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

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)

A slope failure in Pittsburgh caused by low-durability claystone

A slope failure in Ohio caused by low-durability claystone/mudstone

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

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.

Twenty different clay-bearing rocks were selected for the study including:

5 shales5 claystones5 mudstones5 siltstones

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

Slake Durability Test Apparatus

GSD of Slaked Material (Lab)Shale (2)

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Cycle 1

Cycle 2Cycle 3

Cycle 4Cycle 5

Claystone (1)

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Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5

Siltstone (3)

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Cycle 1

Cycle 2Cycle 3

Cycle 4Cycle 5

Shale

Claystone Siltstone

Mudstone

Samples after 5th cycle

Mudstone (3)

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Cycle 1

Cycle 2Cycle 3Cycle 4Cycle 5

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.

Samples Being Exposed to Natural Climatic Conditions

Picture: All samples on roof

Initial Samples

Shale

ClaystoneSiltstone

Mudstone

After 1 Month

Shale

Claystone Siltstone

Mudstone

After 3 Months

Shale

Claystone Siltstone

Mudstone

After 6 Months

Shale

Claystone Siltstone

Mudstone

After 9 Months

Shale

Claystone Siltstone

Mudstone

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

GSD of Slaked Material (Field)After 1, 3, 6, 9 Months

Claystone (1)

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Cycle 2

M1M3

M6M9

Mudstone (3)

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Cycle 2

M1M3

M6M9

Shale (3)

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Cycle 2

M1M3

M6M9

Siltstone (3)

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tCycle 2

M1M3

M6M9

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.

ASTM Description of Retained Material

Type I

Type II

Type III

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

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tShale (3)Claystone (5)Mudstone (3)Siltstone (3)

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

Disintegration Ratio vs. Slake Durability Index (2nd cycle) - Lab Results

y = 0.0001x2 - 0.004xR2 = 0.87

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Slake durability index (Id2)

Dis

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ratio

Disintegration Ratio (Field Samples) vs. Slake Durability Index (2nd cycle)

0.00.10.20.30.40.50.60.70.80.91.0

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Slake durability index (Id2)

Dis

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ratio

(3 m

onth

s)

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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