assessing the slaking behavior of clay-bearing rocks
TRANSCRIPT
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Assessing the Slaking Behavior of Clay-Bearing Rocks
Abdul ShakoorTej P. Gautam
10th Annual Technical ForumGEOHAZARDS IMPACTING TRANSPORTATION IN
THE APPALACHIAN REGIONColumbus, OHIO
Department of Geology, Kent State University
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CLAY-BEARING ROCKS
Because of their low durability, clay-bearing rocks result innumerous problems in engineering construction, especially slope stability
SHALES
CLAYSTONES
MUDSTONES
SILTSTONES
Comprise approximately two-thirds of the stratigraphic column
Cover one-third of the total land area
Deteriorate rapidly upon exposure to atmospheric processes (low
durability)
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A slope failure in Pittsburgh caused by low-durability claystone
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A slope failure in Ohio caused by low-durability claystone/mudstone
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Newly excavated rock mass
One year after excavation
FIELD BEHAVIOR OF CLAY-BEARING ROCKS
Because of extensive disintegration, it is hard to find an intact rock block
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RESEARCH OBJECTIVES
Assess the slaking behavior of clay-bearing rocks under natural atmospheric conditions and quantify the nature of slaked material in terms of grain size distribution.
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Twenty different clay-bearing rocks were selected for the study including:
5 shales5 claystones5 mudstones5 siltstones
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SAMPLE LOCATION AND CLASSIFICATION
Potter et al. (1980) classification was used to classify samples into shales, claystones, mudstones, and siltstones
Laboratory Tests:
Slake durability index (Id2, Id3, Id4, Id5)
Grain size distribution of the slaked material retained in 2 mm-mesh drum after the test
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Slake Durability Test Apparatus
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GSD of Slaked Material (Lab)Shale (2)
0
20
40
60
80
100
110100Particle size (mm)
Perc
ent r
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ned
by w
eigh
t
Cycle 1
Cycle 2Cycle 3
Cycle 4Cycle 5
Claystone (1)
0
20
40
60
80
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110100Particle size (mm)
Perc
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Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5
Siltstone (3)
0
20
40
60
80
100
110100 Particle size (mm)
Perc
ent r
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Cycle 1
Cycle 2Cycle 3
Cycle 4Cycle 5
Shale
Claystone Siltstone
Mudstone
Samples after 5th cycle
Mudstone (3)
0
20
40
60
80
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110100 Particle size (mm)
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Cycle 1
Cycle 2Cycle 3Cycle 4Cycle 5
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Simulation of Field Slaking Behavior Twelve replicate samples of each of twenty clay-
bearing rocks were prepared. Each sample consisted of 10 pieces, weighing 40-60 g, with a total weight of 450 to 550 g. All sample pieces were retained on 1-inch sieve.
Each replicate sample was placed in a 9-inch diameter pan and exposed to natural climatic conditions for 1 year period, from September 2009 to September 2010.
After each month, one sample of each of four rock types was removed and its grain size distribution was determined.
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Samples Being Exposed to Natural Climatic Conditions
Picture: All samples on roof
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Initial Samples
Shale
ClaystoneSiltstone
Mudstone
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After 1 Month
Shale
Claystone Siltstone
Mudstone
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After 3 Months
Shale
Claystone Siltstone
Mudstone
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After 6 Months
Shale
Claystone Siltstone
Mudstone
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After 9 Months
Shale
Claystone Siltstone
Mudstone
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Observations of Slaking BehaviorSample Month-1 Month-3 Month-6 Month-9Shale-3 Highly
fractured; very few pieces remained intact
Most pieces crumbled into smaller particles
All pieces crumbled into smaller particles
All pieces crumbled into small particles (2-6.3 mm)
Claystone-1
Highly fractured; hardly any intact pieces left
Most pieces crumbled into smaller particles
All pieces crumbled into smaller particles
All pieces crumbled into approx. 2mm-size particles
Mudstone-3
Mostly intact pieces; some fractures developed
Numerous fractures developed; slightly-highly fragmented
Most pieces crumbled into smaller particles
All pieces crumbled into smaller, nearly uniform particle size (2-6.3 mm)
Siltstone-3
All pieces remained intact
All pieces remained intact
All pieces remained intact; a few small fractures developed
Some fractures appeared but all pieces remained intact
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GSD of Slaked Material (Field)After 1, 3, 6, 9 Months
Claystone (1)
0
20
40
60
80
100
110100Particle size (mm)
Per
cent
reta
ined
by
wei
ght
Cycle 2
M1M3
M6M9
Mudstone (3)
0
20
40
60
80
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110100Particle size (mm)
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Cycle 2
M1M3
M6M9
Shale (3)
0
20
40
60
80
100
110100Particle size (mm)
Per
cent
reta
ined
by
wei
ght
Cycle 2
M1M3
M6M9
Siltstone (3)
0
20
40
60
80
100
110100Particle size (mm)
Perc
ent r
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ned
by w
eigh
tCycle 2
M1M3
M6M9
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Second cycle slake durability test overestimates the durability for claystones and underestimates the durability for siltstones and shales. For mudstones, it appears to provide a more representative value.
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ASTM Description of Retained Material
Type I
Type II
Type III
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In order to represent a wide range of disintegration behavior of clay-bearing rocks, a new parameter called “disintegration ratio” was used (Erguler and Shakoor, 2009)
Grain Size Distribution
0
20
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Perc
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tShale (3)Claystone (5)Mudstone (3)Siltstone (3)
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DISINTEGRATION RATIO
003.0)(
)()5( abcdAreabciAreaClaystoneD R
315.0)(
)()3( abcdAreabcgAreaMudstoneD R
926.0)()()3(
abcdAreafbceAreaSiltstoneD R
T
CR A
AD )( Ratiotion DisintegraAC = area under any grain size distribution curveAT = total area encompassing grain size distribution curves of all samples
DR = 1, Completely durable
DR = 0, Completely non-durable
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Disintegration Ratio vs. Slake Durability Index (2nd cycle) - Lab Results
y = 0.0001x2 - 0.004xR2 = 0.87
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Slake durability index (Id2)
Dis
inte
grat
ion
ratio
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Disintegration Ratio (Field Samples) vs. Slake Durability Index (2nd cycle)
0.00.10.20.30.40.50.60.70.80.91.0
0 20 40 60 80 100
Slake durability index (Id2)
Dis
inte
grat
ion
ratio
(3 m
onth
s)
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CONCLUSIONS Slake durability test does not predict the field
behavior of clay-bearing rocks.
During the 9-months period of exposure, claystone and mudshale completely disintegrated during the first 3 months, whereas siltstone was found to be the most durable. Mudstone exhibited an average disintegration behavior.
A wide range of disintegration behavior, as indicated by the particle size distribution of slaked material, can be described using the disintegration ratio.
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