Analysis of the Bearing Capacity of Shallow Foundation in Unsaturated Soil Using Monte Carlo Simulation

The ultimate bearing capacity of shallow foundation supported by unsaturated soil depends on the degree of saturation of the soil within the influence zone because the strength and deformation parameters of soil are affected by the degree of saturation. As the degree of saturation varies with rainfall, surface runoff, evapotranspiration and other climatic and geotechnical parameters, these parameters must be systematically incorporated for accurately computing the ultimate bearing capacity. In this study, a framework is proposed to compute the ultimate bearing capacity of a shallow footing in unsaturated soil considering site specific rainfall and water table depth distributions. The randomness in rainfall and water table depth is systematically considered using Monte Carlo method. The infiltration of water through the unsaturated zone is modelled using Richards equation considering infiltration and water table location as the top and bottom boundary conditions, respectively. The results show that the bearing capacity calculated using the proposed method is approximately 2.7 times higher than that calculated using the deterministic approach with fully saturated soil parameters.

Design of Foundations Built on a Shallow Depth (Less than 4 m) of Egyptian Macro-Porous Collapsible Soils

It is nowadays well reported that collapsible soils spread in many countries, including United States, Russia, China, South America (e.g. Brazil), South and North Africa (e.g. Egypt, Algeria), Middle East (e.g. Saudi Arabia) and many countries in Eastern Europe. In general, collapsible soils are located in arid and semi-arid regions around the world. This special type of soil is characterized by abrupt reduction in strength, excessive and sudden settlement when it becomes wet leading to failure of the structure. Construction on such a kind of soil is one of the prominent problems in geotechnical engineering. The main objectives of this study are reporting geological and geotechnical zonation maps for potentially collapsible soils in inhabited areas in Egypt. Furthermore, a design technique for foundations built on a shallow depth of Egyptian macro-porous collapsible soils (less than 4 m) is developed. The design method includes a design chart for soil collapse field classification in terms of the most governing parameters, a method for foundation settlement estimation based on a correlation between the wetting-induced collapse strain and the applied pressure, and a design practice to guide practicing engineers to select the appropriate foundation system to construct on such soil with a great degree of confidence and safety.

平原水库库底浅层气分布特征及其诱发库底渗漏模式初探

本研究以某平原水库为研究区,基于钻探、浅地层剖面结果并结合理论分析,对浅层气致平原水库库底渗漏问题进行了研究。结果显示浅地层剖面法可有效探测平原水库库底浅层气分布。当前库底浅层气在库心沉积物中多个区域独立分布。气温的变化、近三年地下水位及水库蓄水水位的波动均无法引起研究区内水库的库底浅层气突破库底夯实黏土层。但若地下水上升最大幅度由1.1~1.2 m升高到1.4~1.6 m,或水库蓄水最小水深由3.8~4.1 m降低到0.5~3 m时,当前库底浅层气热点分布区的库底黏土防渗层将存在破坏风险。平原水库库底浅层气分布时空演化及其诱发库底渗漏过程可分四个阶段:建成初期,库底浅层气向库心汇聚阶段;极端事件引发库底气突破库底夯实黏土层致浅层气释放阶段;极端事件结束气体排泄通道/渗漏通道被限制或被关闭阶段;前两个阶段反复多次后库底沉积物饱和,库底渗漏点形成阶段。

Slip-line field solution for ultimate bearing capacity of a pipeline on clayey soils

A slip-line field solution is presented for the ultimate bearing capacity of the pipeline on a purely-cohesive clay soil, taking into account the circular configuration of the pipe, the pipe embedment, and the pipe-soil interfacial cohesion. The derived bearing capacity factors for a smooth rigid pipe limit to those for the conventional rectangular strip footing while the pipe embedment approaches zero. Parametric studies indicate that, the pipe-soil interfacial properties have much influence on the bearing capacity for the pipe foundation on clayedy soils.

Numerical Analysis of Settlement Response of Shallow Footing Subjected to Heavy Rainfall and Flood Events

The US and many parts around the world have experienced prolonged periods of heavy rainfall, severe floods, and droughts over the past 50 years. This study investigates the impacts of extreme hydrological events such as heavy rainfall and flood on the settlement behavior of continuous footing installed in unsaturated soil using a coupled Geotechnical-Hydrological finite element software, PLAXIS 2D. Initially, the effect of different degrees of saturation on the settlement behavior of the continuous footing of widths 1.5 m, 3.0 m, and 4.5 m was analyzed by applying a mechanical load. Then the settlement behavior of the footing was analyzed by applying heavy rainfall of intensity 102 mm/day for six days. Finally, the settlement behavior of the footing was analyzed by applying a flood head of 2.5 meters for seven days. The results indicated that the wetting front movement during heavy rainfall and flooding led to the weakening of soil strength and stiffness and induced additional settlements. The additional settlement induced by the flood was significantly higher than the heavy rainfall. The differential settlement was higher when the rainfall was applied on one side of the footing. The rebound of the elastic settlement was uniquely noticed when the flood head receded with time. The results indicated that not all the settlements were induced by the soil saturation but also due to the hydrostatic loading due to the flood head. The settlements induced by the flooding exceeded the allowable settlement of 25 mm, resulting in failure. These additional settlements caused by heavy rainfall and flood will lead to poor serviceability of the structures and cause the failure of the footing.

HVSR analysis of pumice sands for sediment depth characterization:A case study for Guadalajara,Mexico

The horizontal to vertical spectral ratio(HVSR)methodology is used here to characterize pumice soils and to image the three-dimensional surface geometry of Guadalajara,Mexico.Similar to other Latin American cities,Guadalajara is exposed to high seismic risk,with the particularity of being the largest urban settlement in Latin America built on pumice soils.Methodology has not yet been tested to characterize subsoil depths in pumice sands.Due to the questionable use of traditional geotechnical tests for the analysis of pumice soils,HVSR provides an alternative for its characterization without altering its fragile and porous structure.In this work,resonance frequency(F0)and peak amplitude(A0)are used to constrain the depth of the major impedance contrast that represents the interface between bedrock and pumice soil.Results were compared with borehole depths and other available geotechnical and geophysical data and show good agreement.One of the profiles estimated on the riverbanks that cross the city,reveals different subsoil thickness that could have an impact on different site responses on riverine areas to an eventual earthquake.Government and academic efforts are combined in this work to characterize depth sediments,an important parameter that impacts the regulations for construction in the city.

降雨条件下植被根系对非饱和土浅层滑坡的加固作用

为了研究降雨条件下植被根系对非饱和土边坡浅层滑移的稳定性,基于可计算任意时刻任意土层深度处孔隙水压力分布与变化特征的降雨入渗解析模型,通过理论解析方法研究降雨入渗作用、植被根系抗拉强度、根面积指数以及根系生长方向等因素对边坡浅层滑移稳定性的影响。结果表明,植被根系的抗拉强度越大或根面积指数越大,则边坡的稳定性越强。随着边坡坡角的增大,植被根系对边坡稳定性的加固效应越加明显。此外,植被根系的生长方向也会对加固效果产生明显影响。在实际工程中,应在合理控制植被根系生长方向的基础上,选择根系发达且发育较长同时具有较大根面积指数的植被,以达到更好的护坡效果。

渭河流域浅层包气带土体温度分布规律及变化模式

大气温度、太阳辐射强度、降雨量、空气湿度等气象要素决定浅层土体温度的变化.以黄土高原渭河流域西部黄土丘陵沟壑区作为研究区域,建立野外观测站,对该区域浅层包气带土体温度对气候变化的响应过程进行实时监测.通过现场监测和理论分析发现土体温度分布规律及变化模式如下:气象要素只对变温带的土体的温度有影响,且大气温度和太阳辐射强度是影响变温带土体温度的主要因素,土层越深,其影响程度越小;相位随着土体的深度呈线性平移,即土体温度是存在滞后效应的,经分析,土体在地下深度760 cm处的温度曲线的相位与地表处正好相反,此处温度恰好滞后于地表半年时间.通过监测数据预测该地区恒温带深度为1193 cm,恒温带温度为13.152℃;在计算土体温度理论滞后时间时将土体的热扩散系数看作是一常数,导致土体温度滞后时间的理论值与实测值有所差异,随着土层的加深,两者的差距越来越小.

浅层岩土体热物性参数测定的影响因素分析

浅层地热勘查中,岩土体的热物性是区域评价和工程应用的重要参数。为提高热物性参数测定的准确性,建立了具有单项因素误差影响计算功能的传热模型,分别研判岩土体热物性测试的影响因素。结果显示:不同温度的泥浆,成孔停待10d后,热导率值基本稳定;加载功率波动时,可根据波动周期规律选择合适的加载时间与数据舍弃时间,以减少求解误差;如加载过程出现中断,运用线热源简化法计算的热导率有较大误差,中断开始时间越晚、中断时间越长,误差越大;无负荷循环测试时,15min的观测时间可得到岩土体初始平均温度;岩土体温度受四季气温影响深度主要在15m以内,最低平均温度在2、3月份,最高平均温度在8、9月份,均较当地气温约滞后2个月。

浅埋滴灌下玉米秸秆不同还田年限土壤有机碳固存特征及其影响因素

外源有机物(秸秆)添加直接影响耕层土壤有机碳的周转,研究浅埋滴灌条件下玉米秸秆持续还田土壤有机碳固存及其影响因素对西辽河平原灌区玉米可持续生产与耕地地力提升具有重要意义。本文基于连续7年的田间定位试验,设置玉米连作秸秆连年全量还田0年(0a)、3年(3a)、5年(5a)和7年(7a)4个处理,探讨玉米秸秆不同还田年限对土壤结构、胞外酶活性、有机碳组分含量、碳储量、固碳量和碳库管理指数的影响。与0a处理相比,0~30 cm土层3a、5a和7a广义土壤结构指数分别增加1.65%、1.99%和3.33%,土壤三相结构距离分别降低5.55%、6.65%和12.7%;0~30 cm土层3a、5a和7a土壤β-1,4-N-乙酰氨基葡萄糖苷酶和土壤β-葡萄糖苷酶活性分别提高4.34%、13.0%、15.9%和12.1%、16.3%、20.9%,土壤纤维素酶和土壤蔗糖酶活性分别提高5.86%、13.4%、19.2%和12.6%、21.3%、34.1%;不同处理间几何平均酶活性和总体酶活性差异显著,0~30 cm土层3a、5a和7a几何平均酶活性和总体酶活性较0a分别提高8.63%、15.90%、22.25%和9.12%、17.19%、25.09%。土壤有机碳、易氧化有机碳、水溶性有机碳和微生物量碳含量及其储量均表现为3a、5a和7a显著高于0a,0~30 cm土层3a、5a和7a土壤有机碳和土壤易氧化有机碳含量较0a分别提高13.4%、32.7%、42.7%和17.5%、27.5%、42.5%,土壤水溶性有机碳和土壤微生物量碳含量较0a分别提高13.2%、18.5%、28.5%和33.9%、45.3%、56.1%;同时0~30 cm土层3a、5a和7a土壤有机碳和易氧化有机碳储量分别提高11.6%、29.5%、36.2%和14.9%、23.8%、35.5%,土壤水溶性有机碳和微生物量碳储量分别提高10.7%、15.2%、22.3%和31.0%、41.0%、48.5%。0~30 cm土层5a和7a土壤固碳量均显著高于3a;0~30 cm土层各处理碳库指数和碳库管理指数差异显著,5a和7a较3a分别提高17.0%、26.1%和7.00%、19.9%。土壤结构稳定性、胞外酶活性与土壤固碳量和碳库管理指数的相关分析和冗余分析结果表明,土壤β-1,4-N-乙酰氨基葡萄糖苷酶活性、液相体积分数、三相结构距离、纤维素酶活性和β-葡萄糖苷酶活性是影响玉米秸秆不同还田年限土壤有机碳固存的主要因素,其解释率分别为76.0%、4.10%、3.30%、1.70%和3.40%。浅埋滴灌下玉米秸秆连续还田7年改善了土壤结构的稳定性,提高了与土壤碳素相关的土壤胞外酶活性,进而提升了土壤有机碳固存和碳库指数。

较大纵坡浅覆土大断面矩形顶管顶进控制技术

以杭州地铁3号线支线留下站A1-A2出入口连接通道为背景,结合工程地质情况,介绍了始发井及接收井端头加固、较大纵坡及覆土较浅顶进控制等施工技术,对顶管变形和地表沉降监测结果进行了详细分析,阐述了顶管施工引起的周边土体变形特征。现场监测结果表明,采用有效、综合的顶管施工控制措施,避免了顶管下穿范围内的地面沉降问题。

杭州湾富浅层气区工程超前排气井井距优化研究

对遭遇浅层气灾害地质的工程,一般防治措施多为建设前期布设超前排气井进行控制性排气。深入了解排气过程中气藏内部的气水运移规律,对于排气井井距的设计和高效率排气有重要意义。基于非饱和土两相渗流原理和杭州湾富浅层气区的典型概化地质模型,利用数值模拟方法研究了有控排气条件下气藏内部气水界面运移过程,探讨了影响超前排气井布设的因素,提出一种井距优化方法,并在实际工程中应用。结果表明:不同井口排气速率下,气藏内部气水分布形态与气体排放效果显著不同;工程超前排气井井距的设置可以淹井时间最长和剩余气藏厚度最小进行优化;原始气藏压力是影响井距优化的最显著因素,而储气层下部渗水系数、气藏厚度等并不影响最优井距的确定;调节井距对改善竖向排气井的排气效果随原始气藏压力系数减小而减弱;原始气藏压力系数大,井距适当减小,反之,可适度增加;对于气藏压力系数小的工程,建议发展水平排气井布设技术,以改善排气效果。此外,工程超前排气井的井距设置还应考虑与建(构)筑物的合理安全距离,达到高效、安全和避免投资浪费的目的。

盾构掘进对浅基础建筑物的扰动及影响分区研究

软土地层中地铁隧道盾构掘进对上方建筑物的扰动效应明显。本文以典型软土地层为背景,模拟不同空间位置盾构下穿浅基础建筑物的扰动效应,分析地表沉降变化规律,探究盾构下穿独立基础、筏板基础、箱型基础建筑物的内力及变形特征,并基于复合准则和强度折减法研究了软土地层盾构下穿建筑物的影响分区。研究结果表明:建筑物沉降随隧道埋深的增加先增加再减小,在隧道埋深(H)=12 m时达到最大值;隧道埋深增加会导致建筑物的水平倾斜变小,最大倾斜值出现在H=6 m处;可将隧道衬砌及围岩的安全系数比值曲线突变点作为强、弱影响分区的界限。研究结果可为软土区盾构掘进对浅基础建筑物的扰动分析及影响区的划分提供参考。

基于现场原型测试的乔木覆盖滑坡区根-土复合体力学性能研究

为揭示乔木根系力学特性及其对斜坡的力学效应,选取福建武平高植被覆盖区滑坡为研究点,对研究区典型乔木根系按径级分类后进行根系抗拉力学特性测试,并在滑坡现场进行不同根系横截面积比(rootcross-sectionalarearatio,简称RAR)、含水率下的根-土复合体原位直剪试验以及剖面根系分布特征调查,对根系在浅层滑坡的力学效应展开探究。研究结果表明:(1)1~7径级下马尾松、杉木抗拉力范围为12.45~673.09 N,抗拉力与根径呈幂函数正相关;抗拉强度范围为7.16~60.95MPa,抗拉强度与根径呈幂函数负相关。杉木根系平均抗拉力与抗拉强度均大于马尾松。(2)乔木根系显著提高了土体抗剪强度,根系对土体提供的附加黏聚力与剪切面RAR显著正相关。杉木根系构型更接近R型,马尾松为VH型,相近RAR下杉木对土体的加固作用优于马尾松。(3)随着含水率增加,马尾松、杉木根-土复合体抗剪强度显著下降,水分入渗使根系对土体提供的附加黏聚力受到削减。(4)基于Wu模型,考虑水分对土体黏聚力与根系附加黏聚力的影响,建立了考虑含水率的根-土复合体抗剪强度值估算模型,经检验,精度高于Wu模型,结果较为合理。(5)根系对浅层滑坡虽然具有加固作用,但受到根系分布深度、密度以及降雨等因素限制,强降雨作用下根系对浅层滑坡稳定性贡献具有局限性。

软土地层浅埋暗挖矩形大断面隧道施工力学响应特征

[目的]为探究在城市软土地层中安全有效地施工浅埋暗挖矩形大断面隧道的技术方法,以实现施工过程的安全性控制。[方法]以某地铁区间隧道工程为例,提出4导洞开挖和“中导洞”先行6导洞两种开挖方案。利用有限差分软件建立三维数值仿真模型,模拟了隧道施工过程并详细分析了隧道施工过程中的地层竖向位移和初期支护内力变化。通过与现场监测数据分析对比,验证了所提工法的合理性与有效性。[结果及结论]在导洞开挖阶段,隧道顶部和底部的地层竖向位移最为显著,且相邻导洞的开挖对位移影响有限。采用6导洞开挖方案,与4导洞开挖方案相比,底部最大位移值显著减少了21.5%,同时在施工过程中,底部的竖向位移速率也降低了6.1%。特别是在中导洞施工期间,地层竖向位移变化速率是侧导洞变化速率的1.2倍左右,这表明中导洞对地层的影响更为显著。6导洞开挖方案在控制施工期间地层变形和结构受力方面更为有效,特别是在中导洞施工阶段,需要特别关注并加强监控和支护措施。

海洋工程钢结构不规则防沉板基础承载力数值模拟

防沉板基础是海洋工程结构物中常用的浅基础形式,由于结构功能布置需要可能会导致其具有不规则的形状,同时海洋结构物的土壤条件和载荷条件较为复杂,要求防沉板基础具备可靠的承载能力。针对某海洋平台导管架结构的不规则防沉板基础,采用浅层土壤参数根据规范中推荐的理论公式以及面积和惯性矩等效的方法得到承载力;结合深层土壤条件,采用有限元方法和用户子程序对不同空间载荷作用下的极限承载力进行数值模拟。通过两种方法分析并验证了防沉板基础的承载力,结果表明:不规则形状的防沉板基础承载力计算可采用面积和惯性矩等效方法等效为长方形基础,浅层深度范围以内的土壤采用线性变化的承载力计算公式,与三维空间载荷作用下的有限元模型分析结果基本吻合。

浅埋矩形顶管背土效应全过程理论分析模型

针对浅埋矩形顶管顶进的背土效应,提出覆盖背土竖向扩展阶段、水平扩展阶段和整体背土阶段的全过程理论分析模型,确定顶管顶进的松动土压力、管土摩擦力、侧壁剪切约束力和前端阻力的计算方法,得到背土启动和发展全过程的理论计算公式.结果表明:背土是否启动主要由顶管上覆土层的厚度、地层土体特性和管土摩擦特性决定,与顶进长度无直接关系;背土竖向扩展阶段的背土区域厚度和水平扩展阶段的地表扩展宽度都随着顶进长度的增加而非线性增长;理论分析结果与矩形顶管顶进全过程三维有限元模拟结果和现场实测结果吻合较好,理论公式可为矩形顶管背土发展全过程预测分析提供一种有效而简便的方法.

双隧道开挖对浅基础建筑物沉降影响的研究

基于离心模型试验和数值模拟计算,对两条平行开挖的跨江隧道引起的上部浅基础建筑物和地表沉降规律进行了研究。结果表明:浅基础建筑物不同监测点位的沉降量均随土体体积损失率的增大而增大。随着土体体积损失率的增加,上部浅基础建筑物的不均匀沉降增加,隧道上方的土体的竖向位移逐渐增大,沉降槽的范围逐渐增大。本项目中隧道间距为1倍洞径条件下,左右隧道开挖顺序的改变会对上部建筑物沉降量和地面最大沉降量影响较小,但先开挖隧道拱顶处的地面沉降要大于后开挖隧道拱顶处的地面沉降。

浅耕和施肥对茶叶产量、品质及土壤肥力的影响

为分析浅耕和施肥对茶叶产量、品质及土壤肥力的影响,以苍梧县群体种为供试材料,研究浅耕、浅耕+施肥、施肥处理对茶叶产量、品质及土壤理化性质的影响。结果表明,不同处理的发芽密度、芽叶长度、百芽重及鲜叶重之间差异极显著(P<0.01);浅耕+施肥的发芽密度、百芽重和鲜叶重高于其他处理,说明浅耕结合施肥可促进芽叶长度生长。各处理的水浸出物含量差异极显著(P<0.01)、茶多酚含量差异显著(P<0.05);以浅耕+施肥的水浸出物、茶多酚含量最高。各处理的咖啡碱、游离氨基酸、黄酮含量差异不显著(P>0.05),茶氨酸含量差异显著(P<0.05)。浅耕+施肥处理的土壤全氮、全磷、有效磷、速效钾和有机质含量高于其他处理,而施肥处理的土壤全钾含量最高。各处理的土壤水解性氮含量较对照均有不同程度降低。浅耕、浅耕+施肥的土壤pH值显著提高。综合考虑,浅耕+施肥在提高茶叶产量、品质及土壤肥力方面效果明显,适合在六堡茶生产中推广应用。

盾构浅覆土下穿城市河道桥梁影响分析与保护措施研究

为了研究地铁盾构隧道浅覆土且埋深突变情况下下穿河道桥梁及管线的影响规律与对应保护措施的控制效果。依托成都地铁17号线二期清水河站—浣花里站区间隧道工程,利用数值模拟分析了8.7 m直径盾构隧道施工对肖家河桥及大直径中水管的影响规律与极限工况,对保护措施进行了分析,并通过现场监测进行信息化施工反馈。通过施作钢筋混凝土梁和保护箱涵,以及基底注浆加固措施有效减小了桥梁结构最大沉降,分别将桥台、桥面和管道最大沉降控制为8.7 mm,9 mm和7.1 mm,数值模拟结果与现场监测数据在规律与数值方面较为吻合,验证了所采取的控制措施的合理性。