CHARACTERIZATION OF THE MECHANICAL PROPERTIES OF MUSKEG SITE IN PAKDASHT AREA

Jafar Massah¹, Sara Noorolahi^2
1 Department of Mechanical Engineering of Biosystems, College of Aboureihan, University of Tehran, Tehran, Iran
² Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran

ABSTRACT

Muskeg covers a significant portion of the land mass in Iran and elsewhere. It creates considerable difficulties for surface transportation. To evaluate the vehicle mobility over muskeg, it is essential to establish a procedure for identifying and characterizing those mechanical properties of muskeg that are of importance from a vehicle mobility viewpoint. The objective of this study was to determine the mechanical properties of muskeg site at the research farm of College of Abouraihan with a bevameter. Tests were carried out on muskeg with three levels of volumetric moisture content (13%, 34% and 86%) using circular sinkage plates which were mounted on a bevameter. Tests were replicated three times for each of the three levels of moisture content with circular sinkage plates. Results indicated that the size of plates and the moisture content significantly influenced the values of critical pressure (p_cr) and critical sinkage (z_cr).

Key words: Muskeg, Penetration resistance, sinkage, Mat, Peat soil.

INTRODUCTION

Organic terrain is a term used to describe what is commonly known as muskeg. Muskeg is a Canadian term for a swamp or bog containing organic soil with a surface layer of living vegetation and a sublayer of peat of any depth [6]. The surface of this terrain is composed of a living organic mat of mosses, sedges and/or grasses, with or without tree and shrub growth. Underneath the surface there is a mixture of partially decomposed and disintegrated organic material, commonly known as peat or muck [9]. As a rule, this subsurface material is highly compressible compared to most mineral soils. Organic terrain is characterized by its very high water content and its extremely low bearing capacity [4]. Because of its biological origin, organic terrain is extremely complex. To a great extent, the trafficability of muskeg depends on the strength of the vegetative mat overlying the soft peat or muck below, and vehicle mobility depends on the success of the vehicle to utilize the strength of the mat effectively without tearing or breakage [9]. To evaluate the vehicle mobility over muskeg, it is essential to establish a procedure for identifying and characterizing those mechanical properties of muskeg that are of importance from a vehicle mobility viewpoint [12]. To properly identify the mechanical properties of muskeg from a vehicle mobility viewpoint, measurements should be taken under loading conditions similar to those exerted by a vehicle. The vertical load that a vehicle exerts on the terrain results in sinkage giving rise to motion resistance [11]. Various instruments including the cone penetrometer, shear vane, tension apparatus, and the bevameter have been used [11]. Many of the techniques used to assess trafficability of soils have also been used on muskeg with varying degrees of success, depending on how the test evokes the strength of the vegetative mat. Since the vegetative mat overlays very soft peat, the degree of vehicle mobility depends on the flotation and traction provided by the mat. Several instruments have been designed specifically to measure the tensile or tear strength of muskeg and vegetation mats. MacFarlane [5] describes a muskeg "fluke" consisting of several spikes inserted into the vegetation and attached to a cable to which a load is applied. Scholander [8] measured the tearing strength of several forest soils (vegetation-covered mineral soils) by inserting a vertical plate into the vegetation mat and applying a load by pulling the plate with a vehicle-mounted winch. The results show that the vegetation cover provides three to five times greater tearing resistance than bare soil (sand). Bjorkhem et al. [1] used plate sinkage tests to evaluate the effects of roots on compressive strength (or bearing), finding that even though the modulus values are nearly the same, the ultimate strength of the root soil system was 70% greater than for soils without roots. Wong et al. [11] studied results of measurement of the load-sinkage and shearing characteristics of a muskeg in the Ottawa area, and proposed mathematical models for the bearing capacity and shearing characteristics of muskeg. The load-sinkage tests carried out a bevameter which mounted on a tracked vehicle. Based on proposed theory by them, pressure-sinkage relationship for an organic terrain up to the critical sinkage (z_cr) where the breaking of the surface mat is given by:

(1)

where p is the pressure, z is the sinkage, k_p is a stiffness parameter for the peat, m_m is a strength parameter for the surface mat, and D_h is the hydraulic diameter of the plate which is equal to 4A/L, where A and L are the area and perimeter of the contact patch, respectively.

Munasinghe [7] performed consolidation tests with pore pressure measurements and attempted to model the observed consolidation behavior of peaty clay using Terzaghi's one-dimensional model. Karunawardena [2], Karunawardena and Kulathilaka [3] attempted to model the consolidation behavior in the field by back analyzing instrumented earth fill constructed on peaty clay using the model based on Terzaghi's one-dimensional consolidation theory. Stock and Downes [10] studied effects of additions proportions of 0%, 1%, 2%, 3% and 4% by mass of organic matter on the penetration resistance of glacial till for the entire water tension range. Penetration resistance was measured using a steel cone penetrometer fitted in a modified triaxial load frame. Results indicated that at moisture contents greater than field capacity, penetration resistance values were consistent with the observed changes in bulk density, leading to an increase in the samples containing 0–1% organic matter to critical values for root-growth and a decrease for samples containing 2% and more organic matter reaching to values non-critical for roots.

This paper reports the experimental results on the measurement of the penetration resistance characteristics of a muskeg at the research farm of College of Abouraihan in the Pakdasht area with circular sinkage plates which were mounted on a bevameter.

MATERIALS AND METHODS

Description of the test site. Field tests were carried out on muskeg site at the research farm of college of Abouraihan of the University of Tehran in Pakdasht area. The predominant cover class is FI as defined by the Radforth classification system (1952) – Table 1. The surface mat of mosses and shrub roots was about 40 mm in depth. Under the mat there was a peat deposit with depth greater than 300 mm. The different layers of muskeg and relationship between moisture content and depth of the muskeg of college of Abouraihan in Pakdasht area base on the average values of moisture content from the three samples are shown in Figures 1 and 2, respectively. It should be mentioned, however, that additional data would be required to describe different muskeg conditions where the terrain response to vehicle loading varies depending on water content and terrain structure [11].

Test equipment and procedures. To obtain the load-sinkage relationship of muskeg, a tractor-mounted bevameter originally built at the University of Tehran, was used. Figure 3 shows the complete system of the tractor-mounted bevameter unit used in the tests. The overall construction of the complete tractor-mounted bevameter unit was made up of the following main components: the main frame to connect the tractor behind frame; the instrument board contains some parts of the hydraulic and electric circuits; the level system to applied load on soil vertically safely; the sinkage plates with three different shapes (circular, rectangular and oval) to characterize the terrain penetration resistance-sinkage relationships; the carrier to support the bevameter unit and sinkage plates in laboratory; the double acting hydraulic cylinder (AHE01350A-3A-3, Taiyo, Japan) with piston diameter 40 mm, stroke 240 mm to apply the vertical force on the sinkage plates; the pressure, flow and directional control valves; An interface the (CAWL2RG, Camos, Iran) was used: to show the values of force and displacement, to send output load cell and position transducer linear to computer, and to stop the hydraulic cylinder displacement in consequence of overload force; the laptop computer with the controlling software; the S-beam load cell (DBBP, Bongshin, Korea) with capacity 4.9 kN, to measure the applied vertical load; the position transducer linear (KTC 375, Hystar, Taiwan) with resistance 4 kΩ and mechanical travel 378 mm to measure vertical displacement (sinkage). The resolutions of the load cell and position transducer linear were 19.6 N and 0.01 mm, respectively. Sinkage plate was connected directly into the load cell. The double acting hydraulic cylinder applied the vertical force on the sinkage plate and moved them into the terrain. During the field sampling operation, the analog signals from the load cell and linear position transducer were sent to the interface. After acquiring data in the interface, they were sent into the controlling software in the laptop computer and the force-displacement graph was drawn and the collected data was imported to the Notepad software program.

The circular sinkage plates with diameters of 140 and 200 mm and thickness 20 mm, were used. The rate of penetration during tests was 0.025 m s^-1. Tests were carried out on muskeg with three levels of volumetric moisture content (13%, 34% and 86%). Moisture content was determined using an oven-dry technique. Tests were replicated three times for each of the three levels of moisture content with circular sinkage plates. The bevameter unit was mounted on a tractor (M651U, Iran) with mass, 2210 kg and power 48 kW. The tractor hydraulic system delivers 40 liters of fluid per minute in pressure 12 MPa.

RESULTS AND DISCUSSION

To provide a reference, the resistance of muskeg to penetration of two circular sinkage plates with radius of 70 mm and 100 mm was measured continuously and recorded by the automatic data processing system. Tests were replicated three times for each of the three levels of volumetric moisture content with circular sinkage plates. The values of penetration resistance versus the sinkage for each of the three levels of moisture content with circular sinkage plates are illustrated in Figures 4 and 5. It can be seen that, initially, the penetration resistance increases with an increase in sinkage. However, when the applied pressure reached a critical pressure (p_cr), the surface mat was broken. Since the saturated peat beneath the mat is often weaker than the mat and offers lower resistance, the penetration resistance decreased with an increase of sinkage after the surface mat was broken, as shown in Figures 4 and 5. The depth shown in the figure was calculated from the original surface of the muskeg. If the origin for the depth was taken where the cone or circular sinkage plates were fully embedded, then the scale shown in the figure has to be adjusted accordingly. In an approximately constant sinkage, the penetration resistance increased with a decrease in muskeg moisture content. The reason for increasing penetration resistance in the low moisture content of muskeg could be justified by stiffness of terrain. Any decrease in moisture content increased the stiffness of terrain and penetration resistance (Figs. 4 and 5).

Table 2 shows the mean values of p_cr and z_cr for three level of moisture content with circular sinkage plates. It is evident from Table 2 that for each of the three level of moisture content, increasing area of the sinkage plate causes decreasing the value of p_cr. Thus, to utilize success vehicle mobility over muskeg, it is required to decrease the interaction surface between vehicle and muskeg.

The analysis of variance of the data (Tab. 3) indicated that the size of plates significantly influenced the values of p_cr at 1% level. It can be seen that the effect of moisture content of the muskeg on the values of z_cr was significant at 1% level but the effect it on the value of p_cr was not significant. Also the interaction effect of moisture content and size of plates on the value of z_cr and p_cr was not significant.

The results of Duncan's multiple range tests to compare the mean values of soil parameters are presented in Table 4. It is evident that the difference among the mean value of z_cr at level of moisture content of 86% was significant. Also, the effect of the size of plates on the mean value of p_cr was significant.

To determine directly the values of m_m and k_p with circular sinkage plates were used in tests, Equation 1 may be expressed as:

(2)

(3)

where Equations 2 and 3 were used for plates with radius of 70 and 100 mm, respectively. To determine unknown parameters, the measured values of z_cr and p_cr, were replaced in above equations. The calculated mean values of m_m and k_p using Equations 2 and 3 for each of three level of moisture content are given in Table 5.

As shown in Table 5, the calculated values of m_m and k_p using Equations 2 and 3, increase with decreasing of moisture content. The maximum contact pressure is determined with values of m_m and k_p. To evaluate the vehicle mobility over muskeg, the response of the muskeg to maximum contact pressure should also be measured.

CONCLUSIONS

The mechanical properties of muskeg site for three levels of volumetric moisture content (13%, 34% and 86%) at the research farm of college of Abouraihan of the University of Tehran in Pakdasht area was determined with two circular sinkage plates which were mounted on a bevameter. The measured mean values of surface mat stiffness was found to vary from 0.33 to 0.78 and underlying peat stiffness from 1.94 to 5.07 kN mm^-3. Field tests indicated that the size of plates and the moisture content significantly influenced the values of critical pressure (p_cr) and critical sinkage (z_cr) at 1% level.

ACKNOWLEDGMENTS

The authors would like to thank the University of Tehran for supporting of this research project.

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Accepted for print: 6.05.2010

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