ENGINEER Correlation among Index Properties, Strength and Compressibility Parameters of Peat Soils in Muthurajawela Region of Sri Lanka

: Peat is an extreme form of soft soil and its geotechnical properties cannot be determined so easily as it is waterlogged most of the time. Developing correlations among different properties of peat will enable study of geotechnical properties of peat. Hence, aim of this paper was to develop correlations among index properties, unconfined compressive strength (UCS) and compressibility parameters of some peat soils collected from Muthurajawela region, Sri Lanka. Fifteen disturbed and undisturbed samples were collected and index property, UCS and 1-D consolidation tests were conducted for all the samples. Seven correlations were developed based on the experimental results and it was noted that the correlations developed were consistent with similar correlations reported in the literature. The results show that the liquid limit (LL) of peat increases with organic content (OC) and water content (WC). In addition, both the specific gravity (G s ) and UCS values reduce with increase in OC value. When the WC increases, the UCS of peat tends to reduce as the water holding capacity of peat increases with the increase in WC. The compression index (C c ) reduces with the increase in LL and it is due to the increase in compressibility with the increase in LL. The findings of this study revealed that geotechnical engineers can refer to these correlations to study the preliminary behavior of peat soils in Muthurajawela region, where geotechnical data is not readily available.


Introduction
Peat is a soft soil made up of partially disintegrated plant and organic matter that have accumulated underwater and fossilized [1]. The decaying process of plants under acidic conditions without microbial process results in the formation of organic matter in peat [2]. Characteristics of peat are low bearing capacity, low specific gravity, medium to low permeability, high compressibility, high natural water content, high water holding capacity, high rate of creep and difficult accessibility [3,4]. According to Bord and Mona [5], peatlands cover nearly 400 million hectares of the earth which is 3% of the total land surface area. In Sri Lanka, there are 25000 hectares of peatland [6]. Peat is classified into two major types, fibrous and amorphous. Von Post [7] further divided this into ten different ranges using Von Post scale system. In this system, peat is divided from H1 (completely fibrous peat) to H10 (completely amorphous peat) based on the degree of humification, water content, fiber content and botanical composition [8]. According to ASTM Standard D4427, standard peat classification is narrowed down to three classes, namely, fibric (fibrous: least decomposed with fiber content more than 67%), hemic (semi-fibrous: intermediate decomposed) and sapric (amorphous: most decomposed with fiber content less than 33%). As these are extremely soft soils, it is very difficult to determine their physical and geotechnical properties [3]. It is convenient to relate the basic geotechnical properties of peat to few of the easily determined index or physical properties of peat. Hence, development of correlations between different properties will be useful.  Previous researchers [3,9,10,11] have developed correlations among different properties of peat to derive the complex properties of peat from simple parameters. The summary of the findings of these studies are detailed in Table 1.
However, to date, there are very limited studies focusing on the correlation between different properties of peat soils in Sri Lanka. Hence, aim of this research was to develop correlations among index properties, strength and compressibility parameters of peat soils from Muthurajawela, Sri Lanka. The detailed experimental methodology and the results are discussed in the following sections.

Materials
Peat samples for the experimental analysis were collected from two different locations on both sides of Colombo -Katunayake Expressway (CKE) located in the Western Province of Sri Lanka. The depth of sampling was about one foot from the ground surface and the ground water table was found to be at the ground surface. Prior to sampling, surface vegetation was removed to facilitate sampling. Figure 1 shows the location of the sampling along the CKE alignment. These two sampling locations were chosen to cover two representative locations of Muthurajawela region.

Experimental Methodology
Fifteen undisturbed samples, S1 to S15, were collected from both locations using PVC tubes of 150 mm diameter and 400 mm height. One end of the PVC tube was tapered to sharpen the edges to facilitate sampling. Figure 2 shows the tapered PVC tube used and the sampling procedure. Undisturbed samples were waxed immediately after extraction from the field to retain the in-situ moisture conditions of the samples. From each sampling location, disturbed samples were also collected for the determination of index properties.  Figure 4 shows the void ratio vs logarithmic effective stress (e -log σ′) plots for sample numbers 1, 5, 10 and 15. From Figure 4, the compression index (Cc) values of the samples were calculated. Table 2 shows the overall results of index properties, Water Content (WC), Liquid Limit (LL), Specific Gravity (GS) and Organic Content (OC)], UCS and Cc of all the fifteen tested peat samples. Based on Table  2, water content of Muthurajawela peat varies in the range from 12.2% to 164.8%, liquid limit from 41.6% to 96.2%, organic content from 4.3% to 32.9% and specific gravity from 1.73 to 2.59. Based on the literature on peat soils [2,3,9,12,13], water content varies from 140% to 2000%, liquid limit varies from 39% to 550%, specific gravity varies from 0.95 to 2.66, and organic content varies from 0% to 100%. Hence, the index property results are comparable with those reported for peat in the literature. Engineering properties reported in Table 2 were used to develop the correlations for peat.

3.2
Correlation among Index Properties, UCS and Compression Index of Peat Correlations were developed among index properties, UCS and Cc and the developed correlations were compared with similar correlations reported in the literature for peat. Figure 5 shows the plot of Liquid Limit (LL) with Organic Content (OC) and similar plots from literature [3,9,14,15] are also plotted for comparison. The relationship obtained for LL vs OC is given in Equation (1). The coefficient of correlation (R 2 ) for the developed correlation is 0.82 indicating a good correlation. Experimental results plotted, closely fit with similar plots reported in the literature as shown in Figure 5.   Figure 6 shows the correlation developed between LL and water content (WC), and Equation (2) gives the developed correlation. It can be seen from Figure 6 that the developed correlation matches well with similar correlations reported in the literature [3,10,11]. Higher natural water content indicates the higher water holding capacity of peat soil. Higher water holding capacity of peat soil leads to higher LL. Although LL of Muthurajawela peat increases with increase in WC (Figure 6), it does not show a drastic increase as reported in previous studies [10,11].
(%) = 13.703 ln( (%)) − 2.7103 … (2) The relationship between OC and WC is shown in Figure 7. According to Figure  The specific gravity (Gs) of Sri Lankan peat was found to be in the range of 1.73 to 2.59. Equation (4) and Figure 8 show the relationship between Gs with OC. Based on Figure 8, it can be seen that the correlation developed in the present study matches well with similar correlations reported in the literature.  Water Content (%) [10] [3] [15] Present study

Figure 8 -Correlation between specific gravity and organic content
The correlation developed between the Unconfined Compressive Strength (UCS) and LL is shown in Figure 9 and the developed correlation is given in Equation (5). According to Figure 9, increase in LL results in a reduction in UCS value and it is because the rises in LL increase the water holding capacity. As the water content increases effective stress reduces, leading to reduced UCS values.  Figure 11 shows the relationship between the compression index (Cc) and LL and the developed correlation is given in Equation (7). The developed correlation is consistent with similar correlations reported in the literature [11,14,16]. As LL increases, the water holding capacity of peat increases. Higher water holding capacity leads to higher compression leading to increase in Cc values. The Cc values of peats in the present study fall within the range of Cc values reported in the literature. Organic Content (%)

Real Field Applications of the Proposed Correlations
Due to the high water soft nature of peat soils, it is difficult to determine their physical and geotechnical properties. In addition, there is no proper geotechnical database on the properties of Sri Lankan peat. Hence, the developed correlations between index properties, strength and consolidation parameters of Sri Lankan peat will help the geotechnical engineers in design and construction in this region. Estimation of complex properties (UCS, ) using few index properties (LL, OC) will be very useful as the sampling and testing of these softs soils are not easy and consume a lot of time.
Estimation of consolidation settlement requires CC and a quick estimation of CC can be done by knowing the LL. With the estimated CC consolidation settlements can be estimated rapidly to get some idea about the expected primary consolidation settlement before detailed assessments. Proposing ground improvement techniques is much easier when there are correlations among different properties of peat. These correlations can be used to determine various engineering properties of peat and, based on the results, the type of peat can be predicted. Hence, suitable ground improvement techniques for the given peat type can be proposed based on the results. content levels and the

Conclusions
Major aim of this study was to develop correlations among index properties, unconfined compressive strength and compressibility parameters of peat soils in the Muthurajawela region located in the western coast of Sri Lanka. Index property tests (water content, loss on ignition, Atterberg limits and specific gravity), unconfined compressive strength (UCS) and 1-D consolidation test were performed for fifteen peat samples extracted from two different locations along the Colombo Katunayake Expressway (CKE). Based on the experimental outcomes, the following conclusions were drawn: WC. An increase in the OC increases the water holding capacity of peat leading to an increase in LL.  Gs value of peat decreases with OC and it is because peat with higher OC has a greater proportion of voids leading to reduced Gs values.  UCS of peat reduces with both LL and OC and it is because the water-holding capacity of soil increases with increase in both LL and OC. This leads to reduce the strength with the increase in both LL and OC.  Cc of peat increases with increase in LL and it is due to the increased water-holding capacity of peat with increase in LL. An increase in water-holding capacity makes the peat to be more compressible leading to increased Cc values.  On the whole, findings of this study will be useful for geotechnical engineers in the following ways: (i) Geotechnical properties of peat can be better understood; (ii) Complex geotechnical parameters can be obtained using simply determined parameters; (iii) Approximate consolidation settlement can be predicted quickly; and (iv) Suitable ground improvement techniques can be predicted based on the geotechnical properties obtained from these correlations.