Analysis of Cracking Behavior of Drying Soil

Alvis Atique Department of Civil Engineering

University of Strathclyde Glasgow, UK

Email: alvis.atique@strath.ac.uk

Marcelo Sanchez, Ph.D. Zachry Department of Civil Engineering

Texas A&M University Texas, USA

Email: msanchez@civil.tamu.edu

Abstract—This paper reports various experimental observa-tions on desiccation tests performed in the laboratory under controlled conditions over samples of ‘Bengawan Solo soil’, from a defence embankment along the Bengawan Solo River, Indonesia. Data has been reported on the volumetric shrinkage and desiccation cracking exhibited on samples upon air-drying. Water retention curve was obtained to analyse the material properties during shrinkage. A number of tests have been car-ried out to study the desiccation process; in circular desicca-tion plate over 5 mm, 10 mm and 20 mm soil thickness. To characterise the crack patterns quantitatively digital image analysis (DIA) techniques have been used to analyse the desic-cation plate tests. Crack parameters e.g., number of crack in-tersections, number of crack segments, total length of cracks, average width of cracks and crack intensity factor (CIF) have been determined for this purpose.

Keywords- drying soil; crack parameters; digital image analysis; desiccation.

I. INTRODUCTION Cracking is a complex phenomenon in materials like

soils. It is a natural process involving weathering, chemical changes and biological. Desiccation cracking significantly affects soil performance. Cracks create a zone of weakness in a soil mass and reduce its overall strength and stability [1]. Examples can be found in various geotechnical, agricultural and environmental applications e.g. irrigated land, tailing ponds for mining waste, landfill liners, earth embankment, reservoir beds, etc [2]. Considerable work has been done to study experimentally the physical phenomenon of cracking in soils. To explain the development and extent of desicca-tion cracking several factors are considered including clay content and mineralogy, soil thickness, surface configuration, rate of drying, total drying time etc [3]. As soil structure is an important property which affects water storage and move-ment, it is necessary to measure crack size and pattern pre-cisely. Examples of basic crack measurements are reported on several works [4-6]. Imaging techniques have also been developed to basic cracking measurement. Guidi used elec-tro-optical determination to estimate size distribution of crack [7]. Lima also used photographic image analysis to de-termine soil surface cracking [6]. Photographic image analy-sis techniques appeared to be a useful tool to distinguish dif-ferences in crack patterns which may be useful characterising soil cracking.

This study has been conducted on a soil (Bengawan Solo Soil) from a flood protection embankment in Indonesia. The

highly dense desiccation crack network observed in this man made embankment may have an important role in the occur-rence of the small progressive failure frequently detected along the embankment. It is considered that in a drying soil, drying causes shrinkage and a crack initiates when the tensile stresses exceed the soil strength. The tensile strength is de-pendent on the moisture content of the soil and the asso-ciated suction. In this paper the influencing factors behind cracking behaviour of soil such as volumetric shrinkage, wa-ter retention property of Bengawan solo soil has been inves-tigated experimentally under proper laboratory environment.

Accurate measurement of geomaterial parameters of soil shrinkage cracks is not easy by direct measurement. Large measurement error is expectable due to irregular shape and complex cracks pattern. In recent years, image analysis tech-niques have been used extensively to characterise the crack network with improved accuracy [8, 9]. In this work an im-age analysis algorithm has been developed (using Matlab code) to measure the amount of cracking on the surface of the soil. This research work is an attempt of development towards standard and improved procedure of laboratory test-ing using modern technology of cracking behaviour of soil.

II. SOIL PROPERTIES TO BE USED IN THE ANALYSIS The flood defence embankment under investigation is a

section along Bengawan Solo River located in the village of Kedunhardjo, in the region of Bojonegoro. The samples have been obtained at the depth on 0.5 m - 1.0 m from the surface. The soil has been classified as inorganic silts of high com-pressibility and organic clay [10]. Some basic properties of the soil are given in table 1.

From the clay mineralogy of this soil (table 2), it has found that the clay is rich in Smectite minerals (montmoril-lonites) which makes the soil highly attracted to absorb water and causing expansion of the material. These kinds of soils develop significant change of volume (contraction) during drying. The kinds of minerals of the clay fraction play a cen-tral role on the properties of the soil, such as plasticity and strength [11].

TABLE 1. PROPERTIES OF BENGAWAN SOLO SOIL.

Properties Value Liquid Limit 55% Plastic Limit 36% Plasticity Index 19% Linear shrinkage 14.8% Volumetric shrinkage 16.45%

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2011 2nd International Conference on Environmental Science and Technology IPCBEE vol.6 (2011) © (2011) IACSIT Press, Singapore

A

B

Particle Size DSand Silt Clay

TA

Depth

Total PhyllosiQuartz Calcite Plagioclases Mg-hornblend

Smectite Kaolinite

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TABLE 3. SUMMARY OF THE DESICCATION TEST SETUP FOR DIFFERENT PLATE.

Desiccation plate shape

Plate Dimension

(mm)

Base condition

Initial sample thickness

(mm)

Final sample thickness

(mm)

Test duration

(day)

Initial water content (W %)

Cracking water content

(W %)

Circular 175x175 Grooved 5 4.38 3 56.767 52%

295x95 Grooved 10 8.94 5 56.380 47% 125 Grooved 20 16.21 7 56.380 45%

TABLE 4. CRACK NETWORK PARAMETERS FOR THREE DIFFERENT THICKNESS CIRCULAR PLATE (R = 125MM).

Soil thickness (mm)

No. of crack/area

(m-2)

No. of crack intersection/area

(m-2)

No. of crack segment/area

(m-2)

Total length of cracks (mm)

CIF (%) Average width of crack

(mm) 5 23480 16448 10517 7713 21.43 1.36 10 489 1060 1487 1967 19.69 4.91 20 265 122 163 772.5 11.18 7.10 All crack parameters were extracted accurately with

program algorithm. Finally, skeletonisation operation has been performed. Table 3 shows the desiccation test condi-tion parameters.

The information extracted from crack patterns in plates is shown in table 4. The total length of crack was deter-mined by calculating the distance between intersections from skeletonised image. To quantify the amount of crack-ing, Crack Intensity Factor (CIF) was determined, which is the ratio of the surface area of the cracks to the total sur-face area of the soil.

IV. SUMMERY AND CONCLUSION This work focuses on the study of some relevant fac-

tors that affect the behaviour of soils submitted to drying. The experimental research has been performed on samples from Bengawan Solo River of Indonesia. The following conclusions were derived:

1. The volumetric shrinkage of Bengawan Solo soil was found low which is 16.45%. Low shrinkage limit indicates high potential for shrinkage and swelling [1].

2. The visual observation from the circular de-siccation plate test was that all samples pro-duced predominantly sequential, orthogonal crack (cracks that tends to meet at right angles) patterns leading to subdivision of the crack area into smaller cells (Fig. 4a).

3. The shapes of the crack notes at intersection was always ‘+’ or ‘T’ regardless any experi-mental condition (Fig. 4a). Similar observa-tions were found in the work by Tang et al [8].

4. Higher desiccation rate was observed for smaller thickness; non-orthogonal crack pat-terns evolved at the primary cracking stage and they occurred simultaneously and relative-ly fast all over the soil surface

[1] Average length of crack and CIF was varied with the number of crack per area. The higher the number of crack per area the higher was the length and CIF. But

the opposite was found in the case of average width of crack.

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[7] L. A. Lima and M. E. Grismer, ‘Soil crack morphology and soil salinity’, Soil Sci. 153, 149–153, 1992.

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[12] J. K. Mitchell, Fundamental of soil behaviour. New York: Wiley, 1993.

[13] British Standards Institute 1990, Method of test for Soils for Civil Engineering purpose, Part 2: Classification of soil, BS 1377-2:1990.

[14] S. Costa, J. K. Kodikara and N. I. Thusyanthan, ‘Modelling of desiccation crack development in clay soils’, The 12th International conference of International Association for Computers Methods and Advances in Geomechanics (IACMAG), Goa, India, 1-6 October 2008: 1099-1107.

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