HONG KONG UNIVERISITY OF SCIENCE AND

TECHNOLOGY

UNDERGRADUATE RESEARCH OPPORTUNITY PROGRAM

(UROP)

SPRING 2013/2014

THE EXPLORATORY STUDY OF THE APPLICATIONS

OF TRANSPARENT SOILS

By CHOW Jun Kang

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Abstract

The objective of this research is to develop a transparent soil that is environmentally

friendly, economical, easy to replicate and feasible in geotechnical engineering modelling.

Fused quartz was selected as the study material in the combination of sugar solution to

create a realistic physical modelling soil matrix. A benchmark of quantification of

transparency was established in order to serve as a systematic approach to grade the

transparencies of various materials available in the market. Hue-Saturation-Brightness

(HSV) format was selected for the presentation of the colour component to measure how

transparent soil alters and scatters the light passing through it. To examine the feasibility

and usefulness of transparent soil, compaction grouting was studied. Several important

findings were presented, i.e. a) the grouting quality is intimately related to the rate of

inserting and pulling out of the grout tube; b) segregation of grout mixture tends to occur

due to different viscosity of the components; c) preferential flow of grout material along

the shaft of the grout tube was observed; d) various stages of grout growth are observed

throughout the experiment. Applying the transparent soil allows researchers to visualize

each stage of grouting to investigate the seemingly unimportant steps that might lead to

unintended failure of the original foundation work. The future work shall continue onto

other important geotechnical engineering processes, which includes jet grouting and pile

penetration processes.

Introduction

Since time immemorial, all of the geotechnical engineering works performed under the

ground, e.g. pile driving, grouting and etc., cannot be directly observed and measured.

The difficulty of visualizing the mysterious world underground, imposed by nature

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herself, leaves much to desire in regards to the fundamental understanding of soil

behaviors, which is indispensable to the design of foundation. The current practice in the

field, termed the observational method, was suggested by Karl Terzaghi in 1936 for a

workaround, by which he stated that “the accuracy of computed results never exceeds

that of a crude estimate” (Peck 1969). Since then, the foundations are often designed and

constructed based on assumptions (educated guesses and approximations), and empirical

relationship to that observed in the field. The motivation of studying transparent soil (TS)

is driven by the crucial needs in understanding the particle-scale interactions of soil

which is ultimately related to foundation failures; also, the bustling field of

geotechnology in response to increasing demand of new energy source such as methane

hydrate and megatower for growing population escalate the need to understand

geotechnical construction processes.

According to Mannheimer (1989), transparent slurries can be made by dispersing

solid particles that have a refractive index close to a typical glass (1.4 – 1.5) in a liquid

with the same refractive index. However, glass beads are not suitable materials in TS

modelling as they do not represent the geotechnical properties of natural granular soil

(Mannheimer and Oswald, 1993; Sadak et al., 2002). Better success has been achieved by

matching amorphous silica powder, silica gel and Aquabead, Nafion with colourless pore

fluid having the same refractive index as described by Iskander et al. (1994, 2002a,

2002b), Gill and Lehane (2001), Sadek et al. (2002), Liu et al. (2003), Zhao and Ge

(2007), Hird and Stanier (2010) and Downie et al. (2012). However, limitations were

reported at the same time as summarized: (i) Silica gel particles deform plastically ever

under low confining pressure (Iskander, 1998; Iskander et al, 2003; Zhao and Ge, 2007),

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(ii) it is difficult to de-air the internal pores of silica gel particles (Iskander et al. 2002b;

2003), (iii) the presence of air bubbles alters the transparency of TS (Iskander 2010), (iv)

huge difference in refractive index (RI) between container and TS (Ezzein and Bathurst

2011), (v) TS materials are extremely costly (Ezzein and Bathurst 2011). In short, the

core purpose of this research was to develop the environmentally friendly TS with high

stability, non-toxic to human health and chemically inert that is feasible to be applied in

geotechnical engineering modelling.

Material selection

The fused quartz particles are hard, and have good optical transmission and RI of 1.459

compared to that of silica gel. At the same time, fused quartz was found to be a good

material in developing TS as it has the properties similar to granular soil media (Ezzein

and Bathurst 2011). In this project, the fused quartz particles were manufactured by

Lianyungang Fenqiang Trading Co. Ltd, with size ranges 1 to 10 mm.

In combination with fused quartz, sugar solution was selected to model the pore

fluid. Contrary to the liquid and solution used such as paraffin oil, mineral oil and organic

solvents such as toluene and alcohol (Iskander et al., 1994; Zhao et al., 2010), sugar

solution is safe (low toxicity), colourless, stable and odourless. To prepare the sugar

solution of RI 1.459 accurately, RI measurement was taken by Refractometer R5000,

with its accuracy up to 0.001.

Quantification of transparency of TS

Since there is no systematic quantification of transparency of TS has been performed,

effort was taken to measure how TS behaves under different colour background. The

colour component photo captured was then analyzed by software MATLAB. However, it

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was difficult to quantify how the colour deviated from its original component when it was

presented in the RGB form. Therefore, the colour matrix was transformed to HSV form

(Bunks, 2014), whereby (i) hue (H) is the angular dimension representing the colour; (ii)

saturation (S) is the radius of the circle at particular height of a cone showing how pure

the hue is with respect to white colour; (iii) brightness/value (V) represents how much

light is coming from the colour as depicted in Figure 1. By doing so, it was easier to

observe how many degrees (hue) are deviated as well as the extent of light is being

scattered by the TS.

The setup to quantify the transparency of TS is shown in Figure 2. To minimize

the disturbance of light by scattering, bending, reflection and refraction, flat, thin and

transparent container made of acrylic glass with RI value of 1.490-1.492 was used. Also,

lighting was provided to prevent the formation of shadow upon photo taking and video

recording. To investigate the transparency of TS under different background colour, four

colour – red (R), green (G), blue (B) and white (W) were selected, with four sets of

condtions: (i) C – container only; (ii) CQ – container filled with fused quartz only; (iii)

CS – Container filled with sugar solution only and (iv) CSQ – container filled with fused

quartz and sugar solution. As shown in Figure 3, the hue value of blue colour for four

conditions are highest among the other colour, indicating blue in the visible light region

having higher frequency compared to red and green, thus allowing it to penetrate the TS

the most. Since there is no reference or benchmark to calculate how the final colour

deviates from its original, an equation calculating the index of transparency, I , for a

particular colour is proposed as follows:

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3/

222

CCQ

CCSQ

CCQ

CCSQ

CCQ

CCSQ

VV

VV

SS

SS

HH

HHI

where H, S and V represent hue, saturation and brightness respectively, and the subscript

C, CQ and CSQ represent different kind of condition as mentioned above.

Grouting modelling

To determine the feasibility of TS in geotechnical modelling, grouting process was

modelled to observe the internal growth mechanism. Compaction and jet grouting were

modelled by using syringe and brass tube as grout tube respectively in drilling

compaction grouting hole and pumping grout material (gypsum) into the container.

The whole process of grouting was summarized in Figure 4. Based on the

observation, grout mixture was initially injected downwards. However, part of the grout

tended to segregate and disperse into the solution in the sense of the gypsum and water

were not mixed well. Next, the preferential flow changed to the upwards direction along

the shaft as soon as the grout injection started.

In addition, localized shear and stress was believed to have been induced as the

grout tube was pulled out. The grout surrounding the shaft tended to contract as shown in

Figure 9(e) – (h). This illustrates the importance of the rate applied to the grout tube

pulling or inserting during the process as such seemingly unimportant step might defeat

the original purpose of strengthening the ground. All these phenomena have never been

visualized as everything under the ground is opaque but TS has shown its ability in

allowing researchers to understand and re-examine the fundamental behavior of soils.

Therefore, it is crucial to investigate and understand how each geotechnical construction

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process alters the soil behaviors that might potentially lead to catastrophic failure in

foundation design.

Summary and Conclusion

What we assume to have happened is not always what happens in reality. In fact, internal

behavior of soil is not as simple as what we think; guesswork remains guesswork. As

shown in this project, visually monitoring each process in grouting is important to

understand whether there are hidden potential in current practices in leading to the grout

failure. There is no way for us to escape from understanding the fundamental behavior of

soil and its mechanism. In addition, rise in the demand in geotechnical engineering field

such as energy extraction under marine and foundation design of high-rise buildings even

makes the study of soil behaviors more crucial.

Transparent soil turns out to be a timely research to allow geotechnical

researchers to study, observe, and re-examine the principles and mechanisms of soil

behaviors. In order to popularize TS application, robust techniques in material

preparation and quantification have to be achieved and in the meanwhile the objectives of

developing TS with the characteristics of environmentally friendly, non-hazardous to

human health, chemically resistant and easy to be prepared. Then, this technique can be

expanded to many other geotechnical applications modelling, including jet grouting, pile

penetration processes and bridge scouring. Although there are still many parameters to be

verified, the success of TS reported here in modelling geotechnical applications shall

ultimately lead to the greater success at illuminating the internal yet fundamentally

important behavior of soil.

0°

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0°

R

G

B

0°

H

S

V

Saturation

Bri

gh

tnes

s

(a)

(b)

(c)

Figure 1. Presentation of colour scheme: (a) Colour presentation by RGB Cube (R-red, G-

green, B-blue); (b) Colour presentation by HSV Cone (H-hue, S-saturation, V-brightness);

(c) Presentation of HSV hue scale.

Lighting

Container

Background

(a)

(b)

Figure 2. Quantification of transparency of TS under different colour background: (a) a

view of setup for quantification of transparency of TS; (b) Presentation of result for white

background.

(a)

(b)

(c)

Figure 3. Result of quantification of TS under different colour background: (a)

Presentation of result for red background; (b) Presentation of result for green background;

(c) Presentation of result for blue background.

Front Side Top

Front Side Top

Front Side Top

Front Side Top

Front Side Top

Front Side Top

Front Side Top

Front Side Top

t = 0.00 s

t = 11.95 s

t = 23.91 s

t = 39.85 s

t = 0.00 s

t = 0.60 s

t = 1.80 s

t = 2.40 s

t = 3.00 s

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 4. Compaction grouting modelling: Observation of grout injection at (a) 0.00s; (b)11.95s; (c)

23.91s; (d) 39.75s and pulling out grout tube (syringe) at (e) 0.0s; (f) 1.80s; (g) 2.40s and (h) 3.00s

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