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# Frontiers in Earth Science ## Original Research ARTICLE

Front. Earth Sci., 30 April 2020 | https://doi.org/10.3389/feart.2020.00111

# Finite Difference Method for the One-Dimensional Non-linear Consolidation of Soft Ground Under Uniform Load Bin-Hui Ma1,2*, Zhi-Yong Hu1, Zhuo Li1, Kai Cai1, Ming-Hua Zhao3, Cheng-Bin He1 and Xiao-Cheng Huang1,2
• 1School of Civil Engineering, Hunan University of Science and Technology, Xiangtan, China
• 2Hunan Provincial Key Laboratory of Geotechnical Engineering for Stability Control and Health Monitoring, Hunan University of Science and Technology, Xiangtan, China
• 3School of Civil Engineering, Hunan University, Changsha, China

Initial stress and additional effective stress distributions in soil greatly influence the degree of ground consolidation when calculating one-dimensional soft clay ground consolidation in deep soil. The one-dimensional non-linear consolidation governing equations of soft ground under uniform load are derived and solved with the finite difference method. This method is based on the assumptions that the initial stress in soil varies with the ground depth and that the additional effective stress caused by external loads changes with both the ground depth and consolidation time and the hyperbolic model of the soil stress–strain relationship. Formulas for the degree of consolidation and the settlement of the ground are presented. A case study shows that the degree of consolidation in the ground calculated with the finite difference method agrees well with the traditional analytical solution, and the computational efficiency of the finite difference method can be effectively improved when the segmental calculation method is used throughout the consolidation process. The results of another example show that the settlement of the ground calculated with the finite difference method agrees with the in situ data. The suggested method can greatly simplify the consolidation calculation and has a high application value in engineering.

## Introduction

However, due to the small sample used in the test, the applicability of this theory in the calculation of deep soft clay could not be verified. Zhang and Sun (2007) established the one-dimensional consolidation theory of foundations under variable loads based on the hyperbolic model, and Zhou et al. (2013) presented a finite difference model for a one-dimensional electro-osmotic consolidation. However, the theory assumed that the initial effective stress and the additional stress of soil remained unchanged along the depth, which was different from the actual situation. Therefore, this paper combines previous research results with the hyperbolic model of the stress and strain of soil as a starting point. This method assumes that the initial stress changes along the depth of the foundation, and at the same time, the load caused by the additional effective stress changes with time and depth. Additionally, the uniformly distributed load is derived via the control equation of the one-dimensional consolidation of soil, and the finite difference method is introduced into the degree of consolidation and settlement calculation formula.

## One-Dimensional Consolidation Governing Equation

### Basic Assumptions

The distribution of the water pressure in the foundation is shown in Figure 1, where γw(h1 + Hz) is the hydrostatic pressure at depth z, pw is the total water pressure, and u is the pore water pressure generated by the evenly distributed embankment load qtop(t) in the soil.

FIGURE 1

a. The stress–strain relationship of the soil satisfies the hyperbolic model:

where σ′ is the effective stress, E0 is the initial compression modulus of the soil, ε is the strain, and m is the slope of the σ′/ε ~ σ′ curve.

b. Let the permeability coefficient k be proportional to the compression coefficient av:

where γw is the unit weight of water, γw = ρwg. The parameter ρw is the density of water, cv is the consolidation coefficient, e0 is the initial void ratio, and const is a constant.

c. The initial effective stress of the soil ${\sigma }_{0}\prime$(z) changes with depth z.

d. Considering the change in the external load with time (as shown in Figure 2A), the additional stress of the soil caused by the external load also changes with time, and the additional stress will change along the depth, so the additional stress can be expressed by q(t,z) (shown in Figure 2B).

FIGURE 2 Figure 2. Distribution of the additional stress in soil when the external load increases linearly. (A) External load function curve. (B) Curve of the additional stress in the compressed soil layer.

### Derivation of the Consolidation Governing Equation

Based on basic assumptions a and b, the one-dimensional consolidation governing equation can be obtained as:

According to the effective stress principle, the stress expression at depth z is

The two sides of Equation (4) are opposite and partial derivatives, and we obtain:

Substituting Equations (5) and (6) into Equation (3), the one-dimensional non-linear consolidation governing equation of the foundation under uniformly distributed load is

### Boundaries and Initial Conditions

a. At the initial moment, the upper load is conveyed by the pore water pressure, and its mathematical expression is

b. After loading, when the ground surface is a permeable layer, the pore water pressure on the bottom surface is zero.

c. When the ground surface is an impermeable layer after loading, the surface pore water pressure is zero, and its mathematical expression is

d. When the top surface of the foundation is permeable and the bottom surface is impermeable, the pore water pressure of the soil is equal to that of the underlying layer.

## Differential Method for the Governing Equations

### Difference Method Format

According to the basic principle of the finite difference method, with depth z as the vertical axis and time t as the horizontal axis, a calculation area for foundation consolidation analysis is established. The depth z is divided into M equal divisions, the time t is divided into N equal divisions, and a difference method calculation grid is established. As shown in Figure 3, the grid node represents the pore water pressure u, where ua, b represents the value of the pore water pressure at depth b in the ground at the moment of time a.

FIGURE 3

The initial and boundary conditions in the difference equation can be expressed as

For the governing Equation (7), the Crank–Nicolson format is used to discretize the following difference format:

where Δt = T/N, h = H/M, T is the total consolidation time, and M and N are the number of grids of the depth and consolidation time, respectively.

To simplify the calculation, Equation (12) is linearized; then,

Linearization will inevitably bring errors. To minimize the errors, the difference grid can be refined. Assume

Then, the formula for calculating the difference method under two boundary conditions can be written as follows:

When the bottom surface is permeable, A is an (M – 1) × (M – 1) order matrix, u is an (M – 1) × 1 order matrix, and B is an (M – 1) × 1 order matrix. When t = aΔt, then

where ${A}_{b,b}=-\frac{{c}_{v}}{{h}^{2}}-\frac{1}{\text{Δ}t}$

When the bottom surface is impermeable, A is an M × M order matrix, u is an M × 1 order matrix, and B is an M × 1 order matrix. When t = aΔt, then

where ${A}_{b,b}=-\frac{{c}_{v}}{{h}^{2}}-\frac{1}{\text{Δ}t}$

### Calculation Formula of the Consolidation Degree

According to the uniaxial compression theory, the calculation formula of the ground settlement is

The formula for calculating the average consolidation degree of a foundation defined by settlement can be written as

In addition, the average consolidation degree of the foundation, as defined by the average pore pressure, can also be expressed as

### Theoretical Verification

Examples are cited in the literature (Zhu and Yin, 1998) for verification. The calculation parameters are as follows: the thickness of the compressed soil layer is 5 m, and the bottom surface is a permeable layer; the load time linearly increases from qa = 0 kPa to qa = 300 kPa after 50 days and then remains unchanged; the consolidation coefficient of the soil is cv = 5.7888 × 10−3 m2/day, and the initial compression modulus is E0 = 1,687 kPa.

The analytical solution in Zhu and Yin (1998) assumes that the additional stress caused by the external load is linearly distributed with time and depth, and the stress–strain relationship of the soil is a linear elastic model. Corresponding to the special case of the difference method solution in this article, which is m = 0, Equation (1) can be written as

In the following, the finite difference method (hereinafter referred to as the method in this paper) proposed earlier is used to calculate the average consolidation degree of the foundation. To improve the calculation efficiency, the difference calculation is divided into two sections in the time dimension: the first section is 100 days in length, which is divided into 500 parts, Δt = 0.2 day; the length of the second period is the remaining 1,800 days, which is divided into 1,800 parts, Δt = 1 day. The average consolidation degree of the foundation calculated by this method and the analytical solution calculation results in Zhu and Yin (1998) are summarized in Table 1.

TABLE 1

From Table 1, it can be seen that the maximum relative error of the calculation results of the method in this paper and the analytical solution in Zhu and Yin (1998) is only 0.0106%, indicating that the method in this paper is reasonable and feasible and can meet the calculation accuracy requirements.

## Illustrative Examples

In order to verify the feasibility of the method for the calculation of the degree of consolidation of the foundation under the conditions of a variable load and one-dimensional non-linear consolidation, a calculation example in Favaretti and Mazzucato (1994) was selected for analysis. This example is a weak foundation project located under the silo of Ca'Mello. The change law of the load applied to the soft foundation with time is shown in Figure 4. Favaretti and Mazzucato (1994) carried out on-site observations of the settlement of the soft foundation and measured the consolidation coefficient of the soil (cv = 0.1296 m2/day) and the change law of the soil stress and strain during loading by experiments. In this paper, the compression characteristics of the hyperbolic model of the soil (E0 = 270 kPa, m = 0.9) are obtained by fitting the soil properties from on-site observations in Favaretti and Mazzucato (1994). The method in this paper is used to calculate the ground settlement, which are compared with the one calculated by an analytical solution in Rahal and Vuez (1998) and the one measured in Favaretti and Mazzucato (1994). The results are shown in Figure 5.

FIGURE 4
FIGURE 5

It can be seen from Figure 5 that the fluctuation trend of the settlement curve calculated by the method in this paper is basically consistent with the measured results, while the analytical solution settlement curve in Rahal and Vuez (1998) lags behind the measured results, indicating that the method in this paper is more reasonable and feasible.

## Conclusions

a. The presented method can adapt to various variable load situations, can greatly simplify the consolidation calculation process, and can effectively improve the calculation efficiency via a segmented calculation in the time dimension. The calculation results are in good agreement with the traditional analytical solution, indicating that the method is reasonable and feasible.

b. The method in this paper considers the case in which the external load is a function of time and can solve the calculation of the consolidation degree and the settlement of the foundation under the conditions of an impulse load, a simple harmonic load, or multiple step loading in actual engineering. Only the function expression of the external load is input into the program during the calculation, which avoids the steps of other theoretical calculation methods that need to convert the external load. Via this method, human error is reduced, and the calculation process for the degree of consolidation and the settlement of the foundation is greatly simplified, with good engineering application value.

c. The application condition of the presented method is the one-dimensional consolidation of a single-layered saturated soil with a permeable surface. The calculation formulas of the finite difference method should be transformed when analyzing the one-dimensional consolidation of the multilayered unsaturated soil under partially permeable boundary conditions. Besides, a multidimensional consolidation issue cannot be solved by this method.

## Data Availability Statement

The datasets generated for this study are available on request to the corresponding author.

## Author Contributions

B-HM contributed to the study concepts and approved the final version of the manuscript. Z-YH and ZL conducted the literature research. B-HM and Z-YH provided the theoretical analysis. B-HM and ZL conducted the data analysis and interpretation. B-HM and C-BH prepared the manuscript. KC created the tables and figures. B-HM and X-CH edited the manuscript. M-HZ revised and reviewed the manuscript.

## Funding

This research was supported by the National Natural Science Foundation of China (Nos. 51778227, 51308208, and 41372303), the Provincial Natural Science Foundation of Hunan (No. 2015JJ3069), and the Youth Talent Plan Program of Hunan (No. 2016RS3032).

## Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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## Nomenclature

Keywords: soft clay, one-dimensional consolidation, non-linearity, finite difference method, uniform load, hyperbolic model

Citation: Ma B-H, Hu Z-Y, Li Z, Cai K, Zhao M-H, He C-B and Huang X-C (2020) Finite Difference Method for the One-Dimensional Non-linear Consolidation of Soft Ground Under Uniform Load. Front. Earth Sci. 8:111. doi: 10.3389/feart.2020.00111

Received: 11 January 2020; Accepted: 26 March 2020;
Published: 30 April 2020.

Edited by:

Lianyang Zhang, University of Arizona, United States

Reviewed by:

Yunteng Wang, King Abdullah University of Science and Technology, Saudi Arabia
Rui Rui, Wuhan University of Technology, China

Copyright © 2020 Ma, Hu, Li, Cai, Zhao, He and Huang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Bin-Hui Ma, mbh@hnust.edu.cn