吴珺华,杨 松
滤纸法测定干湿循环下膨胀土基质吸力变化规律
吴珺华1,2,杨 松3※
(1. 南昌航空大学无损检测技术教育部重点实验室,南昌 330063; 2. 南昌航空大学土木建筑学院,南昌 330063;3. 云南农业大学水利学院,昆明 650201)
为了获得干湿循环作用下膨胀土基质吸力的变化规律, 首先采用人工模拟降雨和蒸发的方法开展了膨胀土室内干湿循环试验,然后利用滤纸法进行了不同含水率下试样的基质吸力测定试验,获得了干湿循环条件下膨胀土的土水特征曲线,求出了相应的进气值与残余值,结合Fredlund土水特征曲线模型对经历不同干湿循环次数下的土壤土水特征曲线进行了拟合,最终建立了考虑干湿循环效应的膨胀土土水特征曲线模型。结果表明:1)随着干湿循环次数的增加,土壤的进气值呈下降趋势,从循环1次时的134.5 kPa降至循环4次时的58.5 kPa,降幅达56.5%。从循环1次至2次的进气值下降较大,往后降幅明显减小,趋于基本稳定,这表明对土壤进气值的影响以初次干湿循环为主。2)残余值亦呈下降趋势,从循环1次时的1 040.5降至循环4次时的528.5 kPa,降幅达49.2%。每经历一次干湿循环,残余值降幅均较大,尚未趋于稳定,这表明干湿循环效应对土壤残余值的影响比对土壤进气值的影响要大。3)新建土水特征曲线模型中的拟合参数与干湿循环次数成较好线性关系,表明随着干湿循环次数的增加,土壤进气值逐渐减小,水分变化速率有所降低,而残余含水率逐渐增加。该成果可为深入研究土壤基质吸力及其工程应用提供参考。
土壤;裂隙;含水率;膨胀土;干湿循环;滤纸法;基质吸力;土水特征曲线
吴珺华,杨 松. 滤纸法测定干湿循环下膨胀土基质吸力变化规律[J]. 农业工程学报,2017,33(15):126-132.
doi:10.11975/j.issn.1002-6819.2017.15.016 http://www.tcsae.org
Wu Junhua, Yang Song. Changes of matric suction in expansive soil under drying-wetting cycles using filter paper method[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2017, 33(15): 126-132. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2017.15.016 http://www.tcsae.org
土水特征曲线(SWCC)是土壤基质势与饱和度的关系曲线,表示土壤水的能量与数量之间的关系,反映了土壤的持水性能[1]。膨胀土是一种富含亲水性矿物的粘性土,在干湿循环作用下易产生胀缩裂隙,其原有结构明显破坏,持水性能与完整土壤相比大不相同,因此膨胀土的基质势受土壤饱和度和结构的共同影响而变化,导致膨胀土强度、变形、渗流等性质产生较大改变。部分学者对膨胀土进行了相应的试验研究,系统开展了基质吸力测定设备研发[2]、基质吸力的预测方法[3-5]等。此外,膨胀土在水分变化过程中裂隙发育,裂隙形态对膨胀土内部结构影响较大[6-8],进而影响到膨胀土的强度和变形特性[9-11]。张雪东等[12]以概率论为基础,从微观上建立了模拟孔隙率对基质吸力影响规律的经验模型。陈东霞等[13]采用滤纸法测试了厦门地区残积土的土-水特征曲线,认为修正Gardner模型最适合厦门地区残积土的土水特征曲线。张钦喜等[14]将试验测定的土水特征曲线引入至考虑地下水作用时的地表沉降曲线,认为在地表沉降计算中宜考虑土水特征曲线的贡献。胡冉等[15]假定变形后的孔隙分布函数可以从参考状态孔隙分布函数求得,基于此建立了考虑土体变形和滞回效应的土水特征曲线模型。Tarantino[16]通过建立含水率与吸力的关系以间接反映孔隙比对土水特征曲线的影响。刘艳等[17]利用土性参数预测粒状土和黏性土的土水特征曲线。董倩等[18]利用压力板仪测定了非饱和粉质砂土的水土特征曲线,获得了非饱和粉质砂土抗剪强度和基质吸力之间的关系。李涛等[19]采用滤纸法测定了不同干密度下土体的土水特征曲线,建立了考虑基质吸力的破坏包络面。赵天宇等[20]采用非饱和导水仪和压力膜仪测定了重塑黄土的土水特征曲线,获得了黄土土水特征曲线的进气值与残余含水率,但并未考虑黄土湿陷性对黄土基质吸力的影响。张昭等[21]通过引入“水土体积比”对已有土壤的土水特征曲线数据进行分析,并对VG模型进行改进,提出了一种考虑应力引起孔隙比变化的土水特征曲线模型。杨钢等[22]发现SWCC的变化规律与土壤颗粒级配、蒸发效应、防护条件、收缩裂纹等试验条件密切相关。
可以看出,基质吸力的准确测定对研究膨胀土的力学和变形性质十分重要。膨胀土胀缩过程中通常伴随着裂隙的张开与闭合,并且不断有新的裂隙生成。也就是说,不同裂隙形态的土样,其内部结构与含水率均在不断变化,每个时刻的土样状态都不相同。传统测量土壤基质吸力试验方法所获得的试验数据,均是初始状态基本相同的土样测得的,而干湿循环下膨胀土在不同时刻的含水率和裂隙形态均不相同,即试验开始时的初始状态并不相同。若采用传统试验方法测量土壤基质吸力,一方面,试样受到限制难以形成宏观裂隙,不能完全地反映干湿循环产生的裂隙特征;另一方面,若直接采用某一裂隙样进行试验,试验结果只能大致反映初始试样的特征,并不能获得由于含水率变化引起裂隙形态变化这一特征对土壤基质吸力的影响。实际工程中,随着干湿循环的进行,膨胀土裂隙不断开展,基质吸力受土壤含水率及裂隙形态的共同影响,传统测量土壤基质吸力的方法存在缺陷。
滤纸法是一种可测量土壤全程基质吸力的试验方法[23-24],具有价格低廉、操作简单等优点,且可测量较大的基质吸力范围,已广泛应用于土壤学领域[25-26]。该方法遵循热力学平衡原理,当土壤-滤纸-空气间的水气达到平衡时,由滤纸的平衡含水率来反映土壤的基质吸力值[27]。当滤纸与土壤直接接触时,滤纸的平衡吸力相当于土壤的基质吸力;当滤纸与土壤不接触时,滤纸的平衡吸力相当于土壤的总吸力。由于滤纸法可获得任意状态下土壤的吸力,原理清晰,试验设备简单,不干扰土壤的状态,可同时开展大批量试样的吸力测定试验。作者前期开展了基于滤纸法的一次吸湿和脱湿过程中膨胀土基质吸力测定的试验,验证了滤纸法可用来有效测定膨胀土在水分变化过程中的基质吸力[28]。实际膨胀土工程中,往往经历是长期、多次的干湿过程,干湿循环对膨胀土性质的影响非常显著,相关研究也较多,但关于循环过程中基质吸力的准确测定鲜见报道,而这对于完善非饱和土相关理论、处治膨胀土工程问题有积极效应。因此本文在已有研究基础上,采用滤纸法来测量干湿循环下不同含水率的裂隙膨胀土试样的基质吸力,获得了干湿循环下裂隙膨胀土的土水特征曲线,结合Fredlund土水特征曲线模型,建立了考虑干湿循环效应的膨胀土土水特征曲线模型,为研究干湿循环下膨胀土持水性能对土壤性质的影响提供参考。
本文采用美国材料试验学会(ASTM)推荐的Whatman No. 42型无灰定量分析滤纸开展土壤基质吸力测量试验。滤纸典型尺寸为55 mm直径圆状。滤纸法使用的主要设备包括密封容器、绝缘箱、高精度天平、烘箱及干燥器等。试验时将土样与滤纸放在密封容器中若干天以达到平衡,并将密封容器放置于绝缘箱中。绝缘箱必须保持恒温。采用滤纸法测试样的基质吸力,首先要获得滤纸的率定曲线,即滤纸含水率与对应基质吸力之间的定量关系。试验采用Whatman No.42型标准滤纸,其率定关系见式(1)[29],该型滤纸的含水率w与基质吸力us的关系曲线为双折线。
采用轻型击实仪制备的试样,在人工干湿循环条件下,既可以保证裂隙发育良好,又能大大降低制样的工作量。土样基本参数为:液限42.6%,塑限19.6%,塑性指数23,自由膨胀率56.8%,最大干密度1.8 g/cm3,相对密度2.7,试样初始干密度1.68 g/cm3。试样制备过程如下:
1)采用轻型击实仪将配制好的土料制成大圆状样,高度为40 mm,目的是后期切取环刀样时能最大限度地获得完整试样。击实完成后将试样推出,并将表面整平。然后将试样置于抽气饱和装置中进行抽气饱和。至此一个圆状样制备完毕。
2)将饱和试样移出置于室内恒温(22 ℃)环境下,使其在自然状态下蒸发脱湿。考虑到时间因素,试验过程中采用小型电风扇吹试样表面以加速水分蒸发。转面与试样表面平行,距离600 mm。试验过程中定期称量试样质量,当其保持不变时表明一个脱湿过程完成。对于浸水过程,本文采用加湿器喷雾于试样表面的方法模拟降雨过程,当水分在表面有少许残留并不再浸入试样内部时停止喷雾。最后用塑料薄膜将其密封。至此一个干湿循环完成,重复上述步骤即为多次干湿循环。图1为经历不同干湿循环次数后,试样表面裂隙的开展情况。随着干湿循环次数的增加,主裂隙首先生成;当循环至一定次数后,主裂隙边缘土颗粒剥落,形态逐渐模糊,并有新的小裂隙生成,土壤表面破碎程度加剧,这符合实际工程中裂隙的发育过程。
图1 干湿循环下膨胀土裂隙发育过程Fig.1 Crack development process of expansive soil under drying-wetting cycles
3)当试样含水率达到试验指定要求后,采用环刀取样的方法获得试验所需的试样,环刀尺寸为Φ61.8 mm× H20 mm。用螺旋式千斤顶缓慢地将环刀压入试样约30 mm,即环刀顶底面均留余10 mm,以便后期切削时能最大程度地保证试样的完整性。压入速率不宜过大,尤其对于含水率低的试样,其表现出较高硬度和较大脆性,压入太快易使试样产生脆性断裂。对此进行了不同压入速率的取样试验,结果表明对于含水率低的试样,压入速率不宜超过0.3 mm/s;对于含水率高的试样,压入速率不宜超过0.8 mm/s。
为研究不同水分变化路径对膨胀土基质吸力的影响,课题组分别进行了4次干湿循环下试样脱湿和吸湿条件下的滤纸法测基质吸力试验。具体试验方案见表1。需要说明的是,表1中的数据为试验完成后试样的实测体积含水率,可根据试验完成后试样的质量含水率、湿密度和土粒密度等参数换算得到。不同编号分别代表试验不同阶段时所切取的试样。由于试验目的主要是获得整个干湿循环过程中基质吸力的变化情况,因此具体选取哪个阶段时的试样并无严格要求,原则上根据试样颜色、软硬程度等的变化来控制。经历不同干湿循环次数下试样的饱和体积含水率不尽相同,具体见表1中编号11的数据,是由试验完成后取样测得。土粒密度为2.7 g/cm3。采用接触法测量试样的基质吸力,每组取相同试样2个及干滤纸3张。滤纸尺寸小于试样(1张直径为55 mm,另2张直径为58 mm),将小直径的滤纸夹在2张大直径滤纸之间并置于下部试样的顶部,然后将上部试样置于滤纸上方紧密接触(图2),目的是避免测试滤纸(即中间小直径滤纸)沾染土颗粒而影响试验结果。用塑料膜将整体包裹后蜡封,置于恒温环境下至少7 d[26],保证滤纸与试样达到水分交换平衡。然后迅速将滤纸取出并称质量,随后烘干至少6 h并称质量。由此可获得滤纸的含水率,通过式(1)即可计算得该试样的平均基质吸力。同时量测上下试样的体积和质量,以获得试样的体积含水率,最终获得试样的土水特征曲线。
图2 滤纸法示意图[23]Fig.2 Schematic diagram of filter paper method
表1 滤纸法试验方案Table 1 Testing program of filter paper method
非饱和土土水特征曲线通常具有复杂的形态,可用典型特征参数来反映,以吸力进气值与残余值为典型代表:进气值是空气进入土体孔隙并将孔隙中的重力水开始排出时所对应的基质吸力,残余值是重力水开始进入土体孔隙时所对应的基质吸力。在实际应用时,进气值为脱湿曲线在高饱和度下的拐点,残余值为吸湿曲线在低饱和度下的拐点,具体确定方式见图3[30]。
不同干湿循环次数下测得的滤纸含水率见表2,结合式(1)求得相应的试样基质吸力(表3)。结合表1中相应的试样体积含水率,可获得经历不同干湿循环次数下试样的土水特征曲线(图4),其中横坐标为基质吸力的对数值。根据基质吸力进气值和残余值的确定方法(图3),可求出经历不同干湿循环次数下试样的基质吸力进气值和残余值,相应的计算结果见图5。
图3 土水特征曲线的进气值和残余值确定方法Fig.3 Determination of air-entry and residual values of SWCC
表2 滤纸含水率测定结果Table 2 Results of water content of filter papers
表3 滤纸法测定土壤基质吸力试验结果Table 3 Results of soil matric suction by filter paper method
图4 试样土水特征曲线及特征参数Fig.4 Soil SWCC and its characteristic parameters
图5 不同干湿循环次数下试样的进气值与残余值Fig.5 Air-entry and residual values of soil under drying-wetting cycles
随着干湿循环次数的增加,土壤的进气值呈下降趋势,从循环1次时的134.5 kPa降至循环4次时的58.5 kPa,降幅达56.5%。从循环1次至2次的进气值减小较大,往后降幅明显减小,趋于基本稳定,这表明对土壤进气值的影响主要出现在初次干湿循环下;残余值亦呈下降趋势,从循环1次时的1 040.5 kPa降至循环4次时的528.5 kPa,降幅达49.2%。每经历一次干湿循环,残余值的降幅均较大,尚未趋于稳定,这表明干湿循环效应对土壤残余值的影响比对土壤进气值的影响要大。本文中经历4次干湿循环的试样,其残余值仍有较大的下降空间,而进气值已基本趋于稳定。
可以看出,干湿循环效应对膨胀土结构影响十分明显,导致土壤结构松散破碎,裂隙发育,持水能力总体下降。干湿循环效应对土壤残余值影响较大,对土壤进气值的影响相对较小。总体上看,干湿循环对膨胀土持水性能的影响很大,随着循环次数的增加,膨胀土持水能力迅速衰减,遇水后基质吸力迅速降低,工程性质变差。此外,对一些受水分作用影响明显的特殊性土,传统的基质吸力量测方法不能真实完全地反映其持水特性。滤纸法不干扰试样的初始形态,试验结果有效地验证了滤纸法可测定干湿循环下土体的基质吸力,可作为测量该类特殊性土基质吸力的一种有效方法。
计算求出不同体积含水率下试样的基质吸力后,应采用合适的土水特征曲线计算模型进行分析。本文采用Fredlund等[31]提出的三参数模型(式(2))对试验结果拟合
式中ψ为基质吸力,kPa;θw为基质吸力为ψ时的体积含水率;θs为饱和体积含水率;a为与进气值有关的参数;b为与土壤脱湿速率有关的参数;c为与残余值有关的参数。
利用Origin软件的自定义函数拟合功能,对4次干湿循环下的土水特征曲线进行了拟合,拟合结果见表4。可以看出,随着干湿循环次数的增加,a和b值逐渐减小,c值有所增大。这表明,随着干湿循环次数的增加,土壤进气值逐渐减小,水分变化速率有所降低,而残余含水率逐渐增加。这表明,干湿循环效应导致土壤结构逐渐松散破碎,持水能力逐渐下降,土壤中的水分易从孔隙中逃溢,外界水分也易入渗土壤。
表4 土水特征曲线模型参数拟合结果Table 4 Fitted results of parameters in SWCC model
将参数a、b、c与干湿循环次数的关系绘于图6。可以看出,脱湿和吸湿条件下,参数a、b、c与干湿循环次数均呈现较好的线性关系,可采用一次线性关系式进行拟合,结果见表5。
图6 拟合参数a、b、c与干湿循环次数的关系Fig.6 Relationship between fitted parameters a, b, c and drying-wetting cycle time
表5 土水特征曲线模型参数与干湿循环次数关系Table 5 Relationship between parameters of SWCC model anddrying-wetting cycle time
将拟合结果代入式(2)中,即可获得考虑干湿循环效应的膨胀土土水特征曲线模型,见式(3)。
式中ad、bd、cd、aw、bw、cw的具体表达式见表5,其余参数物理意义与式(2)中的参数相同。
1)随着干湿循环次数的增加,土壤的进气值呈下降趋势,初次干湿循环对进气值影响最大,往后降幅明显减小,趋于基本稳定;残余值亦呈下降趋势,每经历一次干湿循环,残余值的降幅均较大,尚未趋于稳定,这表明干湿循环效应对土壤残余值的影响比对土壤进气值的影响要大。经历4次干湿循环的试样,其残余值仍有较大的下降空间,而进气值已基本趋于稳定。
2)采用了Fredlund土水特征曲线模型对干湿循环下膨胀土的土水特征曲线结果进行拟合,建立了考虑干湿循环效应的土水特征曲线模型。模型参数与干湿循环次数均呈现较好的线性关系。随着干湿循环次数的增加,土壤进气值逐渐减小,水分变化速率有所降低,而残余含水率逐渐增加。该成果可为深入研究土壤基质吸力及应用提供参考。
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Changes of matric suction in expansive soil under drying-wetting cycles using filter paper method
Wu Junhua1,2, Yang Song3※
(1. Key Laboratory of Nondestructive Testing , Ministry of Education, Nanchang Hangkong University, Nanchang 330063, China; 2. College of Civil Engineering and Architecture, Nanchang Hangkong University, Nanchang 330063, China; 3. College of Water Conservancy, Yunnan Agricultural University, Kunming 650201, China)
Soil water characteristic curve (SWCC) can show the soil water holding capacity, and is the relationship between the soil matric potential and saturation. The expansive soil has the characteristics of strong swell-shrink, developing fissures and over consolidation, especially under drying-wetting cycles. Its water holding capacity, which has an important impact on the soil properties, is totally different from the capacity of full soil. Therefore, how to exactly obtain the matric suction of expansive soil under drying-wetting cycles is very important to analyze the characteristics of mechanics and deformation of expansive soil. In order to analyze the change rule of matric suction of expansive soil under drying-wetting cycles, the artificial rainfall and evaporation was applied in drying-wetting tests of expansive soil. The specimens sized at 102 mm × 40 mm (diameter × height) were made by compaction apparatus and the initial dry density was 1.68 g/cm3. All the specimens were saturated after compaction and undergoing drying-wetting cycles in constant temperature (22 ℃) environment. The small electric fans were adopted to increase the rate of evaporation until the mass of specimens was kept constant under the condition of drying. Then the water vapours were sprayed to the surface of specimens by humidifiers until the process of infiltration stopped under the condition of wetting. That is one drying-wetting cycle so far. In this paper, 4 set of experiments under drying-wetting cycles were carried out and the cycle times were 1, 2, 3 and 4 respectively. Then 11 specimens sampled by cutting rings (61.8 mm × 20 mm (diameter × height)) in different volumetric water content were obtained in each process of drying and wetting respectively. The matric suction of each specimen was measured with filter paper of Whatman No. 42 and then the SWCCs were obtained under drying-wetting cycles. The corresponding air-entry values and residual values in different drying-wetting cycle times were calculated from SWCC. Ulteriorly, the different SWCCs obtained in different drying-wetting cycle times were fitted by the Fredlund SWCC model. Based on the above analysis, an SWCC model of expansive soil considering drying-wetting cycle effect was established finally. The results showed that: 1) With the increase of drying-wetting cycle time, the air-entry value of expansive soil was evidently reduced. Compared with the air-entry value (134.5 kPa) of expansive soil undergoing one drying-wetting cycle, the air-entry value was 58.5 kPa and reduced by 56.5% undergoing 4 drying-wetting cycles. The most effect of drying-wetting cycles on air-entry value occurred in the first cycle. 2) With the increase of drying-wetting cycle time, the residual value of expansive soil was also reduced. Compared with the residual value (1 040.5 kPa) of expansive soil undergoing one drying-wetting cycle, the residual value was 528.5 kPa and reduced by 49.2% undergoing 4 drying-wetting cycles. Moreover, the residual value is reduced evidently after undergoing one drying-wetting cycle and it cannot yet be stabilized with the increase of drying-wetting cycle time. That means the effect of drying-wetting cycles on ari-entry value is much greater than on residual value. 3) The fitted parameters in the SWCC model of expansive soil considering the effect of drying-wetting cycle have a good linear correlation with the drying-wetting cycle time. With the increase of drying-wetting cycle time, the soil air-entry value and change rate of water content reduce while the residual value increases gradually. The results can provide a reference for the further analysis on soil matric suction and its application in engineering.
soils; cracks; water content; expansive soil; drying-wetting cycles; filter paper method; matric suction; soil water characteristic curve
10.11975/j.issn.1002-6819.2017.15.016
S152.7
A
1002-6819(2017)-15-0126-07
2017-01-12
2017-07-10
国家自然科学基金资助项目(51408291,41662021);南昌航空大学无损检测技术教育部重点实验室开放基金项目(ZD201529002)
吴珺华,副教授,博士,主要从事非饱和土基本性质研究。南昌南昌航空大学土木建筑学院,330063。Email:wjhnchu0791@126.com
※通信作者:杨 松,博士,主要从事非饱和土基本性质研究。昆明 云南农业大学水利学院,650201。Email:yscliff007@126.com