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题目 Correlations between the Resistance to Erosion and Geotechnical Properties of Soil, and the Effect of Cement Treatment

东南大学学士学位论文

Correlations between the Resistance to Erosion and Geotechnical Properties of Soil, and the Effect of Cement Treatment

东南大学交通学院

二零一六年五月

A THESIS Presented to

The Academic Committee of Southeast University

For the Bachelor Degree of Engineering

Correlations between the Resistance to Erosion and Geotechnical Properties of Soil, and the Effect of Cement Treatment

By

BAI Yang

Under the Supervision of

Professor Guo Xiucheng

Transportation College

Southeast University

May, 2016

Abstract

Surface erosion, especially scour which occurs during floods, may cause bridge failure. Chemical stabilization is an effective soil improvement technique. Among various stabilizers, hydraulic binders, such as lime and cement, are very effective to increase the erosion resistance and reduce the erosion rate. This report focus on correlations between the resistance to erosion and geotechnical properties of soil, and also the effect of cement treatment on erosion properties and geotechnical properties. The EFA (Erosion Function Apparatus) was used to determine the erosion properties of soils. The tests showed that the initial erodibility increased as particle size increased, and the initial erodibility increased as plasticity decreased. The resistance to erosion increased as the unconfined compression strength increased. The treatment with 4% cement significantly reduced the plasticity index, increased the unconfined compression strength, and greatly increased the resistance to erosion.

Key words: Soil Erosion, treatment, cement, EFA

Contents

Abstract i

1. Introduction 1

2. Literature Review 2

2.1 Erosion Testing 2

2.1.1 Backgrounds 2

2.1.2 Erosion Categories 3

2.1.3 Completion of Scour 4

2.2 Cement Used 4

2.2.1 Amount of Cement 4

2.2.2 Type of Cement 5

2.2.3 Effect of Cement Treatment 5

2.2.4 Mixing Procedure of Soil, Cement and Water 5

2.3 Correlations between Erosion Properties and Geotechnical Properties 6

2.4 Effect of Hydraulic Binder Treatment on Erosion and Geotechnical Properties 6

3. Experimental Investigation 9

3.1 Materials Used 9

3.1.1 Soil 9

3.1.2 Cement 16

3.2 Experiment Procedure and Sample Preparation 16

3.2.1 Proctor Test 16

3.2.2 Atterberg Limit Test 20

3.2.3 Methylene Blue Test 21

3.2.4 Unconfined Compression Test 22

3.2.5 Sand Castle Test 25

3.2.6 EFA Test 26

4. Results and Discussion 31

4.1 Results 31

4.1.1 Proctor Test 31

4.1.2 Atterberg Limit Test 34

4.1.3 Methylene Blue Test 36

4.1.4 Unconfined Compression Test 39

4.1.5 Sand Castle Test 43

4.1.6 EFA Test 45

4.2 Correlations between Erosion Properties and Geotechnical Properties 51

4.3 Effect of Cement Treatment on Geotechnical Properties and Erosion Properties 52

4.4 Discussion 53

5. Conclusions 55

Acknowledgements 56

References 57

1. Introduction

Earth structures such as dams, embankments, levees and dikes are widely used in hydraulic engineering projects. Soil is the main material in the foundation of hydraulic earth structures. However, hydraulic earth structures are subjected to severe damages by water if their resistance to erosion is not sufficient.

Surface erosion and internal erosion are two important failure modes of hydraulic earth structures. Internal erosion is one of the most significant causes of dam failure, while surface erosion, especially scour which occurs during floods, may cause bridge failure. These failures lead to serious accidents and high economic losses. In USA, there are about 575,000 bridges in the National Bridge Inventory (NBI) and approximately 84% of these are over water. An average cost of $50 million per year is spent on the federal aid system for flood damage repair (Lagasse et al., 1995).

Sometimes in geotechnical projects, the depletion of soils of high geotechnical performance and the higher costs associated with the use of these soils requires the use of local soils, which may cause difficulties in construction due to their poor geotechnical properties. So, it is very important to improve the erosion resistance of erodible soils using economical but effective techniques.

Chemical stabilization is an effective soil improvement technique. In order to increase the erosion resistance and reduce the erosion rate, various admixtures such as lime, cement, fly ash, milled slag, bitumen and calcium chloride have been used to stabilize erodible soils. For example, lime and gypsum treated soils were used at the foundation- embankment interface, on the slope surface of the embankment, and around the rigid structures to reduce the erosion rate of dispersive soils (Biggs, 2004; Cole et al, 1977; Phillips, 1977).

Among these stabilizers, treatment with hydraulic binders, such as lime, cement, fly ash and so on, is a very interesting soil improving solution. This technique involves adding stabilizer and water into soil, and mixing them together to obtain a homogenous condition. Soil treatment with hydraulic binders is used in embankments, subgrade layers and even in foundation layers because of the technical, economical and environmental advantages (LCOC 2000). Lime treatment of soils is a well-known and widely applied technique for soil improvement and stabilization of infrastructure works (roads, highways, airfields, railroad beds) (Little, 1995). However, the use of lime treated soils in hydraulic context is less known, at least in Europe, and existing structures showing the relevance of this application are quite rare.

This report focus on correlations between the resistance to erosion and geotechnical properties of soil, and also the effect of cement treatment on erosion properties and geotechnical properties. Crim et al. (2003) worked on 20 soil samples gathered from bridge locations in Alabama, and studied correlations between soil erosion properties obtained with the Erosion Function Apparatus (EFA) and routine soil classification properties. Properties determined from the erosion test were the critical shear stress and the initial erodibility, and classification properties included particle size distribution, Atterberg limits, and blow counts from the standard penetration tests (blow counts were found not strongly related to erosion properties). Inspired by the works of Crim et al., this report also studied correlations between soil erosion properties obtained with EFA testing and soil geotechnical properties, but worked on only three soils, and one of them was treated with 4% cement and cured for 1 day, 7 days and 28 days). Geotechnical properties considered were: particle size distribution, Atterberg limits, methylene blue value, dispersity and unconfined compression strength.

2. Literature Review

2.1 Erosion Testing

2.1.1 Backgrounds

Most prediction equations to estimate bridge scour depths have been developed on the basis of laboratory flume test results using coarse grained soil. Unfortunately these same equations are also used for fine grained soil which have much lower erosion rate than coarse grained soil. It usually takes less than a day for coarse grained soil to reach the maximum scour depth around a bridge support under a constant flow rate but for a fine grained soil the scour depth developed in a day may be a small percent of the maximum scour depth. The scour phenomenon in fine grained soils is much slower and more dependent on soil properties than that in coarse grained soils. Applying the equations developed to predict depth of scour in coarse grained soils to fine grained soils without the consideration of time yields overly conservative scour depths. Therefore, a scour analysis method for fine grained materials needs to consider the elapse of time and soil properties as well as hydraulic parameters.

In USA, at least till 2003, scour prediction was done using methods described in HEC-18 and HEC-20 (Richardson and Davis, 1995; Lagasse et al., 1995). These are hydraulic engineering circulars that were published by the Federal Highway Administration (FWHA). These methods assume noncohesive behavior and predict the ultimate scour which would occur in a soil and do not consider the rate of scour development which is important in fine grained cohesive soils.

Studies of bridge scour depths in fine grained soils with consideration of soil erodibility and time dependence have been performed at Texas Aamp;M University since 1990. The EFA (Erosion Function Apparatus) and the SRICOS-EFA (Scour Rate In COhesive Soil – Erosion Function Apparatus) method has been developed starting in early 1990s by Briaud and his coworkers for fine grained soils. The EFA can be used for any type of soil which can be sampled with a standard Shelby tube. It has been used for both coarse grained soils such as sands and for fine grained soils such as clays. The SRICOS-EFA method allows the user to predict the scour depth as a function of time. The SRICOS-EFA program can be used to perform the complex pier scour, contraction scour and abutment scour alone, and it can also handle the combined scour of the pier, contraction and abutment scour (integrated SRICOS-EFA method). The program automates the calculations of all the parameters such as maximum initial shear stress, initial scour rate, maximum scour depth, and transformation of the discharge into velocity. It also automates the computations to handle multi-flood hydrograph and multi-layer soil systems (Briaud et al., 2011).

The EFA is used to find the erosion function of a soil. The principle is to go to the site where erosion is being investigated, collect samples within the depth of concern using standard Shelby tubes, bring them back to the laboratory, and test them in the EFA. The tube is placed through the bottom of the conduit where water flows at a constant velocity. After several tests at different flow velocities, the results can be plotted and an erosion rate (ż) versus shear stress (τ) curve can be drawn, which is called the erosion function. The critical shear stress (τ_c) is the shear stress below which no scour takes place. The initial erodibility (S_i) indicates how fast the soil scours at the critical shear stress and is the slope of a straight line tangent to the erosion function at the critical shear stress. Rate of scour at a bridge site can be predicted given the soil erosion function for a sample from the site. In general, knowledge of τ_c and S_i allows estimates for the rate of scour in cohesive soils which is an improvement over ultimate scour methods which were used before.

Figure 2.1: Erosion function obtained from running an EFA test

2.1.2 Erosion Categories

Categories are used in many fields of engineering: soil classification categories, hurricane strength categories, earthquake magnitude categories. Such categories have the advantage of quoting one number to represent a more complex condition. Briaud (Briaud, 2008) proposed Erosion categories in order to bring erodibility down in complexity from an erosion rate vs shear stress function to a category number. Such a classification system can be presented in terms of velocity or shear stress. The categories of erosion rate for different soils are proposed on the basis of 15 years of erosion testing experience using EFA. In order to classify a soil or rock, the erosion function is plotted on the category chart and the erodibility category number for the material tested is the number for the zone in which the erosion function fits. Briaud (2011) pointed out that using the water velocity is less representative and leads to more uncertainties than using the shear stress. In fact, the velocity is not proportional to the shear stress. Nevertheless the velocity chart is presented because it is easier to identify a problem in terms of velocity.

Figure 2.2: Erosion categories for soils and rocks based on velocity (Briaud, 2008)

Figure 2.3: Erosion categories for soils and rocks based on shear stress (Briaud, 2008)

2.1.3 Completion of Scour

For some soils, the scour is usually not uniform and the surface of the soil sample usually becomes uneven through the duration of a test. Some of the exposed sample surface may have scoured more than 1 mm while some of it may have scoured less. When this happens the operator subjectively decides when scour is complete. The operator must decide when the scour is "on average" about 1 mm (Crim et al., 2003).

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