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      增層開(kāi)挖引起既有預(yù)制樁殘余應(yīng)力釋放分析

      2015-10-24 01:09:48茍堯泊夏唐代
      關(guān)鍵詞:靜壓巖土樁基

      茍堯泊,俞 峰,夏唐代

      (1.浙江理工大學(xué)建筑工程學(xué)院,浙江杭州310018;2.浙江大學(xué)濱海和城市巖土工程研究中心,浙江杭州310058)

      增層開(kāi)挖引起既有預(yù)制樁殘余應(yīng)力釋放分析

      茍堯泊1,俞 峰1,夏唐代2

      (1.浙江理工大學(xué)建筑工程學(xué)院,浙江杭州310018;2.浙江大學(xué)濱海和城市巖土工程研究中心,浙江杭州310058)

      為了更好地理解地下室增層開(kāi)挖后既有工程樁的應(yīng)力狀態(tài)和承載性狀,以采用預(yù)制樁基礎(chǔ)的高層建筑為研究對(duì)象,分析增層開(kāi)挖對(duì)既有預(yù)制樁施工殘余應(yīng)力的影響.從施工殘余應(yīng)力簡(jiǎn)化模型和能量法概念出發(fā),提出樁殘余應(yīng)力的計(jì)算模型,再結(jié)合增層開(kāi)挖前后樁側(cè)摩阻力和樁端阻力的計(jì)算模型以及回彈量的計(jì)算方法,確定計(jì)算殘余應(yīng)力所需的模型參數(shù),模擬出增層開(kāi)挖前后樁身殘余應(yīng)力的變化情況.結(jié)果表明,增層開(kāi)挖會(huì)造成樁側(cè)摩阻力及樁端阻力的損失,并引起更高幅度的樁身殘余應(yīng)力釋放.同時(shí),樁殘余應(yīng)力的釋放程度與計(jì)算點(diǎn)到開(kāi)挖面的距離有關(guān),距離越遠(yuǎn)釋放幅度越小,這說(shuō)明殘余應(yīng)力的釋放與樁-土間的相對(duì)位移有關(guān).

      既有建筑;開(kāi)挖;預(yù)制樁;殘余應(yīng)力;模擬

      近年來(lái),“停車難”問(wèn)題日益嚴(yán)重,考慮到大批無(wú)地下室或單層地下室的樁基建筑拆除重建代價(jià)過(guò)大,于是在既有建筑下開(kāi)挖增建地下室成為最佳解決辦法,也演變?yōu)橐环N新興的地下空間開(kāi)發(fā)技術(shù).整個(gè)地下室增建過(guò)程涉及一系列復(fù)雜的技術(shù),其中有關(guān)樁基托換技術(shù)的研究已日漸成熟,如賈強(qiáng)等[1]研究了錨桿靜壓樁在增設(shè)地下空間方面的應(yīng)用,對(duì)錨桿靜壓樁的沉樁阻力、沉樁數(shù)量、承臺(tái)抗剪、沉降規(guī)律等幾個(gè)關(guān)鍵的問(wèn)題進(jìn)行了研究;杜斌等[2]全面介紹了鋼管纖維樁的工法特點(diǎn)、設(shè)計(jì)以及施工;文穎文等[3]則將錨桿靜壓樁技術(shù)應(yīng)用于工程實(shí)踐,在既有建筑下成功新增一層3.6 m高地下車庫(kù).有關(guān)樁基在開(kāi)挖條件下承載性狀的研究也是主攻的方向,如Zheng等[4]研究基坑開(kāi)挖對(duì)樁基承載力產(chǎn)生的影響時(shí)發(fā)現(xiàn),基坑開(kāi)挖會(huì)引起低摩擦樁承載力降低16%~20%,高摩擦樁承載力提高22%~44%;胡琦等[5-7]先后研究了大面積深基坑開(kāi)挖對(duì)坑中樁的影響,認(rèn)為開(kāi)挖條件下既有樁體會(huì)回彈受拉,同時(shí)樁基承載力降低;龔曉南等[8]、伍程杰等[9]用理論分析和數(shù)值模擬的方法,分析了地下室增層開(kāi)挖前后樁側(cè)摩阻力、樁端阻力的損失程度,認(rèn)為損失比隨增層開(kāi)挖寬度和開(kāi)挖深度的增大而增大.然而,現(xiàn)有地下室增層工程相關(guān)研究集中關(guān)注開(kāi)挖對(duì)既有基礎(chǔ)工作階段性狀的影響,尚未將樁基施工階段形成的承載影響效應(yīng)納入到研究體系中.

      實(shí)際上,沉樁完畢后、樁頂荷載施加前的預(yù)制樁并非處于零應(yīng)力狀態(tài).施工殘余應(yīng)力是預(yù)制型樁基的主要施工效應(yīng)之一,它是指沉樁完成一階段后,樁身彈性壓縮變形受樁周土約束無(wú)法完全恢復(fù)而產(chǎn)生的樁身應(yīng)力,忽略其影響會(huì)明顯錯(cuò)估樁的承載力性狀.很早以前就有學(xué)者[10-11]指出殘余應(yīng)力的重要性,近年來(lái),俞峰等[12]通過(guò)對(duì)靜壓樁殘余應(yīng)力的長(zhǎng)期原位觀測(cè)后發(fā)現(xiàn),H型鋼樁在花崗巖殘積土中靜壓沉樁后,樁端殘余應(yīng)力竟高達(dá)57 MPa,這也證明施工殘余應(yīng)力的影響不容忽視.但有關(guān)施工殘余應(yīng)力的研究常集中在沉樁過(guò)程,并未涉及到之后基坑開(kāi)挖的過(guò)程.

      本文以既有建筑下開(kāi)挖地下室增層為工程背景,從施工殘余應(yīng)力簡(jiǎn)化模型和能量法概念出發(fā),首先提出樁身殘余應(yīng)力的計(jì)算模型,再結(jié)合增層開(kāi)挖前后樁側(cè)摩阻力及樁端阻力的計(jì)算模型以及回彈量的計(jì)算方法,確定出計(jì)算殘余應(yīng)力所需的樁側(cè)摩阻力、樁端阻力、埋置深度等參數(shù)的數(shù)值,最終模擬出增層開(kāi)挖前后樁身殘余應(yīng)力的變化情況.

      1 工程背景

      以浙江飯店增層改造工程為參考分析預(yù)制樁在增層開(kāi)挖前后的樁身殘余應(yīng)力.主樓12層,附樓4層,框剪結(jié)構(gòu),整體設(shè)一層地下室,平面呈狹長(zhǎng)L形,占地面積約2 600 m2[13].采用樁筏基礎(chǔ),設(shè)工程樁采用樁徑0.9 m的預(yù)制樁,樁長(zhǎng)34 m,嵌巖1.1 m,既有地下室埋深5.1 m.增層開(kāi)挖寬度18.5 m,增層開(kāi)挖深度6.5 m,增層開(kāi)挖前基礎(chǔ)地板下土體超載p=11 k Pa.其中土層分布及各土層的物理力學(xué)參數(shù)[14]如表1所示.表中:d為層厚,γ為重度, c為黏聚力,φ為內(nèi)摩擦角,κ為滲透系數(shù),E為壓縮模量,ν為泊松比.

      表1 土層物理力學(xué)參數(shù)Tab.1 Physical and mechanical parameters of soil

      2 模擬計(jì)算

      2.1 殘余應(yīng)力計(jì)算模型

      根據(jù)俞峰等[15]提出的施工殘余應(yīng)力簡(jiǎn)化分布模型,繪出如圖1所示的摩阻力折線分布示意圖.

      根據(jù)現(xiàn)場(chǎng)試驗(yàn)的結(jié)果,對(duì)于粗粒土以及黏土、粉土[16],Zn/Lp基本處于0.7~0.9的范圍內(nèi),均值約為0.8,結(jié)合簡(jiǎn)化模型取Zn=0.8Lp,Zlim=0.732Lp,其中:Lp為埋置深度;Zn為中性點(diǎn)的位置;Zlim為與flim對(duì)應(yīng)的深度,flim為殘余負(fù)摩阻力極值.再由能量法[17]中的計(jì)算模型可得

      式中:fu為樁側(cè)極限摩阻力;K為樁側(cè)土壓力系數(shù);δ為樁-土界面摩擦角;σv為樁側(cè)土豎向有效應(yīng)力;β為經(jīng)驗(yàn)系數(shù);γ′為土層有效重度;z為計(jì)算深度;k為摩阻力分布系數(shù);qu為樁端極限阻力;ke為剛度系數(shù);su為發(fā)揮極限端阻所需的樁-土相對(duì)位移,對(duì)于粉土和砂土,可取su=10 mm[18];假定樁端位移在達(dá)到極值之前與樁端應(yīng)力呈線性關(guān)系,可得

      圖1 極限摩阻力及殘余摩阻力分布Fig.1 Distribution of ultimate friction and residual friction

      式中:qr為樁端殘余阻力;se為樁端回彈量;樁端殘余應(yīng)力qr與極限殘余負(fù)摩阻力flim存在以下關(guān)系:

      式中:ζs為樁截面周長(zhǎng)樁身;Ap為截面積;再由樁身殘余應(yīng)力σr與樁側(cè)摩阻力fr之間的微分關(guān)系,最終得出樁身殘余應(yīng)力的計(jì)算式:

      式中:fu、qu、Lp均為計(jì)算樁身殘余應(yīng)力所需的參數(shù),只要分別確定出它們的取值,便可計(jì)算出殘余應(yīng)力σr,增層開(kāi)挖前后樁身殘余應(yīng)力分別為σr1、σr2.

      2.2 樁側(cè)極限摩阻力θus的計(jì)算

      考慮到增層開(kāi)挖前后,土體固結(jié)程度的差異,推導(dǎo)出增層開(kāi)挖前后樁側(cè)單位面積摩阻力fu的計(jì)算公式[8]:

      增層開(kāi)挖前,

      增層開(kāi)挖后,

      式中:φ′為土體有效內(nèi)摩擦角;OCR為土體的超固結(jié)比,等于增層開(kāi)挖前后的豎向有效應(yīng)力之比.

      土體地面下z處的豎向有效應(yīng)力:

      式中:σz為計(jì)算點(diǎn)的豎向附加應(yīng)力.若寬度為a的條形均布荷載p作用在各項(xiàng)同性的均質(zhì)彈性半空間內(nèi)部深度h處,如圖2所示,則邊緣點(diǎn)下的豎向附加應(yīng)力公式為

      圖2 Mindlin應(yīng)力解示意圖Fig.2 Sketch of Mindlin’s stress solution

      考慮到基坑開(kāi)挖的對(duì)稱性,單樁處于開(kāi)挖寬度的對(duì)稱軸上,即開(kāi)挖深度為2a,忽略樁體對(duì)土中應(yīng)力的影響,則整個(gè)樁周土體由于在深度h處施加超載或卸載p引起計(jì)算點(diǎn)z處的豎向有效應(yīng)力增長(zhǎng)或減少為

      開(kāi)挖微小高度d h土體,即p=γ′d h,則開(kāi)挖地面下深度h范圍內(nèi)土體引起計(jì)算點(diǎn)z處的豎向有效應(yīng)力減少量(修正后)為

      基礎(chǔ)底板下土體超載p引起計(jì)算點(diǎn)處的豎向有效應(yīng)力增加,增量pc=σz.

      則增層開(kāi)挖前豎向有效應(yīng)力為

      增層開(kāi)挖后豎向有效應(yīng)力為

      式中:pt1、pt2分別為增層開(kāi)挖前后卸載h1、h2深度范圍內(nèi)土體引起計(jì)算點(diǎn)z處的豎向有效應(yīng)力減少量.將式(15)和(16)分別代入式(8)和(9)即可得出增層開(kāi)挖前后樁側(cè)單位面積側(cè)阻.

      增層開(kāi)挖前后的樁側(cè)極限摩阻力:

      增層開(kāi)挖前,

      增層開(kāi)挖后,

      2.3 樁端極限端阻Qu的計(jì)算

      由推導(dǎo)出的樁端阻力計(jì)算公式[9]:

      式中:Qup為樁端極限阻力,sβ為形狀系數(shù),σh為樁端邊界面上的豎向應(yīng)力,η為強(qiáng)度模量,ρm為平均摩擦角,ρ1、ρ2均為瞬時(shí)摩擦角,ζ為抗拉強(qiáng)度系數(shù).其中

      式中:σc為完整巖塊的無(wú)側(cè)限抗壓強(qiáng)度;m、s為Hoek-Brown常量.

      應(yīng)用Mindlin應(yīng)力解考慮土體開(kāi)挖和土體卸載,可得出樁端到基巖表面的傾斜邊界上的無(wú)量綱化平均上覆壓力hm:

      增層開(kāi)挖之前

      增層開(kāi)挖后

      式中:γR、γS分別為基巖和基巖上覆土體的平均有效重度,HR、HS分別為樁體嵌巖深度和基巖上覆土體厚度,由本節(jié)公式可分別求得增層開(kāi)挖前后樁端極限阻力.

      2.4 增層開(kāi)挖前后樁側(cè)摩阻力及樁端阻力

      計(jì)算出增層開(kāi)挖前后樁側(cè)及樁端的極限阻力如表2所示.

      表2 增層開(kāi)挖前后樁身阻力Tab.2 Change of pile resistances due to excavation

      2.5 計(jì)算埋置深度Lp

      采用基坑回彈的實(shí)用計(jì)算法[19],在考慮開(kāi)挖面下殘余應(yīng)力影響深度的基礎(chǔ)上,將開(kāi)挖面下的土體進(jìn)行分層,通過(guò)分析各層土的卸荷模量系數(shù)Kti,并考慮各種影響因素進(jìn)行修正,最終計(jì)算出基坑開(kāi)挖后的土體回彈量,綜合考慮樁長(zhǎng)及開(kāi)挖深度,確定出參與計(jì)算的埋置深度.回彈量計(jì)算過(guò)程如表3所示.

      表3 增層開(kāi)挖后土體回彈量計(jì)算Tab.3 Calculation of soil heave under excavation

      表中:Rf為土體破壞比為分層厚度,σzi為平均卸荷應(yīng)力,σmi為平均固結(jié)應(yīng)力,Δ為分層土體回彈量,為累計(jì)回彈量為面積修正后的累計(jì)回彈量.

      2.6 殘余應(yīng)力計(jì)算

      增層開(kāi)挖前后樁身殘余應(yīng)力計(jì)算如圖3所示.殘余應(yīng)力的計(jì)算參數(shù)如表4所示.

      圖3 增層開(kāi)挖前后樁身殘余應(yīng)力變化Fig.3 Variation of residual stress in pile due to excavation

      從圖3中可以看出樁身殘余應(yīng)力在增層開(kāi)挖后出現(xiàn)了整體釋放的趨勢(shì),這與樁身殘余應(yīng)力的產(chǎn)生機(jī)理是吻合的.基坑開(kāi)挖必定引起樁周部分土體回彈,樁周土對(duì)樁身的約束減弱,殘余應(yīng)力也得到相應(yīng)的釋放.在增層開(kāi)挖前,樁身殘余應(yīng)力最大值為2.15 MPa,在h=6.5 m,2a=18.5 m的基坑后,樁身殘余應(yīng)力最大值為1.40 MPa,減幅約為35%,而增層開(kāi)挖前后樁側(cè)摩阻力和樁端阻力分別減少了21%和2%,說(shuō)明增層開(kāi)挖時(shí),殘余應(yīng)力的釋放程度更高;但增層開(kāi)挖后樁端的應(yīng)力釋放幅度僅為2%,這可能與樁端嵌巖有關(guān),增層開(kāi)挖后,樁端回彈位移較小,殘余應(yīng)力釋放不多,加之樁端殘余應(yīng)力基數(shù)較大,故樁端應(yīng)力釋放程度較低.增層開(kāi)挖后樁身殘余應(yīng)力釋放程度如圖4所示,其中σr1、σr2分別為增層開(kāi)挖前后的σr取值.

      圖4 增層開(kāi)挖后樁身殘余應(yīng)力釋放程度Fig.4 Release degree of residual stress under excavation

      從圖中可以看出,殘余應(yīng)力的釋放程度與開(kāi)挖面到計(jì)算點(diǎn)的距離有關(guān),距離越遠(yuǎn),釋放的幅度也越小,但在樁端附近有小幅回升.這說(shuō)明殘余應(yīng)力的釋放與樁-土間的相對(duì)位移有關(guān),距離開(kāi)挖面越遠(yuǎn),土體回彈量越少,樁-土相對(duì)位移會(huì)相應(yīng)減少,故殘余應(yīng)力的釋放程度較低.

      表4 殘余應(yīng)力計(jì)算的參數(shù)Tab.4 Calculated parameters for the residual stress

      3 結(jié) 論

      在施工殘余應(yīng)力簡(jiǎn)化模型和能量法概念的基礎(chǔ)上,提出了樁身殘余應(yīng)力的計(jì)算模型,再通過(guò)理論分析,逐一確定出計(jì)算所需的樁側(cè)摩阻力、樁端阻力、埋置深度等參數(shù),最終模擬出增層開(kāi)挖前后樁身殘余應(yīng)力的變化情況.并得出以下結(jié)論:

      (1)增層開(kāi)挖會(huì)造成樁側(cè)摩阻力及樁端阻力的損失,與其減少幅度相比,殘余應(yīng)力的釋放程度更高.但樁端的殘余應(yīng)力釋放程度較低,這與樁端回彈位移小以及樁端殘余應(yīng)力基數(shù)大有關(guān).

      (2)殘余應(yīng)力的釋放程度與開(kāi)挖面到計(jì)算點(diǎn)的距離有關(guān),距離越遠(yuǎn),釋放的幅度也越小.這說(shuō)明殘余應(yīng)力的釋放與樁-土間的相對(duì)位移有關(guān).基坑開(kāi)挖后,樁-土間產(chǎn)生與沉樁時(shí)反向的相對(duì)位移,殘余應(yīng)力得以部分釋放.距離開(kāi)挖面越遠(yuǎn),土體回彈量越少,樁-土相對(duì)位移會(huì)相應(yīng)減少,故殘余應(yīng)力的釋放幅度降低.

      (References):

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      Release of residual stress in existing preformed pile due to further excavation beneath pile raft

      GOU Yao-bo1,YU Feng1,XIA Tang-dai2
      (1.School of Civil Engineering and Architecture,Zhejiang Sci-Tech University,Hangzhou 310018,China;2.Research Center of Coastal and Urban Geotechnical Engineering,Zhejiang University,Hangzhou 310058,China)

      This study aims to improve the understanding on the stress state and bearing behavior of existing pile subjected to excavation beneath the raft or cap of the pile.Concerning high-rise buildings supported by preformed piles,analysis was carried out to observe the change of post-installation residual stress locked in the piles due to such excavation.A proposal for residual stress calculation was proposed by using a simplified simulation model and the principle of energy conservation.The parameters for calculating the residual stress were determined based on the models for pile's shaft and base resistances and the model for soil heave subjected to excavation.The change of residual stress due to excavation was thus derived.The results indicate that excavation leads to the loss of shaft and base capacities of the piles,but the rate of reduction is smaller than that for the residual stress.The release degree of the residual stress is related to the distance between the calculated point and the excavation surface,i.e.,farther distance corresponds to smaller degree of release.This finding is an indicator that the release of residual stress is closely associated with the relative pile-soil displacement.

      existing building;excavation;preformed pile;residual stress;simulation

      10.3785/j.issn.1008-973X.2015.05.023

      TU 473.1

      A

      1008-973X(2015)05-0969-06

      2014-10-10. 浙江大學(xué)學(xué)報(bào)(工學(xué)版)網(wǎng)址:www.journals.zju.edu.cn/eng

      國(guó)家自然科學(xué)基金資助項(xiàng)目(41472284);浙江理工大學(xué)“521”人才培養(yǎng)計(jì)劃.

      茍堯泊(1989-),男,碩士生,主要從事樁基工程方面的研究.E-mail:459807245@qq.com

      俞峰,男,教授.E-mail:pokfulam@163.com

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