Lecture8Deformationbehaviorforunsaturatedsoils

1、Unsaturated soil mechanicsDr. LI, Xu （李（李 旭）旭）Associate Professor, School of Civil EngineeringEmail: 2Deformation behavior of soilsl Loading collapse phenomenonl Shrinkage and swellingl Consolidationl Shearing behavior3Loess4Loess5Loess6濕陷土黃土濕陷土黃土 一、黃土的成因與分布一、黃土的成因與分布 黃土因顏色為黃色而得名，它是由于黃土沉積物黃土因顏色為黃色而得

2、名，它是由于黃土沉積物在干旱性大陸氣候作用下經(jīng)過(guò)風(fēng)化和成土作用而形在干旱性大陸氣候作用下經(jīng)過(guò)風(fēng)化和成土作用而形成的，其顆粒組成以粉粒為主，空隙比在成的，其顆粒組成以粉粒為主，空隙比在1.0左右，左右，肉眼可見(jiàn)，富含碳酸鈣鹽類，垂直節(jié)理。黃土按其肉眼可見(jiàn)，富含碳酸鈣鹽類，垂直節(jié)理。黃土按其成 因 分 為 原 生 黃 土 和 次 生 黃 土 。成 因 分 為 原 生 黃 土 和 次 生 黃 土 。7黃土也是一種三相體。黃土也是一種三相體。黃土顆粒中，粉粒占一半以上，黃土顆粒中，粉粒占一半以上，干容重：干容重：11.416.9kN/m11.416.9kN/m3 3，大于，大于15.0kN/m15.0

3、kN/m3 3時(shí)濕陷性較時(shí)濕陷性較??；??；天然含水率：天然含水率：3.3%40%3.3%40%，當(dāng)含水率超過(guò)，當(dāng)含水率超過(guò)25%25%時(shí)不具有濕時(shí)不具有濕陷性。陷性。黃土的液限是反映黃土力學(xué)性質(zhì)的一個(gè)重要指標(biāo)，當(dāng)液黃土的液限是反映黃土力學(xué)性質(zhì)的一個(gè)重要指標(biāo)，當(dāng)液限超過(guò)限超過(guò)30%30%時(shí)，黃土的濕陷性較弱且為非自重濕陷性，時(shí)，黃土的濕陷性較弱且為非自重濕陷性，而液限小于而液限小于30%30%時(shí)，則濕陷性較強(qiáng)。時(shí)，則濕陷性較強(qiáng)。黃土的物理性質(zhì)黃土的物理性質(zhì)8濕陷性黃土的力學(xué)性質(zhì)濕陷性黃土的力學(xué)性質(zhì)濕陷性黃土的力學(xué)性質(zhì)主要包括壓縮性、濕陷性、抗?jié)裣菪渣S土的力學(xué)性質(zhì)主要包括壓縮性、濕陷性、抗剪強(qiáng)度和

4、透水性，其中濕陷性最為重要。剪強(qiáng)度和透水性，其中濕陷性最為重要。黃土一般具有中等偏低壓縮性，但新近堆積的黃土壓黃土一般具有中等偏低壓縮性，但新近堆積的黃土壓縮性較高。縮性較高。抗剪強(qiáng)度主要取決于土的含水量和密室程度，含水率抗剪強(qiáng)度主要取決于土的含水量和密室程度，含水率越低，密室程度越高，抗剪強(qiáng)度越大。越低，密室程度越高，抗剪強(qiáng)度越大。此外土的顆粒組成、礦物成分、粘粒和可溶鹽的含量此外土的顆粒組成、礦物成分、粘粒和可溶鹽的含量對(duì)抗剪強(qiáng)度也有影響。對(duì)抗剪強(qiáng)度也有影響。目前對(duì)黃土的透水性研究不多，其滲透系數(shù)在目前對(duì)黃土的透水性研究不多，其滲透系數(shù)在1010-6-6cm/scm/s。9黃土的濕陷性黃土

5、的濕陷性1.濕陷的成因濕陷的成因 黃土是由于土中的水分較少，顆粒間的相黃土是由于土中的水分較少，顆粒間的相互粘結(jié)力較大，而上覆土層壓力的增加不足以互粘結(jié)力較大，而上覆土層壓力的增加不足以克服增加的粘結(jié)力，因而處于疏松狀態(tài)，一旦克服增加的粘結(jié)力，因而處于疏松狀態(tài)，一旦水浸入較深，顆粒間的粘結(jié)力減少甚至消失，水浸入較深，顆粒間的粘結(jié)力減少甚至消失，就產(chǎn)生了濕陷性。就產(chǎn)生了濕陷性。10濕陷系數(shù)濕陷系數(shù)001ppppthheehephph0h0epepe式中土樣在壓力p作用下沉穩(wěn)后的高度；上述加壓穩(wěn)定后的土樣在浸水作用下， 下沉穩(wěn)定后的高度；土樣的初始高度；土樣在壓力p作用下，下沉穩(wěn)定后的空隙比；上述

6、加壓穩(wěn)定后的土樣浸水作用，下沉穩(wěn)定后的空隙比；土樣的初始空隙比；11濕陷系數(shù)是判別黃土濕陷性的依據(jù)，一般認(rèn)為濕陷系數(shù)是判別黃土濕陷性的依據(jù)，一般認(rèn)為0.0150.03s0.015s0.030.07s0.07s非濕陷性弱濕陷性黃土中等濕陷性黃土強(qiáng)濕陷性黃土黃土的濕陷性黃土的濕陷性12 （1）重錘表層夯實(shí)法和強(qiáng)夯發(fā)）重錘表層夯實(shí)法和強(qiáng)夯發(fā) （2）土夯法、灰土墊層）土夯法、灰土墊層 （3）土樁擠密法）土樁擠密法 （4）樁基礎(chǔ)）樁基礎(chǔ) （6）化學(xué)加固法，主要用硅化加固法和氫氧化鈉溶液加固法。）化學(xué)加固法，主要用硅化加固法和氫氧化鈉溶液加固法。 （7）預(yù)浸水法）預(yù)浸水法 （8）熱加固法）熱加固法濕陷性黃

7、土地基的處理濕陷性黃土地基的處理13Measurement of Collapse Potential of Soilsl ASTM D 5333 92l Collapse decrease in height of a confined soil following wetting at a constant applied vertical stress.l This test method covers the determination of the magnitude of one-dimensional collapse that occurs when unsaturated s

8、oils are inundated with fluid.141516Loading collapse17collapse index & collapse potential l collapse index (Ie), percentrelative magnitude of collapse determined at 200 kPal collapse potential (Ic), percentrelative magnitude of soil collapse determined at any stress level18Test procedurel Trim n

9、atural samplel Measure water content, sample mass, volumel Applied vertical stress by increments each hour of 12, 25, 50,100, 200, etc.l Inundate the specimen with fluid 1 h after loading to the appropriate vertical stress and after recording the deformationl Record deformation versus time at approx

10、imately 0.1, 0.25, 0.5, 1, 2, 4, 8, 15, 30 min and 1, 2, 4,8, and 24 h after adding fluid.19Cautionsl Fluid shall be distilled-deionized waterl Add fluid to allow for specimen wetting from the bottom only, so that air will not be trapped in the specimen.l Additional vertical stress may be placed on

11、the specimen in increments according to Test Method D 2435 as needed or until the slope of the deformation versus stress level curve is obtained. l Record deformation versus time as in 10.3. Leave each load increment on overnight or until primary consolidation has been completed.20Test Results21Load

12、ing collapse phenomenon22Collapsibility of soil23Collapsibility of soill Handy (1973) suggested that collapsibility could be determined either by the percentage clay content or from the ratio of liquid limit to saturation moisture content24Typical deformation response to wetting of a collapsible soi

13、l (after Houston, 1995).25Collapse potential relationship with relative compaction (after Ng et al., 1998)26Collapse potential relationship with matric suction (after Ng et al., 1998)27Collapse potential relationship with time for a two cycle wetting test (after Ng et al., 1998)28Loading collapse yi

14、eldingl In first cycle, the soil is plasticl In the second cycle, the soil is elastic l Collapse is related to the soil suction state history29膨脹土膨脹土 膨脹土是指粘粒成分主要由強(qiáng)親水性礦物組成，液膨脹土是指粘粒成分主要由強(qiáng)親水性礦物組成，液限大于限大于40%，且漲縮性能較大的粘性土。，且漲縮性能較大的粘性土。膨脹土在自然條件多呈硬塑或堅(jiān)硬狀態(tài)，顏色為黃膨脹土在自然條件多呈硬塑或堅(jiān)硬狀態(tài)，顏色為黃、紅、灰白色，裂隙發(fā)育，常見(jiàn)光滑面和擦痕、紅、灰白色，

15、裂隙發(fā)育，常見(jiàn)光滑面和擦痕。3031中國(guó)膨脹土的分布圖中國(guó)膨脹土的分布圖32 33膨脹土邊坡滑塌膨脹土邊坡滑塌34353637膨脹土坡面防護(hù)膨脹土坡面防護(hù)3839404142434445膨脹土屬于粘性土，具有一般粘性土所共有的物理力膨脹土屬于粘性土，具有一般粘性土所共有的物理力學(xué)性質(zhì)外，最重要的特性是強(qiáng)親水性、多裂隙性、強(qiáng)學(xué)性質(zhì)外，最重要的特性是強(qiáng)親水性、多裂隙性、強(qiáng)膨脹性、強(qiáng)收縮性等。膨脹性、強(qiáng)收縮性等。當(dāng)含水量增加，表現(xiàn)出體積增大的能力，即具有強(qiáng)烈當(dāng)含水量增加，表現(xiàn)出體積增大的能力，即具有強(qiáng)烈的膨脹性；若收到約束，產(chǎn)生的內(nèi)力為膨脹力。的膨脹性；若收到約束，產(chǎn)生的內(nèi)力為膨脹力。相反則產(chǎn)生收縮

16、。具有脹縮特性，而且脹縮變形可隨相反則產(chǎn)生收縮。具有脹縮特性，而且脹縮變形可隨環(huán)境溫度和濕度的變化往復(fù)發(fā)生。環(huán)境溫度和濕度的變化往復(fù)發(fā)生。膨脹土的性質(zhì)及膨脹原理膨脹土的性質(zhì)及膨脹原理46l 粒度組成中，通常黏粒含量大于粒度組成中，通常黏粒含量大于30%。l 粘土礦物成分中，伊利石和蒙脫石等強(qiáng)親水性礦物占主導(dǎo)粘土礦物成分中，伊利石和蒙脫石等強(qiáng)親水性礦物占主導(dǎo)地位。地位。l 具有多裂隙性。具有多裂隙性。l 膨脹，收縮變形可隨環(huán)境變化往復(fù)發(fā)生，導(dǎo)致土的強(qiáng)度衰膨脹，收縮變形可隨環(huán)境變化往復(fù)發(fā)生，導(dǎo)致土的強(qiáng)度衰減。減。l 多數(shù)屬于液限大于多數(shù)屬于液限大于50%的高液限土。的高液限土。l 表現(xiàn)出較強(qiáng)的超固

17、結(jié)特性。表現(xiàn)出較強(qiáng)的超固結(jié)特性。膨脹土的主要物理力學(xué)特征：膨脹土的主要物理力學(xué)特征：47自由膨脹率自由膨脹率V00100%efVVV0V式中試樣的初始體積；膨脹穩(wěn)定后土的體積；48膨脹率膨脹率49Shrinkage and swellingl Some clayey soils undergo slow volume changes that occur independently of loading and are attributable to swelling or shrinkage owing to soil wetting or drying.l The principal ca

18、use of expansive clays is the presence of swelling clay minerals such as montmorillonite. l Differences in the period and amount of precipitation and evapotranspiration are the principal factors influencing the swellshrink response of a clay soil beneath a building.50Expansive soil problemlCracking

19、of house placed on expansive soils is a problem of enormous financial magnitude around the worldlShallow foundations typically undergo 50 to 150 mm of vertical movement subsequent to construction, in western Canada.lThe zone of soil undergoing volume change on an annual basis is called the “ACTIVE Z

20、ONE”.51Swelling potentiall The swellshrink behaviour of a clay soil under a given state of applied stress in the ground is controlled by changes in soil suction.52Swelling potential (Van Merwe 1964)wVO半固態(tài)顆 粒水Vs+VaVs固態(tài)可塑態(tài)液態(tài)wswPwL細(xì)粒土的可塑性：阿特堡界限 (Atterberg limit)粘粒粘 粒強(qiáng)結(jié)合水弱結(jié)合水自由水液 態(tài)可塑態(tài)固態(tài)或半固態(tài)rlSoil suctio

21、n, Normal shrinkageResidualshrinkageAir phasewn, wswlenelVoid ratio, eWater content, w SSCC (intra-aggregate pore structure)e0wr0% air line (1:1)Liquid phaseNon-shrinkageaev, nSoil shrinkage characteristic curve56Shrinkage limit57Volume measurement of samplemercurysoilmercurymercuryMVV58Shrinkage cu

22、rveSuction controlVolume measurementMass of Water exchanged59Shinkage process60Illustration of the water content/void ratio relationship (Tripathy et al. 2002).61swellshrink paths at 6.25 kPa (Tripathy et al. 2002).62Equilibrium swellshrink paths (after Tripathy et al., 2002)63Consolidation64壓縮系數(shù)a：

23、a1-2代表p1=100kPa： p2=200kPa時(shí)割線斜率1212eeeappp65ea 稱為壓縮系數(shù)，稱為壓縮系數(shù)，KPKPa a-1-1a a1-21-2常用作常用作比較土的壓比較土的壓縮性大小縮性大小66e-logp曲線及壓縮指數(shù)曲線及壓縮指數(shù) )/log(loglog12211221ppeeppeeCc67三軸等向壓縮和回彈三軸等向壓縮和回彈l = 0, u = 0l 變形： 塑性變形 vp 彈性變形 ve68正常固結(jié)線正常固結(jié)線l 超固結(jié)土超固結(jié)土l 欠固結(jié)土：不穩(wěn)定態(tài)欠固結(jié)土：不穩(wěn)定態(tài)l 正常固結(jié)線正常固結(jié)線6970If under monofonic stress pathS

24、oil deformation is independent with the stress path7172Cui and Delage 19967374Consolidation for unsaturated soils(Alonso et al. 1990)75不排水三維等向壓縮試驗(yàn)不排水三維等向壓縮試驗(yàn)（初始試樣不飽和）（初始試樣不飽和）浸水階段浸水階段等向壓縮階段等向壓縮階段76Whats the swelling pressure for soil in confining state?7778swelling pressureSwelling pressure79808182P

25、fThe reduction of suction can be considered equally as the decrease of normal stress P0 is the equivalent normal stress of current suctionP083Example 18485Shear behavior (for sand; same )tdxHdzgxzgxz = dx/Hz = - dz/H = vgDense (D)tLoose (L)egDLgvDLDilatancyContractancy86Critical statenCritical states (ultimate states)qFor granular materialsqA constant volume during subsequent shearingqp / q = q/ q = v / q = 0qRegardless of initial statesqRelated to the norm