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1.Introduction
Peat is a type of organic soil that forms in wetland environments, where dead plant material accumulates and decays over long periods of time without fully decomposing. Peat is composed of partially decayed vegetation, including mosses, sedges, and other plant species.
Peat is typically found in areas where water saturates the soil, creating an anaerobic environment that slows the decomposition of organic matter. Over time, the accumulation of partially decayed plant material creates a thick layer of peat that can range from a few centimeters to several meters in depth.
Peat is known for its high water holding capacity, low nutrient content, and acidic pH. It has been used for a variety of purposes over the years, including as a fuel source, a horticultural growing medium, and a soil conditioner.
Peatlands, which are areas dominated by peat, are important ecosystems that provide numerous benefits to the environment and to human society. They can serve as important carbon sinks, helping to mitigate climate change by sequestering large amounts of carbon from the atmosphere. Peatlands also provide habitat for a variety of plant and animal species, and can help to regulate water flow and reduce flooding in downstream areas.
Peat soils occur in many countries and they are often described differently from both a qualitative and quantitative perspective. Peat is formed naturally through the decomposition of plant and animal matter under anaerobic conditions that take place over long periods. It is also given several different names, in order to characterise its differences resulting from the effect of climate and type of plant materials that constitute the peat. Although the same definition of peat may be similar, the characterization of this soil normally is defined by its inherent locality.
In terms of geotechnical engineering, peat is commonly recognised as a material with high compressibility and low bearing capacity and therefore being unsuitable as foundation materials for any construction works (Adnan et al., 2007). However, the rapid development in many countries, coupled with a strong economic performance has resulted in vast infastructure development. These developments are hindered by a dearth of suitable land for development and this provides the motivation for further research into the properties of soil in areas with adverse ground conditions, such as peat, with a view to finding means for infrastructure construction.
Peat in northern temperate regions of the world is formed normally from the remains of grasses, sedges and bog mosses. Hobbs (1986), gave excellent summaries of the development and properties of British peat. There are two types of peat; known as fen and bog peat. The morphological differences between fen and bog peats are attributed to the types of plant remains that occur in the peat and their mode of origin. He explained that the differences lie in the degree of humification, structure, fabric and proportion of mineral material contained in the peat, and this in turn affects their behaviour from an engineering viewpoint.
Peat is an accumulation of partially decomposed and disintegrated plant remains which have been fossilized under conditions of incomplete aeration and high water content. Physico-chemical and biochemical processes cause this organic material to remain in a state of preservation over a long period of time. Macroscopically, peaty material can be divided into three basic groups, namely, amorphous granular, coarse fibrous and fine fibrous peat. The amorphous granular peats have a high colloidal fraction, holding most of their water in an adsorbed rather than free state. In the other two types the peat is composed of fibres, these usually being woody. In the coarse variety a mesh of second-order size exists within the interstices of the first-order network, while in fine fibrous peat the interstices are very small and contain colloidal matter. The mineral material in peat is usually quartz sand and silt. In many deposits the mineral content increases with depth. The amount of mineral content influences the engineering properties of peat. The void ratio of peat ranges from about 9 for dense amorphous granular peat, up to 25 for fibrous types with high contents of sphagnum. It tends to decrease with depth within a peat deposit. Such high void ratios give rise to a phenomenally high water content and most of the peculiarities of peat are attributable to its moisture content. This varies according to the type of peat; it may be as low as 500% while values exceeding 3000% occur. The amount of shrinkage that can occur in peat generally ranges between 10 and 75% of the original volume, and it can involve reductions in void ratio from over 12 down to about 2. Dry densities of drained peat fall within the range 65–120 kg/m3. The dry density is influenced by the mineral content, and higher values than those mentioned can be obtained when peats possess high mineral residues. Apart from its moisture content and dry density, the shear strength of a peat deposit appears to be influenced by its degree of humidification and its mineral content. As both these factors increase, so does the shear strength. Conversely, the higher the moisture content of peat, the lower is its shear strength. In an undrained bog the unconfined compressive strength may be negligible, the peat possessing a consistency approximating to that of a liquid. The strength is increased by drainage to values between 20 and 30 kPa and the modulus of elasticity to between 100 and 140 kPa. When loaded, peat deposits undergo high deformations but their modulus of deformation tends to increase with increasing load. If peat is very fibrous it appears to suffer indefinite deformation without planes of failure developing, whereas failure planes nearly always form in dense amorphous peats. Differential and excessive settlement is the principal problem confronting the engineer working on a peaty soil. When a load is applied to peat, settlement occurs because of the low lateral resistance offered by the adjacent unloaded peat. Serious shearing stresses are induced even by moderate loads. Worse still, should the loads exceed a given minimum, then settlement may be accompanied by creep, lateral spreading or, in extreme cases, by rotational slip and upheaval of adjacent ground. At any given time the total settlement in peat due to loading involves settlement with and without volume change. Settlement without volume change is the more serious for it can give rise to the types of failure mentioned. What is more, it does not enhance the strength of peat.
2.Physical Properties of Peat
The physical study of peat has been done by many researchers, especially engineers, to ensure that any constructions on peat based grounds are safe after completion (Edil, 2003). The main aspects of these studies were focussed on physical characteristics such as moisture content, liquid limit, organic content, specific gravity.
Review of literature indicates that peat soil is very variable in its properties, both from one deposit to another and from point to point in the same deposit. The moisture content for east Malaysian peat ranged between 200% to 2200%. However, Zainorabidin and Ismail (2003), conducted several tests on Johore Hemic peat and found that the natural water content for these ranged from 230% to 500%. Other researchers investigating West Malaysian peat soil include Al-Raziqi et al. (2003), who found some properties of Malaysian peat soils with values similar to those indicated in Table 1. All these figures indicate that the Malaysian peat soil varies with different geographical locations even though the natural water content may be similar. This is due to the influence of different agricultural history of the area and rainfall intensity.
Furthermore, the sampling location might differ as some were taken from coastal areas and others from midlands. As in the case with British peat soil, there are two types of peat, namely bog and fen. The natural water contents for bog seems to be similar to samples taken from East Malaysian peat soil whereas for fen its characteristics, if not the same, are similar to some West Malaysia and Johore peat soil.
3.Shear Strength Characteristics
Shear strength is a fundamental property required in geotechnical design and analysis. Various researchers have studied this property and the reported results indicate important differences in behaviour from inorganic soils both qualitatively and quantitatively. Haan (1997), reported that both undrained shear strength and effective strength parameters of many peats increase with increasing water content or decreasing unit weight. This result seemingly supports the intuitive behaviour that can be attributed to the fiber effects and the fact that the fiber content, in general, increases with increasing water content and decreasing unit weight.
The results showed that the cohesion value, c is very low in the range, 2kPa to 8kPa, while the friction angle is less than 300. However, fibrous peat showed values of c and the angle of friction higher compared than other types. Lechovisz (1993) stated that the results from the simple shear tests give lower strength than triaxial compression testsbecause of the fiber orientation (typically horizontal) relative to the shear plane. He also determined that the reduction due to this effect could be as much as 25%.
Edil (2003), stated that the presence of the fibers can influence the strength of peat in that the shear resistance continues to develop at high strain values without a significant peak behaviour and will exhibit reduced Ko values compared to that of clays. K0 represents the one dimensional lateral earth pressure coefficient under confined conditions at rest condition.
4.Consolidation Characteristics
One dimensional consolidation tests were conducted to assess the compressibility characteristics. The loading was applied in increments with values of 10, 20, 40 to 80 kN/m2 followed by unloading. These stress levels were selected considering the fact that these soils are normally encountered near the surface levels and have a maximum thickness in the range of 10-12m. The capacity of this material to retain high water contents at elevated stress levels is low. They are generally weak in their natural states but can be subjected to significant strain gain with consolidation.
5.Identification
Peat can be identified based on its physical and chemical properties. In the field, peat is often dark brown or black in color, and has a spongy, fibrous texture. It is typically very wet and can have a high water content, which can cause it to bounce or give underfoot when walked on. Other visual cues that can help identify peat include the presence of mosses and other bog plants, as well as the absence of tree roots or other woody debris. Peat may also have a distinctive odor, which can range from earthy and musky to slightly acidic or sour. In addition to its physical properties, peat can be identified based on its chemical composition. Peat has a high carbon content, typically ranging from 45 to 60 percent, which distinguishes it from other types of soil. It also has a low pH, typically between 3.0 and 5.5, which can be measured using a pH meter or by adding a few drops of litmus paper to a soil sample. Peat can be further analyzed in a laboratory setting to determine its nutrient content, water holding capacity, and other physical and chemical properties. These tests can help to identify the specific characteristics of peat and to determine its potential uses in various applications, such as agriculture, horticulture, and energy production.
6.Conclusion
As discussed previously, there remain challenges regarding understanding of peat soils and utilizing it for engineering purposes. In this paper, some pertinent matters have been discussed and emphasized and these are summarised as follows:
