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Investigation of Afram Shale for Road Construction

Yaw A. Tuffour*, Samuel Y. Banini, Charles A. Adams

Department of Civil Engineering, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana

Abstract: Road construction projects in the Afram Plains area in Ghana are characterised by high material haulage cost due to the unavailability of suitable geologic materials local to the area for road work and the need to haul from distant sources. This study investigated the suitability of rock-like shale local to the Plains for road construction in the area. Samples of the rock-like shale from the Plains were evaluated in terms of strength as coarse aggregates and as compacted material before and after being subjected to soaking as well as varying cycles of soaking and drying. The soaking and drying simulated conditions under which pavement structures exist in the area over their design life due to the heavy rainfall in the Plains and the poor drainage characteristics of the terrain. The results indicated that whereas the rock-like shale in the un-soaked state met all the Ministry of Roads and Highways strength specifications for road aggregates, it lost practically all its strength in the soaked state. Water absorption was 900% the specification upper limit. Compacted samples lost close to 90% in CBR value following four days soaking. The high loss in strength under wetting conditions and the material’s ability to imbibe large amounts of water do not favour the use of the Afram shale for road construction in the long-term, given the fact that there are ample opportunities, within the area, for the material to come into contact with water for a long time when existing as part of a pavement structure.

Keywords: Afram Plains, poor drainage, road construction, strength, rock-like shale, wetting and drying cycles

Citation: Yaw A. Tuffour, Samuel Y. Banini, Charles A. Adams, Investigation of Afram Shale for Road Construction. American Journal of Civil Engineering and Architecture, 4 (3). 80-83. January 2016. doi:10.12691/ajcea-4-3-2

1. Introduction

In Ghana, road construction projects in the Afram Plains area (which is located in the country’s Eastern Region) tend to be characterised by substantial material haulage activities and cost due to unavailability of suitable geologic materials in the area for road construction and the need to haul from distant sources. Material haulage cost is especially prominent given the fact that the poor drainage characteristics of some parts of the terrain generally tend to require embankments to be constructed in order to carry pavement structures substantially above the natural ground level. Although some parts of the Plains have rock-like shale deposits, geotechnical appraisal of geologic materials in the area for road projects have tended to be silent about the shale [1]. For most road constructions in the country, lateritic or quartzitic natural gravels, which are obtainable in several regions of the country, constitute the predominant structural layer material used except for heavily-trafficked roads which may require crushed rock in addition as part of layer material. Crushed rock materials supplied by commercial quarries or project quarries are derived mainly from igneous rocks of granitic origin or high grade metamorphic rocks. Where embankments have to be built, fill material from cuts along the alignment supplemented by borrow material from gravel pits are used.

The Afram Plains area, which lies on the southern part of the Voltaian Basin in Ghana, is generally a low-lying peneplain with very gently undulating topography [2]. The Voltaian Basin covers about 45% of the land area of Ghana and consists almost entirely of sedimentary rocks, mainly coarse-grained sandstones, clay shales and mudstones. Within the Afram Plains, sandstones are found mainly along the margins while shales and mudstones outcrop at the central part.

Shales belong to the category of fine-grained siliciclastic sedimentary rocks generally referred to as mudrock which is composed predominantly of clay minerals. Mudrock is a generic name encompassing a variety of clayey rocks including claystones, mudstones, siltstones, shales and slates. Shale, in a more specific context, is a fissile anisotropic mudrock [3], the fissility having developed as a result of the tendency of the micaceous clay minerals present in the material to arrange parallel to the plane of deposition during formation [4]. In terms of mineralogy, petrographic analysis has shown the Afram shale to contain 60-70% quartz and 5-15% feldspar [1].

The importance of shales and other clay-bearing rocks in engineering applications is the wide variation in the material’s engineering properties, including resistance to short-term weathering by wetting and drying phenomenon [5]. In civil engineering practice, clayey soils and clayey rocks in general pose particular problem because they are more prone to swelling and disintegration when exposed to cycles of moisture change than any other types of geologic materials. When freshly exposed, the rock-like character and appearance of clay-bearing rocks in general and mudrocks in particular may give a false impression of durability but they easily degrade and disintegrate to soil-size particles (slake) after a short period of exposure to atmospheric conditions. Such changes in characteristics over a short period have caused highway slope stability and embankment failure problems where the material was present.

According to Solomon et al. [1] the use of shale in road construction is rare due probably to the perceived geotechnical challenges associated with the material in general. While highly indurate shales may be used for road construction, shales in general do not naturally form good pavement layer material [6]. Nevertheless, Gromko [7] investigated oil shale aggregates for road construction and concluded that the material could probably be suitable as a surface course material only in secondary roads as such roads are generally not expected to carry substantial traffic. In Nigeria, Aghamelu and Okogbue [8] have reported of incessant failures in roads and highways for which local shale had been utilised both as part of the pavement structure and as aggregates. Excessive settlements and slides may occur in highway shale embankments due to infiltrating water and poor drainage [9]. Czerewko and Cripps [10] have cautioned that mudrocks, to which shales belong, in general, can undergo troublesome breakdown when exposed to weathering processes or other changes in moisture content. Therefore, the use of such materials in areas subjected to wetting and/or wetting and drying cycles requires extreme caution.

Recently, Solomon et al. [1] initiated consideration of the Afram shale for embankment fill in the Afram Plains area through the construction of trial sections. So far, in the short-term, no adverse performance of the trial sections has been reported yet, but because mudrocks easily change in volume and competence when they come into contact with water, their reliability as a construction material is very low. According to Strohm [9], some rock-like shales when used as embankment rock fills may slake or soften into a weak soil upon wetting. This seems to suggest that even if the short-term performance of the trials as reported by Solomon et al. [1] provides the excitement and motivation for increased use of the shale in road construction in the Afram Plains area, it may be more prudent to proceed on the side of caution as the short-term performance of the embankments may not necessarily be a true reflection of the long-term performance. This study undertook laboratory evaluation of the Afram shale, particularly under wetting and drying cycles, to establish the material’s suitability or otherwise for road construction purposes.

2. Materials and Methods

Samples of the rock-like shale were subjected to the following tests in accordance with the test standards indicated (Table 1).

Strength tests were carried out for samples under wet and dry conditions. Wet conditions involved 1) soaking for a number of days before testing; and 2) subjecting samples to cycles of wetting and drying before testing. A wetting and drying cycle consisted of 24 hours soaking followed by 24 hours oven drying. Thus, samples subjected to wetting and drying cycles were tested in the dry after the process. Compaction was carried out in accordance with ASTM-D1557 [17] using modified effort to establish the optimum moisture content and dry density of the material. CBR tests on the compacted samples were carried out for soaked and un-soaked samples.

Table 1. Schedule of tests and standards

3. Results and Discussion

3.1. Compaction and CBR Characteristics

The compaction test returned a maximum dry density value of 1856kg/m³ and an optimum moisture content of 12.8% for the shale (see Figure 1).

Figure 1. Moisture-density relationship of Afram Shale

Figure 2 is the variation of CBR with period of soaking for the shale compacted at the optimum moisture content and maximum dry density.

Figure 2. Variation of CBR of Afram Shale with period of soaking

The unsoaked CBR was 35% whereas the standard four-day soaked sample value was 4% which represents only 11% of the dry sample value. This portrays a very low sample bearing strength under soaked conditions. Extended period of soaking beyond the standard four days led to only marginal decrease in value to a little less than 3%. In comparison, natural gravel samples used for road construction in the country are required by the MRH specifications to have soaked CBR values falling within the range 30%-80%, depending on the structural layer for which they are used. Compacted crushed rock materials are expected to have higher CBRs than natural gravels. Even though the shale is rock-like in character in the dry state, this character could not be maintained in the wet state. It is to be noted that the compaction process breaks down the depositional structure of the shale and effectively partially pulverizes it. The increased surface area of the partially-pulverised shale favours higher water intake which consequently causes dramatic loss in strength under soaked conditions. As will be seen later, the shale even in the coarse fraction form had a very high water absorption capacity.

3.2. Index Strength under Dry Conditions

The various index strength properties of the dry aggregate samples have been detailed in Table 2 alongside the specification requirements of the Ministry of Roads and Highways (MRH) [18] for road aggregates.

Table 2. Index Strength properties of dry shale samples

It is seen that except for water absorption, all the requirements were fully met by the shale under dry conditions.

3.3. Index Strength under Wet Conditions

3.3.1. Effect of Soaking

Figure 3 shows the effect of soaking period on the Los Angeles Abrasion Value (LAAV), Aggregate Impact Value (AIV) and Aggregate Crushing Value ACV). These index parameters are key to evaluating the suitability of coarse aggregates as road construction material.

Figure 3. Effect of soaking on index strength of Afram Shale

All three strength tests showed increasing measured value with increasing period of soaking, an indication of weakening strength. Averagely, all three indices appeared to show the same rate of change with soaking. Even though most shales are known to disintegrate (slake) completely when exposed or subjected to soaking, this did not happen. In most cases, however, degradation is generally augmented when the depositional structure of the material is disturbed by processes such as compaction. Since the samples were not subjected to such disturbances, no such effects manifested in the strength measurements.

In the case of the 10% Fines value, Figure 4 shows how the parameter was affected by soaking. There was a dramatic loss in strength to the extent that over a period of only five days, the strength reduced to about 17% of its initial value under dry conditions. This is a clear indication of the material’s lack of durability under wet conditions.

Figure 4. Effect of soaking on the 10% Fines Value of Afram Shale

3.3.2. Effect of Soaking and Drying Cycles

The response of the shale to soaking and drying cycles is demonstrated by Figure 5 which shows the variations in LAAV, AIV and ACV with soaking and drying cycles.

Figure 5. Effect of soaking and drying cycles on the index strength of Afram Shale

A close examination of the shale’s behavior trends in Figure 5 shows similarity to those in the soaking tests in Figure 4. It is also clear that each of the index strength was able to adequately portray the material behavior under the soaking and drying cycles fairly well. However, the soaking and drying cycles tended to result in higher strength degradation than the soaking only tests. The 10% Fines values measured at the end of the soaking and drying cycles (see Figure 6) were essentially the same as those obtained under soaking only conditions (Figure 4). This seems to suggest that the test is probably not well suited to fully demonstrating the adverse impact of the drying process on the strength of the shale.

Figure 6. Effect of soaking and drying cycles on the 10% Fines Value of Afram Shale

It is seen from both the soaking only and the soaking and drying tests that beyond a period of three days or three cycles, the shaly material failed completely in respect of the MRH requirements for all the four index strength parameters.

3.4. Water Absorption

The water absorption value of 18% characterising the material is nine times (900%) the upper limit of the MRH specified range of 0.1 to 2.0% for road aggregates. Because shales are composed predominantly of clay minerals, their water absorption capacity generally tends to be high, a fact well epitomized by the results in this study. This great potential for the Afram shale to absorb water may render the material extremely susceptible to changes in engineering properties in the presence of water. This will particularly be the case in situations where the material comes into contact with water for a very long time. Within the Afram Plains area, this would be a daily reality to contend with during the rainy season because of the poor drainage characteristics of the terrain. This calls for the exercise of extreme caution and engineering judgment when the material is to be used in engineering applications in the area.

4. Conclusion

This study investigated the strength properties of rock-like shale material from the Afram Plains under dry and wet conditions to establish the material’s suitability for road construction in the area. The investigation was motivated by the general lack of suitable geologic materials local to the area for road works which requires haulage from distant sources at enormous cost to construction projects. Soaking and drying were used to mimic the worst conditions under which pavement structures may come during the rainy season in the Afram Plains area due to the poor drainage characteristics of the terrain. Whereas the shaly rock in the unsoaked state appeared to meet all strength requirements of the MRH specifications for road aggregates, it failed completely under soaked as well as soaking and drying conditions over a short period. The high loss in strength under soaking and drying conditions and the potential to imbibe large amounts of water characteristic of the shaly rock are a strong indication of the material’s unsuitability as road construction material in the Afram Plains where the opportunity for the shaly material to come into contact with water for a long time is very high.

References

[1]   | Solomon, K. M., Odei, J. K. and Gawu, S. K. (2013). “Performance of shale as fill and embankment material for a trunk road in Ghana.” Proc. 18th Int. Conf. on Soil Mechanics and Foundation Engineering, Paris, 2013.
[2]   | Adu, S. V. and Mensah-Ansah, J. A. (1995). Soils of the Afram Basin, Ashanti and Eastern Regions of Ghana. S.R.I, C.S.I.R., Memoir #12, Kwadaso-Kumasi, Ghana.
[3]   | Grainger, P. (1984). The classification of mudrocks for engineering purposes, Quarterly Journal of Engineering Geology & Hydrogeology, 17(4): 381-387.
[4]   | Ingram, R. L. (1953). Fissility of Mudrocks, Geological Society of America Bulletin, 64(8): 869-878.
[5]   | Singh, T. N., Verma, A., Singh, V. and Sahu, A. (2004). Slake durability study of shaly rock and its predictions. Environmental Geology, 47(2): 246-253.
[6]   | Nwachukwu, M. A. and Osoro, E. (2013). Assessment of Environmental Hazards: Linking borrow pits, gully erosion and pavement failure. Journal of Civil Engineering and Construction Technology, 4(6):199-210.
[7]   | Gromko, G. J. (1975). “Preliminary investigation of the feasibility of spent oil shale as road construction material.” Transportation Research Record 549, pp. 47-54. Transportation Research Board, Washington, DC.
[8]   | Aghamelu, O. P. and Okogbue, C. O. (2011). “Geotechnical Assessment of Road Failures in the Abakaliki Area, Southeastern Nigeria.” International Journal of Civil & Environmental Engineering, 11(2): 12-24.
[9]   | Strohm, W. E. (1980). Design and Construction of shale embankments: Summary. FHWA-TS-80-219, US Department of Transportation. Available at http://www.vulcanhammer.net/geotechnical/FHWA-TS-80-219.pdf.
[10]   | Czerewko, M. A. and Cripps, J. C. (2001). Assessing the durability of mudrocks using the modified jar slake index test, Quarterly Journal of Engineering Geology and Hydrogeology, 34: 153-163.
[11]   | BS 812-110 (1990). Testing Aggregates. Method for determination of aggregate crushing value. British Standards Institution.
[12]   | BS 812-111 (1990). Testing Aggregates. Method for determination of Ten Percent Fines Value. British Standards Institution.
[13]   | BS 812-112 (1990). Testing Aggregates. Method for determination of Aggregate Impact Value. British Standards Institution.
[14]   | ASTM-C 131 (2006): Standard Test method for Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine. American Society for Testing and Materials.
[15]   | ASTM C127 (1993). Standard Method of Test for Specific Gravity and Water Absorption of Coarse Aggregate. American Society for Testing and Materials.
[16]   | ASTM D1557 (2012). Standard Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort. American Society for Testing and Materials.
[17]   | ASTM D1883 (2007). Standard Test Method for CBR (California Bearing Ratio) of Laboratory-Compacted Soil. American Society for Testing and Materials.
[18]   | Ministry of Roads and Highways (2007). Specifications for Roadworks and Bridges, Accra, Ghana.