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Int. j. econ. env. geol. Vol:3(2), 1-4, 2012
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Estimation of Material Loss due to Chemical Weathering of Some
Rocks in Sri Lanka
U. de S. JAYAWARDENA
Department of Civil Engineering, Faculty of Engineering, University of Peradeniya,
Peradeniya, Sri Lanka
E-mail: [email protected], [email protected]
Abstract. The processes of weathering reduce the materials from the parent rocks. The material loss due to
weathering can be estimated by a fundamental method because the remaining residual materials have strong relationship
with their parent rock. The objective of this basic research is to find out the weight loss with the increase of chemical
weathering in Sri lankan rocks. This may be an important data to calculate sedimentation rates in the reservoirs.
Irregular samples from fresh rocks and differently weathered grades of each rock were collected from different
localities without disturbing the structures. Bulk density and the porosity of each sample were measured. The total
number of samples was about 300 from 73 locations. The results show the decrease of bulk density and the increase
of porosity and weathering. It indicates that some materials moving away and separate from the original place create
voids within the same volume during the processes of chemical weathering. The total loss of material is about 18 29 % by weight from the fresh rock stage to highly weathered stage. Increase of porosity goes up to 38 % in highly
weathered rocks.
The river sedimentation may be high in tropical countries
because rock weathering is most intense in the humid
tropical region. Sri lanka is a country in the tropical
region with high humidity. Therefore study on the rock
weathering, river sedimentation and landscape reduction
in Sri lanka may provide useful information to the
scientists and engineers in the country. There are very
few publications available on the study of rock weathering
in Sri lanka (Vitanage etal., 1983; Jayawardena and
Izawa, 1994 a&b; Jayawardena, 2000 and 2003). It is
the continuation of the same research which highlight
the physical condition of rocks during chemical weathering
(Jayawardena and Izawa, 1994a&b; Jayawardena, 2000
and 2003). The aim of this study is to find out the weight
loss of some major parent rocks with the increase of
chemical weathering in Sri lanka.
Introduction
The processes of weathering and erosion change the
physical condition and chemical composition of fresh
rocks. The broken materials from fresh rocks are carried
away as the sediments by the natural transporting agents.
Sediments which are transported by rivers are either from
chemical weathering or physical weathering of fresh
rocks. These sediments are deposited in the lower basins
or finally join with the sea sediments as a part of the
same geological process. Part of this is being used as a
construction material.
The process of weathering depends on the various local
factors within the tropical zone. Thus the rate of
weathering cannot be estimated within a short period.
But the material loss due to weathering (and erosion) can
be estimated by basic experiments because the remaining
residual materials have strong relationship with their
parent rocks. The ions separated after the chemical
reactions in rocks join with the surface water or ground
water. The total dissolved ions and total suspended solids
in surface water and ground water are the result of the
chemical weathering of rocks in the entire region or the
river basin. Change of the chemical composition of the
parent material due to chemical weathering forms
secondary minerals as the residual materials. It also
reduces the bulk density of parent rocks due to loss of
some ions. This is a part of the total process of destruction
of land. This material loss can be estimated by simple
basic methods because the separation of weathered
products from the parent materials changes their original
properties.
General geography and geology of Sri Lanka.
Physiographically Sri lanka consists of a central
mountainous mass or central highland surrounded by a
low, flat plain on all sides and extending to the sea
(Vitanage, 1970). Sri lanka is considered to have a humid
tropical climate. On the basis of rainfall, the dryness and
topography, Sri lanka can be divided into three climatic
zones namely the Dry Zone having rainfall less than
2000mm, the Intermediate Zone having rainfall between
2000mm to 3000mm and the Wet Zone having rainfall
above 3000m (Walker, 1962). Weathering features in
Wet and Dry Zones are different due to this climatic
difference.
Geologically 90% of Sri lanka is made up of high grade
metamorphic rocks of Precambrian age (Fig.1). The
remaining rocks are sedimentary rocks of predominantly
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ÓSEGMITE
U. de S. Jayawardena/Int. j. econ. env. geol. Vol:3(2) 1-4, 2012
Miocene age in the north-west (and very few places of
south east) with some Jurassic sediments preserved in
small faulted basins. There are recent sedimentary
formations, identified as Pleistocene deposits in a few
locations. Intruding the metamorphic rocks of Sri lanka
are some granites, dolerites, pegmatites, quartz veins and
a carbonatite (Cooray, 1967).
Materials and Methods
Engineering classification of weathered rocks (Fookes,
and Horswill, 1969), was used to recognize the different
weathering grades within the weathering profile of the
fresh rock. Different samples from fresh rock and its
each weathering grade were selected future study. Irregular
samples, similar to the same size, were collected from
the different grades of in–situ weathered materials
overlying the parent rocks. The samples represented their
grade of weathering according to the engineering
classification of weathered rocks. Except a few locations,
all samples were collected within the Wet zone of Sri
lanka where the rainfall is generally high. In Dry zone,
the weathering profiles are much thinner and therefore,
differentiation and recognition of different weathering
groups are difficult. About 300 samples were collected
from 73 locations in the country and all are from different
metamorphic rock types (Fig.1).
The land surface of Sri lanka has been subjected to a
prolonged period of weathering and erosion under different
climatic conditions. The secondary formations arising
from weathering such as laterite, ferricrete, calcrete are
found throughout the Island belonging to the younger
Pleistocene formations. Recent deposits include both
residual, alluvial and colluvial deposits. Residual deposits
include the deep weathered zones or soils to be found in
the central hill country and in the intermediate slopes.
These deposits are not uniform in character and contain
fragments of un-decomposed rocks (Herath, 1963a and
1963b). In some areas of the Wet zone, the thickness of
the weathered profile may go up to 50 meters. In general
the thickness may be between 10-15 meters. Residual
shallow weathered zones are mainly confined to the Dry
zone areas and the general thickness is less than 10 meters
(KDWSSP, 1981), The weathering is not uniform in any
place in the country and the thickness change drastically
from place to place.
Irregular fresh and weathered samples were cut from six
sides, using a laboratory rock-cutting machine and prepared
rectangular block specimens in different sizes. The opposite
faces of the blocks are parallel to each other. The block
sizes of different weathered grades were not equal but
the dimensions (length, width and height) are nearly 10
cm each. For weathered rocks, slightly weathered,
Simplified Geological Map of Sri lanka
Quaternary
Miocene
Jurassic
Precambrian
Fig. 1. General geological map of Sri lanka and the location of sampling points
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U. de S. Jayawardena/Int. j. econ. env. geol. Vol:3(2) 1-4, 2012
moderately weathered and highly weathered grade samples
were chosen for sample preparation. Here the samples
were cut very carefully and slowly, keeping parallel faces
without damaging to the softer materials. The porosity
and bulk density for all samples were determined according
to the standard method ISRM Committee on Laboratory
Test, Document No.2. (ISRM, 1972).
The actions of chemical weathering change the
mineralogy of fresh rock and create new secondary
minerals. As a result some elements of the fresh rock
may disappear and then increase the porosity while
absorbing water or moisture. Due to the reduction of
some ions the bulk density also decreases in the weathered
rocks. This process is continues while the degree of
weathering increases. The ions separated from the fresh
rock may join with water as dissolved ions or suspended
ions. Some minerals like quartz and heavy minerals such
as garnets, rutile, ilmenite, corundum, spinel are strongly
resistant minerals to weathering. These minerals separate
from the weathered rocks and join with the large rock
pieces (gravels, and boulders) and moving along the
river flow as the bed load. In addition clay like suspension
material also separate from weathered rocks and join
with water.
Results and Discussion
The reduction of bulk density of the fresh rocks with the
increase of weathering and the increase of porosity with
the increase of weathering of the same rock is given in
Table 1. Table 2 indicates the reduction of density from
18 to 29% and the increase of porosity from 15 to 38
times when a fresh rock becomes highly weathered due
to the reactions of chemical weathering. This may increase
further leading to completely weathered rocks and residual
soils. Fresh rocks occur at the bottom of the weathered
profile and other grades occur above it. Close to the
ground surface level the loose residual soil layer can be
seen. The removal of materials from these loose soils is
generally higher than the other layers. The materials
separated from the fresh rocks due to the weathering and
erosion join with water either as suspension materials or
dissolved materials. Materials separating from the original
rocks occur as bed loads.
Conclusion
The bulk density decreases and the porosity increases
with the increase of the degree of weathering. The bulk
density reduction may be about 18-29% at highly
weathered stage. Increase of porosity goes up to 38 times
than the fresh rocks in Highly Weathered rocks. However
rate of chemical weathering per year cannot be calculated
by this method.
Table 1. The variation of bulk density and porosity of rocks with the increase of chemical weathering.
Rock type
Weathering
grade **
Charnockite
(38 locations)
Fresh rock
SW
MW
HW
Fresh rock
SW
MW
HW
Fresh rock
SW
MW
HW
Fresh rock
SW
MW
HW
Fresh rock
SW
MW
HW
Fresh rock
SW
Fresh rock
SW
MW
Garnet
sillimanite
gneiss
(7 locations)
Hornblende
biotite gneiss (9
locations)
Biotite gneiss
(10 locations)
Migtmatite
(2 locations)
Quartzite
( 3 locations)
Marble
( 4 locations)
Bulk density
3
Kg/m
Maximum
2981
2905
2510
2435
3031
2875
2450
2280
2950
2820
2525
2280
2801
2695
2470
2290
2810
2750
2520
2045
2632
2596
2832
2752
2370
Porosity %
Minimum
2627
2605
2231
2080
2718
2655
2420
1600
2740
2596
2245
2135
2620
2520
2332
2160
2795
2710
2495
2035
2538
2502
2795
2579
2313
Maximum
0.98
2.30
7.90
11.94
0.86
4.20
10.70
26.40
0.45
1.10
6.20
12.80
0.54
3.30
7.50
16.15
0.63
1.50
3.80
12.50
0.49
2.45
0.42
4.6
8.4
Minimum
0.23
0.65
2.32
5.34
0.34
1.40
5.70
11.80
0.32
0.63
3.50
10.20
0.23
0.95
3.50
9.60
0.42
1.20
4.30
13.80
0.35
2.38
0.3
3.5
8.13
SW = Slightly weathered; MW = Moderately weathered; HW = Highly weathered; ** = Weathering grade
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U. de S. Jayawardena/Int. j. econ. env. geol. Vol:3(2) 1-4, 2012
Table 2. Loss of bulk density as percentage and increase of porosity of rocks with the increase of chemical weathering
Rock type
Charnockite
(38 locations)
Garnet
sillimanite
gneiss
(7 locations)
Hornblende
biotite gneiss
(9 locations)
Biotite gneiss
(10
locations)
Migtmatite
(2 locations)
Quartzite
(3 locations)
Marble
(4 locations)
Weathering
Grade
**
FR
SW
MW
HW
FR
SW
MW
HW
FR
SW
MW
HW
FR
SW
MW
HW
FR
SW
MW
HW
FR
SW
FR
SW
MW
Bulk density
Kg/m3
Average
2804
2722
2404
2227
2858
2755
2516
2027
2819
2703
2304
2156
2705
2632
2396
2203
2802
2730
2507
2040
2590
2561
2822
2690
2354
Porosity %
Reduction
00
82
400
577
00
103
342
831
00
116
515
663
00
73
309
502
00
72
295
762
00
29
00
132
468
Loss %
00
2.9
14.2
20.4
00
3.6
11.97
29.07
00
4.11
18.26
23.52
00
2.7
11.42
18.56
00
2.6
10.53
27.2
00
1.12
00
4.67
16.58
Average
0.55
1.81
4.72
8.25
0.53
2.65
7.09
20.30
0.37
1.12
4.76
10.94
0.39
1.50
5.25
12.15
0.52
1.35
4.05
13.15
0.43
2.40
0.33
3.92
8.11
Increase
0
3 times
8 times
15 times
0
5 times
13 times
38 times
0
3 times
12 times
29 times
0
3 times
13 times
31 times
0
2 times
7 times
25 times
0
5 times
0
11 times
24 times
International Association of Engineering Geology,
No. 49, (April), p56-61.
Jayawardena, U.de S. (2000) Geochemistry and
engineering properties of weathered metamorphic
rocks in Sri lanka, DEng Thesis, p231, Kyushu
University, Japan,
Jayawardena, U.de S. (2003) Use of the correlation
between ultrasonic wave velocity and point load
strength of weathered rocks - a new method for site
investigation. Journal of The Geological Society of
Sri Lanka, 11, 107-114.
Kandy District Water Supply and Sanitation Project
(KDWSSP) (1981) Kandy District Water Resources
Potential Study, Interim Report, p90, Plancenter Ltd
Publication, Kandy, Sri Lanka.
Vitanage, P.W. (1970) A study of the geomorphology and
morphotectonics of Ceylon, In: Proceedings of the
Second Seminar on Geological Prospecting Methods
and Techniques, United Nations, p.391-405, New
York, USA.
Vitanage, P.W; Hatva, T; Lumiaho, K. (1983) Some
aspects of weathering in Sri lanka, Rapautuminen
Kallioperassa Symposium, IX/ (1) 23, Nov.9, p123.
Walker, R.C. (1962) The Hydrometeorology of Ceylon,
(Part 1), Canada Colombo Plan Project, Government
Press, Ceylon.
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