呂彩忠
(衢州學(xué)院 建筑工程學(xué)院,浙江 衢州 324000)
基于Mogi-Coulomb強(qiáng)度準(zhǔn)則的隧道圍巖理想彈塑性解答
呂彩忠
(衢州學(xué)院 建筑工程學(xué)院,浙江 衢州 324000)
合理選擇巖石強(qiáng)度準(zhǔn)則對(duì)隧道應(yīng)力及位移預(yù)測(cè)和支護(hù)設(shè)計(jì)都具有重要意義,基于Mogi-Coulomb強(qiáng)度準(zhǔn)則和理想彈塑性模型,通過(guò)中間主應(yīng)力系數(shù)反映中間主應(yīng)力的影響,推導(dǎo)了圓形隧道圍巖應(yīng)力和位移的解析解,并對(duì)所得結(jié)果進(jìn)行比較與驗(yàn)證,得到了中間主應(yīng)力和圍巖抗剪強(qiáng)度參數(shù)的影響特性。研究表明:具有廣泛的適用性和較好的可比性,Mohr-Coulomb強(qiáng)度準(zhǔn)則解答和Matsuoka-Nakai準(zhǔn)則解答均為其特例;結(jié)果關(guān)于中間主應(yīng)力系數(shù)b=0.5對(duì)稱(chēng),較好地反映了巖石強(qiáng)度的中間主應(yīng)力效應(yīng)及其區(qū)間性;粘聚力及內(nèi)摩擦角對(duì)圍巖塑性區(qū)半徑和隧道洞壁位移的影響顯著,應(yīng)充分考慮中間主應(yīng)力影響及圍巖抗剪強(qiáng)度參數(shù)變化對(duì)隧道設(shè)計(jì)與施工的影響。
隧道工程;Mogi-Coulomb強(qiáng)度準(zhǔn)則;理想彈塑性模型;中間主應(yīng)力;應(yīng)力和位移
應(yīng)用能合理反映巖石力學(xué)性狀的強(qiáng)度準(zhǔn)則,可以充分發(fā)揮巖石材料的強(qiáng)度潛能,將給隧道、邊坡等巖石工程建設(shè)帶來(lái)巨大的經(jīng)濟(jì)效益。眾多巖石真三軸試驗(yàn)已表明[1-3]:中間主應(yīng)力σ2對(duì)巖石的強(qiáng)度具有提高作用,并且其影響具有區(qū)間性。隧道開(kāi)挖施工造成的地應(yīng)力重分布,使圍巖處于彈塑性狀態(tài),隧道軸向應(yīng)力對(duì)其橫斷面內(nèi)的應(yīng)力和變形具有重要影響,但當(dāng)前對(duì)隧道圍巖進(jìn)行的彈塑性分析多采用Mohr-Coulomb強(qiáng)度準(zhǔn)則[4-5]或Hoek-Brown強(qiáng)度準(zhǔn)則[6-7],均沒(méi)有考慮中間主應(yīng)力σ2的影響,所得結(jié)果過(guò)于保守,致使隧道施工與設(shè)計(jì)的浪費(fèi)極大;已有學(xué)者將考慮中間主應(yīng)力σ2影響的統(tǒng)一強(qiáng)度理論[8-9]和Matsuoka-Nakai(又稱(chēng)廣義空間滑動(dòng)面SMP)[10-11]準(zhǔn)則應(yīng)用于圍巖彈塑性分析,但所得結(jié)果表達(dá)復(fù)雜不便于工程應(yīng)用。Mogi[12-13]較早開(kāi)展了巖石真三軸儀的研制及多種巖石的真三軸試驗(yàn)研究,并提出了Mogi經(jīng)驗(yàn)強(qiáng)度準(zhǔn)則通式。Al-Ajmi等[14]將線性Mogi經(jīng)驗(yàn)強(qiáng)度準(zhǔn)則和Coulomb強(qiáng)度準(zhǔn)則相結(jié)合,建立了能反映巖石中間主應(yīng)力效應(yīng)及其區(qū)間性影響的Mogi-Coulomb強(qiáng)度準(zhǔn)則,已在井壁穩(wěn)定性分析及巖石真三軸強(qiáng)度預(yù)測(cè)中得到了廣泛應(yīng)用與驗(yàn)證[15-18]。本文將基于Mogi-Coulomb強(qiáng)度準(zhǔn)則和理想彈塑性模型,對(duì)圓形隧道圍巖進(jìn)行彈塑性分析,推導(dǎo)其應(yīng)力和位移的解析解,并對(duì)所得結(jié)果進(jìn)行比較與驗(yàn)證,最后探討中間主應(yīng)力和圍巖抗剪強(qiáng)度參數(shù)的影響特性。
Mogi根據(jù)自己所開(kāi)展的多種巖石真三軸試驗(yàn)數(shù)據(jù),提出的Mogi經(jīng)驗(yàn)強(qiáng)度準(zhǔn)則通式為[12-13]
τoct=f(σ13)
(1)
式中:σ1、σ2和σ3分別為大主應(yīng)力、中間主應(yīng)力和小主應(yīng)力,以壓應(yīng)力為正;τoct為八面體剪應(yīng)力;σ13為與中間主應(yīng)力σ2平行的破裂面上的平均主應(yīng)力;f為單調(diào)遞增函數(shù),可采用一次直線式、二次多項(xiàng)式或冪函數(shù)。
Al-Ajmi等[14]將線性Mogi經(jīng)驗(yàn)強(qiáng)度準(zhǔn)則和Coulomb強(qiáng)度準(zhǔn)則相結(jié)合,建立了以巖石抗剪強(qiáng)度參數(shù)(粘聚力c和內(nèi)摩擦角φ)表示的Mogi Coulomb強(qiáng)度準(zhǔn)則,其表達(dá)式為
τoct=kσ13+d
(2)
由式(2)可見(jiàn),Mogi-Coulomb強(qiáng)度準(zhǔn)則考慮了中間主應(yīng)力σ2對(duì)巖石強(qiáng)度的影響,且其參數(shù)(粘聚力c和內(nèi)摩擦角φ)與Mohr-Coulomb強(qiáng)度準(zhǔn)則相同,物理意義明確且便于由常規(guī)三軸壓縮試驗(yàn)確定。Mogi-Coulomb強(qiáng)度準(zhǔn)則的極限線為外接Mohr-Coulomb強(qiáng)度準(zhǔn)則的曲邊六邊形[18],在軸對(duì)稱(chēng)的三軸壓縮狀態(tài)和三軸拉伸狀態(tài)時(shí)二者重合,如圖1所示。
圖1 Mogi-Coulomb強(qiáng)度準(zhǔn)則的極限線
工程實(shí)踐和真三軸試驗(yàn)研究中,常用中間主應(yīng)力系數(shù)b來(lái)表示中間主應(yīng)力σ2與大主應(yīng)力σ1和小主應(yīng)力σ3的相對(duì)大小關(guān)系,其表達(dá)式為
(3)
中間主應(yīng)力系數(shù)b的取值范圍為[0,1],系數(shù)b=0對(duì)應(yīng)σ2=σ3<σ1的軸對(duì)稱(chēng)三軸壓縮狀態(tài),系數(shù)b=1對(duì)應(yīng)σ3<σ2=σ1的軸對(duì)稱(chēng)三軸拉伸狀態(tài),0
σ2=bσ1+(1-b)σ3
(4)
將式(4)代入式(2),整理得Mogi-Coulomb強(qiáng)度準(zhǔn)則為
σ1=Aσ3+B
(5)
當(dāng)系數(shù)b=0或1時(shí),Mogi-Coulomb強(qiáng)度準(zhǔn)則式(5)退化為Mohr-Coulomb強(qiáng)度準(zhǔn)則,即
(6)
故Mohr-Coulomb強(qiáng)度準(zhǔn)則為Mogi-Coulomb強(qiáng)度準(zhǔn)則的特例,對(duì)應(yīng)系數(shù)b=0或1,沒(méi)有考慮中間主應(yīng)力σ2的影響。
假定圍巖為均勻、連續(xù)、各向同性的理想彈-塑性材料,隧道軸向應(yīng)力σz為中間主應(yīng)力σ2。圓形隧道內(nèi)表面受均勻支護(hù)力pi作用,無(wú)窮遠(yuǎn)處受等值地應(yīng)力po作用,如圖2所示,圖中ri為隧道半徑、R為圍巖塑性區(qū)半徑。
圖2 隧道力學(xué)模型
不考慮圍巖自重,軸對(duì)稱(chēng)條件下平衡微分方程為
(7)
在圍巖塑性區(qū)內(nèi),σ1=σθ,σ3=σr,則Mogi-Coulomb強(qiáng)度準(zhǔn)則式(5)變?yōu)?/p>
σθ=Aσr+B
(8)
將式(8)代入式(7),并以隧道洞壁處r=ri,σr=pi為應(yīng)力邊界條件,求得圍巖塑性區(qū)的應(yīng)力為
(9)
設(shè)ps為圍巖臨界支護(hù)力即彈塑性交界處的徑向應(yīng)力,則圍巖彈性區(qū)的應(yīng)力和位移為[7]
σr=po-(po-ps)R2/r2,σθ=po+(po-ps)R2/r2
(10)
(11)
式中:E和ν分別為圍巖的彈性模量與泊松比。
在彈塑性交界r=R處,圍巖彈性區(qū)的應(yīng)力式(10)應(yīng)滿足Mogi-Coulomb強(qiáng)度準(zhǔn)則式(8),且r=R處的徑向應(yīng)力σr連續(xù),據(jù)此求得圍巖臨界支護(hù)力ps和塑性區(qū)半徑R分別為
(12)
當(dāng)支護(hù)力pi小于臨界支護(hù)力ps時(shí),圍巖才進(jìn)入彈塑性狀態(tài)。在圍巖塑性區(qū)內(nèi),大主應(yīng)變?chǔ)?=εθ,小主應(yīng)變?chǔ)?=εr,不考慮圍巖塑性區(qū)的體積變化,則
εv=εθ+εz+εr=0
(13)
隧道軸向應(yīng)變?chǔ)舲=0,將εr=du/dr,εθ=u/r代入式(13),并以圍巖彈塑性交界r=R處的位移為位移邊界條件,求得圍巖塑性區(qū)的位移為
(14)
將r=ri代入式(14),即得隧道的洞壁位移uo為
(15)
基于Mogi-Coulomb強(qiáng)度準(zhǔn)則,隧道圍巖塑性區(qū)和彈性區(qū)的應(yīng)力可分別由式(9)和式(10)確定,塑性區(qū)和彈性區(qū)的位移則可分別由式(11)和式(14)確定。圍巖位移式(11)和式(14)的表達(dá)式雖相同,但二者的應(yīng)用區(qū)域不同:式(11)只適用于r>R的圍巖彈性區(qū),式(14)適用于ri 此處主要對(duì)本文結(jié)果進(jìn)行比較與驗(yàn)證,同時(shí)探討中間主應(yīng)力和圍巖抗剪強(qiáng)度參數(shù)的影響特性。取文獻(xiàn)[10]中代表性的圓形軟巖隧道:圍巖參數(shù)為c=2.0 MPa,φ=30°,ν=0.5,E=2.0 GPa;隧道半徑ri=3.0 m,初始地應(yīng)力po=20 MPa,無(wú)支護(hù)隧道即支護(hù)力pi為零,圍巖處于彈塑性狀態(tài)。 3.1 結(jié)果比較與驗(yàn)證 不同強(qiáng)度準(zhǔn)則所建立的依據(jù)以及對(duì)中間主應(yīng)力σ2的考慮都不盡相同,因而采用不同強(qiáng)度準(zhǔn)則的計(jì)算結(jié)果之間常有較大差異。將文獻(xiàn)[4]中基于Mohr Coulomb強(qiáng)度準(zhǔn)則的圍巖理想彈塑性解答與本文系數(shù)b=0時(shí)的結(jié)果進(jìn)行比較,如圖3所示;將文獻(xiàn)[10]中基于Matsuoka-Nakai(SMP)準(zhǔn)則的圍巖理想彈塑性解答與本文系數(shù)b=0.285時(shí)的結(jié)果進(jìn)行比較,如圖4所示。 由圖3可以看出,本文系數(shù)b=0時(shí)的應(yīng)力及塑性區(qū)位移和Mohr-Coulomb強(qiáng)度準(zhǔn)則解答的峰值和分布規(guī)律均吻合的極好,二者的圍巖塑性區(qū)半徑R=1.84ri也完全相同。由圖4可以看出,系數(shù)b=0.285時(shí)本文結(jié)果和考慮中間主應(yīng)力σ2影響的Matsuoka-Nakai(SMP)準(zhǔn)則解答吻合的也較好,且二者的圍巖塑性區(qū)半徑R相同并減小為1.53ri。這都說(shuō)明本文基于Mogi-Coulomb強(qiáng)度準(zhǔn)則所得結(jié)果的正確性和合理性,同時(shí)也說(shuō)明基于Mohr-Coulomb強(qiáng)度準(zhǔn)則和基于Matsuoka-Nakai(SMP)準(zhǔn)則的圍巖理想彈塑性解答都是本文結(jié)果的特例,因此本文結(jié)果具有廣泛的適用性和較好的可比性。 圖3 本文結(jié)果(b=0)與Mohr-Coulomb強(qiáng)度準(zhǔn)則解答的比較 圖4 本文結(jié)果(b=0.285)與Matsuoka-Nakai(SMP)準(zhǔn)則解答的比較 3.2 參數(shù)分析 1)中間主應(yīng)力影響 中間主應(yīng)力σ2可在大主應(yīng)力σ1和小主應(yīng)力σ3之間變化,對(duì)應(yīng)的中間主應(yīng)力系數(shù)b在0和1之間變動(dòng)。中間主應(yīng)力σ2對(duì)巖石強(qiáng)度具有增強(qiáng)作用,但巖石強(qiáng)度一般是隨著中間主應(yīng)力σ2的增加先逐漸增大,達(dá)到峰值后又隨中間主應(yīng)力σ2的增加而逐漸降低,即中間主應(yīng)力效應(yīng)具有區(qū)間性[1-3]。圖5給出了圍巖塑性區(qū)半徑R和隧道洞壁位移uo與系數(shù)b的變化關(guān)系,Mohr-Coulomb強(qiáng)度準(zhǔn)則解答也一并標(biāo)于圖中。 圖5 中間主應(yīng)力的影響 由圖5可以看出,Mohr-Coulomb強(qiáng)度準(zhǔn)則解答與系數(shù)b即中間主應(yīng)力σ2無(wú)關(guān),且圍巖塑性區(qū)半徑R=1.84ri和隧道洞壁位移uo=0.029 8ri與本文系數(shù)b=0或1時(shí)的結(jié)果相同且最大;本文基于Mogi-Coulomb強(qiáng)度準(zhǔn)則的結(jié)果關(guān)于系數(shù)b=0.5即σ2=(σ1+σ3)/2對(duì)稱(chēng),且最小分別達(dá)到R=1.47ri、uo=0.021 9ri,較好地反映了巖石強(qiáng)度的中間主應(yīng)力效應(yīng)及其區(qū)間性,相比Mohr-Coulomb強(qiáng)度準(zhǔn)則解答具有明顯的優(yōu)越性。合理考慮中間主應(yīng)力效應(yīng)及其區(qū)間性,可更加充分發(fā)揮圍巖的強(qiáng)度潛能,更經(jīng)濟(jì)安全地進(jìn)行隧道設(shè)計(jì)與施工。 2)圍巖抗剪強(qiáng)度參數(shù)影響 圍巖的抗剪強(qiáng)度包括粘聚力和摩擦滑移兩部分,以粘聚力c和內(nèi)摩擦角φ為代表的抗剪強(qiáng)度參數(shù)的變異性可使圍巖的強(qiáng)度變化很大。圖6和圖7分別給出了系數(shù)b=0.5時(shí),圍巖塑性區(qū)半徑R和隧道洞壁位移uo隨粘聚力c及內(nèi)摩擦角φ的變化關(guān)系,Mohr-Coulomb強(qiáng)度準(zhǔn)則解答也標(biāo)于圖中。 由圖6、7可知,Mohr-Coulomb強(qiáng)度準(zhǔn)則解答均大于本文系數(shù)b=0.5時(shí)的結(jié)果,但隨著粘聚力c及內(nèi)摩擦角φ的增加,二者之間的差異在不斷減小。圍巖塑性區(qū)半徑R和隧道洞壁位移uo隨粘聚力c及內(nèi)摩擦角φ的增加均呈非線性減小,c=3.0 MPa時(shí)塑性區(qū)半徑R和洞壁位移uo分別比c=1.0 MPa時(shí)減小了29.4%、42.2%,φ=40°時(shí)二者比φ=20°時(shí)分別減小了47.1%、53.9%,可見(jiàn)粘聚力c及內(nèi)摩擦角φ對(duì)圍巖塑性區(qū)半徑R和隧道洞壁位移uo的影響顯著,且相比對(duì)隧道洞壁位移uo的影響更明顯。 圖6 圍巖抗剪強(qiáng)度參數(shù)對(duì)塑性區(qū)半徑的影響 圖7 圍巖抗剪強(qiáng)度參數(shù)對(duì)隧道洞壁位移的影響 1)依據(jù)引入中間主應(yīng)力系數(shù)b的Mogi-Coulomb強(qiáng)度準(zhǔn)則,推導(dǎo)了圓形隧道圍巖應(yīng)力和位移的理想彈塑性解答,并利用文獻(xiàn)已有的Mohr-Coulomb強(qiáng)度準(zhǔn)則解答和Matsuoka-Nakai(SMP)準(zhǔn)則解答對(duì)其進(jìn)行了比較與驗(yàn)證,同時(shí)指出這兩個(gè)解答均為本文結(jié)果的特例,表明本文結(jié)果具有廣泛的適用性和較好的可比性。 2)圍巖塑性區(qū)半徑R和隧道洞壁位移uo關(guān)于中間主應(yīng)力系數(shù)b=0.5對(duì)稱(chēng),較好地反映了巖石強(qiáng)度的中間主應(yīng)力效應(yīng)及其區(qū)間性,相比Mohr-Coulomb強(qiáng)度準(zhǔn)則解答具有明顯的優(yōu)越性;粘聚力c及內(nèi)摩擦角φ對(duì)圍巖塑性區(qū)半徑R和隧道洞壁位移uo的影響顯著,且對(duì)隧道洞壁位移uo的影響更明顯。 [1] 許東俊, 耿乃光. 巖石強(qiáng)度隨中間主應(yīng)力變化規(guī)律[J]. 固體力學(xué)學(xué)報(bào), 1985, 6(1):72-80. 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(編輯 胡 玲) Elastic-PerfectlyPlasticSolutionofTunnelSurroundingRocksUsingMogi-CoulombStrengthCriterion LyuCaizhong (College of Civil Engineering and Architecture, Quzhou University, Quzhou 234000, Zhejiang P.R.China) Reasonable choice of rock strength criteria is crucial for stress and displacement prediction and support design in tunnel engineering. Based on Mogi-Coulomb strength criterion and elastic-perfectly plastic model, analytical solutions of stress and displacement for surrounding rocks around a circular tunnel were derived The intermediate principal stress coefficient was used to present the intermediate principal stress effect. The results in this study were compared with the current solutions in the literatures and the influence of intermediate principal stress and shear strength parameters of surrounding rocks was discussed. The results showed extensive applicability and the Mohr-Coulomb strength criterion and Matsuoka-Nakai criterion are two special cases; when the intermediate principal stress coefficient b was equals to 0.5, the results indicated that the intermediate principal stress effect and its range for rock strength; the influence of cohesion and internal friction angle on the plastic radius and tunnel wall displacement was significant; Care should be taken to the effects of intermediate principal stress and shear strength parameter variations of surrounding rocks on tunnel design and construction. tunnel engineering; Mogi-Coulomb strength criterion; elastic-perfectly plastic model; intermediate principal stress; stress and displacement 10.11835/j.issn.1674-4764.2014.06.009 2014-04-18 浙江省自然科學(xué)基金資助項(xiàng)目(Y1100210) 呂彩忠(1964-),男,副教授,主要從事土木工程設(shè)計(jì)與監(jiān)測(cè)研究,(E-mail)lvcz1964@163.com。 TU452 A 1674-4764(2014)06-0054-063 結(jié)果比較及參數(shù)分析
4 結(jié)論