Some Engineering Properties of Muskeg

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Some Engineering Properties of Muskeg

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NATIONAL RESEARCH COUNCIL

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CANADA

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SOME ENGINEERING PROPERTIES OF MUSKEG by 6 . B. Crawford Report No. 47 of t h e D i v i s i o n of B u i l d i n g Research O t t a w a October 1954

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Muskeg is so important a part of the terrain of

Canada, especially in northern ~egions, that the Division

of Building Research has long looked forward to initiating a long term study of its mechanical propesties. The

National Research Council As~ociate Committee on Soil and

Snow Mechanics has already stimulated interest in the subject and supported the work of Dr. N.W. Radforth (of McMaster University) in a botanical study ~f muskeg.

This short report records two sets of teat results on samples kindly supplied to the Division through the good

off ices of Mr. W .G. Manning, Chief of Aids to Navigation

Division, Department of Transport, and Col. E.H. Webb, then

Director of Engineer Development, Department of Nati~nal Defence.

The author of the report is the head ~f the Division of Building Research Soil Mechanics Section who directed the tests described. He joins with the writer in wishing to emphasize that the report is a very preliminary document but it does present these early test results and points the way to the more extensive Division of Building Research program which is to start in the fall of 1954.

Ottawa

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SOME ENGINEERING PROPERTIES OF MUSKEG by C. B. Crawford

A preliminary search of the engineering literature has shown that very few scientific papers are available which describe the physical properties of muskeg. As a matter of fact, there are still a numbes of conflicting opinions on the

definition of the term "muskeg". In recent years Dr, N.W.

Radforth, working through grants of the National Research

Councilts Associate Committee on Soil and Snow Mechanics has

made an extensive study of the many various classes of muskeg. As a result of this work he has published a suggested classifi- cation of muskeg for the engineer (1) and he has suggested methods of recognizing northern organic terrain characteristics from

plant material types (2). With the exception of a publication

from Ireland (36, no reports were found which gave values for

the physical characteristics of muskeg which could be used in an engineering analysis of construction problems on this type of material.

In preparation for an extensive investigation of the properties of muskeg by the Division of Building Research, some preliminary tests were conducted, the results of which are

reported here. These tests were made possible through the

co-operation of the agencies mentioned in relation to the samples. The first samples of muskeg received by the Division of Building Research were obtained at McInnes Island on the coast of British Columbia at the entrance to Millbank Sound, The samples were taken at the site of a proposed structure for the Aids to Navigation Division of the Department of Transport and were shipped to Ottawa in sealed 5-gallon cans, Samples

were taken at the surface, at a depth of 2 feet and at 4 feet in

an area that was, in general, covered by approximately 8 feet of

overburden. At the time of sampling, the water table was at a

depth of 2 feet. Growth at the site varied from bushes and shrubs

to 50-foot trees.

There Had been no sign of moisture loss from the cans of disturbed samples when they arrived in Ottawa, Water contents were det,ermined on samples which weighed in excess of 1,000 grams in the wet state. Teat results are shown in Table 1. Twenty-four

hours of drying at 1 0 5 ~ ~ ~ removed only about one-half of the free

water. Curves in Fig, 1 show that about 60 _{hours of drying in the}

normal laboratory oven is required to remove all free water from these large samples. Burning tests indicated that the material was

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In order to assess this material approximately for construction purposes, two consolidation tests were performed on the sample from the 2-foot depth. Since the material was

disturbed, it was placed in the consolidometer (Fig, 2) in

three *-inch layers and each layer was compacted under a static

load of

73

pounds (equal to

70

p.s.f.). This load was chosen

arbitrarily to obtain a sample density approximately equal to field density, Since field density datawere not available, no accurate comparison is possible. Properties of the test samples

are shown in Table 2.

A typical time compression curve for the material is

shown in Fig.

3.

It will be noticed that, unlike soil, movement

is continuing at an appreciable rate after three days of load application. This indicates that the time required in the field for com lete consolidation of fills over muskeg must be appreciable.

Figure f4 shows the relationship between pressure and compression

for these two tests, Each point on these curves represents the amount of compression after 1,000 minutes of load application.

A second set of samples was obtained through the

co-operation of the Directorate of Engineer Development of the

Canadian Army, from Fort Churchill, Manitoba,and North Bay, Ontario. These samples were sent to the laboratory in their natural frozen condition and it was therefore possible to trim samples for tests in the undisturbed state. Two consolidation tests were performed. In one test the sample was trimmed while frozen, placed in the consolidometer and allowed to thaw before loading. In the second test some of the frozen material was thawed, mixed and inserted in the consolidometer by the same standard method which was used on the previous samples, The rebationshir between compression and

pressure for the two samples is shown in Fig.

5,

The average of

the straight portion of the curves in Fig. 4 and Fig,

5

are shown

in Fig.

6.

This Figure illustrateb preliminary results on the

effect of dry density on compression characteristics.

In addition to the compression tests, several determinations of density, water content and unconfined compressive strength of

undisturbed samples of muskeg were made. These results are shown in

Table

3.

Some preliminary tests were also made on the compressive

strength of large frozen samples. These results are shown in

Table 4. Sample 47 was from Fort Churchill, Manitoba,and Sample

48

was from North Bay, Ontario. These preliminary tests indicated that the rate of strain during testing had a considerable influence on the strength of the material. Where rates of strain were very slow, it was found that the ends of the sample melted and deformed without much increase in strength.

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TABLE 1

Water Content of Muskeg Samples fnom McInnes Island. B.C.

Water Content

k

TABLE 2

Properties of Consolidation Samples from McInnes Island, B.C.

Density

Water in pounhs

Test Wt. of Consolidation Sample Content per cu. ft.

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TABLE 4

Unconfined Compression S t r e n g t h T e s t s of Large Frozen Muskeg Samples Approximate S i z e 7.5

-

8.0" high 3.4

-

3.8" square Sample No * Unconfined Compressive S t r e n g t h a t 10$ S t r a i n ( p a s o i * ) Rate of S t r a i n ( i n c h e s p e r m i n o )

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Conc 1-usions

These Pew tE3ts indicate that the compression charac--

teristics of muskeg depend more on th? dry density of the

material than on whether or not thn sample is disturbed. The

curves of Fig. 4 show thst 2 load _{~ i ?}200 p . s . S . would cause a

seduction in v o l m e of the muskeg to nearly 2 5 _{per cent; a load}

of 1,000 p. s .f

.

would cause about 45 per cent redilct i nn in

volume. For this reason it appears to be quite unsatisfactory

as a foundation material for a structure. Even as the foundation

for a roadway, a settlemznt of two or three feet could be expected after placing a 5-foot fill..

In this type of material the consolidation test seems

to be adequate for esTimatl.ng the amount of settlement that could

be expected in the field. The use of disturbed samples does not appear to ha.ve disadvantages as serious as the use of disturbec! soil samples, since the structure of the material is not so important. It is 3ecessary, however, to compare field and test

densities. It is reasonable to assume that the curves of Fig.

6

would apply generally to muskeg with a dry density of the same order as the samples tested.

The preliminary test results shown in Table 4 indicate

that rate of strai? is a most important consideration in testing

frozen samples. Experience with these tests suggests that it would be most desirable to carry out strength tests on frozen

samples in a refrigerated atmosphere.

It is hoped that these preliminary test results will assist in the planning of a more complete study of this important engineering rnat.eriai.

References

1. Radforth, N . W . A Suggested Clzssification of Muskeg for the

Engineer. Tec.hnica1 Memorandum No. 24, Associate

Committee 3 3 Soil and Snow Mechanics, National Research

Council, Ottawa.

2. Radforth, N .W. mhe TJse of Plant Material in the Recognition

of Northerr. Organic Terrain Characteristics. Technical Memorandum No. 28, Associate Committee on Soil and Snow Mechanics, National Research Council, Ottawa.

3.

Hanrahan, E.'I'. The Mechanical Properties of Peat with

Special Reference to Road Construction. Transactions of

the Institute of Civil Engineers of Ireland, No. 5, Vol.

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- O ! I P R E S S U R E I N T O N S P E R SQ. F T . L E G E N 3 0 _{L I N D I S T U R B E D ,} x D I S T U R B E D , F I G U R E 5 7 D R Y . 10.2 P C F 7' D R Y = 10.2 P C F C U R V E S S H O W I N G T H E R E L A T I O N S H I P B E T W E E N P R E S S U R E A N D C O M P R E S S I O N O F M U S K E G F R O M F T . C H U R C H I L L S A M P L E NO. 4 7

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