循环荷载下滨海风积砂孔压发展规律试验研究

利用GDS动三轴试验系统,在不同的基准应变下对福建漳浦滨海风积砂施加不同幅值的等幅循环应变荷载,分析了滨海风积砂在循环应变作用下的孔压发展规律。试验结果表明:滨海风积砂的孔压在初始段迅速增长,之后循环应变幅值不同,孔压有不同的动力响应。小幅值作用下,过渡段大致为“S”发展趋势,后期试样结构崩溃,孔压值迅速达到围压值。密实砂抗液化能力随着基准应变的增大和循环应变幅值的减小而增大,但是中密砂受基准应变的影响很小。根据实验数据建立孔压增长对数模型,模拟结果与试验结果一致性较好。     

前期动载对低塑性粉土静态和动态强度的影响

 低塑性粉土广泛存在于世界范围内,在地震中容易产生液化现象,然而一些基础设施破坏不仅见于地震中也发生在地震后,这就决定了研究低塑性粉土震后行为的必要性。以美国中部密西西比河沿岸低塑性粉土为试验材料,研究动载对低塑性粉土静态和动态强度的影响。在动三轴仪上对试样施加动载引起超孔隙水压力,排水重固结后,分别对2组震动后试样进行静态和动态三轴强度试验。试验结果表明,当液化水平小于0.70时,前期动载对粉土的不排水剪切强度影响不大;相反地,只有当液化水平大于0.70,密西西比河沿岸粉土的震后重固结体积应变和不排水剪切强度才伴随着液化水平的提高显著增加,但相对于砂土而言,重固结体积应变在较低的液化水平时即有明显增加。与前期动载对不排水剪切强度的影响不同,当动载所引起的液化水平为0.35或轴向应变为0.2%时,抗液化强度达到最大值,若液化水平大于0.35,抗液化强度伴随液化水平提高而降低。如果前期荷载引起较大的压应变,在重固结后第二次动载循环中,轴向压力相比轴向拉力引起较小的超孔隙水压力。     

循环荷载下饱和南沙珊瑚砂的液化特性试验研究

针对饱和南沙岛礁珊瑚砂,开展了一系列不排水循环加载试验,研究了相对密度D_r和初始围压■对饱和珊瑚砂的超孔隙水压力、应变发展、有效应力路径及动强度特性的影响,并比较了珊瑚砂与福建砂的液化特性差异。试验表明,珊瑚砂的超静孔压Δu的发展模式与石英砂的有较大区别,可用修正后的Seed模型进行表征。珊瑚砂液化时的累积能量耗散远比福建砂的大。珊瑚砂的轴向应变ε_(DA)随着循环振次增加而逐渐变大,不会发生急剧增大的现象。在有效应力路径接触相转换线后,珊瑚砂会发生剪胀和剪缩交替出现的现象,仍然会存在有效应力。较之福建砂Δu的波动特征,珊瑚砂Δu的波动更大,且当Δu接近■时波动明显增大,产生"瞬时液化"现象。珊瑚砂的动强度随着D_r以及■的增大而增大。珊瑚砂的动强度大于石英砂的动强度。     

Effects of Pore Fluid Compressibility on Liquefaction Resistance of Partially Saturated Sand

It has been recognized that the soil resistance to liquefaction increases significantly as the degree of saturation decreases. However, the effect of the degree of saturation reported in the literature varies widely between researchers. In this study, influential factors of the liquefaction resistance of partially saturated sand are derived from theoretical consideration and effects of the factors are examined through a series of triaxial tests. It was confirmed that the degree of saturation has a significant effect on the liquefaction resistance. It also appeared that the liquefaction resistance depends on the initial confining pressure and the initial pore pressure; the higher the confining pressure and the lower the initial pore pressure, the higher the liquefaction resistance of partially saturated sand. A unique relationship between liquefaction resistance ratios and the potential volumetric strain was found, which enable to estimate the liquefaction resistance of partially saturated sand with the effects of the three factors taken into account.     

Liquefaction and post-liquefaction resistance of sand reinforced with recycled geofibre

The present study provides an insight into the effect of recycled carpet fibre on the mechanical response of clean sand as backfill material subjected to monotonic loading and cyclic loading as well as post-liquefaction resistance of both unreinforced and carpet fibre reinforced soils. To achieve these goals, a series of multi-stage soil element tests under cyclic loading event resulting in liquefaction followed by undrained monotonic shearing without excess pore water pressure dissipation as well as a series of monotonic undrained shear test is conducted. All the specimens are isotropically consolidated under a constant effective confining stress of 100 kPa by considering the effect of cyclic stress ratio and carpet fibre content ranging from 0.25% to 0.75%. The obtained results revealed the efficiency of carpet fibre inclusion in increasing the secant shear modulus and ductility of clean sand under monotonic shearing without previous loading history. The impact of carpet fibre inclusion on the trend of cyclic excess pore water pressure generation and cyclic stiffness degradation was minimal. However, adding carpet fibre significantly improved both liquefaction and post-liquefaction resistances of clean sand. The liquefaction resistance of clean sand, at a constant 15 loading cycles, improved by 26.3% when the soil was reinforced with 0.75% recycled carpet fibre. In addition, the initial shear modulus of the liquefied specimen significantly increased by adding recycled carpet fibre.     

Influence of strain histories on liquefaction resistance of sand

The liquefaction resistance of sand increases with cyclic pre-shearing and pre-shaking as a result of earthquakes if the strain level in the pre-shearing is small. When larger shear strains are imposed, liquefaction resistance decreases. These complicated effects of pre-shearing histories on the liquefaction resistance are investigated in this study through a series of cyclic triaxial tests. Various combinations of cyclic stress amplitude and number of cycles of pre-shearing are examined. The tested sand is Toyoura Sand at 45% relative density, under a confining pressure of 50 kPa. Test results indicate that for the range of shear strain amplitude in pre-shearing smaller than 0.35%, the liquefaction resistance increases with pre-shearing. The increase in the liquefaction resistance depends strongly on the volumetric strain in the pre-shearing, and several effects of the shear stress amplitude and number of cycles can be negligible. Small volumetric strain of the order of 1% doubled the liquefaction resistance. Meanwhile, in the range of shear strain amplitude larger than 0.6%, the liquefaction resistance decreases. The liquefaction resistance decreases as the shear strain amplitude increases. Shear strain amplitude is one of the factors dominating this degrading effect, and the volumetric strain exerts beneficial effects to a certain extent. In this study, another series of tests are conducted to investigate the combined effects of small and large strain amplitude pre-shearing. It is observed that small shear strain pre-shearing cycles subsequent to large shear strain cycles erased the degrading effect of the latter. However, a large shear strain pre-shearing after small strain cycles degrades the beneficial effect of the small shear strain pre-shearing cycles previously applied to the specimens; however, the effects of the former small strain pre-shearing remains.     

不规则动荷载作用下砂土孔压特性试验研究

以多组人工调制的不规则正弦波荷载作为输入荷载,对饱和松砂开展一系列动三轴试验,探讨应力时程中不同脉冲幅值和脉冲顺序对砂土孔压特性的影响。试验结果表明,出现更早、幅值更大的应力脉冲更易导致孔隙水压力的发展,同时也更易诱发砂土液化。进一步地,峰值应力之前和之后的脉冲波组对孔压发展的影响有所不同。此外,不同荷载模式下,饱和松砂动孔压比与振次比存在近似唯一关系。根据砂土孔压特性,引入单位体积剪切功并考察其与超静孔压的内在关联,得到了孔压比与正交化剪切功的归一化关系。这一关系具有应力路径无关性,受动荷载不规则性影响较小,但与材料特性即土体种类有关。     

Cyclic resistance of two unsaturated silty sands against soil liquefaction

The changes of the cyclic resistance of two silty sands under unsaturated, partially saturated and fully saturated conditions are examined based on a series of undrained cyclic tests conducted using triaxial test apparatus specially equipped for testing unsaturated soils as well as ordinary triaxial test apparatus for testing partially saturated and fully saturated soils. Based on the observations of volumetric strain, pore air and pore water pressures of unsaturated soil specimens, the possibility of soil liquefaction triggering under different degrees of saturation is examined and discussed. The changes in the cyclic resistance under different degrees of saturation are then examined. Those two unsaturated silty sands with different grain size compositions are found to give rise to different responses on the volumetric strain as well as pore air and pore water pressure developments during undrained cyclic loading, leading to different relations between cyclic resistance and degree of saturation, covering unsaturated, partially saturated and fully saturated conditions.     

循环加载频率对砂土液化模式的影响试验研究

为了研究循环加载频率对饱和砂土的液化特性的影响,针对密实度为35%、50%、70%的福建标准砂进行了振动频率为0.05Hz、0.1Hz、0.5Hz、1Hz、2Hz的循环扭剪试验,并对密实度为50%的珊瑚砂和细砂进行了振动频率为0.1Hz和1Hz的循环扭剪试验。结果表明：无论是松砂还是密砂,其剪胀剪缩特性与加载频率密切相关,在低频荷载作用下表现出显著的剪胀特性,达到初始液化后孔隙水压力波动,土体仍具有抵抗液化能力,呈现“硬化型”液化模式;在高频荷载作用下表现出显著的剪缩特性,达到初始液化后孔隙水压力保持稳定,循环液化模式呈现“软化”特征,珊瑚砂和细砂的孔隙水压力特征和液化模式也同样受加载频率的影响,说明循环加载频率显著影响饱和砂土的剪胀剪缩特性,进而影响液化模式;液化阶段产生的流滑变形大小与加载频率密切相关,低频荷载作用下所产生的流滑变形显著大于高频荷载作用下的流滑变形。     

Liquefaction Resistances of Unsaturated Non-Plastic Silt

It has been pointed out that there are two possible mechanisms that enhance liquefaction resistances of unsaturated sand. The first mechanism is where air in a partially saturated sand mass plays a role of absorbing generated excess pore pressures by reducing its volume. Okamura and Soga (2005) derived the influential factors of the liquefaction resistance for partially saturated sand from theoretical consideration and effects of the factors were examined through a series of triaxial tests on a clean sand. They found a unique relationship between liquefaction resistance ratios and the potential volumetric strain, which allows the estimation of the liquefaction resistance for partially saturated sand. The second is the matric suction of unsaturated sand which increases the effective stress and thus the strength of the soil mass. In this study two series of cyclic triaxial tests on non-plastic silt were carried out to observe the liquefaction resistance in both mechanisms. In the first series, a top cap with an accumulator tank was used to study the effect of compressibility of pore fluid on the liquefaction resistance. The empirical relationship derived by Okamura and Soga is found to be valid even for the silt provided that the matric suction is negligible. In the second test series an ordinary cap was used. The liquefaction resistance increased linearly with the matric suction, with the increasing ratio being higher than that for the net stress. A unique linear relationship is found between the normalized liquefaction resistance and the matric suction. Results are summarized in the form which can be easily applied to evaluate the liquefaction resistance of a partially saturated soil.     

砂土液化后再固结体变规律表征与离心模型试验验证

针对一定相对密度的饱和砂土,首先开展单元体K₀固结试验和振动液化试验研究,发现饱和砂土液化后体变规律受再沉积和再固结两种机制制约：其中再沉积部分与所受振动历史密切相关,尤其是液化触发后的应变历史,土骨架累积剪应变比越大、再沉积体变越大;而再固结部分受先期固结历史和循环振动历史影响显著,再固结曲线会沿原有正常固结曲线趋势发展,其稳定段再压缩指数比相同条件下的正常固结曲线的压缩指数稍大。据此提出了考虑先期固结和振动历史的砂土液化后体变模型和简化算法,将再沉积和再固结两者统一表达成再固结体变,并建议了再固结压缩指数和假设起始应力的确定方法。进一步开展了水平场地地震液化离心机模型试验,监测模型固结和振动液化过程的沉降,从模型尺度进一步揭示砂土液化后体变规律,并初步验证了本文模型与简化算法的有效性。     

液化及液化后砂土的流动特性分析

将液化后土体视为流体是一种较新的地震液化研究思路。根据液化后砂土变形试验结果,从流体力学的观点,分析了液化及液化后砂土的流动特性,包括应力-应变率关系和表观粘度的变化,试验中考虑了相对密度、固结压力和液化度三个参数的影响。分析表明,在一定条件下液化砂土的表观粘度随应变率的增大而减小,是一种“剪切稀化非牛顿流体”,且在“剪切稀化”状态下可发生较大的应变。随着液化后孔压的降低,砂土的表观粘度随应变的增大而增大,随孔压比的减小而增大。     

Experimental study on suffusion under multiple seepages and its impact on undrained mechanical responses of gap-graded soil

An experimental investigation of multiple seepage-induced suffusion and its impact on the mechanical responses of internally unstable gap-graded soil, with a fines content of 25%, is presented in this paper. Using a modified triaxial erosion apparatus, with a redesigned seepage control system, erosion tests under multiple seepage conditions, as well as undrained monotonic and cyclic compression tests, are performed. It is found that multiple seepages cause an eroded mass without a marked change in volume and with a change in hydraulic conductivity as the number of seepage cycles increases. The monotonic compression tests show that eroded soil presents a smaller peak strength, residual strength, and a greater contractive response than non-eroded soil. The peak strength and stiffness of eroded soil are seen to decrease considerably as the number of seepage cycles increases. Multiple seepage-induced suffusion may create a collapsible soil structure in eroded soil, as shown by sudden decreases in deviator stress and stiffness, coupled by sharp increases in pore water pressure at small strain levels. As eroded soil might already be unstable, cyclic loading causes it to collapse, revealing a sudden increase in the generation of cyclic pore water pressure and a decrease in liquefaction resistance. The results highlight the importance of conducting laboratory tests to determine the impact of internal erosion on the strength and liquefaction resistance.     

珊瑚礁钙质沉积物液化特性及其机理研究

针对南沙群岛永暑礁小澙湖珊瑚礁沉积物-钙质砂土的多孔隙及颗粒破碎特点,利用现场原位扁铲侧胀试验和室内动静三轴-扭剪试验手段,开展不同条件下钙质砂土的液化特性分析。试验结果表明,在7度地震烈度下浅海钙质砂土会发生液化;在波浪荷载作用下,饱和钙质砂土峰值孔压随振动次数的增加可分为抛物线型增长、线性增长、指数型增长和稳定状态等四个阶段,而每一阶段均对应有钙质砂颗粒间孔隙和内孔隙的不断调整和破碎过程;在均压固结条件下其液化机理归结为流滑破坏。     

Liquefaction behavior of fiber-reinforced calcareous sands in unidirectional and multidirectional simple shear tests

A study on the liquefaction resistance of calcareous sands reinforced with polypropylene fibers was reported. Stress-controlled cyclic simple shear tests were conducted on specimens prepared at a relative density of 50%, with and without fiber reinforcements. The liquefaction behavior was investigated by considering the effects of fiber contents ranging from 0% to 1%, fiber lengths varying from 3 mm to 12 mm and loading patterns. The results indicated that increasing fiber content and fiber length resulted in a decrease in the deformation, a reduction in pore pressure accumulation rate, and improved the liquefaction resistance of calcareous sands. Additionally, the risk of soil liquefaction could be significantly reduced when the fiber content was greater than 0.8%. The multidirectional loading had a considerable effect in reducing the liquefaction resistance compared to unidirectional loading. Further, the stiffness degradation of calcareous sands decreased with increasing fiber content and fiber length. The pore pressure generated in the cyclic tests was analyzed and was found to be affected by fiber content. A pore pressure prediction model was proposed to obtain the pore pressure characteristics of fiber-reinforced calcareous sands under various fiber content conditions.     

微生物处理砂土不排水循环三轴剪切CFD-DEM模拟

微生物诱导碳酸钙沉积是一种新型的地基处理技术,处理后的土体可以看成一种结构性土。首先,在已有三维含颗粒抗转动和抗扭转模型及三维胶结破坏准则的基础上,通过考虑颗粒碰撞接触过程中颗粒本身的塑性变形及率相关性的接触黏滞阻尼,建立考虑循环荷载作用下的三维胶结模型;然后,参考已有研究,建立了反硝化反应在加固砂土中的时效性关系。并引入CFD-DEM耦合程序,用以模拟分析不同胶结含量以及不同气泡含量下,微生物处理砂土在固结不排水循环剪切试验中的力学特性;最后,从宏微观角度分析生物胶结与生物气泡对砂土抗液化性能的影响及其作用机理。研究表明,胶结和气泡共同存在对抗液化能力的提升并没有起到“1+1=2”的效果;胶结的存在提高了非饱和砂土的抗液化能力,明显抑制孔压比和轴向应变的发展,力学配位数得到了提升;而气泡的存在却降低了胶结砂土的抗液化能力,使得胶结砂土达到初始液化的振次减少,轴向应变向受拉方向大幅增长,力学配位数下降明显。     

Modelling the Undrained Response of Fibre Reinforced Sands

Mixing a loose clean sand with random discrete flexible fibres has been found beneficial in decreasing the susceptibility to the phenomenon of liquefaction under monotonic loading. The addition of fibres can convert the strain softening response, typical of a loose unreinforced sand, into a strain hardening response by affecting the pore pressure generation and the effective stress path response. A new constitutive model based on the rule of mixtures has been used to simulate the undrained response of fibre reinforced sands. The model superimposes the individual contributions of the sand and the fibres according to their volumetric fraction. An apparent densification of the sand matrix induced by the presence of the fibres is accounted for in the model by assigning some of the void space to the fibres. This apparent densification is considered responsible for the observed strain hardening behaviour of reinforced sands. The proposed model is able to accommodate any distribution of fibre orientation: the orientation of fibres plays a key role in explaining the experimentally observed effective stress paths.     

Centrifuge model tests on large-diameter monopiles in dense sand subjected to two-way lateral cyclic loading in short-term

To evaluate the lateral resistance of rigid monopiles for wind turbines in dense sand under lateral cyclic loading, centrifuge model tests are performed, focusing on the base resistance and degradation of the soil resistance under two-way lateral cyclic loading in the short-term. The slenderness ratio (embedded pile length to diameter) is varied from 3.75 to 8 and the loading frequency is in the range of 0.002 to 0.4 Hz in the prototype scale. Under cyclic loading with a maximum horizontal displacement of 5% of the pile diameter, the build-up of excess pore water pressure is observed, but the maximum value of the average excess pore water pressure ratio is around 50% in the steady-state for dense sand whose relative density is 80%. A simple analytical model for the rigid piles, considering the base resistance, is derived and then used to quantify the significance of the resistance at the pile base and the degradation of the soil resistance under cyclic loading. When the slenderness ratio is less than 5, a significant contribution of the moment resistance at the base is confirmed. The estimation of the degradation of the horizontal subgrade reaction coefficient using the simple analytical model suggests that, through cyclic shear tests for the determination of the deformation properties of the soil in a laboratory, it is possible to estimate the degradation of the soil stiffness and the parameters for the reduced sway-rocking type of foundation model.     

双向耦合循环剪切条件下饱和砂土体应变发展规律试验研究

砂土的剪胀与循环应力路径密切相关。针对饱和南京细砂,利用空心圆柱扭剪仪(HCA)进行了一系列均等固结条件下轴向–扭转耦合循环剪切排水试验,研究了复杂应力路径下饱和砂土的剪胀性及其体应变量化方法。研究表明:双向耦合循环剪切条件下饱和砂土的剪胀由一个完全可逆的循环体应变分量和一个不可逆的累积体应变分量构成,循环应力路径对累积体应变发展规律影响显著;以等效循环应力比ESR作为表征复杂应力路径下动应力大小的指标,饱和砂土累积体应变与ESR值具有事实上的唯一性关系,累积体应变随ESR的增加而线性累积;通过引入参数ESR,提出了双向耦合剪切条件下饱和砂土累积体应变规准化方法。验证性试验表明新的体应变增量模型的预测值与试验结果的吻合度较高,而基于循环直剪试验结果建立的Byrne模型对双向耦合剪切条件下饱和南京细砂的体应变预测偏小。     

水泥固化漳州滨海风积砂液化特征试验研究

利用GDS动三轴仪对水泥固化漳州滨海风积砂进行液化强度试验,分析水泥固化漳州滨海风积砂在不同水泥剂量、不同龄期、不同基准应变和不同等幅循环应变幅值下的液化特性。试验结果表明:砂样的抗液化能力随水泥掺量的增加近似呈线性提高;随龄期增长而增强,但基准应变越高,增强越弱;随循环应变幅值的减小而显著增强;基准应变对砂样抗液化能力的影响受水泥掺量的制约。孔压半对数模型可以较好的反应水泥固化漳州滨海风积砂土的孔压发展规律;存在一个临界基准应变,使得抗液化能力随水泥剂量的增加得到最大化提高。