Correlation between Durability and Geotechnical Properties of Compacted Shales

L. S. Bryson1, Ph.D., P.E., M. ASCE, I. C. Gomez-Gutierrez2, A.M. ASCE, and T. C.

Hopkins3, P.E., M. ASCE 1Assistant Professor, Department of Civil Engineering, 161 Raymond Bldg., University of Kentucky, Lexington, KY 40506, USA, Phone: 859.257.3247, Fax: 859.257.4404, email: bryson@engr.uky.edu 2Research Assistant, Department of Civil Engineering, 161 Raymond Bldg., University of Kentucky, Lexington, KY 40506, USA. 3Chief Research Engineer (ret), Kentucky Transportation Center, 176 Raymond Bldg., University of Kentucky, Lexington, KY 40506, USA ABSTRACT

In many areas of the United States, shale is used in the construction of highway embankments and pavement subgrades due to the lack of economical alternate materials. Immediately after construction, compacted shale exhibits an acceptable range of engineering properties and behaviors. Unfortunately, with time and in the presence of wet conditions, compacted shale becomes less durable. This reduction in durability translates to the reduction of stability and loss of bearing capacity.

This paper presents the results and analysis of several engineering tests performed on a number of different unweathered and weathered shale samples in Kentucky. Slake-durability tests were performed on unweathered shale samples while consolidated undrained triaxial compression tests were performed on compacted weathered shale samples. The test data suggest that the clay fraction of crushed unweathered samples correlates with the consistency index. Also, the ratio between the slake-durability index and clay fraction correlates with natural water content. Finally, this durability ratio can be used to predict the effective critical state friction angle of compacted shale. In general, correlations between the durability ratio and geotechnical properties provided a means to estimate the suitability of compacted shale typically used in embankments and pavement subgrades. INTRODUCTION

Shale is extensively used in the construction of highway embankments and is present in most roadway cuts in the United States and particularly in Kentucky. Unfortunately, numerous problems have been encountered when making highway cut and fill sections through different shale formations under wet conditions. Such problems include embankment failures due to large settlements and slope instability. Large expenditures have been required for excessive maintenance and in some cases

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major remedial work. The main causes of these problems are the use of shale with low durability as rock fill, the formation of voids in the embankment fill due to the weathering of shale, and inadequate benching and drainage of underlying slopes (Strohm, 1980).

This paper presents the results of several engineering tests performed on a number of different unweathered and weathered shale samples found in Kentucky. This study identified basic physical and engineering properties related to the durability of shale. Also, empirical relationships were developed between basic soil parameters and shale durability, and between shale durability and shear strength of compacted shale. SHALE USED IN THE STUDY

Shale is a sedimentary rock formed by the lithification and diagenesis of clay or silt. The consequent rock is characterized by a finely stratified structure approximately parallel to the bedding planes. Generally, shale includes all weak sedimentary rocks such as claystones, siltstones, and mudstones. Shale makes up the largest portion of all sedimentary rocks. Shale is composed of about one-third quartz, one-third clay minerals, and one-third of miscellaneous substances (Yonekura et al., 2006).

The shale samples investigated for this study were taken from various physiographic regions of Kentucky and belong to varied geologic periods. The samples represent both hard, durable shale with low susceptibility to weathering and soft, less durable shale with high susceptibility to weathering. Table 1 lists the specific shale specimens selected for this study. Descriptions in Table 1 were made by subjective observation.

Table 1. Shale selected for study.

Geologic Formation Geologic Period Description of Specimen

New Albany Devonian Hard black shale Hance Pennsylvanian Hard gray shale Upper Drakes Ordovician Hard gray shale Osgood Middle Silurian Hard gray shale Nancy Mississippian Medium hard gray shale Kope Ordovician Soft gray clay shale New Providence Mississippian Soft greenish-gray shale Crab Orchard Silurian Soft greenish-gray shale Newman Mississippian Soft gray shale

SAMPLING PROCEDURES

A detailed description of sampling and laboratory testing procedures can be found in Hopkins and Deen (1984). Hower, the procedures are summarized herein. Unweathered shale samples were obtained using hand tools and a drill rig. Shale pieces for bag samples were loosened using a rock hammer and mattock. Most of the

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bag samples were obtained at a distance of at least 0.3 m measured horizontally from the face of the shale formation. This distance was usually sufficient to obtain samples free from the effects of surface weathering, although some fractures penetrated to greater depths and were coated with iron precipitates. In certain instances, softer shale had to be sampled at horizontal distances greater than 0.3 m to obtain unweathered samples. The harder shale was usually sampled at horizontal distances of less than 0.3 m because of difficulties in excavating such samples. However, shale samples obtained at the shallow distances were essentially unweathered.

Core samples were obtained using a double tube, NX-size (54-mm) core diameter, M-series core barrel. During sampling, the drill rig was positioned as close as possible to the bag-sampling site. The core barrel was advanced through the overlaying material to an elevation equivalent to the elevation of the bag sample digging. Hence, the core samples and the bag samples were essentially the same material. Selected sections of the extracted core were waxed or thoroughly wrapped with cellophane to protect the sample and to retain the moisture. The waxed samples were stored in an environmental room (humidity controlled) for future testing.

Weathered shale samples were obtained from talus piles that had accumulated near the bottom of highway cut sections to simulate as closely as possible the potential condition of a particular shale formation after several years in an embankment. Each highway cut section selected for sampling consisted essentially of only one type of shale. The highway cut sections selected were as near as possible to the sampling sites previously chosen for obtaining unweathered samples for slake-durability and physical testing. Also, several disturbed bag and bucket samples of each type of shale were obtained. The gradation of the weathered shale was assumed to represent to some degree the natural condition of the shale after exposure to weathering agents. LABORATORY TESTING

The natural water content, ωn of unweathered shale samples were determined by averaging approximately 20 tests of samples of the same shale. These tests were performed according to ASTM D2216. Unweathered hand samples were crushed to pass the No. 10 sieve. A mortar and rubber-covered pestle, recommended by the ASTM D421 and D2217 standards, were used to crush the softer shales. A porcelain-tipped pestle was used with harder shale samples. Particle size distribution (i.e. mechanical and hydrometer tests) was performed. The clay fraction from crushed shale was obtained from hydrometer tests. In the same way, weathered samples were crushed following the same procedures. For Atterberg limits determination, the shale samples were crushed to pass the No. 40 sieve, according to ASTM D4318. As with the unweathered samples, index properties and grain size distribution were evaluated for the weathered shale.

Slake-durability tests were performed on unweathered shale specimens according to ASTM D4644. Also, consolidated undrained triaxial compression, CIU tests were performed on weathered shale samples according to ASTM 4767. Triaxial specimens were compacted to the maximum dry unit weight and optimum moisture contents determined from the standard compaction test. The effective critical state

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friction angle, φ’cs of compacted shale was calculated from the results of the CIU triaxial tests. RESULTS

Table 2 presents the index properties of the unweathered shale samples and Table 3 presents the index properties of the weathered shale samples. The ωn of the unweathered shale ranged from 1.7 to 11.6 percent. The liquid limit, LL of these specimens ranged from 20 to 35 percent and the plasticity index, PI ranged from non-plastic NP to 11.1 percent. The LL and PI of the weathered shale increased, ranging from 24 percent to 40 percent and 7.1 percent to 15.4 percent, respectively. However, it was observed that New Albany and Hance shale were less plastic with weathering. It is noted that these two shale units have the lowest percentage of clay fraction, CF of crushed unweathered samples. Thus, the degradation product of these two shale is most likely a fine sand, as opposed to a silty or clay material. Table 2. Index properties of unweathered shale.

Geologic Formation

natural water

content (%)

Liquid Limit (%)

Plasticity Index (%)

Specific Gravity

Particle-size analysis (Percent finer by weight)

No. 10 No. 200 < 0.002mm

New Albany 1.7 21.5 NP 2.24 100 70.8 3.3 Hance 2.9 21.0 5.0 2.73 100 27.5 14.0 Upper Drakes 4.5 20.0 7.0 2.83 100 89.2 18.2 Osgood 4.9 24.0 7.0 2.77 100 95.2 20.1 Nancy 4.6 20.2 4.0 2.75 100 99.0 29.8 Kope 8.3 28.0 3.0 2.73 100 71.5 22.3 New Providence 11.0 33.0 8.6 2.78 30.3 Crab Orchard 8.4 35.0 11.1 2.68 100 96.0 28.4 Newman 11.6 24.0 6.8 2.71 100 94.2 35.8

Table 3. Index properties of weathered shales.

Geologic Formation

Liquid Limit (%)

Plasticity Index (%)

Specific Gravity

Particle-size analysis (Percent finer by weight)

No. 10 No. 200 < 0.002mm New Albany - NP 2.52 100 88.0 13.5 Hance - NP 2.74 100 87.0 8.0 Upper Drakes 24.0 15.0 2.85 100 85.0 21.0 Osgood 26.0 7.1 2.74 100 96.0 26.0 Nancy 31.0 11.0 2.73 100 98.0 25.0 Kope 30.0 8.3 2.83 100 95.6 34.2 New Providence 40.0 15.4 2.61 100 97.2 31.0 Crab Orchard 38.0 14.0 2.78 100 97.5 32.5 Newman 35.0 12.1 2.73 100 93.4 32.0

Figure 1 shows ωn as a function of CF of the unweathered crushed shale. It

can be seen that ωn increases with increasing CF. The trend increases significantly as

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the CF becomes greater than 20 percent. The observation appears to coincide with the rule-of-thumb that regardless of the parent matrix, a material will behave as clay once the clay content exceeds approximately 20 percent. The relationship between the ωn and CF is described by an exponential function given as

( )058.0exp502.1 CFn ⋅=ω (1)

This type of relationships between ωn and CF are generally a good predictor of the engineering behavior of fine grain soils (Mitchell, 1993). For example, soils with high CF tend to hold more pore water. Consequently, these soils tend to exhibit lower shear strengths than soils with lower clay content.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0 10 20 30 40

Natural water content (%)

Clay fraction (%)

Unweathered shale

 

( )058.0exp502.1 CFn ⋅=ω

Figure 1. Natural water content as a function of clay fraction of the unweathered

shale.

Figure 2 shows the consistency index, CI as a function of the CF for the study shale. The CI describes the relative consistency of cohesive soils (or in this case, compacted shale) in their natural state. It is given as

PILL

CI nω−= (2)

For conditions in which ωn is approximately equal to LL, CI is approximately zero. This condition would imply that the natural state (behavior) of the geomaterial is similar to a viscous fluid. A CI equaled to unity implies that ωn is at the PL. Shale with a CI between 0 and 1 will behave like a stiff-to-hard clay. If the CI exceeds unity, the geomaterial is in semi-solid state and will be characterized as behaving more like a rock.

In Figure 2, the harder, more durable shale tends to coincide with the higher CI. This harder shale is characterized by low CF and natural water contents less than the PI.

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0

1

2

3

4

0 10 20 30 40

Consistency Index

Clay fraction (%)

Unweathered shale

Figure 2. Consistency index as a function of the clay fraction.

Influence of weathering

Mechanical weathering includes processes that fragment and disintegrate the shale into smaller pieces without changing the mineral composition. Chemical weathering is the alteration of the shale minerals into new minerals. Both mechanisms (mechanical and chemical) constitute weathering, but one process may dominate over the other. This study was performed under the assumption that mechanical weathering is the dominant mechanism of weathering.

Shale composed of minerals with high plasticity had high susceptibility to weathering. Thus, after degrading into their constitutive minerals, these shale specimens became more clay-like, whereas the shale that became less plastic after weathering had higher percentages of particles sized greater than 0.002 mm (i.e. clay fraction).

Figure 3 shows the fines ratio of the test shale before and after weathering. The fines ratio is defined as CF divided by the fines (percent passing the No. 200 sieve). Shale that showed an increase in the percentages of silt and clay-sized particles after weathering are plotted above the unity line. Shale that showed a decrease in percentages of particles smaller than silt-sized are plotted below the unity line. Three notable exceptions to the aforementioned trends are the Nancy, Newman, and Hance shale samples. Although these three shale samples are plotted below the unity line (i.e. fines ratio decreased after weathering), all three specimens became more plastic after weathering. This was most likely a function of the mineralogy of these shale samples, or due to some chemical weathering mechanism not taken into account. Further research is required to ascertain the reason for this discrepancy.

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0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

(CF/

Fine

s)w

eath

ered

(CF/Fines)unweathered

Hance

Newman

Nancy

Figure 3. Comparison of the fines ratio before and after weathering.

Factors affecting durability

The durability of shale refers to how well the shale material can withstand changes in the environment and still retain its initial strength and stiffness. The most common method of quantifying shale durability is through the use of the slake-durability test described in ASTM 4644. This test involves placing about ten oven-dried pieces of unweathered shale (each piece weighing approximately 40 to 60 g) in a spinning wire mesh drum, which is submerged in a water bath. The drum is rotated at 20 revolutions per minute for 10 minutes. After the spinning cycle is complete, the drum and the material is oven dried. The remaining material is put again in the drum and rotated another 10 minutes. The final oven-dried mass for the second cycle is obtained. The slake durability index, Id2 is calculated as

%1002 ×=i

fd W

WI (3)

where Wf = oven-dried weight of material retained in the drum for the second cycle; Wi = initial total dry weight.

Figure 4 shows the results of modified slake durability tests performed on the unweathered shale specimens. The figure presents the Id2 as a function of the CF. As shown in Figure 4, the Id2 appears to be heavily influenced by the CF. The relationship between the two parameters can be described via a second order polynomial given as

( ) ( )2

2 093.0156.1375.95 CFCFI d −+= (4) Because of the strong influence of the CF on the Id2, a new parameter was

developed, called the durability ratio given as

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CFIH d 2

10 = (5)

NewAlbany

Hance

UpperDrakes

NancyOsgood

CrabOrchard

Kope

Newman

NewProvidence

0

20

40

60

80

100

0 10 20 30 40

Id2

Clay fraction (%)

Unweathered shale

 

( ) ( )22 093.0156.1375.95 CFCFI d −+=

Figure 4. Slake durability index as a function of the clay fraction.

Figure 5 shows how the durability ratio varies ωn. For the study shale types,

the durability ratio varies over a couple of orders of magnitude. Therefore, it is convenient to show the durability ratio on a square root scale as opposed to an arithmetic scale. It is noted that the durability ratio requires hydrometer analyses to be performed on the shale specimens. Figure 5 allows for the determination of durability ratio using the following empirical expression:

( )nH ωln88.2061.5510 −= (6)

0

10

20

30

40

50

60

0 5 10 15

Sqrt H10

Natural water content (%)

Unweathered shale 

( )nH ωln88.2061.5510 −=

Figure 5. Durability ratio as a function of natural water content.

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Correlation with shear strength

In developing a simple means for estimating φ’cs of compacted shale, it was decided to develop correlations between the φ’cs and other variables. Several correlations of φ’cs with simple index parameters were attempted. Correlations between LL, PI, or CI did not exhibit distinctive or reliable correlations. Traditionally sin(φ’cs) has been shown to correlate with the PI of fine grain soils (Bjerrum and Simons, 1960). However, this was not the case for the study shale types.

A desired outcome of this study was to develop a simple means to estimate the shear strength (i.e. effective shear strength) of weathered, compacted shale using durability characteristics. Correlations between φ’cs and H10 produced good results. In Figure 6, φ’cs is shown as a function of H10.

25

30

35

40

45

0 20 40 60

φ'cs

sqrt (H10)

06.2821.0' 10 += Hcsφ

Figure 6. Critical state friction angle as a function of durability ratio.

The expression that describes the relation between φ’cs and H10 is given as

06.2821.0' 10 += Hcsφ (7)

The implication of Figure 6 is that harder, more durable shale exhibits higher

weathered shear strengths. Thus, the aforementioned empirical expression (Equation 7) allows for the prediction of long-term behavior from simple short-term durability procedures. CONCLUSION

Clay fraction obtained from crushed unweathered shale samples shows important correlations between the index properties of crushed material and the mechanical properties of compacted shale. Also, the slake-durability test provides a means of distinguishing and characterizing different types of shale. Therefore, the

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durability ratio, H10, was defined as a simple and effective parameter for estimating the critical state effective friction angle of compacted shale.

The natural water content of shale is a strong indicator of slake-durability characteristics. This simple and inexpensive test may be used to predict the clay fraction for crushed Kentucky shale and also the durability and the critical state effective friction angle of compacted shale. As natural water contents of undisturbed shale increase, the clay contents after the crushing process increase. No correlations between plasticity indices were observed.

A simple inexpensive method, based on two simple index test, is proposed for estimating the shear strength, φ’cs, of compacted shale. The method makes use of results from the slake-durability test and particle-size analysis of shale after the crushing process. To make use of the method proposed herein, particle-size tests based on the hydrometer method are required. REFERENCES Bjerrum, L., Simons, N. E. (1960). "Comparison of shear strength characteristics of

normally consolidated clays." Proceedings of the ASCE Research Conference on the Shear Strength of Cohesive Soils, Boulder, Colorado: 711-726

Hopkins, T. C., Deen, R. C. (1984). "Identification of Shales." Geotechnical Testing Journal 7 (1): 10-18.

Mitchell, J. K. (1993). Fundamentals of Soil Behavior, 2nd ed., John Wiley & Sons, Inc., New York.

Strohm, W. B. (1980). "Design and construction of shale embankments: summary." U.S. Department of Transportation Report No. FHWA-TS-80-219.

Yonekura, K., Hasegawa, H., Suzuki, T. (2006). "Mineral Compositions, microstructures, and mechanical properties of primary materials from the Paleolithic age." Materials Characterization 56 (2): 165-168.

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