Effect of Cyclic Loading on Post-Cyclic Behaviour of Parit Nipah Peat Soil

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Effect of Cyclic Loading on Post-Cyclic Behaviour of Parit Nipah Peat Soil

Habib Musa Mohamad^1*, Adnan Zainorabidin²

1 Faculty of Engineering, Universiti Malaysia Sabah (UMS), Jalan UMS, 88400, Kota Kinabalu, Sabah, Malaysia

2 Faculty of Civil Engineering and Built Environment, Universiti Tun Hussein Onn Malaysia (UTHM), 86400 Parit Raja, Batu Pahat, Johor, Malaysia

*Corresponding Author: [email protected]

Accepted: 15 March 2021 | Published: 1 April 2021

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Abstract: The post-cyclic behaviour of a peat soil after subjecting to cyclic loading is a major topic in this study and interdisciplinary structured. A series laboratory monotonic and cyclic triaxial test followed by post-cyclic monotonic tests carried out to determine the effect of soil behavior when subjected to cyclic loading. Tests were carried out on the undisturbed samples taken from Parit Nipah, Batu Pahat, Malaysia. The cyclic triaxial tests were performed on consolidated undrained specimens with 50mm diameter by 100mm height consequently subsequently followed by monotonic loading. A dynamic triaxial testing system was used to investigate the undrained shear resistance of undisturbed soil samples and tests were performed under different cyclic effective stress ratios from 50 and 100 kPa with various amplitudes from 0.5, 1.0 1.5 and 2.0Hz. Monotonic tests were conducted on soil specimens by controlling the strain rate in order to determine the pre-cyclic and post-cyclic shear strength.

The index properties of peat with 650% moisture content and fibre content consist of 27.17%

and indicate that Penor peat is classified as Hemic.

Keywords: peat, monotonic loading, dynamic triaxial, pre and post-cyclic

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1. Introduction

In present situation, due to demand and human civilization necessity, large areas of peat soil forest have already been cleared and due to conversion to other uses. The conversion of it aimed to change completely alter the landscapes for development policies to take advantages of land use to more productive uses for human activities, settlement and industry as a result of economic development for greater Malaysia. Unfortunately, the utilization of peatland in Malaysia is an optional and compromised because, construction on marginal land such as peat soil has high economic factor to consider. The behaviour of peat soil itself inviting future major problem in the construction industry and related to the post-construction quality and productivity.

Peat soil is an organic soil which consists more than 70 per cent of organic matters. Peat deposits are found where conditions are favourable for their formation (Youventharan et. al.

2007). Peat has compared to clay a complex mechanical behaviour, high compressibility, predominance of creep and shear strength properties which are strongly stress depended and anisotropic (Termaat, 1999). This paper investigates the behaviour of post-cyclic specimens subjected to cyclic loading in a triaxial cell. A knowledge of shear behaviour is however, also required in the design of embankments and other structures (Cola, 2005).

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The post-cyclic monotonic shear strengths were evaluated after various numbers of cycles of dynamic loading (Erken and Ulker, 2008). Cyclic and Monotonic tests were conducted on undisturbed Penor peat specimens and the post-cyclic behaviour was determined based on the results of post-cyclic monotonic shear strengths that evaluated after various numbers of cycles of cyclic loading. A review on the performance of road settlements after subjected to dynamic loading or post-construction should be investigated. It therefore necessitates and great importance to understand the post-cyclic behaviour of any structure or infrastructure build up on peat soil. The excess pore pressure generations of peat soils mainly depend on the strain level reached and produce a small proportion of excess pore pressure with the increasing in the number of cycles (Rodríguez and López-Molina, 2008). The amount of effective stress increase, the deviator stress also increases with the increasing in axial strain (Paikowsky, 2003).

2. Methodology and Laboratory Testing

This paper had conducted investigation of the post-cyclic shear strength of peat soil. All tests have been conducted using undisturbed peat soil specimen. The sampling location was located in Parit Nipah, Batu Pahat and tagged as PNpt. Ground water table was found at the depth of less than 1 meter during the sampling. The soil was excavated to a depth of 0.5 m below the ground surface and numbers of tube sampler with the size of 50mm diameter and 100mm height were pushed slowly into the soil. The undisturbed peat soils were waxed both at the end of the tubes and sealed with the aluminums and plastics to prevent the loss or gain of moisture. Jolting during transport was avoided. The samples were kept in the laboratory under constant temperature in the air-conditioned room. Cyclic Triaxial machine or popularly known as Dynamic Triaxial Testing System (ELDYN) was used in the determination of shear strength of PNpt using a electronic controlled system. Sample preparation for the cyclic triaxial test is similar to that of the monotonic test. The specimens were mounted on the base of the pedestal sealed with a rubber membrane and ends with filter paper and porous stone at each end.

All samples were consolidated to 50 and 100 kPa effective confining stress and cyclic tests were performed under different frequencies range of 1.0Hz to 2.0Hz in order to determine the shear strength by differentiate the cyclic loading and frequencies effects. Index properties tests conducted on undisturbed specimens. The cyclic tests followed by static loading of all specimens in order to determine the effect of cyclic shear stresses on the undrained static shear strength of Parit Nipah peat soil (PNpt). Monotonic loading was seriate applied at a 0.10mm/minute loading rate and lasted till the soil specimens exhibited a shear strain of 20%.

The specimens were subjected to 100 cycles for all specimens with different frequency for each sample. The frequencies applied between 1.0 Hz and 2.0 Hz respectively. After cyclic loading, the specimens were immediately subjected to standard consolidated undrained triaxial monotonic loading to failure. Since the focus of this study is not on strain rate effects, all tests were conducted at relatively slow rates. Peat sample was obtained from Parit Nipah, Batu Pahat near, Malaysia. Under reserved land area and far from agricultural activity and exposed to the burning peat in natural heat condition. As seen in Table 1, the index properties of PNpt fairly significant that natural moisture content for PNpt is 593%. The natural water content of peat in Malaysia ranges from 200 % to 700 % and with organic content in the range of 50 % to 95 % (Huat, 2004).

Therefore, the recorded values for PNpt fulfill this statement. Specific gravity recorded 1.3 were within the range (Huat, 2004). In addition to basic characterization tests, the Parit Nipah peat identified as Hemic.

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0 10 20 30 40 50 60

0 2 4 6 8 10 12 14 16 18 20 22

excess pore pressure,u (kPa)

Axial Strain, _a(%)

PNpt - 50kPa - Static PNpt - 50kPa - 2.0Hz PNpt - 50kPa - 0.5Hz PNpt - 50kPa - 1.0Hz PNpt - 50kPa - 1.5Hz

0 20 40 60 80 100 120

0 2 4 6 8 10 12 14 16 18 20 22

excess pore pressure,u(kPa)

Table 1: Index properties of PRpt

Properties PNpt

Natural moisture content m, % 593

Liquid limit wl, % 243

Specific Gravity, Gs 1.3

pH test 4.0

Organic Content, % 95.6

Fiber Content, % 28.5

Effect of Cyclic Loading of Various Frequencies to the Post-cyclic Behaviour

In order to evaluate the post-cyclic behaviour, an extensive series of monotonic and cyclic tests were carried out on large 50 mm cylindrical specimens of peat soil materials using triaxial tests and followed by post-cyclic monotonic shear test after cyclic loading. This paper presented the behaviour of PNpt subjected to monotonic loading. Frequencies and undrained condition effects on specimen have been studied under initial stress conditions 50 and 100 kPa.

(a) (b)

(c) (d)

Figure 1: Effect of cyclic loading on post-cyclic behaviour of specimens at ’ = 50 and 100 kPa (a) (c) Deviator stress versus axial strain and (b) (d) excess pore water pressure versus axial strain

Effect of cyclic loading of various frequencies and effective stresses on post-cyclic behaviour have been monitored in these cases. A comparison of the deviator stress behaviour during the post-cyclic tests with the stress-strain behaviour during monotonic tests as presented in figure 1 (a, b, c and d) and figure 2 (a, b, c, and d) as well. Figure 1 shows the effect of cyclic loading on post-cyclic behaviour of specimens at σ’ = 50 and 100 kPa. Figure 1 (a) and (c) represent for PNpt at 50 kPa and (b) and (d) for 100 kPa. As observed in figure 1 (a) and (b), 50 kPa of effective stress and 100 kPa, it is showed that high similarity resulted in a significant decreases of deviator stress after cyclic loading in initial or static tests result. The shear strength of peat decreased after cyclic loading and nearly 60% loses strength where initial static test for σ’ 50

0 50 100 150

0 2 4 6 8 10 12 14 16 18 20 22

Deviator Stress , q (kPa)

0 20 40 60

0 2 4 6 8 10 12 14 16 18 20 22

Deviator Stress , q (kPa)

Axial Strain, a(%)

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0 5 10 15 20 25 30

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

Shear stress (kPa)

Shear strain, %

kPa and 100 kPa are 53.36 kPa and 92.08 kPa respectively. The shear strength decreases notched in sequenced patently similar. 0.5 and 1.0 Hz noted higher than 1.5 and 2.0 Hz, the effect of various loading frequencies shows gradually decreases after 1.0 Hz loading applied.

This is showed the higher loading frequencies, the lower shear strength in resulted. Destruction of fibre in peat soil confiscated of peat strength.

Therefore, the recorded values for PNpt fulfil this statement (Moreno and Edgar, 2004) that different confinement pressures exhibited in a cyclical shear strain range between 0.3% to 1%

a growth on the damping ratio, and a shear modulus degradation of 10% at the end of the first 5 cycles of each strain level, and of 20% after 10 cycles, due to the gradual destruction of its structure. While this PNpt samples was applied with 100 number of cycles, and these results override the statement.

Figure 1 (c) and (d) represented excess pore water pressure versus axial strain. The value of excess pore water pressure during the post-cyclic tests are significantly lower than induced in static result. The excess pore water pressure decreases as increases of loading frequencies.

This reduction of excess pore water pressure in line with the increase of loading frequencies and decreasing trend with increasing axial strain up to the end of test. Additionally, it was noticed that in PNpt the excess pore-water pressure generation influenced by loading frequencies and generated by strain level that showed decreases trend in figure 2 as well.

(Moreno and Edgar, 2004) highlighted the generation of excess of pore water pressure depends mainly on the strain level reached and on the cyclic degradation produced, and on a small proportion, on the increase on the number of cycles. A substantial decrease in the shear resistance for cyclic loading. Increasing number in cycles of stress application will result in decreases of post-cyclic shear stress (Habib et. al. 2004).

(a) (b)

(c) (d)

Figure 2: Post-cyclic undrianed monotonic shear test results,’ = 50 and 100 kPa.

(a & b) Shear stress versus shear strain and (c & d) pore pressure versus shear strain.

0 10 20 30 40 50 60

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

Pore pressure (kPa)

Shear strain, %

0 20 40 60

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

Shear stress (kPa)

Shear strain, %

0 50 100

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

Pore pressure (kPa)

Shear strain, %

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Variation of loading frequencies results in decreases of the undrained shear strength of peat soil. Stress strain behaviour of PNpt shows in figure 2 (a) and (b), the behaviour of the pot- cyclic tests on PNpt specimens keep shown decreases trend. The pore pressure generation seems to depend on the different loading frequencies and rely on shear deformation levels that generated in cyclic loading stage in figure 2 (c) and (d). From figure 1 and 2 respectively, it was determined that the undrained monotonic shear strength of the peat decreased significantly due to the cyclic loading and behaviour of peat on post-cyclic shear strength shows a significant deformation.

3. Discussion and Conclusion

The undrained behaviour of peat soil subjected to cyclic loading significantly show the correlation of static and dynamic loading behaviour of undisturbed soil sample. The following results have been observed and obtained.

a. The shear strength decreases notched in sequenced patently similar. The effect of various loading frequencies shows gradually decreases after 1.0 Hz loading applied.

This is showed the higher loading frequencies, the lower shear strength in resulted.

Destruction of fibre in peat soil confiscated of peat strength.

b. The excess pore water pressure decreases as increases of loading frequencies.

c. The excess pore-water pressure generation influenced by loading frequencies and generated by strain level that showed decreases trend.

d. The pore pressure generation seems to depend on the different loading frequencies and rely on shear deformation levels that generated in cyclic loading stage.

e. The post-cyclic monotonic tests show that the effect of cyclic loading degrades undrained shear strength.

f. The excess pore water pressure in post-cyclic considerably lower than values developed in monotonic tests.

References

Youventharan D., Bujang B.K. H. and Azlan A. (2007) American Journal of Applied Sciences Publisher: Science Publications Audience: Academic.

Termaat R.J (1999). The stability of constructions on peat and organic soils: Accuracy, reliability and reality. Road and hydraulic engineering division, Ministry of transport, public works and water management, Delft, Netherlands.

Cola S., Cortellzzo G. (2005). The shear strength behaviour of two peaty soils. Geotechnical and Geological Engineering (2005) 23: 679-695. DOI 10.1007/s10706-004-9223-9.

Erken A. and M.B.C. Ülker. (2008). The post cyclic shear strength of fine-grained soils. The 14th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China.

Rodríguez J. A. and J.A. López-Molina (2008). Strain thresholds in soil dynamics. The 14th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China.

Paikowsky, S.G., Elsayed, A. A. and Kurup, P.U. (2003). Engineering Properties of Cranberry Bog Peat. 2nd International Conference on Advances in Soft Soil Engineering and Technology, Putrajaya, Malaysia, pp 153-171.

Huat, B.K. 2004. Organic and Peat Soils Engineering. Serdang: University Putra Malaysia Press.

Moreno C. A and Edgar Rodriguex E. R. (2004). Dynamic behaviour of Bogota’s subsoil peat and it’s effect in seismic wave propagation. 13thWorld Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 2004 Paper No. 2632.

Habib M. M., Zainorabidin A. and Siti N. A., (2014). Post-Cyclic Behaviour of Soil – A Critical Review. 8th MUCET 2014, Date: 10-11 November 2014, Melaka, Malaysia.