孔擴(kuò)張理論研究及自鉆式旁壓試驗(yàn)數(shù)值分析

1、孔擴(kuò)張理論研究及自鉆式旁壓試驗(yàn)數(shù)值分析        旁壓試驗(yàn)的基本原理是利用旁壓儀對(duì)鉆孔壁施加橫向均勻應(yīng)力,使孔壁土體發(fā)生徑向變形直至破壞,量測(cè)壓力和徑向變形的關(guān)系,或加壓使孔壁膨脹至某孔徑時(shí)做保持試驗(yàn),觀測(cè)超孔隙水壓力的消散過(guò)程,然后可根據(jù)試驗(yàn)結(jié)果(壓力-體積曲線或超孔隙水壓力消散曲線)確定土體的物理力學(xué)性質(zhì)、區(qū)分土類(lèi)及確定地基承載力等。其中,自鉆式旁壓試驗(yàn)(SBPT)因具有擾動(dòng)小、測(cè)試深度大等優(yōu)點(diǎn)成為確定土性參數(shù)有效的原位測(cè)試手段之一。在飽和軟黏土層中宜采用SBPT,但利用常用的步驟確定的黏土不排水剪切強(qiáng)度及

2、滲透特性參數(shù)與其它高質(zhì)量的試驗(yàn)結(jié)果有差別。因此,分析其在確定黏土不排水剪切強(qiáng)度和滲透性參數(shù)時(shí)可能產(chǎn)生的誤差及修正研究是本論文的研究目的之一。大多數(shù)旁壓試驗(yàn)的分析方法基于柱形孔擴(kuò)張理論,其中包含材料特性、旁壓儀幾何特征和排水條件等重要假設(shè)。除了對(duì)旁壓試驗(yàn)機(jī)理解釋及反分析確定土體特性參數(shù)外,孔擴(kuò)張理論還是許多其它巖土工程問(wèn)題研究的基礎(chǔ)理論。目前利用較符合實(shí)際地描述土體的非脆性軟化及剪脹,并考慮中主應(yīng)力影響的模型進(jìn)行的孔擴(kuò)張分析尚少。因此,發(fā)展和完善孔擴(kuò)張理論是本論文研究的另一目的。具體地,論文主要研究?jī)?nèi)容及所取得的研究成果包括以下方面:1、回顧SBPT在確定土性參數(shù)上的應(yīng)用,討論了以孔擴(kuò)張理論為基

3、礎(chǔ)確定的土性參數(shù)的誤差來(lái)源,總結(jié)了現(xiàn)有的誤差分析方法及誤差修正。2、針對(duì)中主應(yīng)力對(duì)孔擴(kuò)張問(wèn)題的影響,采用SMP破壞準(zhǔn)則及非關(guān)聯(lián)流動(dòng)法則,并結(jié)合Bolton簡(jiǎn)化的應(yīng)力-剪脹關(guān)系及Bolton模型,以反映砂土的剪脹和應(yīng)變軟化特性,運(yùn)用塑性區(qū)離散分析方法,對(duì)不同初始狀態(tài)下典型石英砂(Ottawa砂)中的柱形孔擴(kuò)張問(wèn)題進(jìn)行了分析及數(shù)值計(jì)算,不僅得到了砂土中柱孔擴(kuò)張的應(yīng)力場(chǎng)、應(yīng)變場(chǎng)、極限擴(kuò)孔壓力、塑性區(qū)半徑,還可獲得塑性區(qū)土體的內(nèi)摩擦角、剪脹角和孔隙比的變化。該數(shù)值方法可同時(shí)用來(lái)分析零和非零初始孔徑的柱形孔擴(kuò)張問(wèn)題。通過(guò)與基于Mohr-Coulomb破壞準(zhǔn)則的計(jì)算結(jié)果比較分析,探討了孔擴(kuò)張問(wèn)題中的中主應(yīng)

4、力效應(yīng),并闡述了砂土的相對(duì)密度、初始應(yīng)力、臨界狀態(tài)摩擦角對(duì)塑性區(qū)半徑比、極限擴(kuò)孔壓力、剪脹等的影響。比較結(jié)果表明基于Mohr-Coulomb破壞準(zhǔn)則的解答偏于保守。繪制了一系列不同初始狀態(tài)和臨界狀態(tài)內(nèi)摩擦角時(shí)Ottawa砂的孔擴(kuò)張結(jié)果圖,為確定具有相似性質(zhì)的砂土中柱孔擴(kuò)張的極限擴(kuò)孔壓力和塑性區(qū)半徑提供直接的參考。3、針對(duì)非脆性軟化及剪脹的摩擦黏性巖土材料中的柱形孔擴(kuò)張問(wèn)題,將土體損傷后的軟化特性用材料的力學(xué)性能參數(shù)下降來(lái)描述,引入損傷軟化參數(shù)?？紤]土體的剪脹與內(nèi)摩擦角、黏聚力以及平均有效應(yīng)力的相關(guān)性和結(jié)構(gòu)性衰減(軟化特性)對(duì)應(yīng)力-剪脹關(guān)系的影響,將反映結(jié)構(gòu)性衰減的黏聚力下降公式代入Rowe(1

5、971)的應(yīng)力-剪脹方程,修改了Rowe(1971)的應(yīng)力-剪脹關(guān)系。在塑性區(qū)采用對(duì)數(shù)應(yīng)變描述土體的大應(yīng)變特性,在彈性區(qū)采用小應(yīng)變理論,同時(shí)使用廣義SMP準(zhǔn)則和修改的應(yīng)力-剪脹關(guān)系,結(jié)合平面應(yīng)變軸對(duì)稱(chēng)問(wèn)題的控制方程,推導(dǎo)并給出完全排水條件下摩擦黏性巖土材料中的柱形孔擴(kuò)張問(wèn)題的求解步驟,得到柱形孔擴(kuò)張的孔擴(kuò)張率、塑性流動(dòng)參數(shù)(剪脹)、極限擴(kuò)孔壓力、塑性區(qū)變化規(guī)律等。進(jìn)一步,通過(guò)損傷軟化參數(shù)、內(nèi)摩擦角和黏聚力的變參數(shù)計(jì)算,探討了它們對(duì)土體剪脹性、塑性區(qū)應(yīng)力、塑性區(qū)變化規(guī)律及極限擴(kuò)孔壓力的影響;并與基于Mohr-Coulomb準(zhǔn)則的解答進(jìn)行對(duì)比分析,研究了中主應(yīng)力對(duì)孔擴(kuò)張結(jié)果的影響。4、利用通用有限

6、元分析軟件ABAQUS模擬黏土中不排水條件下的自鉆式旁壓加載試驗(yàn),針對(duì)旁壓儀探頭長(zhǎng)徑比及應(yīng)變區(qū)間的選擇對(duì)確定土體不排水剪切強(qiáng)度的影響進(jìn)行了分析。使用修正劍橋模型模擬土體應(yīng)力-應(yīng)變特性,采用低滲透系數(shù)控制加載過(guò)程中的不排水條件,通過(guò)數(shù)值計(jì)算確定不同長(zhǎng)徑比、不同應(yīng)力歷史下的旁壓加載曲線。根據(jù)Gibson等建議的方法,選擇幾個(gè)應(yīng)變區(qū)間,用最小二乘法確定該區(qū)間曲線的斜率,該斜率為不排水剪切強(qiáng)度值。當(dāng)只考慮長(zhǎng)徑比對(duì)確定不排水剪切強(qiáng)度的影響時(shí),以從L/D=的旁壓曲線確定的不排水剪切強(qiáng)度作為真實(shí)值S_u,不同應(yīng)力歷史(OCR)下土體的不排水剪切強(qiáng)度真實(shí)值與不同長(zhǎng)徑比時(shí)確定的不排水剪切強(qiáng)度的比值S_u/S_u

7、(L/D)作為考慮L/D影響的修正系數(shù)。通過(guò)OCR在120,L/D在6范圍內(nèi)的計(jì)算,確定了從正常固結(jié)到重超固結(jié)土體,5種長(zhǎng)徑比時(shí)不排水剪切強(qiáng)度修正系數(shù)。應(yīng)變區(qū)間對(duì)修正系數(shù)有一定影響,對(duì)于同一旁壓曲線,選擇的應(yīng)變區(qū)間越大,修正系數(shù)會(huì)有所減小,不排水剪切強(qiáng)度的高估量增加。而應(yīng)變區(qū)間的選擇對(duì)確定不排水剪切強(qiáng)度值的影響顯著,隨著應(yīng)變區(qū)間的增大,不排水剪切強(qiáng)度明顯增加。因此,選擇合適的應(yīng)變區(qū)間對(duì)測(cè)定較準(zhǔn)確的不排水剪切強(qiáng)度值至關(guān)重要。通過(guò)將從有限元模擬的平面應(yīng)變和不排水條件下的旁壓加載曲線確定的不排水剪切強(qiáng)度和Cao等解析解比較,建議了不同應(yīng)力歷史下確定不排水剪切強(qiáng)度的應(yīng)變區(qū)間。5、以Ducker-Pra

8、ger理想彈塑性模型為基礎(chǔ),模擬了旁壓應(yīng)變保持試驗(yàn)(SHT),分析旁壓儀幾何尺寸、加載應(yīng)變水平、滲透系數(shù)、應(yīng)變加載率對(duì)孔擴(kuò)張過(guò)程中排水情況、超孔隙水壓力分布及土體固結(jié)行為的影響,建議了應(yīng)變加載率,給出了Randolph解的適用條件,并考慮不同滲透系數(shù)、不同應(yīng)變加載率引起的部分排水的影響,對(duì)Randolph解進(jìn)行修正,建議了誤差曲線。為分析土體流變性對(duì)SHT過(guò)程中超孔隙水壓力消散及推求土體固結(jié)系數(shù)的影響,采用Drucker-Prager理想彈塑性與土體流變的耦合模型描述土體的應(yīng)力應(yīng)變強(qiáng)度關(guān)系及其對(duì)時(shí)間的依賴(lài)性,利用有限元法模擬SHT過(guò)程,分析土體流變性對(duì)超孔隙水壓力消散的影響。其中,流變時(shí)間硬化

9、模型參數(shù)由土體流變?cè)囼?yàn)確定。分析發(fā)現(xiàn),土體的流變對(duì)超孔隙水壓力消散的影響,除了通過(guò)應(yīng)力松弛改變應(yīng)力水平而使超孔隙水壓力下降,還通過(guò)改變超孔隙水壓力的分布而改變固結(jié)速度?？紤]流變使不同滲透系數(shù)時(shí)超孔隙水壓力消散過(guò)程差異變大。滲透系數(shù)越小,土體流變性對(duì)超孔隙水壓力的消散速度影響越明顯。以s_u=48kPa,I_r=156為例,在流變?cè)囼?yàn)確定的參數(shù)變化范圍內(nèi)進(jìn)行變參數(shù)計(jì)算,分析流變參數(shù)變化對(duì)超孔隙水壓力消散的影響Self-boring pressuremeter(SBPT) has been effectively utilized to determine in-situ properties o

10、f soils in terms of overcoming soil disturbance to a large extent,especially in saturated soft soil for determination of permeability characteristics and undrained shear strength.However,it is recognized that the undrained strength value and horizontal coefficient of consolidation derived from SBPT

11、are different from those obtained by good laboratory tests and other in situ tests.In order to find the reasons of inaccuracy,the possible factors are analyzed by using finite element methods and quantified by comparing with analytical solutions of cylindrical cavity expansion that are useful interp

12、retation methods which involves important assumptions about material behavior,pressuremeter geometry and drained conditions.Many other geotechnical engineering problems also rely on cavity expansion theory for analyzing,such as pile driving,CPT and anchor working mechanics. And the theory has been p

13、rogressively refined over the past years.Many research results indicate that it is important for study on cavity expansion theory to choose the reasonable yield criterion and to consider the strain-softening and shear dilatation behavior of geomaterials.Though achievements have been made on the theo

14、retical study,the cavity expansion in non-brittle softening and dilatant soil is in the stage of development.Thus the other purpose of this paper is to present cylindrical cavity expansion analysis in dilatant and elastic-plastic soil,taking non-brittle softening and dilatancy behavior of geomateria

15、ls,and the effect of intermediate principle stress on soil strength into account.The main investigations consist of the following portions:1,The current use of fundamental mechanics in developing rational interpretation methods for deriving soil properties from SBPT is reviewed,and the sources of in

16、accuracy and modified methods are analyzed and summarized.2,Considering the effects of intermediate principal stress on material yielding strength, SMP yielding criterion is selected in analysis of cylindrical cavity expansion.Meanwhile, combining Bolton's simplification on the stress-dilatancy

17、relationship and Bolton model to reflect soil shear dilatancy and strain softening behavior this paper analyzes the problem of cylindrical cavity expansion in typical quartz sand(Ottawa sand) by discretizing the plastic zone to determine stress field,strain field,limit cavity pressure,and variation

18、of friction angle, dilatancy angle and void ratio.It is noted that this analysis is applicable to cavity expansions from zero initial radius and finite initial radius simultaneously.The impacts of intermediate principal stress on cavity expansion are examined by comparing the present solutions with

19、results based on Mohr-Coulomb criterion.And the effects of initial state and critical friction angle on ratio of plastic radius,limit pressure,dilatancy etc.are explored.A set of charts have been provided for use in direct estimate of limit pressure and ratio of plastic to cavity radius as a functio

20、n of soil state(relative density and initial stress state) for soils with the similar properties to Ottawa sands.3、The extended spatial mobilization plane theory(SMP) is adopted in consideration of the effect of intermediate principal stress on soil shear strength.Damage softening parameter, taken a

21、s degradation grads of material mechanics capacity,is introduced to depict the soils softening behavior after yielding.Rowe's Stress-dilatancy equation in combination with cohesion degradation expression by damage softening parameter is rewritten.It can simultaneously consider the bonding compon

22、ent in dilatant response of elastic-plastic soils and softening behavior due to structure degradation for cemented soils during cavity expansion.The cavity expansion problem is formulated in small strain in the elastic zone and large strain in the plastic zone.Based on the extended SMP criterion and

23、 stress-dilatancy relation,the governing equations of axisymmetric problem in the plane strain condition and the partial differential equations for the boundary-value problem of cavity expansion in frictional cohesive soils are established.Solutions of radial and hoop stresses and strains around an

24、expanding cavity are obtained by recursive computations.The significance of consideration of the effect of intermediate principal stress is demonstrated by a comparative study between the present solution and current solution based on Mohr-Coulomb criterion. Also the influence of damage softening pa

25、rameter,cohesion and friction angle is examined by a parametric study.The proposed solution presented here can be of interest for the interpretation of pressuremeter tests or pushing-in piles carried out in cohesive-frictional materials under drained condition.4,Numerical simulations of self-boring

26、pressuremeter loading tests using ABAQUS are performed to assess the influence of limit length of SBP and strain range on undrained shear strength derived from pressuremeter curves.The numerical models are built where soils behavior is depicted by modified Cam model,and low coefficients of permeabil

27、ity are chosen to control undrained conditions during expansion.The pressuremeter curves are obtained from numerical simulation of SBPTs for ratio of length to diameter L/D values of 6,10,15, 20 and various overconsolidation OCR range from 1 to 20.According to Gibson's analysis, the undrained sh

28、ear strength using the least squares method is determined from the several chosen strain ranges.The ratio between the undrained shear strength obtained with infinite L/D and the value with various L/D is defined as correction factor,which are obtained from several strain ranges.It is observed that s

29、train range over which the pressuremeter curve is fitted has little influence on derivation of the correction factors,while it has significant influence on undrained shear strength.And further,strain ranges of deriving undrained shear strength are proposed for different overconsolidation ratio by co

30、mparing finite element results for L/D=with analytical solution(Cao et al.) based on modified Cam clay model.5,Based on elastic-perfectly plastic Drucker-Prager model,analysis of holding tests using FEM is carried out to illuminate the applied conditions of commonly used a closed-form solution propo

31、sed by Randolph and Wroth through investigating the effects of pressuremeter geometry,the strain rate,permeability coefficient and the cavity strain level on partial drainage during cavity expansion,distribution of excess pore pressure,dissipation of excess pressure during SHT.Moreover,based on the results of numerical analyses,strain rate is proposed and the values of time factor T_(50) provided by Randolph el al.to estimate horizontal consolidation coefficient are modified in view of the effects of soil permeabili