振冲碎石桩处理软土地基

振冲碎石桩处理软土地基导言碎石桩法是用振动水冲法在软土中挤入碎石形成密实桩体,从而提高软土的承载力。振动水冲法是软土加固中主要处理方法之一，利用Plaxis软件对振冲法进行数值模拟分析，以确定处理后复合地基的承载能力。通过数字模拟莫尔库仑，改善土壤的性质是首要被考虑的。从数值模拟的结果中可以得到软土弹性系数的改善程度是可以估算的。此外，复合地基的影响范围也是可以估算的。关键词：巩固；复合地基；弹性模量；改进；数字模拟；软粘土；碎石桩；振冲法1、绪论碎石桩改善后的软土提高程度取决于两个因素：一、软土中碎石桩部分是由比较坚硬的材料（碎石、砾石等）组成。这些主要在[1-6]的文章中报导过。二、在振冲法形成碎石桩的过程中对桩体周围的土体不断挤密压实，使桩体周围土体本身强度提高，最终以提高复合地基的强度。Wattes[7]和Vautrain[8]等人实验证明，振冲而成的碎石桩复合地基改善了原来软土的一些性质，从而提高了加固土壤的排水性能。另外，从原位测试中还可以得到，碎石桩体周围土的固有属性（弹性模量，不排水固结系数，等）比处理前有很大的提高。Vautrain[8]提出了不排水抗剪强度值的规范标准,Al-Khafaji和Craig[9]提出了桩体周围各土层的强度测量方法。Alamgir和Zaher[10]和Sanglerat[11]提出自然土和加固土之间的差异，自然土的标准贯入阻力在碎石桩处理后比之以前有着明显提高。天然地基的N值范围在2到7之间，而经碎石桩加固后的地基可以提高到5到12左右。wattes等人[7]测得，碎石桩处理的复合地基承载力有着显著提高，桩体周围单位面积内平均提高了3倍左右。碎石桩处理砂土地基也有着显著的效果。十字剪切试验表明，碎石桩处理软土地基的前后，地基土的物理力学性质有着类似的提高。从试验结果可得，碎石桩处理地基的效果以桩体为中心呈放射状，随距桩体轴心的距离增加面减弱。术语符号弹性模量系数有效平均压力偏应力,应力差有效径向应力有效垂直应力径向应变总容重排水固结系数内摩擦角渗透系数静止土压力系数如图1所示，z和Hc分别表示深度（距水平地面的高度）和桩的长度，碎石桩处理范围内距桩体中心（处理范围内某点到桩心的距离）处土的物理性质随的变化而不同。处理后的软土平均强度提高了1.5到5倍左右,改善的效果主要取决于土壤的类型.不采取这种改进到可能导致中，在相应的设计方法，高估了所需数量的柱材料。虽然改善软土的特性是由野外数据确定，但从实验室中的测试结果也可推出结果。事实上，大量的实验室测试结果证明:例如Biarez等人[13]的体侧限压缩和三轴试验，证明其增加的杨氏模量作为一个函数的固结应力。阿尔梅达等人[12]和沈等人[14]在岩土文学曾说过:土壤改良效果并没有引起广泛的研究。其中一个原因可能是，无论是何种数值方法，也无法存准确计算出振冲法改善软土特征后的土壤性能。不过，格林伍德[15]根据经验提出了振冲法成碎石桩的一种设计估算方法。本文主要描述了振冲法形成碎石桩后土壤弹性模量的变化。为此，在振冲过程中于碎石桩体上按置模拟器,利用数值模拟分析软土的弹性模量的变化。利用这个方法进一步推出振冲法设计时的桩体改善效果的计算方法。首先，改善软土性质的处理方法是振冲法形成碎石桩。接下来，改善后土体弹性模量的估算。随后，模拟土壤性质提高的情形。得出结论，取得了一定的成效。2.加固原理由各种实验证明,由于成桩过程中桩对周围土层产生很大的横向挤压力,通过成桩机械将不良的地基土强制排开并置换，在地基中形成具有密实度高和直径大的桩体，桩径向扩张在地基中是一个良好的排水通道，大大缩短了孔隙水的水平渗透途径，增加了有效应力。由Wattes等人[7]记录。实际上碎石桩的安置，形成了碎石桩和软土共同作用的复合地基，改变了桩体周围软土的剪切模量等性质，Vautrain[8]。此外，Biarez[13]和Long[16]等人由室内土工实验得到之间的关系。例如：突尼斯工科学校对突尼斯软粘土进行三轴试验，观察应力状态和刚度模量的变化关系。如图2所示（作为初始边坡的应力应变曲线），Tounekti等人[17]排水三轴试验证明刚度模量随着有效围压的巩固而增加。碑图攻3.遗清楚地说谢明了软粘嚷土危中屡压力和稻固结系数绵的馋关系殃。案挑和改分别表示等固结系数病和阵巩固压力湿。角畏标招“摆re咬t槽”缎表示根据漏哪国标准作得来。如撤图畅3播誉，增加体否侧限压缩及模量作为居一个功能滋的固结应局力可近似近地装上，饰在各种小醉变形，有浸良好的线落性回归。重根据这些略实验观察智推断，要岗提高土壤珠刚度模量蜂，在其中移间相互依叨赖的刚度旧模量和应球力状态，挣是由以下后公式计算猫[1汇3让]杀哗：宫僚偿（域1肚）兵公式顶（迁1慌）脱中蝶E(E只0)锣喊和喝(锦)伤分别表示钉弹性模量鱼和有效平真均压力。斜土壤悬中惭=(劈+丈+乞/3生)障，渴指罪数晨m舅代表正常掌弹性模量冈有效应力卷均值游的关系。呼对嫂砂和泥沙市不同的参榜量祝m帝的邻范累围顽0.5袍<裤m农<1帖违。对于一混般粘土和闹砂将，用多代替颗杨氏模量蜂作为碎参考琴值介[18生]多，雷从应力应绢变曲线写可得玻，从三轴美测试割线惧模量各对盒应于偏应糟力的一半伙。对于一仔般粘土辟，浮plax殿i屠s拳用户手册桥指明债，廊建欠议乐m你取期1狱。此时，瞧由公式至（泼1冠）可得刚婚度模量和狗有效平均霸压力之间茶是线性关谋系。逮因此，在禽软粘土加阿固会中搏的杨氏模拦量改可以笛从有效平滴均应力分南布夹中得出结停果凑。德在此基础景上推统导镇，模拟柴在宾软粘土渴中汉安标置振冲碎凭石桩赖，是用一桶种复合脆地基舍。届时，舅振冲碎石胃桩加固地牙基的有效炕平均压力阵就可以推霞算出来。咏数值分析隶的是用计乏算机软菊件奋plax盆is[识18盈]续如碎石桩她装置是模愤拟作为扩严大悠桩间蚕的软粘土锋。拟议的夫分析是普土壤桃的改善继。币土壤中，默一方面膜，桩体强患度比较大朱，另一方罢面，冈在振冲过界程中桩间控土也得以隔改善迷。明3卵．复合地超基壮复合地基部已被许多凭研究人员火研究饶：西可受增加承载爹力，减少靠沉降和土妨体固影结动[4,登5,19膛,20殊]底演。在本研前究中，苹N栽ama更a衬工程冒和程S哑.渡A劝.烂E汇协畅会衔[21唐]词在研究锯调查碎石冬桩工程芬资料拍。土壤剖提面是一形层捞12.2泛5讨米厚软粘截土勒，慰一兔层快12.7堪5绿米厚沙层稻。数据林在旱[21匹塌]税显示较高迎的力学特龄性，梳砂层释超出了软偏粘土。因聋此，假定恳土壤剖面麻是为刚性睡地层（见壤图稳4狭邪）馅明。馒在拌软粘土，傻初步定器振动压实离碎石桩直睛径删1.贪1碰米。展则原土壤补和加固桩晚体以及砂艳石的性质别见贪表得1俩。在为构柱材搅料的便捷叹选彼取荷就是立处理后级睁配征砾石证明新其低排币排水固结个和可靠的巷摩擦声系数渣。新Brin诸kgre敢ve牵们和喇Ver欢meer变[1佣8呼]举建议箱，亿软粘土蜜内摩擦么角通常取妨零。呈Dami道ett珍e询的设计就顶是如膀图团4胃所模拟的剂土壤剖面宜（参见教图庆4日百）饿锹。边界条善件是堵固结丽试验偿来决定龄。科振冲法形坝成复合地优基中桩体榴和周围土南壤的边界晨假定为理极想的（全搞部粘结）邀。这就意迹味着剪切周应力在柱架和土的接膜触面上。琴复合地基尺中右plax朱i六s绝软件是用盗来进行局轴对称旅数值计算望改良土壤罢中悦1烛5欺个节点三傅角形有限炊元素。从钉数值结果创，我们着盗眼于正桩体的有明效份影响范围眠和初步改踪善后的土许体变化。批预测结果啦是在软粘平土层（参啦见佣图钢4条虚线）在任距离金轴心嫌0.辰6恋米波，奸1.舒5霞米尽和钩2.贝4咱米售。皇图凑5西详苏振冲碎石乒桩复合地历基的模拟叫：（一）愿模惰拟招改良春土美;炼敏（二）模俱拟桩体扩筑张妥;往贪（三）改露良土离散志性贱。植在软土中亚桩体的横望向影响段占肥45泡%垮，疲虑到采佩用域''逆有限元龟网召格则'浅'吉所规定的母选款择福plax币is币软件灰（参见被图闻5谱C尿）拼。栗为了分析佳在软土中蛙安置碎石器桩的衬加固效果印，弃D呈ebat软s业等颜人膜[22痰]效对其数值矩模拟分析贺。星它包映括榆3骑个阶段：浆软粘土茂中桩体不假排水固结密，砂石的逢挤密压实座和裕1壤1云个月的复胃合地基土筝壤的提高厘。对改良抹后土壤可忌用目plax臣i敲s伟软件才的各种方页法对基进照行模拟分惑析。章事实上，裤桩径鉴超出这个废值，它已宫表示，愁桩体已经毛不影响土任壤改良。当改良后土述壤中的桩沫体和周围值土体都可惭以进行数冤值计算：孝为侧向土淡压力系数梨。兽4肯、霉结果和烫建议把振冲碎石该桩改良地遍基土分两军步进行：瞎。一是鄙桩体对土挽体的改良非。二酒是原土壤僻改变后土宰的改变。揭桩体对周伪围土挤压必产生的偏接应初力拨q旺表明桩对恨周围土体棍产生的影挣响范围，截可以计算手出最大径吗向应变（馆见弦图证6愈，阶腔段合1代）。骡这个结果踏和假定的缓结果不是伙一致的繁，而周围虑的软土仍挪停留在弹询性阶锋段橡[23裳,3]歪忙。想图浊6沃已清楚显且示办当距离桩蜜心轴线越己远（挪从友0.6素m粱至较2.4稿m众），偏应丝力越小，举加固效果术越弱愈。唉沿桩在形成盟过程中，闪靠近桩体策周围效果古最明显回（浆r=0.混6明m例，参见植图宾6国），以桩土心轴线向跑外呈射线探状辐射。值因此，横努向举压缩（围点压建）奉冒，哀是与周围耳土有关的序。但是，江有基本上掏不变（参忆见张图张7扛、阶巧段妥2会），而在昨土壤固结屋过程中有岁效平均应烈力才开始俱增加炸（参见罗图抬7蜻，阶祖段萄3羊棋）料。誓5血、桩体的难影响效果挤由于碎石轻桩形成过梯程中在原静软土中置撇入一个石控桩，是黏跃土中一个败良好的排暖水通道。肢桩体对周露围土体进革行挤压产挤生超孔隙垮压力，其护随着径向住距离增大殊而减小（经见虏图盐8圆）。当径吗向应力增冲加使垂直杏应力减小贴（大咐约廉20男%你）时，而威有效平均令压力基本验上不变（杠见卡图夹7窗，阶绳段登1僚）。悄例如数值拦预报，欠乔联大当天盗通过的莫令尔库仑的地行为，仍宵是值得商卷榷的，特多别是减少屯了垂直应姓力在软粘膝土。畏图逃9哭显示变异与的归一径桥向，纵向纱和平均有柱效压力，随在中厚度愧的软粘土奋层。止振冲成桩姿后土体的氧有效平均品压力基本地没有改变尼（见嫂图透9针），而垂菌直应力减绑小使径向墙应力增大墓，切应力负或多或少牺的有些变麻化。如罚图菜1县0阶所示，桩来形成序约森1可1哲个月后，赌有效平均枣压缩应力锹进行重新宿分配（阶甩段荣2泪和惰3亮比较明显侮），大部塔分荷载由临桩承担，莲桩体应力牌和桩间土谨应力之比晓约怜为纺3~坛4水。复合地丹基的弹性驳模量也随缩之增加。膊处理后的构地基沉降炎减小。命6童、周围土佳体的改善貌效果称在第二部终分中曾提耻到过，在号地基上振厉冲碎石桩形后，复合课地基中桩晴体周围存缸在着超孔衡隙水压力晓导致的一盲些结果（察见券图但1惨2预）剩。另Dami那ett蛮e名的调查研彼究方案中侨，在安置特桩体后大欣约有暴1蒙1述个月蜡（牙33馋5核天）田的改巩固期步。这一时盐间段比预盆想的时间劫要长，也恼比固结时衣间长，有炊助于提高血软粘土强垮度特性葵。为消除姻那些过剩鄙的孔隙水愁压力，桩戒体的有效国影响半径监平均这有烈效应石力里60邻%蔑（陶图屿1配3灵），有效婆半径嫂在阳5涂m衬范围内的金实验结果震，筒Deba沙t背s礼等人实验岛[22炎]优。处理后封软土的正乐压力，纵代向平均压六力与半养径如r预之间的堂函数累关系由洋图恐1某3麦给出。恼数值计算慈结果表明哥，桩的影略响半径（继从桩轴心棋到影响点胳距）大约戚是桩径倘的俭6我倍。在这香个有效影妖响范围内谋有效平均屋应力平均索增遭长腿30多%币（见这图椒1行3盾）。定超出这个宅区域的有问效平均应舌力是几乎眨不变。旱W脉atte丰s省等父人梨[7荒]隐类似的别实验说烫明谋:捷假定显着扮增加应力俭测量在距榨离相当寻于故5挤倍柱的半是径。从摧图廊13工困，它也可愉以得出结洲论认为，唐增加有效骄平均应力趟，主要是喇由于增加逆的径向应妙力深。设冠由压力传驻感器可得六，随着距劫离桩体轴汤心距离的此增加，其滤影响效果糖在减小。奥初步加固粪之后，处宝理后复合荣地基的土霜比原软土换的有效平糠均压力有以明显的增法长缩（复图膊14顷也）娱。其绝实是由于足杨氏模量悟实提高了污很多。驾7皮.缺结束语节安装碎石赌桩在软粘桃土采用手数值私模拟怀分析，弹固有的数佣值绪可以率使傍用打plax顿i疏s秋软件计算手。模拟显系示，软粘烘土镰性的显著独改善，受拌到瘦振冲碎石夺桩安置的比影响麻。梢Mohr姑Cou世lom叼b痛对这主面会有着重大秘的贡献，甚其主要结旗论如下：字1旁，短期内团，劲振冲碎石版桩形成过繁程中，超族高孔隙压叔力，使周沙围的软粘受土排水固陷结，有效屈平均应力拜增大划约村4忘倍左右。夹2纹，之后脾的扒1姓1晶个月，碎乳石桩安装或其间软粘桂土的倡固结盐预测如下毕：宣尼-凯肆超伍孔隙水压填力所造成床的有效径仅向应力增榆加范导致拼有效平均梅应力显着型增加。义-慈有效的垂笼直应力几鸟乎不变，皮其发系数荡随着购侧向土压普力拨变化旷。骄秩-玉从这种初熟步的土壤订改良令的天距离相当胖于弄6没倍柱半径爸，改善后板的贩软土杨氏汁模量可以括通过片加固土来室估算币，誓吃G可ueti编f昌等予人矿[24缎]翠钟。羊勉-驾饥依桩体的影吗响半径和终桩间距有景着很大的得关系，还气要考虑其阶群桩效应累。绵-惜Mo夕hrC勺oulo抱m没b弟说直这些结果治的长预测蚀都通过实竿验试试验册的。土3扒.腾议在设计振呆冲碎石桩安形成复合议地的过程鄙中，就考至虑改善添软粘土杨嚷氏模量迫。因此减赏小沉降的嫂设计相对耐于以前只陪是改变原锄土壤的特铅性更为重直要。宇现在只是胁安置碎石系桩体后形甚成复合地救基以改善啊软土的特率性，提高绣土体承载揭力。将来骑我们可以氧用更好的临材料来安萍置到土壤敞中，以承弟载其上的装永久荷载旗。川Impr翠oved织sof次tcl仆ayc尖hara阿cter行isti版csd寇uet鸟ost在one高colu岗mni摧nsta映llat途ion何Z.G聋ueti专fa,叨M.携Boua约ssid皮aa,傲*,J公.M.精Deba筑tsb鲜aGe秩otec富hnic驻alE秤ngin后eeri认ngR岔esea狠rch例Team刺,05或/UR/高11-0基6,N宴atio妨nal修Engi够neer倘ing妨Scho际olo押fTu什nis,汗BP营37L层eBe惧lv迁e房′咱de`r计e10喝02T湖unis屈,Tu挺nisi轨a谋bTh界evi醉brof妈lota评tion鸦Gro退up,滚1445晴Che期min献des呈Lauv急es,估1310厨0Ai弓xen救Pro鞠venc刺e,F削ranc兽e虽Rece犯ived托12线July暖200完5;r蛛ecei淹ved状inr滑evis骄edf周orm限20S态epte伯mber据200桥6;a冬ccep吃ted慰29S堆epte柿mber香200硬6催Avai材labl拢eon戚line胆18垂Janu做ary桌2007色Abst夺ract泥Ame尚thod跨is牢prop腿osed叮for沾eva繁luat柔ing雁the于impr允ovem隔ent啊oft燕heY育oung北mod拳ulus吨of营soft撑cla请yin蒙whi威cha牌vib兼roco议mpac台te炸dst党one苦colu誉mni猫sin稠stal际led.桂By定cons羡ider君ing猾aco逮mpos肆ite泥cell层mod林ela洲num欣eric傻ala陆naly酷sis灶isc加arr霞ied俊out涨usin银gPl熄axis牢sof考twar安eto辟sim仰ulat富eth潜evi禾broc枝ompa虑ctio报nte选chni卵que,拣whi呢chl婚eads村to阳afo唤rmo迫fpr景imar窑yco滴nsol慌idat弓ion疗oft秆hes顽oft衫clay歼.Fo拨rth记enu最meri摊cal廊simu军lati引onM引ohr醉Coul标omb硬perf册ect失plas昨tic贡beha滩viou桂ris没con肆side洒red报for道the塔impr胞oved掌soi厅lco纽nsti困tuen僻ts.对From识num敢eric州alr届esul右tst政hed店egre摊eof贿imp批rove宋ment柏of历the和Youn简gmo础dulu落sso别ftc另lay凝has店been袜est拘imat啊ed.窄Also闹,th陡ezo字neo梨fin事flue沫nce轮oft将hei燃mpro然ved陷soft职cla碗yha脸sbe张enp锦redi衡cted瞒.盆Keyw校ords美:Co祸nsol太idat累ion;宣Com片posi室tec滩ell;谈You午ngm戚odul敌us;子Impr寄ovem消ent;耻Num双eric福als淡imul封atio鄙n;S偶oft猛clay敬;St池one胁colu培mn;椅Vibr田ocom裁pact裁ion伪1.I裳ntro名duct财ion慎The娇degr下eeo鬼fim穷prov绒emen盘tof煮as执oft岸soil盆by反ston搂eco队lumn课sis跳due秀to毯two究fact喝ors.壤The贡fir仅sto威nei思sin押clus靠ion欢ofa惜sti悲ffer颤col橡umn致mate阁rial红(su底cha摇scr歪ushe剂dst桶ones教,gr辽avel轨,an洗dso由ali擦ke.骄..)鞋in奶the余soft属soi垫l.T顷his盟isl挣arge让lyr飘epor蓬ted躺int黄hel秀iter油atur辞e[佣1意–唇6].尤The茅seco川ndf朝acto劣ris立the我den离sifi通cati赴ono拥fth瓣esu者rrou挺ndin勺gso攻fts造oil留duri方ngt腰hei宪nsta般llat勤ion扰oft庄hev蠢ibro统comp涉acte枪dst呼one姥colu威mni炼tsel沫fan姑dth赶esu后bseq船uent源con满soli柜dati遥onp俗roce阿sso坦ccur权ring墓in前the卖soft顾soi超lbe型fore盘the弦fin瓣all栋oadi谨ngo名fim待prov币eds三oil.夏The剃exp争erim淘enta剖lwo地rkp产erfo劈rmed虏by剩Watt爬ese本tal散.[7武],a浓ndV号autr维ain胀[8]域veri化fies耕tha县tth碗ein济stal层lati汁ono海fvi格broc净ompa蜡cted赞sto半nec慰olum娘nsl涛eads姑to框ani正mpro野veme稍nto滔fth仪ein核sit歪uso陈fts族oil装char帅acte获rist携ics就and,岂con钱sequ广entl娘y,e呈nhan蝶ces随the税load转dis旗plac叮emen劝tre滴spon犹seo堆fth锐ere恩info饱rced幼soi漆l.I翅nad津diti没on,为from还fie罚ldo猴bser良vati辽ons,茧it配was被repo栏rted献the串mec惠hani董cal况char爪acte祖rist疫ics浸(You里ngm冬odul暴us,取undr丙aine泛dco虑hesi缓on,床etc.课..男.)青oft基hei育nsi饰tus辈oft震soil汪sur盾roun亩ding战sto原nec中olum见nsw忽ere凡much结hig这her悟than锣tho允sem图easu岗red库befo漂ret域reat强ment农.As搜exa其mple盐sme荷asur爪edu徒ndra晨ined魂she睡ars短tren扑gth帮valu延esw宁ere截prov裁ided端by妄Vaut落rain蜓[8]施,an裙dre借cord兆edm艰easu拆reme牢nts橡att行hev给icin戒ity柿oft芹hec召olum兔nat宗var风ious炕dep握ths答oft慌hes摔urro塌undi冰ngs甚oft失clay纤wer虏ere拢port视edb寒yAl团-Kha至faji右and妖Cra华ig[渔9].倦Alam戴gir焦and持Zahe宫r[1替0]a领ndS藏angl将erat养[11凯]il查lust劳rate军d,i期nna择tura需lan暖dre童info万rced岩sof雄tgr怪ound至,th猛att种hes裹tand泛ard那pene赠trat惯ion界resi嫂stan瓶ceo盯fth交eso半ftg鹿roun崇dha兽sbe幕eni恼ncre歼ased追sig奋nifi史cant程lya非fter芝as事tone台col猾umn椒inst凡alla兔tion想.兴N-va染lue辅rang歪esf乏rom混2to脆7f勉orn内atur华alg怒roun铲dbu订tit们inc址reas晕esf加rom裂5to兄12榜int盼her浊einf首orce翼dso血ftg陕roun达d.A恨sig鸭nifi誉cant贴inc院reas史e,a撑vera雷ging销thr友eet肤imes屡pre紧-tre任atme略ntv嗓alue酱sin溉num于ber漠ofb殊lows凉,wa喘sme闭asur控edi骆na绞cohe至sive心soi事lat猛one隔met触erd码ista殖nce抢from暮the订col备umn甜cent题re,授Watt摇ese炕tal补.[7趴].A益lso蚂ahi秧gher翅inc雷reas假eha夕ppen拣edw品ithi抬na傍gran奥ular室soi撒lin喊sim溜ilar院con宾diti驻ons.描Van者ete围str离esul者tsr点ecor绳ded绑befo么rea抓nda平fter劈an俯emba亏nkme芳ntc胆onst呼ruct币ion牺ons土oft伪clay首rei扯nfor配ced参byc举olum贪nss堡howe槽dup彩sim杂ilar婚imp馋rove扎ment诉of眼soft舌soi割lch润arac妇teri鉴stic简s[1恼2].浊From古mea隆sure固ment买sre固cord塑eda阀tdi布ffer驴ent钢dist查ance邀sfr不omt闸hec虽olum邀nce冤ntre皱it挑has孩been惧che当cked够the凝dec约reas哥eof炉col矮umn对inst蜡alla矩tion倦eff须ect旱ast糟her碧adia亮ldi六stan巷cei夜ncre亩ases嗽.局For通abe取tter岛ill睁ustr养atio货nfi落eld乞data需are筝plo眉tted所in云the帅diag斜ram国皇‘返‘排norm政aliz追eds圈oil季char电acte右rist任ics术vers冤ust供hen遮orma膊lize父dde欣pt缺h花’晒’堪.Fo挽cusi恢ngi倚nFi蚂g.1渠,wh赖ere翻zan烧dHc井den石ote,志res储pect将ivel俭y,t秋hed牛epth绩,fr撇omt粱het膜opo什fle序vel幻grou攀nd,伸and寄colu棕m短n好’卷sle册ngth改,it突is睛show于nth颈era偷nge戏wher性eim透prov层emen老tof尚sof志tso未ilc膜hara扫cter偿isti望cst呀akes落pla晚cea炼tdi师ffer症ent恐loca君tion构sar掩ound休the蚂col穷umn搜with侨rad诉ius没rc.梁Int离erms抢of材mean间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pondingeffectivemeanstress.Forsandsandsiltsdifferentvaluesofparametermaresuggestedintherange0.5<m<1.FornormallyconsolidatedclaysandsandsinsteadofE0itisrecommendedtousethesecantmodulusE50asreferencevalueofYoungmodulus[18].Fromstress–straincurverecordedfromaclassicaltriaxialtestthesecantmodulusE50correspondstothedeviatoricstressequalsthehalfoffailurestress.Fornormallyconsolidatedclays,asspecifiedinPlaxisusers’manual,itisrecommendedtotakem=1.Inthiscase,fromEq.(1)alinearrelationshipisobtainedbetweennormalizedstiffnessmodulusandeffectivemeanstress.Therefore,theevolutionofYoungmodulusduringconsolidationofsoftclayisthatpredictedfromtheeffectivemeanstressdistribution.Basedonthisreasoning,thesimulationofvibrocompactedcolumninstallationwithinsoftclayisinvestigatedusingacompositecellmodel.Then,theevolutionsoftheeffectivemeanstressimmediatelyafterthecolumninstallationandduringtheperiodofprimaryconsolidationtakingplaceinthesoftclayarepredicted.AnumericalanalysisisundertakenbyusingthecomputersoftwarePlaxis[18]wherethestonecolumninstallationissimulatedastheexpansionofacylindricalcavitywithinsoftclay.Theproposedanalysisisdevelopedtoexaminethebehaviourofimprovedsoil,ononehand,immediatelyaftercolumninstallationand,ontheotherhand,duringagivenperiodoftheprimaryconsolidationprocessinvolvedinthesurroundingsaturatedsoftclaybeforefinalloading.3.CompositecellmodelThecompositecellmodelhasbeenwidelyconsideredbymanyresearcherstoinvestigateseveralaspectsofreinforcedsoilsbycolumns:increaseofbearingcapacity,predictionofsettlementreductionandevolutionofsoilconsolidation[4,5,19,20].Inthepresentstudy,thedataaretakenfromthestonecolumnprojectinvestigatedbyNamaaEngineeringandConsultationS.A.E,[21].Thesoilprofileismadeupofasoftclaylayerwith12.25mthicknesscoveredbyasandylayerof12.75mthickness.Thedatapresentedin[21]revealedhighermechanicalcharacteristicsfortheunderlayerlocatedbeyondthesoftclay.Therefore,thislowerbaseofthesoilprofileisassumedasrigidstratum(seeFig.4).Plaxissoftwareisusedtocarryoutnumericalcomputationsintheframeworkofanaxisymmetricstudywhichwellmatcheswiththecompositecellmodel.Theimprovedsoilismodelledwith15nodestriangularfiniteelements.Fromnumericalresultswefocusontheeffectoftheexpansionofthecolumnandtheprimaryconsolidationinducedwithinthesoftclay.Predictedresultsarepresentedatthemid-thicknessofsoftclaylayer(cf.dashedlineinFig.4)atdistances0.6m,1.5mand2.4mfromtheaxisofthecolumn.Fig.5.Simulationbythecompositecellmodelofstonecolumnexpansion:(a)modelofimprovedsoil;(b)modellingcolumnexpansion;(c)discretizedimprovedsoil.Thelateralexpansiongenerateslargestrains,estimatedaboutof45%,insoftclayneighbourhoodthecolumn.Thisfactistakenintoaccountbyadoptingthe‘‘updatedmesh’’optionprovidedbyPlaxissoftware(cf.Fig.5c).Inordertoanalyzethereinforcingeffectduringthecolumninstallationandtheconsolidationoccurringinsoftclay,amultistagemodellingisperformed.Itincludesthreestages:undrainedexpansionofthecolumnwithinthesoftclay,compactionoftheoverlyingsandlayerplayingtheroleofsurcharge,andconsolidationoftheimprovedsoilduring11months,Debatsetal.[22].ModellingthebehaviourofsoftclayandcolumnmaterialcanbedonebymeansofvariousconstitutivelawsprovidedinPlaxissoftware.Indeed,beyondthisvalueithasbeenindicatedthatthecolumninstallationhasnoeffectontheresponseoftheimprovedsoil.Thestateofstressatresthasbeenadoptedasinitialconditionfornumericalcomputationsbothinthecolumnandinsoftclayi.e.=1-.K0=coefficientoflateralearthpressureatrest.4.ResultsandinterpretationsAfterthecolumninstallationtwostages,intime,arediscussed.Thefirstonecorrespondstotheimmediatetimeaftertheexpansionofcolumnmaterial.Thesecondstagecorrespondstotheendofprimaryconsolidationwhichtakesplaceinthesoftclay.Asaresultoftheexpansionofthecolumnthedeviatoricstress,denotedq,isincreasedatthevicinityofthecolumnuntilthefailurestateisreachedinthesurroundingsoftclaywherelargeplasticradialstrainsarepredicted(seeFig.6,phase1).Thisresultisnotinaccordancewiththeassumptionadoptedinseveralcontributionspostulatingthatayieldingstateisreachedonlyinthecolumnmaterial,whilethesurroundingsoftsoilremainsintheelasticphase[23,3].Fig.6clearlyindicatesthattheeffectofthecolumnexpansiondecreasesintermofdeviatoricstresswhileplasticstrainsbecomesmallerandsmallerasthedistance,fromthecolumn’saxis,increases(from0.6mto2.4m).Immediatelyaftertheexpansionofthecolumntoitsfinaldiameterandthesubstitutionofthe‘‘dummymaterial’’forthegranularcolumnmaterial,thesurroundingsoftclayisrelaxedandasignificantdecreaseofthedeviatoricstressisnotedclosetotheinterfacebetweenthesoftclayandcolumn(r=0.6m,cf.Fig.6).Consequently,alateralcompression(confiningpressure)isexertedbythesurroundingsoftclayonthecolumn.However,theeffectivemeanstressiskeptunchanged(cf.Fig.7,phase2)inthesoftclaywheretheconsolidationprocess(cf.Fig.7,phase3)isjuststarting.Thestresspathspredictedfromthenumericalsimulation(cf.Fig.7)demonstratetheevolutionofstressinvariantsduringtheexpansionofthecolumnandtheconsolidationphasetakingplaceinthesoftclay.5.EffectoftheexpansionofthecolumnSincethereinforcingcolumnisinstalledbyalateralexpansiontheexcessporewaterpressuresaregeneratedinthesaturatedinsitusoftsoil.Immediatelyaftertheexpansionofthecolumn,thevariationofexcessporepressureswithradialdistance(seeFig.8)illustratesverylargepredictedvaluesnearthesoil–columninterface.Thepredictedporewaterpressuresdecreasewhenthedistancefromtheedgeofthecolumnincreases.Whilethedistributionofeffectivemeanstressremainsunchangedinsoftclay(cf.Fig.7,phase1)theradialeffectivestresslargelyincreasestothedetrimentoftheverticaleffectivestress,whichdecreasesuniformlybysome20%.Suchnumericalprediction,owedtotheadoptedMohrCoulombbehaviour,remainsdebatableespeciallythedecreaseofverticalstressinsoftclay.Fig.9showsthevariationsofnormalizedradial,verticalandmeaneffectivestressesatthemid-thicknessofthesoftclaylayer.Immediatelyaftertheinstallationofthecolumn,itisshownthateffectivemeanstressesarenotmodifiedinthesurroundingsoftclay(cf.Fig.9).Theincreaseofradialstressiscompensatedbyadecreasedverticalstress,whilevaluesoftangentialstressremainmoreorlessconstant.However,itshouldbenoted(seeFig.10)thatthecompressiveeffectivemeanstressinducedinthedrainedcolumn(whichincreasessignificantlybetweenphase2andphase3)ispredictedaboutthreetofourtimestheeffectivemeanstressgeneratedinthesoftclayduringtheconsolidationperiodof11months.FromthelatteranincreaseoftheYoungmodulusofcolumnmaterialisalsoexpected.Aftertheexpansionofthecolumnanewdistributionofstressestakesplaceinthesurroundingsoftclay(seeFig.11)thatcanbequantifiedbytheratiobetweeneffectiveradialandverticalstresses().Fromthepredictedincreaseofeffectiveradialstressthelateralstrainsinsoftclayaresignificantlyreduced.Consequentlythecoefficientoflateralearthpressureatrest,denotedbyK0,increasesfromitsinitialvalue(=1-=0.64)insoftclaytovaluesexceedingoneatthevicinityofthecolumn.Itshouldbementionedthatahydrostaticstate(K0=1)beinganeffectofstonecolumnsinstallationintheinitialsoilwasassumedbyPriebe[3]forsettlementcalculation.6.EffectoftheconsolidationinthesurroundingsoftclayAsmentionedinSection2,upontheinstallationofavibrocompactedstonecolumn,thereisasubsequentconsolidationprocessinvolvedinthesurroundingsoftclaythatresultsfromthedissipationofexcessporewaterpressures(seeFig.12).InthecaseofDamietteprojectinvestigatedhere,theconsolidationperiodof11months(335days)aftercolumninstallationcorrespondstotheactualtankconstructionschedule.Thisperiodappearstobelargerthantherealtimeneededtoachievetheprimaryconsolidationwhichcontributestotheincreaseofsoftclaystrengthcharacteristics.Consequently,duetothisquasi-totaldissipationofexcessporewaterpressureanimportantincreaseoftheeffectiveradialstress,averaging60%(seeFig.13)withinaradiusof5mfromtheaxisofthecolumn,isrecorded,Debatsetal.,[22].Thenormalizedradial,verticalandmeaneffectivestressesevolutionsasafunctionofradialdistance,atthemidthicknessofthecompletelyconsolidatedsoftclaylayer,areplottedinFig.13.Thus,numericalresultsshow,aftertheprimaryconsolidationofsoftclay,theexistenceofazoneofinfluencesurroundingthecolumnuptoadistance(fromthecentreofthecolumn)estimatedbysixtimestheradiusofthecolumn(i.e.3m).Withinthiszoneofinfluence,theeffectivemeanstressincreasesbysome30%inaverage(cf.Fig.13).Beyondthiszonetheeffectivemeanstressispracticallyunchanged.AsimilarobservationhasbeenreportedbyWattesetal.[7]postulatingasignificantstressincreasingmeasuredatadistanceequivalenttofivetimesthecolumn’sradius.FromFig.13itcanbealsoconcludedthattheincreaseofeffectivemeanstressismainlyduetotheincreaseofradialstress.Thediminishingeffectwithincreasingdistancefromthecolumncentrewasalsoobservedaftermeasurementsrecordedfrompressurecellslocatedatdifferentdistancefromthecolumnaxis,Wattesetal.[7].Attheendofprimaryconsolidation,itispredictedanincreaseoftheeffectivemeanstressinthereinforcedsoilwithhighervalueswithinthecolumn