Recent reports prepared by the Geological Research Authority of the Sudan
(GRAS) and other workers in Khartoum State showed that there is a shortage
in construction material (mainly natural coarse aggregates) due to the
rapid development in Khartoum State. Tackling this issue by focusing mainly
on searching for building material is the main objective of this study,
putting in consideration all the economic and quality factors. To render
this objective, characterization of the field occurrence, geology, geomorphology
and local environment were carried.
In field, the rock mass designation (RQD) was determined in accordance
with Hudson and Priest (1979) formula for economic factor and lack of
the rock core. Also joint set number was obtained. RQD and joint set number
for each rock mass unit were determined in many traverses and the average
The standard aggregates quality test were determined for samples collected
from the study area in accordance with appropriate standards as mentioned
later. The tests include Specific Gravity (SG), water absorption, Los
Angeles abrasion test value (LAA), Aggregate Impact Value (AIV), Aggregate
Crushing Value (ACV) and soundness test. For accuracy of tests results,
each test was carried three times and the average was recorded.
MATERIALS AND METHODS
The study area includes Khartoum area and Sabaloka region which is bounded
by latitudes 15° and 16° N and longitudes 32° and 3°30"
E (Fig. 1). Geologically, Delany (1955), Kheiralla (1966), Whiteman (1971),
Ahmed (1968, 1977), Rabba (1976) and Vail (1982) have studied the area
and mentioned many sequences. From these sequences the following rock
types which were very significant due to the aim of the study were studies
in details. These rock types are basalts, granites, ignimbrites, rhyolites,
syenites and sandstones which were shown in Table 1.
Closed field observation showed that Babados and Suleik granites reflect
great similarity and characterized by intact homogeneous rock mass with
two set of joints. The good-looking appearance of the large K-feldspar
crystals and the intactness behaviour provided a good condition for utilizing
this rock as ornamental slabs (Fig. 2, 3).
Part of Suliek granite was quarried as facing stone three decades ago
(Kheirelseed, 2002); from the appearance of the Fig. 3
some quarrying marks are obviously seen.
In Silietat Es-Sufur Complex, the fresh sample is very hard, massive,
slightly weathered, with two horizontal joints and easily excavated. The
trachy basalt occupies the basal part of the volcanic succession and extends
discontinuously along the eastern margin of the volcanic plateau with
thickness of few meters and four sets of joint and highly altered.
||Geological map of the study area (After Awad, 1994)
The lower and upper rhyolite block is controlled by two major set of
joints and random joints give rise to irregular block shapes of different
sizes. In micro-granite, spheroidal weathering is selectively affecting
numerous parts of the intrusion particularly at the upper section of the
rock masses. The alteration zone penetrates deeper into the rock mass
and associate with discoloration. The Tertiary basalt was found in three
quarries, the basaltic mass always demonstrates three set of joint filled
with clay and rock fragments and it was intensively quarried for crushed
aggregates (Fig. 4).
In this study, the rock masses were divided into different domains depending
on their weathering condition. Every joint set of the examined rock masses
was individually treated. Due to the absence of the rock core, the Rock
Quality Designation (RQD) values were estimated from the average frequency
of the discontinuities. Consequently, Hudson and Priest (1979) formula
for estimation of RQD was applied.
where, λ is the average discontinuity frequency per meters.
Usually the RQD is in percentage and high RQD indicates the intactness
of the rock mass and few discontinuities per length (Table 2). Due to
the significant of the joint set number (Jn) in rock masses,
for each rock type was calculated and presented as ratio with RQD, RQD/Jn.
This ratio gives the block size of the rock mass (Hoek and Brown, 1980).
The results were show in Table 2.
||Summary of the geological characteristics of lithological unit in
the study area
||Results of field descriptions and measurements of RQD and Jn
in the study area
|*The samples were collected from Sabaloka Region
||Large block of Babados foliated granite (Sabaloka region)
The degree of weathering was obtained as a descriptive term in the field
and categorized into five groups; non-weathered, slight weathered, moderately
weathered, highly weathered and earth like materials. Similarly, the texture
described in five groups; very coarse, coarse, medium, fine and glassy texture.
Intact large block of Suliek granite (the light marks depict the
ancient quarried mass more than three decades ago)
Toryia basalt with dirty appearance showing false severe weathering
which in fact only few mm deep
The experiments were done in accordance with different
standard methods. Specific gravity and water absorption test has been
carried for 14 rock samples (Table 3) to determine the bulk and apparent
specific gravity and absorption ratio (AASHTO Designation, T 85-88 (1990),
American Society for Testing and Material, 1997a).
Measurement of resistance of mineral aggregates of standard grade to
the action of abrasion and impactness were identified by Los Angeles abrasion
test (9 samples) based on AASHTO Designation 96-87 (1990). Aggregate Impact
Value (AIV) and Aggregate Crushed Values (ACV) were determined according
to British Standard 812 part 112-1990 and British Standard 812 part 110-1990,
respectively (Table 4).
Soundness test for determination of the resistance of aggregates to disintegration
under the action of saturated solution of sodium sulfate were carried
for 12 samples (American Society for Testing and Material, 1997b).
From the fieldwork and tests results which depicted in the above tables,
ratings of the parameters were derived (Table 5) and
used in accordance with desired required materials (Table
6, 7 and 8) in the discussion
In the field work results which are summarized in Table 2, from the texture,
the rock masses can be grouped into three: fine (6 samples), medium (5
sample) and coarse (3). As known the fine texture material, are having
higher strength than the coarse one and it is costly in excavation process.
Texture result and others results were weighed and summed to have complete
picture in the following paragraphs. Degree of weathering result shows
only one sample was highly weathered (trachy basalt) and the rest slight
The RQD results of the rock masses (Table 2) depict that most of the
samples attained high RQD values which correspond to excellent quality
(Barton et al., 1975). Only one rock mass is very poor which is
trachy basalt. This result is due to the numerous columnar joints in basalt.
The joint set number is ranging from few joints (1.5), to four joint
sets or more (15), (Barton et al., 1975). Generally, the studied
rock masses were between three to four joint sets from which we concluded
that some rock types can be excavated by loader without blasting. The
ratios of RQD/Jn is directly showing that the smallest block
size showed by trachy basalt and the largest size depicted by Babados
||Specific gravity and water absorption test results
The RQD/Jn parameter was utilized to give an abstraction about
the crude block sizes of the examined rock masses. The higher RQD/Jn
value, the more massive rock and can be used as slabs. Hence, it can be
taken as primary criterion for the quarrying method. In other words, the
small RQD/Jn is preferred for crushed stone.
Concerning the laboratory tests, the Specific Gravity (SG) is often considered
to be a good indicator of the strength or quality of an aggregate type.
Geotechnically, in pavement, SG of aggregates is between 2.6 to 2.7 up
to 2.9. From the results of the studied samples (Table 3), about five
rock masses can be used in pavement.
The water absorption values which are indirectly measure of porosity,
ranging from 0.68% (trachy basalt) to 6.90% (sandstone of Jebel Aulia).
Owning to this result, all samples of low water absorption can be used
as building stone in the rainy area whereas those with high value are
suitable for the arid zone.
Los Angeles Abrasion (LAA) test for the selected samples showed that the syenite
samples reflected a high percentage whereas the upper and lower rhyolites depicted
low value. LAA values are specified for subbase and base material in addition
to wearing surfaces. Low value of LAA indicates harder aggregates. It is internationally
accepted that the LAA should not exceed 30% in the engineering uses. In the
study all the rock types can be used with exception of syenite.
The Aggregate Impact Value (AIV) test measures the resistance of aggregate
to resist sudden impact. It was performed for the same samples that have
been subjected to LAA test. The higher value was revealed by syenite while
the lower value recorded by the trachy basalt. As fact, the lower AIV
value is more resisting sample. Thus, trachy basalt is the premium one
to be used in engineering works that require great impactness. It has
been recommended that AIV greater than 30% should not be used in concrete
mix. Regarding the tested samples, all are usable.
||Los Angeles abrasion, aggregate crushing value, aggregate impact
value and soundness tests results
|* The samples were collected from Sabaloka Region
||Ranges and rating of essential parameters that used in evaluation
of rock for crushed, dressed and ornamented stones
||Total rating scores of some rock types for crushed rocks
||Total rating scores for some rock types for dressed stone
||Total rating scores for some rock types for ornamented slabs
In Aggregate Crushing Value (ACV) test, the higher value was scored by syenite
and the lowest value obtained from trachy basalt. The result is equivalent
to that obtained by AIV test. Also all the samples could be utilized in
different uses. In addition, the selected samples can be used in all asphalt
and concrete works since the ACV values are less than 45% (Fattohi et
The soundness test of the tested samples showed that the grey gneiss
scored the highest value while syenite recorded the lowest value. Generally
speaking, most of the examined samples obtained low soundness value in
particular the sandstones, which indicate that no constituents reacted
with the solution. The rest of the samples more or less are resistant
to the disintegration.
From the above discussion, it is obvious that one parameter is not enough
for determining the quality of rock mass for certain engineering use.
According to this fact, the relative quality of the individual rock type
for specific purpose is determined by compilation of the available geological
and geotechnical parameters. The compilations were carried out by weighing
these parameters through a simple rating system. The rating procedure
is as following:
||Each parameter is individually treated where its range
value of the entire examined rock is determined primarily.
||The acquired total range is divided into five sub-ranges that initiated
from the lowest value and gradually increase with constant rate, which
is equal to about 20% of the total range.
||A weighed numerical value is given to each sub-range, which orderly
raised or reduced with constant rate.
||The total rate score being the sum of weighed value of the individual
||The higher the total rate scores the better the rock quality for
the desired usage.
The weighing results of the parameters for crushed and cut stones are
presented in Table 5. Rock masses desired for the purpose of the crushed
aggregates were weighed using four geological factors (RQD/Jn,
degree of weathering, water absorption and texture) and four geotechnical
parameters (ACV, AIV, LAA and soundness).The rating output (Table 6) showed
that the total score ranging between 10 and 26.The lowest score is registered
by Babados granite (foliated granite) and the highest rating is depicted
by trachy basalt, dark ignimbrite and Toryia basalt which are ranging
between 24 to 26 The rest of the examined rock masses more or less scored
similar value ranging between 19 up to 21. The abundance of dark ignimbrite
and lower rhyolite in the study area could be considered as alternative
future source of crushed aggregates Table 7 showed the parameters used
for classification of the rock masses for the purpose of dresses stones
in which three geological factors (RQD/JN, texture and degree of weathering)
and one geotechnical parameter (soundness) were used to achieve this goal.
The high ratings were recorded by four rock types including dark ignimbrite,
Babados granite, Toryia basalt and Syenite. Porphyritic micro-granite
and the sandstone of Jebel Aulia came in the second order.
Ornamented slabs were determined by totaling rate of five parameters
including RQD/Jn, soundness, texture, water absorption and
degree of weathering. Babados granite and syenite are the best rock quality
within the examined types. Also porphyritic micro-granite, Basalt (J.
Toryia) and the dark ignimbrite can be utilized when the best quality
consumed. The rest of the examined suites seem to be inappropriate as
ornamented slabs since they show a remarkable low score (Table 8).
With the exception of texture, the same parameters and factors used for
dressed stone and decorative slabs could be used in evaluation of the
rock masses as source of building materials. Babados granite, dark ignimbrite,
Toryia basalt and syenite are recommended to be used as building stone
for heavy construction since they reveal the highest score among the examined
suite. Lower rhyolite, light ignimbrite and sandstones are suitable for
light building. The trachy basalt is extremely unrecommended due to the
intensive jointing which resulted in small block sizes.
CONCLUSION AND RECOMMENDATION
This research depicts the methods by which artificial coarse aggregates
and cut stones could be assessed by using geological factors and geotechnical
parameters. The field and laboratory results which were weighed and compiled
together revealed the engineering performance of the different rock masses
in term of crushed aggregates, dressed stones, ornamented slabs and building
stone. The outputs show that the majority of the rock masses are ranging
from high to very high quality in term of rock quality designation.
The best rock types for crushed aggregates are the dark ignimbrite, Toryia
basalt, trachy basalt, lower rhyolite and syenite, respectively. Due to
the overuse of the near sources, syenite is preferable with reference
to the economic factors. In future, the great quantity of ignimbrite and
lower rhyolite could be considered as alternative sources of crushed aggregates.
Suitability of the rock masses as decorative slabs were shown by Babados
granite and syenite and to some extends by dark ignimbrite.
Babados granite, Toryia basalt and syenite are recommended to be used
as building stones for heavy constructions types. Also it was found that
all the rock masses of low water absorption ratio can be used as base
building materials in the rainy area. With exception of sandstones all
the rock types could be used in concrete mix since their crushing value
less than 45% and the impact value less than 30%. In addition, most of
the tested masses more or less resistant to disintegration, showing no
constituents reacted with the used solution in particular the sandstones.
For searching of building materials whether natural aggregates or rock
masses, attention seriously should be paid to the geology, paleoenvironment
and the sedimentological system of the area under study.
I would like to thank Dr. Salah for his great helping in performance
of this work and my colleagues in the field and laboratory works. Also
thanks due to the University of Dongola for the financial support of this