Consolidation mechanism of vacuum preloading for dredged slurry and anti-clogging method for drains
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摘要: 针对吹填淤泥真空预压地基处理效果差的关键问题,全面总结了笔者及其课题组多年来的研究成果,主要包括4个方面:①采用超低压固结-渗透仪进行了大量的吹填淤泥固结、渗透试验,发现了初始含水率和液限对压缩-渗透性状的影响规律,建立了压缩过程中渗透系数的定量演化规律。②吹填淤泥真空预压过程中的位移、应变发展规律和土柱形成机理。基于粒子图像测速(PIV)技术和粒子追踪测速(PTV)技术,获得了吹填淤泥真空预压固结过程中的土体位移、应变发展规律,揭示了吹填淤泥真空预压固结机理,获得了真空预压作用下不同初始含水率和不同级配吹填淤泥的土柱形成过程。③吹填淤泥真空预压固结计算方法。基于不均匀应变假定提出了土柱区内、外固结计算公式,获得了不同边界真空度时空变化下的固结规律。以土柱形成过程为基础,建立了考虑土柱影响的吹填淤泥真空预压固结计算方法。④通过室内和现场试验,从增强真空渗流场、延缓土柱形成和改良土性等方面,优化排水板滤膜孔径,提出了分级、增压式、絮凝-真空预压等多项防淤堵处理技术,缓解排水体淤堵,提高固结速率,加速吹填淤泥处理,增强加固效果。研究成果对吹填淤泥场地开发利用,缓解城市土地资源紧张,促进城市可持续发展具有重要意义。Abstract: Considering the key problem that the vacuum preloading method cannot efficiently improve dredged slurry, the research findings over the years obtained by the author’s research team are comprehensively summarized, including the following four aspects: (1) A large number of consolidation-hydraulic conductivity tests are performed using the modified consolidation apparatus under low initial stress. Based on the test results, the influences of the initial water content and the liquid limit on the compression-permeability properties are discussed. The evolution laws of the permeability coefficient during the compression are obtained quantitatively. (2) The development laws of displacement and strain of soils and the formation mechanism of soil column during the vacuum preloading consolidation of the dredged slurry are illustrated. By utilizing the particle image velocimetry (PIV) and the particle tracking velocimetry (PTV) techniques, the development laws of the displacement and strain of soils during the vacuum preloading consolidation process are obtained. Furthermore, the consolidation process of the dredged slurry is revealed. Besides, the formation process of the soil columns under different initial water contents and particle-size distributions is obtained. (3) The analytical and numerical methods are proposed to calculate the consolidation of dredged slurries improved by vacuum preloading. Based on the uneven strain assumption, the consolidation solutions are derived for the clogging zone and the intact zone, respectively. Subsequently, the consolidation behaviors are studied under various distributions and development laws of vacuum preloading at the permeable boundaries. Besides, the consolidation model considering the formation of the soil columns is also established to investigate the relevant influences. (4) Based on the laboratory and field tests, several methods are proposed by enhancing the vacuum seepage fields, slowing the formation of soil columns, and improving the soil properties, to increase the consolidation efficiency. These methods include applying staged vacuum preloading, booster PVDs, multi-flocculant treatment, and appropriate pore size of PVD filters. Using these methods, the clogging of the dredged slurry during the treatment process can be reduced, and then the consolidation speed and improvement quality can be increased. The research findings are of great significance to developing and utilizing dredging slurry sites, alleviating the shortage of urban land resources, and promoting the sustainable urban development.
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Keywords:
- dredged slurry /
- vacuum preloading /
- consolidation /
- clogging /
- vacuum gradient /
- flocculation
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0. 引言
高压实膨润土作为高放废物深地质处置库首选缓冲/回填材料,具有维持结构稳定、延缓地下水入渗、阻滞核素迁移和传导核衰变热等重要缓冲功能(图 1)。在处置库长达上万年的服役期间,高压实膨润土将长期受到围岩地下水及其化学成分的侵入作用,导致矿物相态和微观结构不断演化,缓冲性能逐渐衰减,最终威胁处置库的长期稳定与运营安全。因此,深入剖析膨润土与孔隙溶液的相互作用机制是研究处置库复杂近场环境下膨润土缓冲性能长期演化的关键。
为此,众多学者针对高压实膨润土在不同孔隙溶液条件下缓冲性能的演化规律开展了大量试验研究,并建立孔隙溶液对膨润土膨胀力、渗透系数等水力-力学特性和吸附量、扩散系数等化学阻滞特性影响的理论模型[1-4]。化-水-力膨胀模型方面,Guimarães等[5]以阳离子交换量作为孔隙溶液的化学行为描述,对巴塞罗那膨胀模型(BExM)[6]进行了耦合改进;Lu等[2]、Dominijanni等[7]分别以渗透吸力和迁移特性参数变化作为孔隙溶液的化学行为描述,改进了有效应力的表达。Bennethum等[8]引入化学势,利用自由能和热力学第二定律建立了膨胀多孔介质的本构框架;徐永福[9]基于膨润土吸水膨胀特性,用分形表面吸附理论建立了膨润土膨胀变形的统一理论。渗透模型方面,Ruan等[10]对Kozeny-Carmen模型进行双孔修正,提高了在压实膨润土中的适用性;Komine[11]基于阳离子交换量提出了多组分阳离子在膨润土中等效渗透系数的计算模型。吸附扩散模型方面,Torstenfelt等[12]将菲克第二定律对多孔介质的扩散模型进行了吸附耦合修正;Muurinen等[13]排除层间孔隙影响,以有效孔隙代替总孔隙进行修正,将孔隙溶液对吸附扩散的化学行为简化为对有效孔隙和线性吸附分配系数的影响。
然而,现有研究表明,膨润土晶层胀缩和扩散双电层厚度受孔隙溶液中阳离子交换的影响[14],且交换难易程度与其干密度和水化约束条件有关[15-16],并表现出对不同价态阳离子的选择性[17-18]。由于膨润土对阳离子的响应明显大于阴离子,因此,以吸力或阳离子交换作为膨润土在溶液中有效应力和水化响应的修正依据,忽略了缓冲性能随溶液扩散的演化过程、各参数互相影响和溶液类型的差异性,也缺乏描述溶液作用下孔隙结构变化的过渡模型和化学吸附模型,尤其是微观吸附模型、细观孔隙迁移模型和宏观力学体变模型在尺度上存在极大差异。至今尚未建立起跨尺度和溶液作用描述的缓冲性能模型。
事实上,高压实膨润土在溶液中的响应十分复杂,既有晶层胀缩、集合体裂解、扩散双电层水化、阳离子交换等不改变矿物晶胞结构的物理作用[19-20];也有产生矿物晶胞相变、化学胶结的化学作用[20-21]。本文在阐述溶液对高压实膨润土缓冲性能影响的基础上,总结分析了与孔隙溶液的物理作用机制和仍待研究的重点方向,以期为高放废物深地质处置库工程屏障在近场复杂环境下长期缓冲性能演化评估、设计建造和运营管理提供依据。
1. 膨润土结构与孔隙溶液
1.1 膨润土基本结构
膨润土是以层状蒙脱石为主要矿物的高膨胀性黏土。在蒙脱石晶体结构中,硅氧四面体中的四价Si可被三价Al同晶置换[22],铝氧八面体中的三价Al又可被二价Mg,Ca,Fe等同晶置换,使得每个单位晶胞有0.2~0.6的永久负电荷[23]。为了平衡晶层负电荷,晶层间吸附有大量可交换金属阳离子和极性水分子。蒙脱石特殊结构为吸水膨胀和离子吸附提供了强劲动力;其膨胀挤密和吸附黏滞效应也使膨润土具有高愈合性、低渗透性、强阻滞性等重要特性。
高压实膨润土可分成单元晶层,层叠体(准晶体)和集合(聚)体3个结构层次,如图 2所示。其中,层叠体是由数个至数百个单元晶层层状堆积形成,其数量与蒙脱石层间的阳离子类型、饱和度和孔隙溶液有关;集合体是由若干个层叠体聚集组成,尺度一般在微米级别。同时,这3种基本单元也构成了高压实膨润土的3种主要结构层次的孔隙,即晶层间孔隙、层叠体间孔隙和集合体间孔隙[24]。
通常,层叠体结构内晶层间可吸附1~4层极性水分子。这些水分子一部分被强吸附在晶层表面形成氢原子向内的定向排列,另一部分则以水合阳离子形成层内溶液[25]。这些定向排列的强吸附水通常密度大于1.0 g/cm3,黏滞性比自由水大100倍左右,活动度、扩散性和氢键作用强度远比自由水弱,一定程度上表现出固体性质[26],使得水分子难以穿越晶层。因此,层叠体作为基本微观结构单元,也是与孔隙溶液直接作用的基本受力单元。层叠体内晶层膨胀和层叠体外扩散双电层膨胀共同构成了膨润土的总体膨胀。
1.2 孔隙溶液的形成
膨润土工程屏障中的孔隙溶液主要由近场围岩地下水侵入形成,其化学成分十分复杂,与处置库围岩和场地环境密切相关。比利时、瑞士和法国为代表的黏土岩型处置库主要考虑黏土岩孔隙水,如法国处置库围岩地下水TDS约为5700 mg/L,主要阳离子中Na+含量高达1035 mg/L,Ca2+含量为280 mg/L[27];日本沉积岩型处置库主要考虑海水的侵入,其地下水TDS高达42400 mg/L,主要阳离子中Na+含量高达11040 mg/L,Ca2+含量为440 mg/L[28];西班牙、韩国和中国的花岗岩型处置库主要为花岗岩裂隙水,以中国北山地下水为例,其TDS值介于1793.3~3594.5 mg/L,pH介于7.5~8.5,化学相为Cl·SO4-Na型,其中Na+含量高达494~1036 mg/L,Ca2+含量为90.2~183.0 mg/L [29]。
同时,膨润土中的孔隙水长期遭受衰变热、气体运移、围岩应力、膨润土吸水膨胀等复杂多相多场耦合作用后,会因干湿循环作用而产生盐分浓缩积聚,形成浓度更高、化学成分更复杂的孔隙溶液[20]。此外,混凝土衰解和膨润土溶蚀转变等化学作用也会对孔隙溶液的组成产生重要影响[30]。
2. 膨润土水化膨胀过程
处置库中拼砌的高压实膨润土屏障服役后,将长期处于非饱和状态,并在吸力作用下从围岩中吸收水分,发生膨胀愈合形成机械屏障和化学屏障阻滞核素迁移。然而,缓冲屏障建设时不可避免地在块体间、块体与围岩间留下大量施工接缝,成为地下水及化学成分入渗的优势通道,使部分膨润土很快达到饱和状态。
2.1 非饱和吸湿膨胀
处置库服役初期,高压实膨润土主要通过水汽吸湿产生水化膨胀,典型的膨胀力时程曲线见图 3 [31]。当吸力大于149.6 MPa时,膨润土处于初始水化阶段,主要发生蒙脱石晶层膨胀,膨胀力大小取决于晶层间阳离子类型,不受干密度(初始孔隙率)影响。随着水化的持续进行,吸力逐渐降低,膨胀力大小与干密度呈现正相关,增长趋势因干密度不同而表现出显著差异。当吸力低于38 MPa时,膨胀力因层叠体大规模裂解而发生累积“楔”力释放和孔隙坍塌而表现出减小,并因层叠体表面积增多使得扩散双电层总膨胀增大而再次增长,呈现出“先增加—后减小—再增加”的双峰演化特征[32]。
此外,膨润土吸湿过程中形成的晶层水化厚度、层叠体裂解后的晶层数与水化吸力或含水率对应。Saiyouri等[25-26]测试了不同吸力下MX80-Na基膨润土的晶层衍射数据,并基于Pons等[33]统计得到层间吸附水分子层数和层叠体内晶片层数随吸力变化规律(图 4)。结果表明,吸力在7~50 MPa时,层间含有2层水分子;吸力小于0.06 MPa时,层间最多含有4层水分子。相应地,当吸力从50 MPa降至7 MPa时,层叠体内晶层从350层裂解至150层;吸力降至3 MPa时,晶层数量骤减至3层。可见,蒙脱石层叠体在吸力低于50 MPa时开始大规模裂解,在吸力低于7 MPa的高饱和阶段,层叠体已基本裂解分散。
根据水化膨胀主导机制差异,通常将膨润土由非饱和向完全饱和的水化膨胀过程分为3个阶段(图 5)。①第1阶段,蒙脱石层间吸水膨胀;②第2阶段,层叠体膨胀裂解,并填充层叠体内孔隙;③第3阶段,集合体逐步崩解、大孔隙坍塌并被裂解的薄层叠体重新构建,此时,薄层叠体迅速增加,并在薄层外表面快速形成扩散双电层继续水化膨胀[34]。
2.2 溶液水化膨胀
溶液中高压实膨润土水化膨胀同样经历膨胀力快速发展的晶层膨胀、膨胀力不断调整的层叠体裂解和膨胀力再增加的扩散双电层膨胀3个阶段。溶液入渗初期,由于水分运移速率大于离子,当离子进入膨润土时,膨润土已水化膨胀。因此,当离子依次通过大小孔隙时,势必打破不同尺度孔隙溶液的吸力平衡,引起土颗粒表面阳离子竞争吸附和孔隙结构不断调整,尤其以溶液浓度对膨胀力演化影响最为显著。如图 6所示[30],NaCl浓度越高,晶层膨胀力越小,孔隙塌陷时膨胀力下降越大,扩散双电层膨胀越小。更多研究表明[35-39],溶液浓度增加对最终膨胀力既有促进作用,又有抑制作用,并与离子类型有关。此外,孔隙溶液引起的膨润土化学体变也因阳离子类型不同而呈现差异性(图 7)。Na基膨润土在NaCl溶液的盐—淡循环中,化学体变具有可逆性;而在KCl或CaCl2溶液的盐—淡循环中却呈现出不可逆收缩变形[40-41]。可见,溶液离子对高压实膨润土膨胀的影响机制是多因素共同作用的结果,物理作用的可逆性也会因干密度和水化约束条件不同而不同。
3. 孔隙溶液对晶层膨胀的作用机制
3.1 对晶层膨胀的促进作用
孔隙溶液对晶层膨胀的作用与渗透吸力有关。蒙脱石晶层表面定向排列的强吸附水分子一定程度上表现出固体性质[26],使得水分子难以通过晶层。当层叠体外溶液的溶质吸力小于层内溶质吸力时,层间吸附阳离子和水分子不能自由扩散至层外,仅能通过层叠体外水分子向内迁移达到吸力平衡。当晶层受到约束,限制水分子向内迁移时,层叠体将产生向内的渗透力Oπ,大小与吸力差成正比。当晶层在约束力Pc作用下达到平衡时,膨胀力Ps等于约束力和渗透力之和,即Ps = Pc + Oπ,如图 8所示[42]。
由图 8可知,膨润土最大膨胀力和膨胀变形并不是发生在浓度为零的去离子水环境,而是蒙脱石晶层内外溶质吸力差降为零的临界吸力溶液中[36, 38]。当孔隙溶质吸力低于临界吸力时,孔隙溶液浓度的增加会促进晶层膨胀。进一步分析图 7,9可知[15, 43],临界吸力的大小与约束力(如恒体积、恒定荷载或自由膨胀等水化边界条件)有关,约束力越大,层间膨胀量越小,此时层间溶质吸力越大,对应的临界吸力也越大。恒体积条件下临界吸力对应的浓度小于0.05 mol/L,而自由膨胀条件下普遍小于0.02 mol/L。
3.2 对晶层膨胀的抑制作用
当层叠体外溶液的溶质吸力大于临界吸力时,层叠体内水分子可向层外运移、层叠体外离子也向层内扩散以达到吸力平衡。因此,孔隙溶液对晶层膨胀的抑制包括阳离子交换和层间排水,分别对应渗透固结和渗透诱导固结两种体变机制。
阳离子内迁交换对晶层的影响与层间溶质吸力密切相关。发生阳离子交换时,晶层更倾向选择高价态或强吸附性的低水合吸力离子,以降低整体水化势能。如图 10(a)[45],层间相同正价浓度中,阳离子水化吸力大小为Na+ > K+ > Ca2+,相应地,膨润土膨胀性能相对大小为Na基膨润土>K基膨润土>Ca基膨润土。此外,层外阳离子内迁还与层叠体约束力密切相关。约束力增加在提高临界浓度的同时,也抑制了晶层膨胀间距和扩散孔径,增加阳离子交换难度,从而使层叠体内外吸力差更倾向于通过层间排水达到平衡。在恒体积膨胀时,相同浓度条件下,阳离子溶质吸力越大的溶液对膨胀力的抑制作用越强,高价态阳离子的抑制作用程度低于低价态阳离子,且水合离子半径越小,影响越大(图 11)[16],各阳离子影响程度依次为:Na+ > K+ > Ca2+ > Mg2+ > Al3+ > Fe3+。恒定荷载条件下(图 12),土体膨胀使孔隙增大,当K+进入层叠体,因其离子半径与晶层中硅氧四面体六联环空间相近,形成K-O配位多面体而具有更强的交换优势,表现出比Na+更强的抑制作用[15]。自由膨胀条件下(图 9),层叠体不受约束,晶层完全膨胀,孔隙进一步增大,高价态Ca2+也能进入层叠体内,从而表现出比Na+更强的抑制作用[15, 43]。
4. 孔隙溶液对扩散双电层的作用机制
膨润土在孔隙溶液作用下,层叠体表面将产生扩散双电层膨胀。扩散双电层厚度可为[14]
1K = √ε0εRkBT2n0e2v2。 (1) 式中:1/K为双电层厚度;ε0为真空介电常数,取8.854×10−12 C2·J−1·m−1;εR为孔隙溶液相对介电常数;kB为玻尔兹曼常数(1.38×10−23 J·K−1);e为单位电荷带电量(1.602×10−19 C);n0为电解质浓度;v为离子价态。
由此可知,扩散双电层厚度主要与孔隙溶液浓度和离子价有关,浓度越大,离子价越高,扩散双电层厚度越小。如图 12所示,相同浓度Na+溶液中,SO42-比Cl-对膨胀抑制作用更强[15]。而且低价阳离子的抑制作用强于高价阳离子[35, 37],但这一现象与粉末状膨润土的自由膨胀刚好相反[15, 43]。可见,高压实膨润土中晶层膨胀量大于扩散双电层膨胀量,且高价阳离子很难进入晶层微孔,多处于层叠体大孔中。因此,孔隙溶液对扩散双电层膨胀影响较晶层膨胀更为显著。进一步研究表明,膨润土完全水化后,集合体裂解产生的层叠体厚度也不均一。土体内存在层间微孔(约2 nm)、薄层叠体间小孔(10~150 nm,峰值孔径20 nm)和厚层叠体间大孔(大于150 nm)[24]。孔隙溶液浓度相同时,离子类型对膨润土膨胀力和饱和渗透系数大小的影响规律依次为Na+ > K+ > Ca2+ > Mg2+ > Al3+ > Fe3+[16]。这一现象表明,离子很难在微孔中运移,主要通道为层叠体间孔隙,且受水化双电层阻滞效应的影响,实际有效孔隙更小(图 13)。实际上,扩散双电层中的水分子和离子越靠近层叠体表面,吸附固定作用越强[45],提供给溶液运移的有效孔隙直径更小。由此可见,孔隙溶液对晶层和扩散双电层膨胀的抑制作用,显著增加了有效孔隙通道并减小弯曲度,使膨润土渗透性能(图 14)和核素离子扩散性能(图 15)明显提高[16, 46]。高压实膨润土力学缓冲性能和化学缓冲性能也因其它离子的加速侵入而衰减。
5. 孔隙溶液对膨润土吸附的作用机制
5.1 膨润土表面物理化学吸附机制
膨润土对溶液离子吸附作用包括表面络合(化学)吸附和静电(物理)吸附,分别对应两类表面活性位点。如图 16所示,表面络合吸附主要是离子与蒙脱石矿物表面硅/铝原子形成羟基络合位点(Si/Al―OH)形成共价键结合的内层配合物,化学反应式为
n≡Si/AlOH+Mn+⇌(≡Si/AlO)nM+nH+。 (2) 式中:≡Si/AlOH 为羟基络合位点,Mn+为吸附阳离子。
静电吸附主要是水合离子与矿物表面在范德华力作用下形成外层配合物,也叫离子交换吸附。发生在层间的静电吸附(离子交换反应)可表示为
n≡XNa+Mn+⇌(≡X)nM+nNa+。 (3) 式中:≡X−为极性负电荷位点。
表面络合吸附与吸附离子形成的共价键比静电吸附形成的范德华键的亲和力高出一个或几个数量级[48-49]。因此,静电吸附的离子在环境条件改变时容易发生解吸附和置换,而络合吸附的离子不易解吸附。
5.2 孔隙溶液pH对膨润土吸附的作用机制
溶液中H+或OH-的浓度不仅影响膨润土表面吸附位点的活性,也会改变离子水解结合形式,其在表面吸附位点的反应机制见表 1[50]。通过平衡常数可知,酸性环境中,H+率先与蒙脱石矿物边缘的铝羟基(Al―OH)发生质子化反应而带正电;酸性更强时,H+又可作为竞争阳离子吸附占据阳离子交换位点,影响膨润土对目标离子的吸附活性。膨润土易与异性电荷离子产生静电吸附,而与同性电荷离子发生络合吸附。当膨润土表面由负电荷变为正电荷时,静电吸附发生吸附反转,表现出对阴离子的吸附活性,并抑制对阳离子的吸附作用。碱性环境中,OH-对蒙脱石表面羟基络合位点(Si/Al―OH)的去质子化反应(表 1),会使原本羟基络合位点带负电,进入准络合状态,增强络合活性。如图 17所示[51-55],核素离子吸附率在强酸环境时,均因强竞争吸附和质子化而随pH减小而迅速降低。
除对膨润土吸附位点的活性影响外,pH对核素水合离子形态的影响也极为复杂。核素离子由于价态高,通常存在多级水解反应,离子形态随pH升高会结合多个OH-而减小正电荷,从而提高被吸附量(图 17),也可能因带负电而减小吸附量,更有如U(Ⅵ)存在多种同价异构离子而表现出特殊吸附特性(图 18)[53]。此外,碱性环境下核素离子的沉淀作用也会导致核素离子去除率迅速升高的重要原因之一。膨润土对核素离子静电吸附和络合吸附的主次关系与孔隙溶液pH密切相关。以阳离子形态存在的核素离子在弱酸溶液中主要以静电吸附为主,在强酸和碱性溶液中主要以络合吸附为主[50, 56]。
5.3 孔隙溶液离子对膨润土吸附的抑制作用
GMZ-Na基膨润土表面活性位点的分布密度见表 2,其中蒙脱石矿物表面阳离子交换位点的分布密度约为羟基络合位点的5倍(络合位点仅占17.27%)[50]。两类吸附位点对离子吸附响应有着显著的活性差别,Na+,Ca2+,Mg2+,K+等金属离子水解能力较强,与羟基的络合能力弱,吸附主要发生在阳离子交换位点,对羟基络合位点影响较小。存在多种离子时,有限吸附位点对目标离子的吸附量会因竞争吸附而降低[17-18, 57],并表现出选择吸附性。
研究表明,离子在黏土矿物表面的竞争吸附优势主要与其带电性、一级水解常数、离子半径、离子强度和体系pH等因素有关。对相同正价电荷的金属离子,一级水解常数越小,水合离子半径越小,与膨润土竞争吸附亲和力越强[58]。而核素离子与背景离子的竞争吸附机制会因该核素离子水解价态的变化而出现不同的促进或抑制结果。如图 19所示,酸性环境下,U(Ⅵ)以阳离子形式存在,离子强度增加会导致竞争优势减弱而吸附减少[59];碱性环境下,U(Ⅵ)离子正价减小反而促进吸附,当水解为阴离子时,则抑制吸附[54]。因U(Ⅵ)水解特殊性,阴离子和阳离子都会对吸附产生较大影响(图 18,20),其中K+影响大于Na+和Li+,HCO3−影响则因pH变化而变化。离子类型方面,Ca2+会显著降低膨润土对U(Ⅵ)吸附容量,而Na+,K+,Mg2+对吸附影响较小;阴离子中,CO32−对吸附影响最大,HCO3−次之,而SO42−,NO3−对吸附影响较小(图 21)[60]。
6. 结论及建议
孔隙溶液对高压实膨润土的物理作用机制包括晶层膨胀、扩散双电层膨胀和吸附作用,相应的内在影响机理如下所示:
(1)层叠体是膨润土与孔隙溶液直接发生吸力平衡和吸附平衡的基本单元,且与水化约束力有关。孔隙溶液在低浓度时能够促进晶层膨胀,在高浓度时抑制晶层膨胀,取决于与临界吸力的差;孔隙溶液对层叠体外扩散双电层膨胀具有抑制作用。
(2)层叠体间孔隙是离子运移的主要通道,孔隙溶液对层叠体膨胀的力-化作用伴随着层叠体裂解和孔隙坍塌调整。孔隙溶液对扩散双电层的抑制作用是导致有效孔隙通道扩大、渗透性和扩散性增大的主要原因。
(3)膨润土对溶液离子吸附作用包括表面络合作用的化学吸附和静电作用的物理吸附。孔隙溶液对吸附作用的影响与其pH、离子成分及浓度有关。pH对膨润土表面活性位点和核素水合离子带电性质的影响是引起吸附特性变化的主要原因;背景离子的竞争吸附作用致使膨润土对核素离子的吸附量显著减少。
由于高压实膨润土缓冲性能受蒙脱石矿物层间阳离子、孔隙溶液成分、干密度和孔隙结构的影响十分复杂。现有溶液对膨润土缓冲性能的量化表征研究仍十分有限,国内外学者仅通过考虑化学作用效应对现有模型进行修正,无法从膨润土多尺度结构与孔隙溶液相互作用机理出发构建微宏观跨尺度的缓冲性能预测模型。因此,笔者认为以下工作仍需进一步研究:
(1)开展蒙脱石层叠体与孔隙溶液相互作用的系统性试验,研究不同孔隙溶液、干密度和水化约束条件下,蒙脱石层叠体膨胀裂解、吸力平衡和吸附平衡的作用特性,量化孔隙溶液对膨润土水化膨胀的影响程度,构建与宏观本构模型的参数关联。
(2)开展压实膨润土中不同尺度孔隙的等效量化研究,探究不同尺度孔隙对压实膨润土膨胀、渗透、扩散等模型的溶液响应特性,确定孔隙的参数选用。
(3)开展约束条件下膨润土在动态、多组分溶质中的竞争吸附研究,构建约束条件下高压实膨润土的竞争吸附模型。
(4)在此基础上,提出考虑化学效应的高压实膨润土缓冲性能演化长期评价方法,为中国深地质处置缓冲/回填材料的选取及设计提供理论和技术支撑。
致谢: 感谢土力学和岩土工程界各位同行的信任,让我有幸成为今年黄文熙讲座的主讲人;本文研究和撰写过程中得到了浙江工业大学土木工程学院孙宏磊教授、曾玲玲教授、潘晓东副教授、史吏副教授、徐山琳博士后,温州大学建筑工程学院王军教授、王鹏教授、符洪涛副教授等的大力帮助,也部分反映了课题组的研究成果。 笔者的相关研究生论文成果是本文的主要基础,这些研究生包括翁振奇、刘斯杰、何自立、陆靖凌、陆逸、张皓、朱彦臻。感谢国家自然科学基金委对笔者相关研究的持续支持。 -
表 1 淤泥的初始含水率
Table 1 Initial water contents of sludges
试验编号 w0 /%wL /%w0 /wL 1 106 53 2.0 2 133 53 2.5 3 159 53 3.0 表 2 淤泥的初始含水率
Table 2 Initial water contents of sludges
试验土样 w0 /%wL /%w0 /wL 温州土 106 53 2 台州土 80 40 2 广州土 54 27 2 表 3 试验方案
Table 3 Test schemes
试验编号 加载方式 真空梯度/kPa 加载模式/kPa 1 一级 80 80 2 二级 60 20→80 3 三级 30 20→50→80 4 四级 20 20→40→60→80 -
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