Analysis on the condition stability of the existing tunnel condition

Analysis Of Failure Base Plate Anchor Flare Stack Foundation and Repair Method

5. Analysis on the condition stability of the existing tunnel condition

89

7. Result of analysis to draw conclusion between the Mohr Coulomb model and

Hardening Soil model.

ANALYSIS AND RESULT Analysis Mohr Coulomb Model

The analysis was done started from the use of the element method to the use of plaxis software. The result obtained from the Mohr Column model is presented as follows.

Figure 10 Result of total displacement without reinforcement of tunnel using mohr coulomb model

Figure 11 Result of total displacement in alternative reinforcement wiremesh+RB5 of tunnel using mohr coulomb model

From the analysis on the tunnel using Mohr coulomb model, it has been found the result of

safety factor at excavation stage construction is 2,353. And the result of safety factor with

90

RB3 = 4,303; and RB5 = 4,647.

Meanwhile, for the displacement in the condition shown without reinforcement 0,02065 m;

with wiremesh 0,01831 m; and with alternative rockbolt in RB1 displacement shown 0,01709 m; RB 3 = 0,01551 m; and RB 5 = 0,01413 m.

Analysis Harderning Soil Model

The analysis was done started from the use of the element method to the use of plaxis software. The result obtained from the Hardening Soil model is presented as follows.

Figure 12 Result of total displacement without reinforcement of tunnel using hardening soil model

Figure 13 Result of total displacement in alternative reinforcement wiremesh+RB5 of tunnel using hardening soil model

From the analysis on the tunnel using hardening soil model, it has been found the result of

safety factor without reincforcement is 2,3378. And the result of safety factor with wiremesh

91

3,8436; and RB5 = 4,653.

Meanwhile, for the displacement in the condition shown without reinforcement 0,02901 m;

with wiremesh 0,02114 m; and with alternative rockbolt in RB1 displacement shown 0,01773m; RB 3 = 0,01537 m; and RB 5 = 0,0146 m.

Result of Mohr Coulomb Model and Harderning Soil Model approach

From the analysis results obtained can be seen in the following graph between mohr coulomb model and hardening soil model

Table 2. Result of safety factor using mohr coulomb model and hardening soil model

Figure 14 Grapf of safety factor using mohr coulomb model and hardening soil model

While the displacement shown in figure 15:

92

Figure 15 Grapf of safety factor using mohr coulomb model and hardening soil model

CONCLUSION

From the analysis on the tunnel using Mohr coulomb model, it has been found the result of safety factor without any reinforcement at 2,352, and after giving the reinforcement, the wire mesh safety factor experienced an increase to be2,428 With the alternative of the addition of rockbolt RB1, RB3, and RB5 safety factor respectively experienced the increase of 3,0217; 4,303; 4,647. Meanwhile, for the displacement in the condition without any reinforcement showed the value of 0,02065 and with the wiremesh reinforcement is 0,01831;

and with addition alternative rockbolt of RB1, RB2, and RB 3 respectively showed 0,01709;

0,01551; and 0,01413 Thus, it can be concluded that the use of analysis on the finite element of Mohr Coulomb model, the tunnel experienced the increase of safety factor level and the decrease in the soil deformation surrounding the tunnel.

For the hardening soil model, it can be seen that safety factor without any

reinforcement was at 2,3378, and after given the reinforcement of wire mesh, safety factor

experienced an increase to be 2,3017. Given the alternative of addition of rock bolt RB1,

RB3, and RB5 the safety factor respectively experienced the increase of 2,4261; 3,8436; and

4,653 For the displacement surrounding the tunnel, in the condition without any

reinforcement the level was at 0,02901 and when given the addition wiremesh is 0,02114 of

93

be concluded that using the analysis of finite element,hardening soil model on tunnel experienced an increase of safety factor and the decrease in the soil deformation surrounding the tunnel. But the value of the safety factor is still smaller than the mohr coulomb method.

As for the displacement value is still greater than the mohr coulomb method as shown in the figure 14 and 15.

In the analysis on the value in the Mohr coulomb model and hardening soil model, though not showing a significant difference, it can be seen that the value of hardeningsoil model was lower than that of Mohr Coulomb Model. This was in relation to that the analysis with hardening soil model, the measurement considered the soil stiffness for the primary load, in which the value of 𝐸

_𝑜𝑒𝑑^𝑟𝑒𝑓

(tangent stiffness for primary odometer loading) approaching to the existing field condition. In addition, there were the parameters of𝐸

₅₀^𝑟𝑒𝑓

and 𝐸

_𝑢𝑟^𝑟𝑒𝑓

in which

𝐸_𝑢𝑟^𝑟𝑒𝑓= 3. 𝐸₅₀^𝑟𝑒𝑓. The measurement of stiffness was assumed in the unloading or initial

condition. This then made the result of the analysis on hardening soil more approached to the carefulness more/critical condition of tunnel stability.

REFERENCES

Brinkgreve, R.J.B. 2002.Reference Manual V.8 Plaxis (manual Plaxis). A. A Balkema.

Netherlands.

Das, Braja M. 1995. Mekanika Tanah (Prinsip-Prinsip Rekayasa Geoteknis).

Erlangga. Surabaya.

Hudson, John A. 1997. Rock Mechanics. London: Pergamon

Minister of Public Work and Housing.(2015). Metode Perencanaan Penggalian dan Sistem

Perkuatan Terowongan Jalan Pada Media Campuran Tanah-Batuan. Jakarta:

Ministry of Public Work and Housing

Effect of Matric Suction Change on Pile Foundation Capacity in Unsaturated Soils

H. Pujiastuti

Doctoral Candidate at Department of Civil and Environmental Engineering, Universitas Gadjah Mada, Yogyakarta, INDONESIA

heni.pujiastuti@ugm.ac.id A. Rifa’i

Department of Civil and Environmental Engineering, Universitas Gadjah Mada, Yogyakarta, INDONESIA ahmad.rifai@ugm.ac.id

A. D. Adi

Department of Civil and Environmental Engineering, Universitas Gadjah Mada, Yogyakarta, INDONESIA adhadhi2@yahoo.com

T. F. Fathani^1,2

1Department of Civil and Environmental Engineering, Universitas Gadjah Mada, Yogyakarta, INDONESIA

2Center for Disaster Mitigation and Technological Innovation (GAMA-InaTEK), Universitas Gadjah Mada, Yogyakarta tfathani@ugm.ac.id

ABSTRACT

The capacity of pile foundation driven into unsaturated soils is controlled by undrained cohesion. The hydrological process and seasonal changes lead to the changes in the degree of saturation, matric suction, and cohesion of the soil. This research analyzes the influence of the change of undrained cohesion due to the change of matric suction to the pile capacity on unsaturated sandy clay. The calculation of pile capacity is also applied for unsaturated kaolinite clay as a case study. The evaluation is then conducted on these two soil types i.e. unsaturated kaolinite clay and unsaturated sandy clay. Matric suction is measured using the filter paper method, whereas the undrained cohesion is obtained from a laboratory bearing test for unsaturated kaolinite clay and unconsolidated-undrained triaxial test for unsaturated sandy clay. The calculation of pile capacity was simulated on the pile foundation with 0.4 m in diameter, 5 to20 m in length, with the matric suction varied from 10 to 1000 kPa. The results show that the skin friction component of kaolinite clay increased significantly more than sandy clay, while the end bearing component of sandy clay increased significantly more than kaolinite clay. The total pile capacity increased non-linearly with the increasing of matric suction. The total pile capacity increased significantly until the matric suction of 50 kPa, furthermore the curve of the relationship between total pile capacity and matric suction begins to curve on the matric suction of 200 kPa. The total pile capacity driven into unsaturated sandy clay are influenced by both the cohesion and internal friction of the soil.

Keywords

: unsaturated clay, undrained cohesion, filter paper, skin friction, end bearing

1 INTRODUCTION

The hydrological processes including the precipitation, evaporation, evapotranspiration, and the seasoning change from wet season to dry season, resulting in the decrease of groundwater level. This phenomenon may cause the decrease of the degree of saturation and the increase of matric suction of the soil above groundwater level.

The pile foundation is generally used to support the building or transmit the upper structural load to the hard soil that is very deep (Gaaver, 2013). The soil which is located above the groundwater level is known as unsaturated soil (Fredlund and Rahardjo, 1993). The pile foundation capacity (includes skin friction, end

bearing and total pile capacity) that is driven into unsaturated soils, is controlled by the undrained cohesion (Uchaipichat, 2012), whereas unsaturated cohesion is controlled by matric suction (Pujiastuti et al., 2018).

Several studies have been conducted to calculate the pile capacity driven into unsaturated soils. Fattah et al.

(2014) used the numerical methods, while Vanapalli and Taylan (2012) analyzed the equation and then validated the pile capacity by laboratory testing.

Furthermore Chung and Yang (2014) used the laboratory test and then verified the calculation of pile capacity by finite element method. Carvalho and Rocha (2013) implemented several theories and validated the results by the field load test.

The behavior of single pile on unsaturated clay by incorporating the finite element method was investigated by Fattah et al. (2014). The soil parameters were taken from the laboratory testing. The measurement of matric suction was conducted using the filter paper method. The data fitting was evaluared using the Soil Vision to describe Soil Water Characteristic Curve (SWCC) and H-modulus function. Clay soil samples were taken from three locations in the city of Baghdad, Iraq. The results showed that the proposed method to define H-modulus obtained a satisfactory result on the analytical procedure of pile foundation capacity in unsaturated soil.

The skin friction capacity of a small scale single pile on compacted unsaturated clay due to static axial load was evaluated by Chung and Yang (2014) using finite element approach. The disturbed laterite soil samples were taken from 1 m below the ground level on the hillside of Hsinchu in northern Taiwan. It was found that the loading behavior of a small-scale single pile on unsaturated soil indicated a nonlinear response. The ultimate skin friction capacity of the pile has decreased from 52 % to 5 % when the water content was increased from 15 % to 21 %.

In this paper, laboratory tests were conducted to obtain the soil properties. The filter paper contact method was used to measure the matric suction, meanwhile the shear strength of unsaturated sandy clay was examined based on a series of unconsolidated-undrained triaxial test in order to obtain the relationship between the cohesion, internal friction angle, and matric suction of the soil. A laboratory bearing test was performed to obtain the relationship between undrained shear strength and matric suction for unsaturated kaolinite clay. The relationship between shear strength parameters and matric suction was used as the input to the formula for calculating the ultimate pile capacity.

2 DETERMINING THE ULTIMATE TOTAL PILE CAPACITY

Ultimate total pile capacity is calculated from the sum of ultimate skin friction and ultimate end bearing capacity, which are determined as follows.

2.1 The Contribution of Undrained Cohesion

For the cohesive soil having no internal friction, the ultimate skin friction capacity of pile foundation is calculated based on the contribution of undrained cohesion expressed in Equation (1).

s u

s S A

Q =



(1)

where 𝑄_𝑠 is the ultimate skin friction capacity of the pile, 𝑆𝑢 is

undrained shear strength

, 𝐴𝑠 is the area of shaft in contact with the soil , and 𝛼 is the adhesion factor. American Petroleum Institute (API, 1984) suggests the relationship between 𝛼 dan the

undrained cohesion

(𝑐_𝑢) as expressed in Equation (2). This formula can be used as an alternative to estimate the adhesion factor (𝛼).

= 1



for cu < 25 kPa



 



 −

−

= kPa

kPa c_u

50 5 25 , 0

 1 for 25 kPa< cu < 75 kPa

5 .

= 0



for cu > 75 kPa (2)

where 𝑐_𝑢 is similar to 𝑆_𝑢. The contribution of undrained cohesion to the ultimate end bearing capacity of the pile expressed in Equation (3).

b c u

b S N A

Q = (3) where 𝑄_𝑏 is the ultimate end bearing capacity, 𝐴_𝑏 is cross sectional area, 𝑁_𝑐 is bearing capacity factor (equal to 9 for pile).

2.2 The Contribution of Undrained Cohesion and Internal Friction Angle

For the soils having both the undrained cohesion and internal friction angle, the ultimate skin friction capacity of the pile considers the contribution of undrained cohesion and internal friction angle, as expressed in Equation (4).

) tan (

)

(

_{u s s 0 s}

s c A K p A

Q =



+



(4)

where 𝐾_𝑠 is a coefficient of horizontal soil stress depends on soil condition, 𝑝̅_𝑜 is the average of effective overburden pressure at pile base, 𝛿 is angle of interface along the pile and soil. The contribution of undrained cohesion and internal friction angle to the ultimate end bearing capacity of the pile is expressed in Equation (5).

) 3 . 0 3

. 1

(

c N P^'N



dN_ A

Q_b = _{b u c} + _{b q} + (5) where 𝑃′_𝑏 is the effective vertical pressure at pile base, 𝑁_𝑐, 𝑁_𝑞, 𝑁_𝛾 is bearing capacity factor,  is unit weight of soil, d is pile diameter.

3 EXPERIMENTAL STUDY 3.1 Soil Properties

In this study, the ultimate pile foundation capacity is estimated in two soil types, i.e. unsaturated kaolinite clay and unsaturated sandy clay. A series of laboratory tests on kaolinite clay have been carried out by Uchaipichat and Man-koksung (2011). Meanwhile, a series of laboratory tests on unsaturated sandy clay have been carried out by Pujiastuti et al. (2018). The results of the laboratory tests on both type of soils are shown in Table 1.

Table 1. The properties of kaolinite clay and sandy clay Properties Kaolinite clay Sandy clay

Specific Gravity, Gs 2.72 2.62

Liquid Limit, LL (%) 52 39.83

Plastic Limit, PL (%) 31 24.07

Maximum Dry Density, MDD (kN/m³)

14.1 15.1

Optimum Moisture Content, OMC (%)

27.5 23

3.2. Calculation of the Pile Capacity

A circular pile foundation with a diameter of 0.4 m, is driven into unsaturated kaolinite clay and unsaturated sandy clay. The pile length variations are 5 m, 10 m, 15 m, and 20 m. The variations of matric suction are 10 kPa, 50 kPa, 100 kPa, 200 kPa, 400 kPa, 600 kPa, 800 kPa, and 1000 kPa. The matric suction and undrained shear strength of soils is assumed to be constant throughout the pile. The ultimate skin friction capacity and the ultimate end bearing capacity of the pile are calculated by using Equations (1) and (3) for kaolinite clay soils, whereas Equations (4) and (5) are for sandy clay.

4 RESULT AND DISCUSSION

4.1 Soil Water Characteristic Curve (SWCC)

The Soil Water Characteristic Curve (SWCC) is a curve which states the relationship between the degree of saturation and matric suction of the soil. The filter paper method is used to measure the matric suction on both types of soil. The unsaturated kaolinite clay has the air entry value (AEV) of 700 kPa and the residual suction of 7.000 kPa, whereas the unsaturated sandy clay has the AEV of 21.5 kPa and residual suction of 70.000 kPa.

4.2 Undrained

Cohesion

The undrained shear strength in an unsaturated clay was obtained from back analysis base on the laboratory bearing tests conducted by Uchaipichat and Man- koksung (2011). The result provides the relationship

between the undrained shear strength (Su) and matric suction (s), which can be expressed by Equation (6).

3488 .

026 0

.

43 s

S_u = (6) The undrained cohesion in unsaturated sandy clay is obtained by UU Triaxial Test conducted by Pujiastuti et al. (2018). The result shows that the relationship between undrained cohesion (𝑐_𝑢) and matric suction (s) can be expressed by Equation (7) and Figure 1.

1438 .

767 0

.

23 s

c_u = (7)

Figure 1. The relationship between undrained cohesion and matric suction based on laboratory test

4.3 Internal Friction Angle

The internal friction angle (𝜑) in unsaturated sandy clay is obtained by UU Triaxial Test conducted by Pujiastuti et al. (2018). The relationship between internal friction angle (𝜑) and matric suction (s) is expressed by Equation (8) and Figure 2.

3211 .

339

0

.

3

s



= (8)

Figure 2. The relationship between internal friction angle and matric suction based on laboratory test.

0 10 20 30 40 50 60 70 80

0.1 1 10 100 1000 10000

Undrained cohesion (kPa)

Matric suction (kPa)

0 5 10 15 20 25 30 35 40

0.1 1 10 100 1000 10000

𝜑

Matric suction (kPa)

Furthermore, the relationship between tangent of internal friction (tan 𝜑) with matric suction (s) can be expressed by Equation (9) and Figure 3.

3336 .

0575

0

. 0

tan 

= s (9)

Figure 3. The relationship between tan 𝜑 and matric suction based on laboratory test.

4.4 Pile Capacity

For pile foundation driven into unsaturated kaolinite clay, the ultimate skin friction capacity was calculated using Equation (1), while 𝑆_𝑢 as a function of the matric suction is determined by Equation (6), and α is determined by Equation (2). The ultimate end bearing capacity was calculated using Equation (3) and the ultimate total pile capacity is the sum of the result of calculation by Equations (1) and (3).

For pile foundation driven into unsaturated sandy clay, the ultimate skin friction capacity was calculated using Equation (4), while 𝑐_𝑢 as a function of the matric suction is calculated by Equation (7), 𝛼 is determined by Equation (2), 𝐾_𝑠 is assumed to be equal to 1 (Tomlinson, 2004), 𝛿 is determined in the same way as 𝜑 (Tomlinson, 2004). In this case, tan 𝛿 is a function of matric suction and determined using Equation (9).

The ultimate end bearing capacity was calculated using Equation (5). The bearing capacity factors (𝑁_𝑐, 𝑁_𝑞, 𝑁_𝛾) are determined by inputting 𝜑 value on the Terzaghi graph (Tomlinson, 2004). The internal friction angle as a function of matric suction is calculated using Equation (8). The calculation results of ultimate skin friction, ultimate end bearing and ultimate total capacity of the pile are shown in Figure 4-7.

(a)

(b)

Figure 4. Ultimate pile capacity with the variation of matric suction on L = 5 m; (a) Kaolinite clay; and (b) Sandy clay.

(a)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.1 1 10 100 1000 10000

Matric suction (kPa)

0 500 1000 1500 2000 2500

0 200 400 600 800 1000 1200

Ultimate pile capacity(kN)

Matric suction (kPa) Skin friction (clay)

End bearing (clay) Total pile capacity (clay)

0 500 1000 1500 2000 2500

0 200 400 600 800 1000 1200

Ultimate pile capacity(kN)

Matric suction (kPa) Skin friction (sandy clay) End bearing (sandy clay) Total pile capacity (sandy clay)

0 1000 2000 3000 4000

0 200 400 600 800 1000 1200

Ultimate pile capacity(kN)

Matric suction (kPa) Skin friction (clay)

End bearing (clay) Total pile capacity (clay)

(b)

Figure 5. Ultimate pile capacity with the variation of matric suction on L=10 m; (a) Kaolinite clay; and (b) Sandy clay.

(a)

(b)

Figure 6. Ultimate pile capacity with the variation of matric suction on L=15 m; (a) Kaolinite clay; and (b) Sandy clay.

(a)

(b)

Figure 7. Ultimate pile capacity with the variation of matric suction on L=20 m; (a) Kaolinite clay; and (b) Sandy clay.

Based on the calculation results on the ultimate pile capacity ( Figure 4-7), it is clarified that the ultimate skin friction, the ultimate end bearing and the ultimate total pile capacity increased with the increasing of matric suction on both types of the soil. The ultimate total pile capacity and the ultimate skin friction on unsaturated kaolinite clay is higher than in unsaturated sandy clay.

The physical behavior of the long piles (L = 15 m and L = 20 m) are presented in Figure 6-7. The skin friction is more dominant than the end bearing on both the types of the soil. With the matric suction of 200 kPa, the skin friction began to curve significantly on the kaolinite clay, while on the sandy clay is rather significant. The end bearing capacity tends to be flat on the kaolinite clay but on the sandy clay increased rather significant.

The difference between the total pile capacity and the

0 1000 2000 3000 4000

0 200 400 600 800 1000 1200

Ultimate pile capacity(kN)

Matric suction (kPa) Skin friction (sandy clay) End bearing (sandy clay) Total pile capacity (sandy clay)

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500

0 200 400 600 800 1000 1200

Ultimate pile capacity(kN)

Matric suction (kPa) Skin friction (clay) End bearing (clay) Total pile capacity (clay)

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500

0 200 400 600 800 1000 1200

Ultimate pile capacity(kN)

Matric suction (kPa) Skin friction (sandy clay) End bearing (sandy clay) Total pile capacity (sandy clay)

0 1000 2000 3000 4000 5000 6000 7000

0 200 400 600 800 1000 1200

Ultimate pile capacity(kN)

Matric suction (kPa) Skin friction (clay) End bearing (clay) Total pile capacity (clay)

0 1000 2000 3000 4000 5000 6000 7000

0 200 400 600 800 1000 1200

Ultimate pile capacity(kN)

Matric suction (kPa) Skin friction (sandy clay) End bearing (sandy clay) Total pile capacity (sandy clay)

end bearing capacity is high. It is stated that the dominant role of skin friction a long pile capacity.

The physical behavior of the short piles (L=5 m dan L=10 m) driven into the kaolinite clay show that the skin friction is more dominant than the end bearing.

With the matric suction of 200 kPa, the skin friction began to curve rather significantly, while the end bearing capacity tends to be flat. The difference between pile capacity and end bearing capacity is rather high. This states the role of the skin friction is stronger than the end bearing. The physical behavior of the short piles driven into sandy clay shows that on a low matric suction, the value of the skin friction tends to be higher than the value of the end bearing. When the matric suction value of 200 kPa, the skin friction tends to be flat while the end bearing value tends to increase and exceed the skin friction value that occurs when the matric suction greater than 400 kPa. The difference between the total pile capacity and the end bearing capacity is slightly different. This states the role of end bearing is very dominant on a short pile capacity.

In this research, there is a necessity to validate the calculation results using the same soil, however this research compares the calculation of pile capacity on kaolinite clay and sandy clay. In the future, an experimental testing of sandy clay is required to validate the results of proposed estimation.

5 CONCLUSIONS

This research examines the calculation of the circular pile foundation driven into unsaturated soils. The ultimate skin friction, the ultimate end bearing and the ultimate total pile capacity were calculated using the equations considering the undrained cohesion for kaolinite clay and incorporating the cohesion and internal friction angles for sandy clay.

The undrained cohesion was obtained from laboratory test using the UU Triaxial Test and the laboratory bearing test. Matric suction was measured using the filter paper method for determining Soil Water Characteristic Curve (SWCC). The relationship between undrained cohesion and matric suction yields the undrained cohesion equation as a function of the matric suction as well as the internal friction angle of soil and matric suction.

The results show that the ultimate skin friction, the ultimate end bearing and the ultimate total pile capacity increased with the increasing of matric suction. In the kaolinite clay, the ultimate total pile capacity is dominated by the skin friction contribution from undrained cohesion components both on long piles and short piles. In the sandy clay, the ultimate total pile

capacity is dominated by the skin friction. A significant contribution from undrained cohesion and internal friction components can be found on the long piles. For the short piles with low matric suction (below 400 kPa), the ultimate total pile capacity is dominated by the skin friction, but in higher matric suction, the ultimate total pile capacity is dominated by the end bearing capacity with the dominant constribution of the internal friction angle of the soil.

ACKNOWLEDGMENTS

The authors would like to express their sincere gratitude to the Directorate General of Science and Technology and Higher Education, Ministry of Research, Technology and Higher Education, of the Republic of Indonesia for the financial support by the Education Scholarship of Postgraduate Domestic Programs.

REFERENCES

API, 1984, Recommended practice for planning designing and construction fixed offshore platforms, 14th Edn. APIRP2A, American Petroleum Institute, Dallas, TX.

Carvalho D. and Rocha A.P.J., 2013, Uplift behavior of bored piles in tropical unsaturated sandy soil, Proceedings of The 18^th International Conference on Soil Mechanics and Geotechnical Engineering, Paris.

Chung S.H. and Yang S.R., 2014, Loading behavior of small scale single pile in unsaturated clayey soil, Material Research Innovation, Vo.18.

Fattah M.Y., Salim N.M and Mohsin I.M., 2014, Behavior of single pile in unsaturated clayey soils, Eng.

& Tech. Journal, Vol. 32, Part (A), No. 3.

Fredlund D.G. and Rahardjo H., 1993, Soil mechanics for unsaturated soils, John Wiley & Sons, Inc., New York.

Gaaver K.E., 2013, Uplift capacity of single piles and pile groups embedded in cohesionless soil, Alexandria Engineering Journal, 52, 365-372.

Pujiastuti H., Rifa’i A., Adi A.D. and Fathani T.F., 2018, The Effect of Matric Suction on The Shear Strength of Unsaturated Sandy Clay, International Journal of GEOMATE, Jan., 2018 Vol.14, Issue 42, pp.112-119.

Uchaipichat A., 2012, Variation of Pile Capacity in Unsaturated Clay Layer with Suction, EJGE, Vol.17 Uchaipichat A. and Man-koksung E., 2011, Variation of Ultimate Bearing Capacity of Unsaturated Clay with Suction, ARPN Journal of Engineering and Applied Sciences.