Download 1 Contributed by - K.Raju and NJSumanth, North Eastern Region

Geological Survey of India
STUDY OF HARD SANDSTONE OCCURRENCES IN SOUTH MIZORAM FOR THEIR
SUITABILITY AS BUILDING STONES
Abstract
The state of Mizoram is not endowed with any major mineral deposit. Absence of industrial
minerals has hampered the industrial growth and progress of the state. Geologically the area is
comprised of repetitive sequence of argillaceous and arenaceous lithounits of deep sea flysch type
assemblages belonging to Barail and Surma Group (Middle Bhuban Formation of Bhuban Subgroup) of rocks of Upper Oligocene to Middle Miocene age. The regional trend of the rock types
varies from NNE-SSW to N-S, with moderate to steep dips on either side. The focus point of the
study was to spot out the hard sandstone occurrences in South Mizoram. A total of 15 zones of hard
sandstone of thickness up to 30m were identified in the study area. Geo-Technical studies such as
compressive strength, tensile strength, young’s modulus etc, of representative samples of such zones
have brought to light the quality and suitability of atleast three zones which are suitable for building
stone purpose.They are, a 10m thick zone of fine to medium grained, grey coloured, hard, massive
and compact sandstone located near Bualpui(H) village and a 5m thick grey coloured, fine grained,
hard massive and compact sandstone with occasional black streaks of iron minerals, located near
Cherhlun village in Lunglei district and a major occurrence of 30m thick very hard and massive, fine
to medium grained, light grey coloured sandstone located near Zawngling village, Saiha district.
Introduction
The study area is located in the interior parts of Lunglei and Saiha districts in the southern
parts of Mizoram state. In general the topography of the area is very youthful and is represented by
linear ridges running in almost N-S direction. It is a rugged mountainous terrain. The intervening
valleys also comprise gently sloping low level mounds. The area has very poor communication and
transport facilities, since it lies in the extreme interior near Mizoram – Myanmar border. The
mapping in parts of Lunglei district, and reconoitry traverses carried out in parts of Saiha district , in
South Mizoram (T.S. Nos 84E/4 & 84F/1) have brought to light the occurrences of hard sand-stone
bands belonging to Barail Group.
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Contributed by - K.Raju and N.J.Sumanth, North Eastern Region
Geological Survey of India
Geology
The Mizoram area belongs to Assam-Arakan geological province consisting of great flysch
facies of rocks. The rocks of the Lushai hills are the southern continuity of the rocks of the Cachar
hills, (La Touche, 1891). The mapped area is comprised of repetitive sequence of argillaceous and
arenaceous rocks of flysch type sediments belonging to Barail Group of Oligocene age and MiddleBhuban Formation of Bhuban Sub-group of Surma Group of rocks of Miocene age (Fig-1). The
Barails and the Surmas (Middle Bhuban) are brought into contact by N10o E – S10
o
W trending
major fault, recorded in the western part of the study area.
The area falls within Tripura-Mizoram miogeosynclinal basin (southern extension of Surma
Valley) which evolved after the regional uplift of Barail Group of sediments and thus was related
with the plate behaviour in the subduction zone west of Arakan-Yoma, after the spreading of Indian
Ocean. The regional structure is represented by a series of approximately N-S trending anticlines and
synclines, largely affecting the repetitive succession of Neogene arenaceous and argillaceous
sediments. The major structural trend coincides with the regional tectonic lineaments. Structurally
the rocks of the state lie on a highly compressed asymmetrical anticlinal ridge and narrow synclinal
valleys, with parallel to sub-parallel sub-vertical axial planes.
Hard sandstone occurrences
Special focus was given to the occurrence of hard sandstone zonations in the mapped area
(Plate-1). A total of 15 zones of very hard sandstone bands with thickness ranging from 1m to 10m,
have been located in the area. During reconnoitry traverse mapping in Zawngling area of Saiha
district, a very hard variety of light grey coloured , fine to medium grained sandstone of about 30 m
thickness has been located. Due to the concealed nature of the sandstone bands in to the steep slopes
of the mountains the length and breadth of the hard rock zone of workable quantity could not be
deciphered.
Table-1 shows the thickness, location and characteristics of the hard sandstone bands.
Sl.
No.
1
Rock
Type
Sandstone
2
Sandstone
2
Table-1
Description
Greenish grey, medium grained, hard,
massive, compact, with specks of iron
minerals.
Greenish grey fine to medium grained, hard,
compact with specks of iron minerals.
Thickness
(m)
5m
10m
Contributed by - K.Raju and N.J.Sumanth, North Eastern Region
Location
2.5km SSW of
Bualpui.
2.25km SSW
of Bualpui.
Geological Survey of India
3
Sandstone
4
Sandstone
5
Sandstone
6
Sandstone
7
Sandstone
8
Sandstone
9
Sandstone
10
Sandstone
11
Sandstone
12
Sandstone
13
Sandstone
14
Sandstone
15
Sandstone
Dark grey, very fine grained, hard, massive,
compact with conchoidal fractures, quartzitic.
Dark gray, fine grained, hard, massive and
compact.
Grey, fine to medium grained, hard, compact
with conchoidal fractures and shells of
lamlibranchs.
Dark grey, fine grained, hard, massive,
compact, ball and pillow structure, silica rich
with black iron specks.
Dark grey, fine grained, hard, massive and
compact with occasional black iron
specks(quartzitic)
Gray, fine grained, hard, massive and
compact with occasional black iron streaks
Light grey, fine to medium grained, hard,
compact.
Dark grey, fine grained, hard, massive and
compact.
Greyish coloured, medium-grained, hard with
slicken sides.
Dark greenish grey, fine to medium grained,
hard, massive, quartz rich.
Dark grey, fine to medium, quartz rich,
conchoidal fractures.
Dark grey, bluish ash colour, fine grained,
hard with lamellibranch shells
Grey, fine-medium grained, hard and
compact.
2m
10m
5m
3m
1m
5m
30m
1m
2.5m
5m
1m
4m
5m
4km SSW of
Bualpui.
400m SE of
Bualpui.
2.5km East of
Thingsai.
1.5km NNW
of
Thingsai.
(ThingsaiCherhlun road)
1km SSW of
Bualpui
2.5km SSW of
Cherhlun.
500m NNE of
Zawngling.
4.5km SSW of
Bualpui
2.9 km SSW of
Bualpui
7km NE of
Bualpui.
(western bank
of Tiova river)
6.5km NE of
Bualpui.
1 km SE of
Khawhri
1 km S of
Cherhlun.
Out of 15 zones six selective samples were studied for their geo-technical characters such as
specific gravity, absorption ratio, porosity, density, compressive strength, tensile strength, primary
wave velocity, secondary wave velocity, Poisson’s ratio and young’s modulus.
Table-2 shows the results of the geo -technical study of the samples.
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Contributed by - K.Raju and N.J.Sumanth, North Eastern Region
Geological Survey of India
Table-2
TABLE SHOWING THE GEO-TECHNICAL CHARACTERISTICS OF SANDSTONES
Sl.
No
1
2
3
4
5
6
Samp
le No
Location
GT–1 2.5 km
west of
Bualpui
GT–2 2.25 km
SSW of
Bualpui
GT–4 400 m SE
of Bualpui
GT–5 2.5 km SE
of
Thingsai
GT–8 2.5 km
SSW of
Cherhlun
GT–9 500 m
NNE of
Zawngling
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Results of Geo-Technical studies
Compress
Tensile
Primary
ive
Strength
wave velocity
strength
Kg/Cm2
Km/sec
Kg/Cm2
675
82
3.962
Sp.G
r
Absor
ption
%
Poros
ity%
Density
gm/cc
2.62
1.12
2.92
2.53
2.72
1.43
3.88
2.51
1052
58
2.62
0.94
2.47
2.57
734
2.55
1.31
3.33
2.50
2.53
1.86
4.70
2.59
1.15
2.99
Secondary
Poisson’s
wave velocity ratio
Km/sec
Young’s
Modulus
2.33
0.24
3.36
3.848
2.203
0.30
2.69
102
3.864
2.221
0.26
3.18
841
87
3.985
2.213
0.28
3.13
2.46
906
60
3.718
2.291
0.20
3.07
2.54
1173
81
3.512
2.037
0.25
2.63
Contributed by - K.Raju and N.J.Sumanth, North Eastern Region
Geological Survey of India
The specific gravity of a material is measured as the ratio of the rock to the weight
of an equal volume of water. The samples have given specific gravity values ranging
from 2.53 to 2.72.
Absorption value of a rock is the amount of water absorbed by the rock and is
expressed in percentage as compared to oven dry weight of the rock. In most of the poly
crystalline rocks, the moisture content is less than 1% and more than 10% in porous rocks
such as sandstones. In underground works the rocks are nearly saturated as a result, the
water drips, seeps leaks out from the rocks. Also, alternate wetting and drying causes
some rocks to expand and contract, thus affecting their properties inducing weathering.
The increased moisture content adversely affects electronic and electrical installation
equipment in the engineering structure. The samples from the study area have given
absorption values ranging from 0.94 % to 1.86 %, on the whole not more than 2 % . This
indicates the highly compact and indurated nature of the sandstones and also a slight
degree of metamorphism, which the rocks might have undergone.
Porosity can be defined as the ratio of volume of voids to the total volume of the
rock. Usually the porosity is expressed in percent of the total volume. Porosity depends
upon the grain size, shape and the kind of interlocking cement and matrix. The presence
of pores effect the strength properties negatively, if the pores are well connected the rock
is more porous and water or solutions can pass through the rock and its permeability is
high, which is an important parameter for determining the water tightness of foundation
grade rock. The six samples, studied for their geo–technical characters have given
porosity values ranging from 2.47 % to 4.70 % which implies the more compact nature of
the rock compared to the normal sandstone variety.
The bulk density values of the samples which range from 2.46 to 2.57 gm/cc
indicate they fall into the category of sandstone of very hard nature.
Compressive strength may be defined as an ultimate stress at which failure occurs
and the specimen cannot take any more load or pressure. It is always expressed per unit
area. The compressive strength depends on textural / structural elements which determine
the anisotropy of the material and ultimately the load bearing capacity of the rock. The
tensile strength is also a useful parameter in comparison with uniaxial compressive
strength. The samples have given values of compressive strength and tensile strength
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Contributed by - K.Raju and N.J.Sumanth, North Eastern Region
Geological Survey of India
ranging from 675 kg/cm2 to 1173 kg/cm2 and ranging from 58 kg/cm2 to 102 kg/cm2
respectively.
Conclusion
Out of a total 15 zones of sandstone occurrences three zones have been identified
as suitable for building construction work. They are:
1. 10m wide sandstone located at 2.25 km SSW of Bualpui (H) village in Lunglei
district. The rock is greenish grey in colour, fine to medium grained in nature, and
compact containing fine specs of iron minerals. The rock has specific gravity of
2.72 and density of 2.51 gm/cc. The value of compressive strength of the rock is
1052kg/cm2.
2. A 5m wide zone of grey coloured fine grained, hard, massive and compact
sandstone, located at 2.5 km SSW of Cherhlun village. The specific gravity and
density values of the rock are 2.53 and 2.46 gm/cc respectively. The compressive
strength of the rocks is 906 kg/cm2, and
3. A 30m wide light grey coloured fine to medium grained hard and compact
sandstone which appears to have under gone a slight degree of metamorphism.
This zone is located at 500m NNE of Zawngling Village in Saiha district. The
rock has got the specific gravity of 2.59 and density of 2.54 gm/cm. The
compressive strength of the rock is 1173kg/cm2.
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Contributed by - K.Raju and N.J.Sumanth, North Eastern Region
Geological Survey of India
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Contributed by - K.Raju and N.J.Sumanth, North Eastern Region
Geological Survey of India
Thick bedded sandstone
Loc: Lunglei-Lawngtlai road section
Chevron folds in thin bedded shale and sandstone
Loc:East of Mullalet Tlang
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Contributed by - K.Raju and N.J.Sumanth, North Eastern Region
Geological Survey of India
Linguoid ripple marks in sandstone
Loc: NE of Cherhlun
Symmetrical ripple marks in sandstone
Loc: NE of Cherhlun
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Contributed by - K.Raju and N.J.Sumanth, North Eastern Region