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American Journal of Engineering and Applied Sciences
Year: 2009  |  Volume: 2  |  Issue: 1  |  Page No.: 184 - 188

Experimental Investigation on Geomechanical Properties of Tropical Organic Soils and Peat

Bujang B.K. Huat, Afshin Asadi and Sina Kazemian    

Abstract: Problem statement: Organic soils and peat were believed to be geotechnically problematic due to their very high compressibility, very low shear strength and difficult accessibility. Although conventional soil mechanics theory could be applied to the soils, it was found that important anomalies existed which required special considerations. Correlations between geomechanical parameters for the soils were known to be important for geotechnical engineers to be able to obtain suitable design parameters, as well as to find suitable construction techniques on these soft materials. Approach: To evaluate the geomechanical characteristics of the soils, field and laboratory investigations were carried out according to the organic contents. To achieve such purpose, the soils samples having different organic contents from several locations in Malaysia were collected to determine the correlations of various geomechanical properties of the soils. The classifying tests were determined based on the test procedures according to the British Standard Institution. The compressibility behavior of the soils was determined by Rowe cell consolidation test. Results: The test results indicated that the natural water contents, organic contents, liquid limits, specific gravities and bulk densities ranged from 150-700%, 50-95%, 180-500%, 1.05-1.9 and 0.8-1.2 Mgm-3, respectively. The compression indexes of the soils were higher than Hobbs and Skempton’s approximations. Conclusion/Recommendations: The soils properties were highly dependent on the organic contents. With an increase in organic content, the natural water content, liquid limit, compression index and void ratio increased and the specific gravity and bulk density decreased. Furthermore, the hemic and sapric peat had lower shear strength than the fibrous peat. The first-of-its-kind study was the first step on the road to persuade researchers to improve these problematic soils.

Figure 1 shows plot of organic content with natural water content of tropical organic soils.

The natural water contents of these soils were found to the range from 150-700%, with organic content in the range of 50-95%. Empirical relationship between organic content with natural water content of tropical organic soils could be written as:


Where,
m = Water content
OC = Organic content

Organic content-liquid limit: Figure 2 shows the graph of organic content versus the liquid limit.Empirical relationship between organic content and liquid limit was found as:


Where,
LL = Liquid limit
OC = Organic content


Fig. 1: Water content-organic content

Fig. 2: Organic content-liquid limit

Fig. 3: Natural water content-liquid limit

Fig. 4: Specific gravity-organic content

Based on this study, the range of liquid limit was from 180-500% for the Malaysian peat soils.

Water content-liquid limit: Figure 3 shows a plot of water content versus the liquid limit.

Specific gravity-organic content: Figure 4 showed a graph of specific gravity with organic content, for peat soils.

The average specific gravity of organic soil particles could be calculated from:


Where,
Gs = Specific gravity
OC = Organic content

Specific gravity of the peat was in the range of 1.05-1.9, in about the same range to that of Den Haan[8] observations.

Fig. 5: Bulk density-organic content

Based on the experimental data obtained in the laboratory, our observations approximately agreed with correlation proposed by Den Haan[8], which is:

Bulk density-organic content: Figure 5 shows a graph of bulk density and organic content of the tropical peat and organic soils of this study.

The following relationship was derived from tropical peat soils:


Where,
OC = Organic content
gb = Bulk density

The bulk density of organic soil was in the range of 0.8-1.2 Mg m-3 compared with the bulk density of mineral soils which is in the range of 1.8-2.0 Mg m-3.

Compressibility:
Correlations of compressibility properties:
Compression index-liquid limit: The plot of found compression index (Cc) versus the liquid limit of the organic soils (Fig. 6) shows that (Cc), increased with increase in liquid limit of the soils.

Void ratio-liquid limit: Figure 7 shows a plot of the initial voids ratio versus liquid limit values obtained for tropical peats from Malaysia together with those normally consolidated peat found by[2]. The general trend was the void ratio that increased with increase in liquid limit.

Fig. 6: Compression index-liquid limit

Fig. 7: Void ratio-liquid limit

Its relationship is:


Where,
e = Void ratio
LL = Liquid limit

Compression ratio-liquid limit: The values of (Cc/1+e0) determined from the laboratory test on tropical organic soil samples from Malaysia are shown in Fig. 8.

Shear strength:
Vane shear strength-moisture content: The plot of moisture content against vane shear strength was shown in Fig. 9.

At the same moisture content, the high fibrous peat (H1-H3) gave higher strength compared with the medium and low fibrous peat (H4-H10).

Fig. 8: Compression ratio-liquid limit

Fig. 9: Vane shear strength-moisture content

DISCUSSION

Peat and water: For organic soil, the water is held in the organic matters and cells of the plant remain. Organic soils had very high natural water content, which could be in excess of 700%, compared with mineral soils (sand, silt and clay), whose values in the field may range between 3-70%, but with values of greater than 100% are sometimes found in soft soils below ground water table.

These observations agreed with what have been carried out by McBrierty[7], but the maximum water content, found in tropical organic soils with high organic content is less than Irish peat[7].

Organic contents and peat index parameters relationships: The natural water contents and liquid limits were found to increase with increase in organic content. The specific gravity of peat decreased, with increase in organic content. According to the test results, the bulk density increased with decrease in organic content. This was due to the lower specific gravity of the particles and the higher water holding capacity in peat compared to mineral soils. The results revealed that organic content played a key role in the soils properties.

Deformation characteristics of peat: The empirical relationship between the compression index and liquid limit suggested by[2] for organic soils (Cc = 0.009(LL-10)) gave a reasonable approximation of these parameters. Hobbs[4] estimated the compression index of fen peat was about 0.007(LL-10), which gives slightly lower value of (Cc).

Compression index of tropical peat samples tested (Fig. 6) however, were apparently little higher than the above two relationships.

Hobbs[4] found the ratio of (Cc/1+e0) was small. Our observation is in agreement with Hobbs[4] study. The trend is similar to other researches but slightly higher, which is likely due to higher in situ void ratio (e0). The value of (e0) depends on the in situ vertical stress; hence, (e0) should be that appropriate to the very low effective stress conditions.

Correlations between shear strength properties: Figure 9 showed a decreasing behavior of shear strength with increasing water content. Determination of shear strength parameters for organic soils, as with other soils, is important and somehow a difficult job in geomechnical engineering. For organic soils, several methods have been used to determine the undrained shear strength namely; laboratory Swedish fall-cone test, triaxial test, shear box test and vane shear test. Presence of fibers and their varying interaction within the shearing mode imposed by the particular testing procedure however creates difficulties in assessing the true operating strength value. The fibric peat (H1-H3) had higher shear strength than hemic (H4-H6) and sapric (H7-H10) peat.

CONCLUSION

The natural water content was found to increase with increase in organic content. The natural water contents of organic soils were found to range from 150-700%, with organic content in the range of 50-95%
The liquid limit of the organic soils increased with increase in organic content. The range of liquid limit was between 180-500% for the Malaysian peat soils
The liquid limit of tropical peat and organic soils of this study was found to increase with increase in water content
The specific gravity of peat decreased, with increase in organic content and Specific gravity of the peat was the range of 1.05-1.9
The bulk density increased with decrease in organic content. The bulk density of organic soils was in the range of 0.8-1.2 Mg m-3
The compression index (Cc), increased with increase in liquid limit of the soils. The void ratio increased with increase in liquid limit
Despite the large variations, which occurred within organic and peat soils, the variation in the ratio of (Cc/1+e0) was relatively small
In general, fibric peat (H1-H3) had higher shear strength than hemic (H4-H6) and sapric (H7-H10) peat

ACKNOWLEDGEMENT

Sincere appreciation is attributed to the Ministry of Science, Technology and Innovation Malaysia for providing a fund (cement column stabilization of peat soils, ref. 5450411) to make the research on the peat soil possible.

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