Due to its low hydraulic conductivity, high swelling capacity, and good adsorption properties, the Gaomiaozi (GMZ) bentonite has been selected as potential buffer/backfill materials for construction of engineered barriers in the deep geological repository for disposal of high-level nuclear waste (HLW) in China. Investigation of salt solution effects on the water retention properties of compacted bentonite is of great importance in the context of geological disposal of HLW based on the multibarrier concept. In this study, amended specimens were obtained through a spray of different concentrations of salt solutions to reach target salt contents, respectively. With employment of the vapor phase technique for suction control, water retention tests were conducted on densely compacted GMZ bentonite (1.7 Mg/m3) with different salt contents under confined conditions. Corresponding soil water retention curves (SWRCs) were obtained. Analysis indicates that, for a given suction, the measured water content of GMZ bentonite specimen increases as the salt content (or pore fluid concentration) increases. The influencing rate depends on suction. For lower suctions (lower than 38 MPa), the water retention capacity increases as the salt content increases, while for higher suctions (higher than 38 MPa), the influence can be negligible. Based on the Fredlund and Xing (1994) equation, a soil water retention model was proposed for simulation of the SWRCs of compacted GMZ bentonite with consideration of pore fluid chemistry. Parameters were analyzed and determined with consideration of influences of the pore fluid concentration. Verification indicates that the SWRCs simulated by the proposed model are well agreed with the measured ones.
Compacted bentonite has been recommended as a buffer/backfill material in many countries for engineered barriers in the deep geological repository for disposing high-level nuclear waste (HLW) [
The resaturation of the initially unsaturated repositories is predicted to last hundreds of years after the closure of a repository [
The soil water retention curve (SWRC) has been widely used to evaluate unsaturated soil properties, including saturated and unsaturated permeability functions, shear strength parameters, and volume change [
It is generally agreed that direct measurement of SWRC in the laboratory requires hard work and is time-consuming [
Lots of factors can influence the SWRC of geomaterials, including soil type, initial water content, mineralogy, density, temperature, texture, stress history, method of compaction, and net confining stress. In this regard, various investigations [
China’s deep geological disposal program for HLW began in the 1980s. Beishan of Gansu has been selected as the preferred site for construction of repository in China [
Efforts have been made to thoroughly understand the water retention property of GMZ bentonite in recent years [
According to in situ investigations in Beishan, the average dissolved solid (TDS) concentration of groundwater is in the range of 3 g/L to 12 g/L, with a maximum up to 50 g/L [
In this work, GMZ bentonite powder with different salt contents was obtained after being sprayed with different concentrations of NaCl solutions. The specimens were prepared by compacting the amended bentonite powder to target dimensions and dry density. Suction-controlled water retention tests were carried on with the compacted amended GMZ bentonite specimens under confined conditions. Based on these, a modified soil water retention model for the compacted GMZ bentonite with consideration of salt solution concentration effects was established and verified.
The material tested in this work is GMZ bentonite. The GMZ bentonite was sampled in the northern Chinese Inner Mongolia autonomous region, 300 km northwest from Beijing. There are 160 million tons with 120 million tons of Na-bentonite reserves in the deposit (Figure
Open pit of GMZ bentonite deposit.
Basic properties of the GMZ bentonite.
Property | Description | References |
---|---|---|
Specific gravity of soil grain | 2.66 | Wen [ |
pH | 8.68−9.86 | Wen [ |
Liquid limit (%) | 276 | Ye et al. [ |
Plastic limit (%) | 37 | Ye et al. [ |
Total specific surface area (m2/g) | 570 | Ye et al. [ |
Cation exchange capacity (mmol/100 g) | 77.06 | Wen [ |
Exchangeable cation (mmol/100 g) | E(Na+): 37.52 | Wen [ |
E(1/2Ca2+): 23.18 | ||
E(1/2Mg2+): 10.17 | ||
E(K+): 0.55 | ||
Main minerals | Montmorillonite (75.4%) | Wen [ |
Quartz (11.7%) | ||
Feldspar (4.3%) | ||
Cristobalite (7.3%) |
XRD pattern for GMZ bentonite powder (M: montmorillonite; Cri: cristobalite; Q: quartz; Al: albite) [
For most investigations involving chemical effects on the hydromechanical behaviours of soils in the laboratory, various methods were developed for specimen preparation: (1) mixing the soil powder with desired solution, (2) immersing a soil specimen in a solution, and (3) percolating a soil specimen by solution [
Following this aspect, a pretreating course was conducted. The GMZ bentonite powder with an initial water content of 11.7% was dried in an oven at 105°C for 24 h. After that, distilled water and 0.5mol/L, 1.0mol/L, and 1.5mol/L NaCl solutions were sprayed on the bentonite powder to achieve a target water content of 20%, respectively. The mass ratio of salt and bentonite was defined as
Specifications of the compacted specimens with NaCl solution.
Specimen | Salt (NaCl)/bentonite ratio |
Dry density (Mg/m3) | Total suction (MPa) |
---|---|---|---|
0 | 1.70 | 139 | |
0.101 | 1.70 | 139 | |
0.204 | 1.70 | 139 | |
0.310 | 1.70 | 139 |
It should be noted that, as the total salt quantity in the specimen is kept constant during the wetting/drying processes, the current salt concentration
In this work, the vapor phase technique was employed for suction control (Figure
Schematic setup for the vapor-phase technique (a) desiccator, (b) specimen, and (c) cell.
Saturated salt solutions and corresponding suctions (20°C) [
Saturated salt solutions | Total suction (MPa) |
---|---|
K2SO4 | 4.2 |
ZnSO4 | 12.6 |
(NH4)2SO4 | 24.9 |
NaCl | 38 |
Mg(NO3)2 | 82 |
CaCl2 | 139 |
The wetting course from 139 MPa to 4.2 MPa, including 6 suctions listed in Table
The gravimetric water content
For determination of the influence of pore fluid chemistry on the saturated (“zero air voids” [
The measured SWRCs of confined compacted GMZ bentonite with different salt contents in wetting path are shown in Figure
SWRCs of compacted GMZ bentonite specimens with different salt contents.
According to Figure
For the confined compacted GMZ bentonite at a dry density of 1.7 Mg/m3 with different salt contents, measured saturated water content
Water content
It is obvious that water content
As mentioned before, due to difficulties and labor costs for measuring SWRC, many empirical models for SWRC have been developed. Actually, among all the models available, the commonly used ones are those proposed by van Genuchten [
The Fredlund and Xing model [
Parameter
It can be observed from Figure
Integrating equation (
With salt concentrations 0.01, 0.384, 0.735, and 2.0 mol/L and the corresponding measured saturated water contents in Figure
With equation (
In this respect, Karagunduz et al. [
The fitted relationship between the air entry value-related parameter
Relationship between the air entry value-related parameter and the pore fluid concentration.
With the relationship between the air entry value-related parameter and the pore fluid concentration in Figure
The concentration of infiltration pore fluid will influence pore size distribution, which results in variation of the
The influence of pore fluid concentration on water retention properties of clay has succeeded to attract sufficient attention by researchers [
Relationship between the slope value of SWRCs and the pore fluid concentration.
Based on the fitted results in Figure
With SWRCs of confined compacted GMZ bentonite specimens with dry density of 1.7 Mg/m3 tested using distilled water and different concentrations of NaCl solutions in this work, the relationship between the slope values of SWRCs and the corresponding pore fluid concentrations is shown in Figure
It can be observed that the value of the concentration-related parameter of 1.5 Mg/m3 (
Comparison results indicate that the proposed exponential function (equation (
Substituting equations (
It should be noted that, considering that the influence of the pore fluid concentration on the residual water content of compacted clays is insignificant (Figure
Deduced from the Fredlund and Xing [
It should be noted that, in equation (
Figure
Relationship between water content and suction of the compacted GMZ bentonite.
Equation (
With equation (
In this work, the SWRCs of confined compacted GMZ bentonite with different salt contents were attained by a series of suction-controlled tests. The effects of pore fluid chemistry on the water retention properties of GMZ bentonite were analyzed. A modified model considering the pore fluid chemistry effects for describing the SWRCs of confined compacted GMZ bentonite was proposed and verified. The main conclusions can be drawn as follows.
For the compacted GMZ bentonite, the effect of pore fluid chemistry on the water retention properties depends on suction. For a given suction, the measured water content of specimens increases with the increase in pore fluid concentration. Furthermore, the influencing rate depends on suction.
With the Fredlund and Xing equation, parameters including the air entry value-related parameter
It should be noted that the proposed model only verified with test results of confined compacted GMZ bentonite. Further investigations should be done in the coming work for extending its applications to related geomaterials.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors are grateful to the National Natural Science Foundation of China (Project Nos. 41807253, 41672271, and 41527801) for the financial support. The authors also wish to acknowledge the support of the European Commission via the Marie Curie IRSES project GREAT—Geotechnical and geological Responses to climate change: Exchanging Approaches and Technologies on a world-wide scale (FP7-PEOPLE-2013-IRSES-612665). The authors are also grateful to the Research Fund Program of the Key Laboratory of Geotechnical and Underground Engineering (Tongji University), Ministry of Education (Project No. KLE-TJGE-B1803).