Summary of doctoral dissertation: Research on the vacuum consolidation method for soft soil improvement in works construction

Purposes of the topic: To define the variation pore water pressure and deformation (settlement) of soft soil layer during vacuum consolidation. To establish the relationship between Plasticity Index (PI), degree of consolidation (U), thickness of improved soft soil layer (H), and consol.

MINISTRY OF

EDUCATION AND

TRAINING

MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT

THUYLOI UNIVERSITY



PHAM QUANG DONG

RESEARCH ON THE VACUUM CONSOLIDATION METHOD

FOR SOFTSOIL IMPROVEMENT IN WORKS

THUYLOI UNIVERSITY



Promotors: 1 PROF DR TRINH MINH THU

2 PROF DR NGUYEN CHIEN

The thesis could be found at: National Library

Or Water Resources University Library - 175 Tay Son – Dong Da –

Hanoi

1

PREFACE

1 Background of the topic

The vacuum consolidation method has been applied successfully all over the world, and now it is chosen to improve the softsoil in Vietnam because this method has advantages as: shorten construction time, save loading materials, easily remove loading material after improvement, no environment pollution

in construction, especially appropriate for ground improvement in a large and/or long area

Vietnam has applied this method to improve the ground for some works, with technology and equipment managed by foreign partners However, the knowledge and mastering of technology and equipment, and building of relationship among parameters of soft soil layer during vacuum consolidation process for soft soil in Vietnam are topical and essential Therefore, the topic

“Research on the vacuum consolidation method for soft soil improvement

in works construction” has large scientific and practical significance

2 Purposes of the topic

- To define the variation pore water pressure and deformation (settlement)

of soft soil layer during vacuum consolidation

- To establish the relationship between Plasticity Index (PI), degree of consolidation (U), thickness of improved soft soil layer (H), and consolidation time (t) when improving the soft soil by vacuum consolidation method

3 Object and Scope of research

- The softsoil in different projects at Dinh Vu – Hai Phong, Duyen Hai – Tra Vinh, Nhon Trach – Dong Nai, Thai Binh Thermal Power Plant – Thai Binh

- The softsoil in some another areas which have similar physical and mechanical behaviors

4 Contents of research

(1) Literature reviews the vacuum consolidation solutions for improving soft soil foundation all over the world and in Vietnam Evaluate the existing problems in technics and point out the matters that the thesis is focusing on solving (2) Apply the background theory to figure out parameters of vacuum consolidation process and compare with physical model test in laboratory and with the field treatment (3) Study the installation and the operation procedures

to be well controlled the vacuum consolidation technology (4) Studying the variation pore water pressure and the deformation of soft soil layer in vacuum consolidation process by physical model The study results are compared with calculated results from numerical model to see the conformity of digital

calculation model (5) Determining the rationality of the numerical model to calculate and formulate the relationship between plastic index, consolidation, depth of softsoil improvement and consolidation time when apply the vacuum consolidation method

5 Research Method

(1) Theory calculation and analysis method: research the vacuum consolidation problem, study the contents related to solving the vacuum consolidation problem (2) Practical method: physical model experiment to determine the variation process of pore water pressure and deformation at interested locations and depths during the application of vacuum consolidation (3) Statistics method: Processing the experimental and statistic data to establish and build the relationship between the parameters of vacuum consolidation process (5) Expert method: organize workshops, scientific reports to collect comments from experts and scientists in the researched field

6 Scientific and practical meanings

a) Scientific meaning

There are few studies on factors that affect the vacuum consolidation process

in Vietnam, therefore the results of this thesis on the variation pore water pressure and settlement in the softsoil of Vietnam can be used to predict the behaviors of softsoil when improving by vacuum consolidation

Currently, there is no software applied for the vacuum consolidation problems, therefore, the selection of the appropriate finite element software has great scientific meaning

To have basis for giving initial forecast on the consolidation process when improving the soft soil layer using vacuum consolidation method, the ability of building a relationship between consolidation parameters, consolidation time, plasticity index and the depth of soft soil layer for improvement is essential

b) Practical meaning

The research results will determine the variation law of soft soil layer, and establish its relationship when improving the softsoil by vacuum consolidation, give a tool for geo-technician to give initial forecast on consolidation process when improving the softsoil by this method

7 Contributions of the thesis

(1) Establish a large scale physical model which is the first model which applies the vacuum consolidation method to improve the soft coastal underlying soil has been performed at the Geotechnical Laboratory, ThuyLoi University to study the variation process of pore water pressure and the deformation of softsoil at different interested locations and depths

3 (2) Select appropriate software to calculate the vacuum consolidation together with loading for the problem in the laboratory and at site

(3) Compute the relationship between plasticity index, degree of consolidation, thickness of soil layer for improvement and consolidation time when improving the soft soil by the vacuum consolidation method

8 Layout of the Thesis

Chapter 3: Calculation model for vacuum consolidation problems

Chapter 4: Establishment of a relationship between parameters of vacuum consolidation problems

Conclusion and recommends

List of publications

References

Chapter 1 OVERVIEW OF VACUUM CONSOLIDATION METHOD FOR SOFTSOIL IMPROVEMENT AND THEORY OF THE METHOD 1.1 Softsoil

There have been many different definitions about softsoil, but it could be summarized that softsoil is the soil not suitable for works construction The building in softsoil requires very careful improvement to ensure the structrures will be safe during the construction and operation time

1.2 Overview of research and application of vacuum consolidation method

1.2.1 The application of vacuum consolidation method for improving softsoil in the world

The vacuum consolidation method for improving softsoil was introduced for the first time in 1952 by W Kjellman, in 1980, the vacuum consolidation

was improved by a combination of loading and prefabricated vertical drain In

1989, Menard Construction Company (from France) applied this improvement, since then, the method has been widely applied in many countries Since 1997, Cofra Construction Company of Netherland had an improvement is to remove the protective membrane which made the construction difficult and vulnerable, however, a loading layer must be used to compensate the removed atmosphere pressure difference Basically, the

vacuum construction method could be divided into 2 main types which are sealed air membrane and no sealed air membrane construction

1.2.2 The situation of study on vacuum consolidation method

There are many factors that will create effect on the vacuum consolidation method process, these factors have been researched by many authors from

practical results in the lab and at site of actual works The results of these

researches also point out that depending on the type of soil, pressure level, type of prefabricated vertical drain, distance of prefabricated vertical drain affects the consolidation level of the ground However, in addition to the above conditions which create the differences in research results; the horizontal consolidation coefficient and the disturbance level around the prefabricated vertical drain are major factors that affect the consolidation

process and resulting in this difference

1.2.3 The research and application of vacuum consolidation method for softsoil improvement in Vietnam

The application of vacuum consolidation for improving softsoil has been applied in some works in Vietnam Based on the application result, this method proved itself as a new and an effective one However, up to now, the design and construction using this method hold by foreign partners only Therefore, it is essential to have research on the nature of consolidation process and understand the appropriate construction technology in the condition and actual situation to widely apply in the softsoil improvement in Vietnam

There is not many research on vacuum consolidation method in Vietnam, and the researches are often based on the results of works improving at site There is no practical model in the laboratory to study the variation parameter

of pore water pressure and deformation variation of soil, as well as proper numerical model to check and compare

1.3 Theory of vacuum consolidation method

1.3.1 Consolidation of softsoil

The nature of consolidation is the decreasing of void ratio of soil due to the release of pore water by permeability process, as such, the soil particles transfer stress directly on each other, thus improve the connection of soil structure

If the principles of mechanical compaction uses the loading to increase total stress, thus increase the effective stress On the other hand, the vacuum consolidation will decrease the residual stress in pore, thus increase the effective stress without changing the total stress (fig 1.12)

øng suÊt khi b¬m hót

øng suÊt hiÖu qu¶ khi kh«ng b¬m hót øng suÊt tæng øng suÊt hiÖu qu¶

khi b¬m hót øng suÊt d- tr-íc khi b¬m hót Khi kÕt hîp hót ch©n kh«ng

~100 kPa

Fig 1.12 Vacuum consolidation principles

1.3.2 Basic differential equation

Terzaghi suggestted the basic differential equation of consolidation phenomenon as follows:

2 2

z

u C t

u C z

u C t

u

r v

1

2 2 2

2

(1-2)

consolidation coefficient; u – residual pore water pressure

1.3.3 Theory of consolidation

To solve the consolidation problem with similar compression consolidation

method and Barron – Terzaghi method, the scientists re-solved the problem of Barron – Terzaghi applying for the vacuum consolidation problem, in which

there is solution of Wollongong University

Average value of pore water pressure at time t is determined by the formula 1-33:

2 0

0 0

8 1 2

1 2 exp 1 2

8 1

m

h vh

va va

L c

m m

u

p u

m m

1 2 exp 1 2

(1-35)

1.4 Settlement prediction method

1.4.1 Asaoka method

According to this method, the final settlement value will be predicted based

on actual monitoring data This data could modeled approximately as a straight line (Fig 1.20)

In which:

β – slope of the most accurately stimulated regression line;

A – extended intersection of line stimulating the vertical axis;

After determining the stimulated

regression line, the final settlement

value in Asaoka is measured as:

Fig 1.20 Asaoka line

1.4.2 Inflection point method

The inflection coefficients in theory and field are determined as the following equations:

T d

to have studies on variation law of parameters of underlying soil using

7 physical models In this thesis, the author suggests using large physical model

to investigate the variation pore water pressure and the deformation of soil at interested points and depth

Chapter 2 EXPERIMENTAL STUDY ON VACUUM CONSOLIDATION METHOD FOR SOFTSOIL IMPROVEMENT IN LABORATORY

2.1 Purposes of research

The purposes of practical research in laboratory are to determine the variation of settlement and pore water pressure for softsoil at different points and depth during vacuum consolidation process

2.2 Experimental model

2.2.1 Introduction of model

The experiments model is a physical model which was built in the laboratory of Geotechnical Engineering, ThuyLoi University The model includes the rectangular cube, size (2.0x1.0x1.2) m and monitoring equipment

of pore water pressure, settlement deformation, vacuum pump system… The layout of the experimental model is in figure 2.1

1200 600

1 2 3 4 5

7

Máy bơm chân không

Đồng hồ đo lún Giá đỡ thiết bị Màng kín khí

8 9

10 4

Fig 2.1 Simple scheme of the model

The physical model stimulates a block of soft soil for research In which, the prefabricated drain type CT-D910 has size 100x4m is located at effective distance 1.0x1.0m with the length is through the depth of soil block Piezometer (PIE) monitoring pore water pressure is located at interested depth next to the prefabricated drain and between two prefabricated drains, the Tenxomets (TEN) monitoring are located right on the surface near two prefabricated drains The

sand layer 0.2m on the surface of soil surface has both drainage and loading functions

The physical models are built for the case of softsoil improvement with prefabricated vertical drain (Physical model 1, physical model 2) and without vertical drain (physical model 3) The layout of monitoring equipment for settlement and pore water pressure at interested points of physical models are shown in Fig 2.2, 2.3 and 2.4

Vải địa

Bấc thấm Bấc thấm

TEN 1-1 TEN 1-2

ống thu n-ớc

Màng kín khí Vải địa ống thu n-ớc

Fig 2.2 Layout of locating physical model 1

Màng kín khí Vải địa ống thu n-ớc

TEN 2-1 TEN 2-2

Màng kín khí Vải địa ống thu n-ớc Thiết bị đo lún

Fig 2.3 Layout of locating physical model 2

500 500

Màng kín khí Vải địa ống thu n-ớc

500 500

Màng kín khí Vải địa ống thu n-ớc Thiết bị đo lún

Fig 2.4 Layout of locating physical model 3

2.2.2 Experimental soil

The experimental soil is taken in the coastal area of Pvtex Dinh Vu – Hai Phong The experimental soil sample (in the physical models) is prepared from this type of soil which has the similar physical and mechanical properties of the original soil (very soft sandy clay) To investigate the effectiveness of

9 improvement method, it is necessary to determine the physical and mechanical properties of the original softsoil

The undrained shear resistance in depth before experiment is shown in fig 2.7

2.2.3 Experiment equipment

The model experiment equipment (Fig

2.1) includes: pump system to create

vacuum pressure, pore water pressure

deformation measurement equipment,

membrane for air sealing, prefabricated

connecting to the prefabricated vertical

drain and pump This equipment is

almost used for works at site

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

1.0 1.2 1.4 1.6 1.8 2.0 Søc kh¸ng c¾t Su (kPa)

Fig 2.10 Datalogger - Geokon

2.4 Experimental results

2.4.1 Experimental results of model 1

The relation between the settlement and time of physical model 1 in the vacuum consolidation process is shown in Fig 2.21 Due to the equipment conditions, the physical model 1 can only create the vacuum at maximum 36 kPa

Thêi gian (ngµy)

between 2 prefabricated vertical drains is lower than next to the prefabricated vertical drain, however, this difference is very small

To predict the final settlement based on the practical figures of physical model 1, it is used Asaoka forecast method The predicted results show the agreement of this method with minor difference between the predicted and experiment settlement next to the prefabricated vertical drain and between 2 prefabricated vertical drain are 0.9% and 0.7% accordingly

11 The relationship between pore water pressure and time of physical model 1 during the vacuum consolidation process is shown in Fig 2.24

0 2 4 6 8 10 12 14 16 18 20 22 24 26 -40

Thêi gian (ngµy)

prefabricated vertical drain is larger than between 2 prefabricated vertical drains in the first day of loading, after that, this difference becomes smaller and tends to be asymptotical The nearer of the soil surface, the more decrease

of the residual pore water pressure

2.4.2 Experimental results of model 2

The relation between the settlement and time of physical model 2 in the vacuum consolidation process is shown in Fig 2.25 Due to the equipment conditions, the physical model 2 can only create the vacuum at maximum 41kPa

Thêi gian (ngµy)