Advances in Civil Engineering

Volume 2018, Article ID 9213674, 7 pages

https://doi.org/10.1155/2018/9213674

## Estimation of Soil-Water Characteristic Curve for Cohesive Soils with Methylene Blue Value

^{1}National Engineering Laboratory of Highway Maintenance Technology, Changsha University of Science and Technology, Changsha 410114, China^{2}Changsha Commerce and Tourism College, Changsha 410001, China

Correspondence should be addressed to Junhui Peng; moc.361@400986673q

Received 1 March 2018; Accepted 10 June 2018; Published 5 July 2018

Academic Editor: Qiang Tang

Copyright © 2018 Junhui Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This study described a new methylene blue test to measure the methylene blue value (MBV) for 15 cohesive soils and established the relationship between MBV and plasticity index (PI) and between MBV and percent passing No. 200 sieve (P_{200}), respectively. Thereafter, the soil-water characteristic curves (SWCCs) for 15 cohesive soils based on Fredlund and Xing’s model were generated by the pressure plate test. Then, regression equations for determining the four fitting parameters in a previously developed SWCC equation by using the measured MBV were utilized to generate the SWCC for the cohesive soils. At the same time, the slope parameter, *b*_{f}, in the SWCC equations was found to be associated with the moisture susceptibility of cohesive soils. A higher *b*_{f} value indicates that the material is more moisture susceptible. In addition, a lower MBV/PI/P_{200} shows a lower suction at the same degree of saturation; on the other hand, a higher MBV/PI/P_{200} presents a higher suction. Therefore, the moisture-holding capacity of cohesive soils increases with increasing MBV, PI, and P_{200}. Finally, the proposed estimation method was validated by a comparison between the four determined fitting parameters from MBV and the pressure plate test.

#### 1. Introduction

The soil-water characteristic curve (SWCC) is a graphical relationship between the matric suction and the water content. It is one of the basic characteristics of partially unsaturated soils, and as such, it is useful for estimating the other properties of soil, when solving engineering problems in these three classic areas: fluid flow, compressibility, and shear strength [1]. For example, when modeling unsaturated moisture flow beneath a highway pavement, the hydraulic conductivity of the base course and subgrade materials, as a function of moisture content, must be known. Since the experimental procedures, in which a filter paper or pressure plate test, adopted for determining the matric suction-water content relationship, is time-consuming and cost-intensive [2, 3], recent research has placed a major focus on an estimation method to predict the SWCC using some mathematical functions [1, 4, 5]. However, the shape of the curve depends on many basic soil properties, such as the percent passing No. 200 sieve (P_{200}), plasticity index (PI), and environmentally induced factors that determine the stress state, compaction level, and temperature. It is difficult to find a valid and convenient mathematical expression to describe it. However, several analytical functions for predicting the SWCC can be found in some literatures [6–9]. The predicting variables, including sieve analysis and index properties, display extensive variability in those literatures [10]. Some time-consuming and material-consuming experiments are still necessary, including sieve analysis and Atterberg limits. Under this circumstance, Hakan Sahin et al. proposed a new estimation method to determine SWCC for unbound aggregate mixtures based on the methylene blue value (MBV) and percent fines content (PFC) [11–13].

The methylene blue has a large polar organic molecule C_{16}H_{18}N_{3}S^{+} that can be adsorbed onto the negatively charged surfaces of clay minerals. The amount of adsorbed methylene blue depends on the amount of the surface area of the clay particles. The more the methylene blue adsorbed by the clay particles, the brighter the methylene blue solution will be. The adsorbed methylene blue is able to be quantified by assessing the color change of the methylene blue solution. At the same time, SWCCs for cohesive soils reveal their water-holding capacity which depends on the specific surface area of the clay particles [9,14–18]. Based on the above description about the methylene test, MBV reflects the specific surface area of soil particles. Therefore, SWCCs for cohesive soils can be predicted using the methylene blue value. Once the relationship between the four fitting coefficients of Fredlund and Xing’s model, which are shown in (1), and the MBV is built, SWCCs for cohesive soils will be determined:where is the volumetric water content; is the saturated volumetric water content; is the matric suction; and are fitting coefficients, which are primarily a function of the air entry value, rate of water extraction from the soil, residual water content, and suction at which the residual water content occurs, respectively. Once these four fitting parameters are determined, the SWCC for a specific soil can be established automatically.

This study is organized as follows: The forthcoming section introduces a new methylene blue test method, and the methylene blue tests of 15 cohesive soils were completed. Subsequently, the correlation between PI and MBV and between P_{200} and MBV was proposed and analyzed, respectively. The next section builds the correlations between the four fitting parameters of Fredlund and Xing’s model and the MBV, which were validated subsequently. The final section summarizes the major findings of this study.

#### 2. Experiments and Materials

Based on the preceding discussions, this section presents the laboratory experiments and materials required to develop the fitting models for the SWCCs.

##### 2.1. Laboratory Experiments

The sieve test and Atterberg limit test were employed to determine the particle distribution and plasticity index, respectively. At the same time, the maximum dry density and optimum moisture content, which were utilized to mold the soil samples for the pressure plate test, were gained according to the Proctor test. Thereafter, the pressure plate test was used to measure the matric suction for different moisture contents. In addition, the methylene blue test was used to detect the amount of fine particles in 15 cohesive soils. The pressure plate test and methylene blue test are briefly introduced in the following sections.

##### 2.2. New Methylene Blue Test

A traditional methylene blue test, specified in ASTMC837 [19], was used to determine the active clay content in fine materials by measuring the methylene blue dye content adsorbed by clay particles. This traditional test method contains an empirical check criterion, in which the test procedures need to be repeated until a light blue ring is found. It is time-consuming and requires experienced personnel to operate the test, which is similar to the method of the current specification of *Test Methods of Aggregate for Highway Engineering* in China. Recently, a new test method, which measures the MBV of soils by using the methylene blue solution with a colorimeter, was proposed by W.R. Grace Inc. The advantage of this new test method is that it is relatively simple, inexpensive, and repeatable. Figure 1 shows the apparatus which consists of a colorimeter, a 150 *μ*L pipette with a resolution of 1 *μ*L, a dropper, a 3 mL syringe, two 50 *m*L plastic bottles, two sample bottles, methylene blue solution, and distilled water. In addition, a 0.20 *μ*m filter of the syringe, a portable balance with a resolution of 0.01 g, a standard sieve, and a small glass tube are also needed.