Cone morphology
A combination of remote sensing and field observation was used to derive simple geometric parameters for the sampled cones, maars and related features. These included base width and length (or diameter), height above ground level, side slope angle and diameter of summit (either with or without a crater). The remote sensing was based on Google Earth interpretation plus stereo analysis of black and white aerial photographs taken between 1965 and 1986 at a scale of 1:50,000. These aerial photographs pre-date almost all of the material extraction from the cinder cones and, therefore, record the original cone morphology. Comparison between these parameters and the field strength descriptions, simplified into
low,
moderate and
high, failed to yield any definitive relationships and so the various landforms were classified into four main groups, as listed in Table
2. Recorded field strengths were averaged for each of these four landform categories, giving the distributions shown in Table
2. The strongest material is generally found in maars (less vesicular and with greater lithic content) and steep-sided, well-defined cones, with or without a summit crater. The weakest materials appear to occur in pyroclastic ridges or flow features.
Table 2
Field strengths of cinder gravels according to landform type
Pyroclastic ridge or flow feature extending from cinder cone or volcanoa | 80% | 20% | 0% |
Dome – low-amplitude, mostly circular raised ground with shallow side slopesb | 50% | 25% | 25% |
Well-defined, steep-sided, cone, with or without crater | 11% | 48% | 41% |
Maar | 28.5% | 28.5% | 43% |
Comparison of field and laboratory strengths
The geological assessment of field strength, using a geological hammer, is a qualitative method, but is rapid and obviously does not require the collection of samples. For prospecting purposes it is of interest, therefore, to compare the method with the AIV test, which provides a quantitative assessment of particle impact strength. AIV was used in the comparison, and not CBR, as CBR measures bulk bearing strength and not particle strength. The range and average of geological field strength, AIV and bulk specific gravity (BSG) were determined for each of the cluster areas shown in Fig.
1, and comparisons are made in Table
3. The field strength was again summarised into
low,
moderate and
high, and the average for each cluster was determined. The variability in field strength for each cluster was also assessed and categorised into
low,
moderate and
high. From Table
3 there is less than a clear relationship between field strength and either AIV or BSG. There are several reasons for this. First, there is evidently high variability within tephra, and especially cinder gravel deposits, both within cones and between cones in the same cluster. Second, the field description of strength is to some extent influenced by judgement, despite the guidelines set out in BS5930. Third, the field description was based on an assessment of a range of material sizes, including especially medium and coarse gravel and cobble-sized clasts, while the AIV test focuses on a narrow size range of medium gravel. Given that the strength of smaller-sized clasts may be influenced by weathering to a greater extent than the larger material, this is a potentially significant source of error, and therefore future studies should also determine the field strength of the actual AIV sample in addition to that of the in situ material.
Table 3
Comparison between field strength, AIV and bulk specific gravity according to cluster area (Rank 1 = best; 9 = worst)
Butajira | 4 | Mod | 5 | Low | 1 | 4 | High | 1 | 37–39 | 38 | 5 | 1.85 | 1 |
Alemgena-Tuludimptu | 5 | Mod | 5 | High | 6 | 7 | 37–47 | 42 | 8 | 1.69 | 5 |
Tulubolo | 2 | Mod- High | 1 | Mod | 3 | 2 | 38–40 | 39 | 7 | 1.76 | 4 |
Hawassa | 3 | Low-Mod | 8 | Mod | 3 | 6 | Low-Mod | 9 | 44–62 | 51 | 9 | 1.50 | 9 |
Asasa | 4 | Mod-High | 1 | Mod | 3 | 2 | High | 1 | 25–54 | 37 | 4 | 1.60 | 8 |
Adama-Dera | 6 | Low-Mod | 8 | High | 6 | 9 | Low-High | 8 | 17–50 | 35 | 2 | 1.67 | 6 |
Bishoftu | 13 | Mod-High | 1 | High | 6 | 5 | Low-High, mostly High | 6 | 9–71 | 38 | 5 | 1.77 | 3 |
Bahir Dar | 6 | Mod | 5 | High | 6 | 7 | 23–47 | 32 | 1 | 1.63 | 7 |
Injibara | 8 | Mod-High | 1 | Low-Mod | 2 | 1 | High | 1 | 25–45 | 36 | 3 | 1.78 | 2 |
Adama-Metahara | 5 | Low-Mod | | Mod-High | | | Low-High | | | | | | |
Geographical distribution of cinder gravel strength
Materials with the highest average field strength and lowest variability offer the greatest potential for use in road construction, and samples from Injibara, Tulubolo and Asasa were optimum in this regard. In terms of maximum strength, samples from Injibara, Asasa and Butajira were among those ranked highest. The Bishoftu cluster also yielded material of moderate-high strength, but because of the large number of sampled locations, the variability was also high. For bulk specific gravity, Butajira, Injibara and Bishoftu gave the best results for the sampled materials. Samples from Bahir Dar, Adama Dera and Injibara had the lowest average AIV values, i.e. highest strengths. Injibara samples therefore appeared to contain consistently the better material. In contrast, samples from Hawassa ranked the lowest in terms of field-derived strength, AIV strength and bulk specific gravity. This geographical distribution of material strength appears to reflect the vesicularity of the cinder gravel: generally, the lower vesicular materials were found in the Injibara area and the higher in the Hawassa area.
Cinder gravel mineralogy and material strength
In addition to vesicularity, another possible explanation for geographical variation in cinder gravel strength is geochemical composition. Several publications describe the geochemistry of lavas in Ethiopia (e.g. Barberio et al.
1999; Boccaletti et al.
1999a,
b; Chernet and Hart
1999; WoldeGabriel et al.
1999; Ayalew et al.
2003; Abebe et al.
2007; Meshesha and Shinjo
2007), but in the present study no clear relationship could be found between these descriptions and observed geographical variation in cinder gravel strength.
X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses were undertaken in order to study the mineralogy and chemical composition of cinder gravel samples collected by the present study. Results were averaged for each of the geographical cluster areas and are shown in Table
4. The average alkali (Na
2O+ K
2O) versus silica (SiO
2) contents correspond to predominantly basaltic materials. Note that the results have not been normalised for volatile content or the oxidation state of iron and, therefore, a TAS (total alkali versus silica) Classification (e.g. Le Bas and Streckeisen
1991; Verma and Rivera-Gomez
2013) was not carried out.
Table 4
Average percentage major element and mineral content of cinder gravel samples for each geographical cluster (Fig.
1) from XRF and XRD analyses. Leucite, ilmenite, zeolite and magnetite contents were zero to trace, and are not shown)
XRF major element | Na2O | 2.92 | 2.27 | 2.25 | 2.86 | 3.22 | 3.04 | 3.26 | 2.44 | 2.43 |
MgO | 8.35 | 7.71 | 6.82 | 5.77 | 3.53 | 5.94 | 5.73 | 9.16 | 11.72 |
Al2O3 | 15.33 | 15.51 | 16.27 | 13.87 | 17.12 | 16.31 | 16.43 | 15.49 | 14.39 |
SiO2 | 46.96 | 47.00 | 45.64 | 44.58 | 48.62 | 49.21 | 48.91 | 45.50 | 44.20 |
P2O5 | 0.52 | 0.42 | 0.60 | 0.72 | 0.56 | 0.50 | 0.48 | 0.42 | 0.39 |
SO3 | 0.05 | 0.04 | 0.02 | 0.06 | 0.03 | 0.03 | 0.03 | 0.03 | 0.01 |
K2O | 1.05 | 0.82 | 0.82 | 0.70 | 1.27 | 1.08 | 1.45 | 1.24 | 1.19 |
CaO | 9.50 | 9.80 | 10.03 | 8.98 | 7.25 | 8.52 | 8.91 | 9.63 | 9.29 |
TiO2 | 2.40 | 1.86 | 2.14 | 4.29 | 2.43 | 1.74 | 1.94 | 1.71 | 1.54 |
Mn2O3 | 0.20 | 0.20 | 0.19 | 0.28 | 0.24 | 0.22 | 0.21 | 0.23 | 0.23 |
Fe2O3 | 11.96 | 11.62 | 11.93 | 16.77 | 12.23 | 11.33 | 11.11 | 11.58 | 12.09 |
BaO | 0.07 | 0.05 | 0.04 | 0.06 | 0.05 | 0.04 | 0.08 | 0.07 | 0.07 |
Loss on Ignition | 0.50 | 2.20 | 2.80 | 0.25 | 3.00 | 1.70 | 1.00 | 2.00 | 2.10 |
Na2O + K2O | 3.97 | 3.09 | 3.07 | 3.56 | 5.00 | 4.11 | 4.71 | 3.68 | 3.62 |
XRD mineral phase | Total claya | 0.00 | 7.50 | 1.50 | 0.00 | 1.67 | 0.20 | 0.67 | 1.60 | 0.33 |
Plagioclase | 27.67 | 24.50 | 30.50 | 39.00 | 37.00 | 36.00 | 44.33 | 25.40 | 21.67 |
Pyroxene | 12.33 | 13.50 | 25.00 | 16.00 | 9.33 | 8.20 | 15.67 | 28.80 | 29.33 |
Forsterite | 12.67 | 8.50 | 8.00 | 4.00 | 1.67 | 4.40 | 4.67 | 13.60 | 16.33 |
Haematite | 0.00 | 7.50 | 7.50 | 20.50 | 8.67 | 7.20 | 8.00 | 7.60 | 6.33 |
Goethite | 0.67 | 0.00 | 0.00 | 0.00 | 13.33 | 0.00 | 0.00 | 0.00 | 1.33 |
Nepheline | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 2.60 | 0.00 |
Amorphous | 44.00 | 38.50 | 27.50 | 18.50 | 41.33 | 43.80 | 27.00 | 20.20 | 24.67 |
Table
4 indicates that the geochemistry of the samples did not vary greatly between cluster areas, however there are some important differences. In the XRF data, these relate to the high MgO values for Injibara, Bahir Dar and Butajira, which correspond to their high forsterite contents (forsterite is the magnesium-rich end-member of the olivine mineral group), and the low Al
2O
3 and high Fe
2O
3 values for Hawassa. In the XRD data, important mineralogical differences include high and low values for plagioclase at Bishoftu and Injibara, respectively, high forsterite values at Butajira, Bahir Dar and Injibara (referred to above), high pyroxene values at Bahir Dar and Injibara, the very high value for haematite (Fe
2O
3) at Hawassa and the presence of nepheline at Bahir Dar. Nepheline is a felspathoid mineral typically found in silica-undersaturated igneous rocks. Compared with other areas, a high average goethite content was recorded in samples from Asasa. Goethite is an iron hydroxide derived from the weathering of other iron-rich minerals.
Ferromagnesian minerals (represented by pyroxene and forsterite) and calcium-rich varieties of plagioclase feldspar are particularly vulnerable to chemical weathering, while sodium-rich plagioclase is moderately susceptible. Samples with high contents of ferromagnesian minerals or plagioclase, such as those from Bahir Dar, Injibara and Bishoftu, may have been less affected by weathering and, therefore, potentially stronger. Samples with high haematite and goethite contents, such as those from Asasa and Hawassa, might represent more weathered materials that are potentially weaker. From Table
3, the Bahir Dar and Injibara materials were among the strongest in AIV tests, and those from Injibara were the strongest according to the geological field description. Hawassa samples were found to be the weakest in both AIV tests and field descriptions. Therefore, there is an apparent relationship between mineralogy, as an indicator of weathering state, and strength.
The Chemical Index of Alteration (CIA, Eq.
1, Nesbitt and Young
1982) measures the extent to which feldspar has been converted to aluminous weathering products, and is commonly used to characterise weathering profiles. CIA values typically range from 40 to 50 for weathered igneous rocks containing fresh feldspar to near 100 for kaolinite-rich residual soils. On this basis, the CIA values for the study samples, that ranged between 53 and 59, would indicate a low to moderate degree of chemical weathering.
$$ \mathrm{CIA}=\left[{\mathrm{Al}}_2{\mathrm{O}}_3/\left({\mathrm{Al}}_2{\mathrm{O}}_3+\mathrm{Cao}+{\mathrm{Na}}_2\mathrm{O}+{\mathrm{K}}_2\mathrm{O}\right)\right]\times 100 $$
(1)
The loss on ignition (LoI) provides an indication of the volatile content of a sample, principally clays (hydrous aluminium or magnesium silicates) and hydroxides (including goethite), together with carbon dioxide released from carbonates. Given the temperatures at which the cinder gravel was ejected (approximately 1000 °C) it is likely that all the weight loss is from secondary clay minerals produced by weathering and alteration. Fresh basalt has LoI values of about 1, while intensely weathered basalt has values of about 20. On this basis, the values in Table
4 suggest a low to moderate degree of weathering or alteration, consistent with the CIA values. The highest LoI value (3.00 for Asasa) is clearly related to the high goethite content. The reason for the next highest value (2.80 for Tulubolo) is unclear, but the relatively high CaO content could possibly indicate the presence of calcium carbonate as a secondary mineral. The lowest LoI (0.25) for Hawassa suggests that the high haematite content might be the product of late-stage volcanic activity (rather than chemical weathering) where the temperature-pressure conditions did not favour magnetite-ilmenite crystallisation.
The amorphous (non-crystalline) content of the samples was high, ranging from 18.5% (Hawassa) to 44% (Butajira), making any attempt at mineralogical analysis incomplete. Amorphous phases are not well understood, but in the present materials they probably consist mainly of volcanic glass together with varying amounts of secondary products, including hydrous iron oxides, colloidal silica and hydrous aluminium silicates (allophone). High volcanic glass contents reflect low crystallisation, resulting from effervescence (frothing) and sudden chilling of the magma. This may be the result of quenching during phreatomagmatic eruptions, but most of the sample locations are considered to have been subaerial. Moreover, quenching would have formed hydrated glass which probably would have resulted in higher LoI values than those found in this study.
The crystalline clay content of the cinder gravel samples was very low (approximately 1%), but samples from Alemgena-Tuludimptu had a higher content (7.5%). For blending purposes (see below), other samples were taken of material from the near-surface weathered horizon (upper 1-2 m) and these yielded combined illite, smectite and kaolinite clay contents of up to 30% (not represented in Table
4).
Regional variations in the degree of chemical weathering and alteration indicated by the geochemical data appear to both support and contradict the observed variability in average strength. This can probably be explained by the complex and variable sequence of emplacement and the post-formational processes that have determined the chemical and physical composition of these materials. The comparisons made using Tables
3 and
4 may, therefore, be petrologically and mechanically too simplistic and indeterminate, especially given the high amorphous contents recorded.
To conclude, XRF major element geochemistry does not appear to control the strength of the cinder gravel. The XRD data, however, may provide some insight into the weathering state of cinder gravel and, therefore, its likely strength and durability characteristics. The total clay, and in particular the expansive clay, content is of considerable significance to the engineering behaviour of the material and, from the limited data available, it would appear that material extracted from below the weathered horizon contains negligible expansive clay. However, blending operations, whereby the plasticity of the cinder materials is increased through the addition of fines, need to be careful not to introduce any expansive clays from the weathered horizon.
The 110 km Alemgena-Butajira road is the oldest road in Ethiopia known to possess a cinder gravel sub-base. Approximately half of the road length was constructed with a blended cinder gravel sub-base and the other half with conventional weathered basalt, i.e. very different materials. The entire road has a crushed basalt base course. The road was constructed 15 years ago as a low-volume road, but traffic volumes have increased substantially since then. Two sections were selected for performance measurement and sampling. Rutting was measured, a traffic classification count made, and sub-base samples were taken for laboratory testing. The rut depth for both sections was regarded as
fair (i.e. 5–15 mm) by Ethiopian Roads Authority standards. Moreover, the road carries an estimated 2.7 million ESAs of traffic in the heaviest loaded direction and has, therefore, become a relatively high-volume road. The blended cinder gravel has, therefore, performed very well as a low-cost sub-base material, despite it having a liquid limit of 50% and a PI of 26%, both of which are higher than the maximum recommended in the ERA (
2017) Low-Volume Roads Design Manual (45 and 12%, respectively). Instead of using the PI as the defining parameter for cinder gravel sub-base suitability, it is recommended that the plasticity modulus (PM) be used instead. PM is a measure of the quantity of plastic fines in a material, it is expressed as plasticity index multiplied by the percentage of particles passing the 425 μm sieve, whereas plasticity index (PI) is only a measure of the magnitude of the plasticity, i.e. material
stickiness.