed to predict the duration of uniform rainfall required for the propagation of wetting front to a certain depth for a given rainfall intensity. The proposed equation was subsequently tested against field-monitored advancements of the wetting front during real rainstorms with variable rainfall intensity.     

Large-scale experimental investigation of the installation of suction caissons in silt sand

Offshore wind power is a rapidly growing area of electricity in China. In the present paper, interaction mechanisms between the caisson for wind turbines and saturated silt sand are investigated with laboratory tests based on two different installation methods, jacking installation and suction installation. For the jacking installation process, the results indicate that the soil pressures inner and outer the skirt of the caisson vary with a similar feature and the magnitudes of the two are nearly balanced. The tip resistance plays a key role in the total jacking installation resistance. This paper examines the predictive performance of q_c method and API approach for jacking installation resistance. It is demonstrated that the q_c method provides better predictions. The resistance coefficients are recommended. For the case of suction installation, however, the changes of soil pressures inner and outer the skirt are contrasting. Specifically, the inner pressure and tip resistance fall dramatically, but the outer pressure increases when suction is applied. Seepage effect is found to be an important mechanism for the installation of suction caisson. The reduction ratios of the inner friction and tip resistance follow a power-function with the normalized suction. Based on the test results, a prediction method for the required suction has been developed and evaluated.     

Numerical Simulation of Installation Process and Uplift Resistance for An Integrated Suction Foundation in Deep Ocean

A concept design, named integrated suction foundation, is proposed for a tension leg platform (TLP) in deep ocean. The most important improvement in comparing with the traditional one is that a pressure-resistant storage module is designed. It utilizes the high hydrostatic pressure in deep ocean to drive water into the module to generate negative pressure for bucket suction. This work aims to further approve the feasibility of the concept design in the aspect of penetration installation and the uplift force in-place. Seepage is generated during suction penetration, and can have both positive and negative effects on penetration process. To study the effect of seepage on the penetration process of the integrated suction foundation, finite element analysis (FEA) is carried out in this work. In particular, an improved methodology to calculate the penetration resistance is proposed for the integrated suction foundation with respect to the reduction factor of penetration resistance. The maximum allowable negative pressure during suction penetration is calculated with the critical hydraulic gradient method through FEA. The simulation results of the penetration process show that the integrated suction foundation can be installed safely. Moreover, the uplift resistance of the integrated suction foundation is calculated and the feasibility of the integrated suction foundation working on-site is verified. In all, the analysis in this work further approves the feasibility of the integrated suction foundation for TLPs in deep ocean applications.     

西藏贡觉县泥石流灾害研究

西藏自治区贡觉县境内对居民存在影响的泥石流沟有3条，即者龙洼Ⅰ沟，者龙洼Ⅱ沟，克西林沟，均位于该县东南部的高山峡谷区。泥石流沟长度均超过2km，沟床平均纵坡降130‰，地表松散物质的厚度都在6．5m以上，其来源有所不同，克西林沟泥石流的固体物质来源于沟谷两侧的也古、拉巴滑坡堆积物；者龙洼Ⅰ、Ⅱ泥石流的固体物质来源主要为岩石风化物质及崩、坡积物。固体物质颗粒粒径分布范围很广，分布曲线均呈三峰型。说明该区泥石流中固体物质含量较高。贡觉县境内泥石流的诱发因素是降雨，为降雨型泥石流。文章根据非饱和土强度理论对贡觉县降雨型泥石流的成因机理进行了研究，提出降雨型泥石流的形成过程可以划分为2个阶段，第1个阶段为基质吸力引起的抗剪强度丧失阶段，与前期实效降雨量有关；第2个阶段为泥石流发生阶段，与短历时强降雨有关。运用上述理论方法，对降雨型泥石流的预测模型进行了探讨。建议用临界雨量线模型框架来建立贡觉县泥石流的预测模型，并依据者龙洼Ⅱ沟泥石流爆发前20d的降雨量数据进行拟合分析，结果表明：文中建议的2个临界雨量线模型框架均适于用建立贡觉县降雨型泥石流暴发的预报模型。     

Failure mode and pullout capacity of anchor piles in soils with cohesive and cohesionless properties

Abstract

The present work develops a theoretical model based on a rational mechanical model and the failure mechanism of anchor piles in the seabed, by which the failure mode and pullout capacity of anchor piles under inclined loading can be predicted in the soils with both cohesive and cohesionless properties. Experimental and numerical results are employed to validate the theoretical predictions. Parametric studies are performed to investigate the effects of different parameters on the failure mode and pullout capacity of anchor piles, to demonstrate the applicability and efficiency of the theoretical model and to gain further knowledge of the anchor properties. An analytical method is also proposed to evaluate the optimal position of the attachment point of anchor piles, and confirmed by relevant studies in either cohesive or cohesionless soils.

1. Highlights

2. A novel theoretical model is proposed to analyze the failure mode and pullout capacity of anchor piles.

3. The model is applied to inclined loading and to soils with both cohesive and cohesionless properties.

4. Efficiency and applicability of the model are validated through comparative and parametric studies.

5. A simple expression is proposed to predict the optimal position of the attachment point for anchor piles.

     

数值优化方法在确定张力计反应时间上的应用性

张力计的反应时间可用于估算非饱和土的渗透系数,并可作为张力计选型的评判依据。为了更加合理且便捷地确定张力计的反应时间,先通过不同含水量条件下的泾阳黄土、定边黄土以及毛乌素砂土的土柱试验测定张力计的响应曲线,再利用半经验图解法与结合通用全局优化(UGO)算法的数值优化方法来确定反应时间,并探讨了数值优化方法的实用价值及其局限性。研究结果表明,张力计的响应曲线可分为张力计安装扰动段、压力反应滞后段以及稳定平衡段,其中,压力反应滞后段建议采用4次多项式函数拟合,而稳定平衡段则采用线性函数拟合。数值优化计算结果表明,在一般情况下,Levenberg-Marquardt优化法(LM+UGO)、BFGS准牛顿优化法(BFGS+UGO)与共轭梯度优化法(CGM+UGO)的拟合程度高,用时短,但对于数据抖动幅度较大或存在突变型曲线特征等复杂形式的张力计响应曲线,数值优化算法不易获得较理想的结果,此时仍需借由经验判断才能确定合宜的反应时间。     

地震泵抽吸作用与油气运聚成藏物理模拟

地震泵抽吸作用是在断裂活动开启过程中,引发流体进入断裂带,并发生垂向运移的作用方式。在裂隙递进扩张形成优势裂隙并进一步发展成大的断裂面的演化过程中,地震泵抽吸作用发生,围岩中的流体在相对负压抽吸作用下进到断裂带中,之后主要在构造应力和异常高孔隙流体压力的共同作用下向上运移,并向着邻近断裂的封闭程度差、体积大、物性好的储层中充注运移,运移效率远远高于达西流和扩散流。断裂周期性破裂活动的特征决定了油气沿断裂的垂向泵吸运移呈周期性、间歇性和脉动的特点,同时也决定了油气易于在断裂错开的最新层位聚集成藏。通过克拉2气田天然气运聚成藏过程物理模拟实验结果表明,紧邻圈闭的盐下断裂为有效的天然气运移通道,穿盐断裂为无效的天然气运移通道,天然气在地震泵抽吸作用下沿断裂运移速度快、效率高,在断裂活动的短时间内可以大规模运聚成藏。     

冻土/岩的液相吸力与固相冰压作用机制

为研究冻土/岩内液相吸力和固相冰压在冻结过程中的作用机制,首先根据热力学基本原理,统一了不同边界条件下的理论冰压方程;然后,联立广义Clapeyron方程和Gibbs-Thomson方程,在阐释液相吸力和固相冰压物理意义基础上,给出了弯曲界面液相水的冻结温度方程;最后,将冻结温度方程引入冻结毛细管模型,将液相吸力方程代入达西定律,分别对固相冰压和液相吸力作用机制进行验证。研究表明：（1）平衡状态下的理论冰压仅与温度线性相关,与边界条件无关;（2）液相吸力来源于理论吸力与固相冰压抵消因子之差,为水分迁移统一驱动力,当固相冰压趋近于理论冰压时,液相吸力趋近于0;(3)固相冰压为绝对压力,可抵消液相吸力,而液相吸力为相对吸力,不可抵消固相冰压;（4）孔隙内的总压力仅为固相冰压,在毛细管冻结时,可达到平衡状态下的理论冰压,故遵循冰压法分凝冰形成机制。研究结果揭示了冻土/岩的液相吸力与固相冰压作用机制,对完善现有冻胀理论具有较高的理论价值和科学意义。     

吸力式沉箱底部土体循环特性及其等效循环蠕变模型研究

吸力式沉箱基础作为张力腿平台(TLP)的锚固基础,其底部土体不仅存在轴向卸荷作用,而且还存在循环荷载作用。现有研究很少涉及在卸荷条件下进行的软黏土循环累积变形研究。因此,通过循环三轴试验研究了轴向卸荷条件下软黏土的应变累积变形及应变软化特性。试验结果表明:在低静偏应力以及动偏应力条件下,土体应变累积及软化程度较低;随着动偏应力增加,循环累积变形逐渐变大且前期发展较快后期趋于稳定,同时应变软化程度逐渐增大;土体循环累积变形随动偏应力提高而增大,其循环累积应变曲线呈衰减-稳定的蠕变特性。在试验基础上,提出与静偏应力和动偏应力有关的软化系数公式。构建了考虑软化系数与静偏应力、动偏应力等条件下软黏土等效循环蠕变模型。将该模型推广至三维,并开发了有限元子程序。建立了循环荷载作用下吸力式沉箱基础循环抗拔承载力有限元分析法,将其用于吸力式沉箱基础循环累积变形分析中,并与沉箱模型循环试验结果进行了对比验证。