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Technical Note (National Research Council of Canada. Division of Building Research), 1970-03-01
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Churchill Falls Dykes - Unstable Glacial Tills
Eden, W. J.
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DIVISION OF BUILDING RESEARCH
No.
NATIONAL RESEARCH COUNCIL OF CANADA
545
NOTlE
TEClHIN ][CAlL
-PREPARED BY W. J. Eden CHECKED BY L.W.G. APPROVED BY C.B.C. March 1970 PREPARED FOR Re stricted Circulation
SUBJECT CHURCHILL FALLS DYKES - UNSTABLE GLACIAL TILLS
On 17 December 1969, Messrs. Bhusari, Brunet and Martin of Miron Company LiInited visited the Geotechnical Laboratory of the Division of Building Research to discuss problems encountered in construction of the Sandgirt-Lobstick dykes at the Churchill Falls
project. The material used for dyke fill had proven, in some instances, to be extremely unstable when disturbed by construction activities.
Eleven samples of the material taken from three test holes in one borrow pit were displayed as being typical of the borrow material. These samples were taken from depths of from 5 to 20 ft. Topog-raphical maps of the work area indicated that the borrow pit areas occurred in drumlin-like hills which were a cormnon feature of the landscape.
SOIL TESTS
All eleven saznples were examined and found to be a sandy silt glacial till. Grain size tests showed that all eleven samples fell within the envelope of grain size curve s found for tills on the Quebec North Shore and Labrador Railway from M. P. 201 to M. P. 267 (1). Two grain size curves and the envelope are shown
2
-the "as received" condition, i. e. in bags with a plastic liner. Due to densification of the samples by vibrations during transport, the measured water contents are probably less than the in situ water contents. Free water was noted on some of the samples. The water contents are listed in Table 1.
To indicate the behaviour of the soils under compaction, four standard Proctor tests were conducted to determine the optimum water content under a standard compactive effort. The results are given in Table 1 and two moisture density curves are shown in Figure 2. The average optimum density was found to be 126 lb/ cu ft at a water content of 9.2 per cent using the standard Proctor test. It should be noted that this water content is less than that measured for most of the samples.
DISCUSSION
As indicated in Figure 1, similar glacial tills were encountered over a considerable length of the Quebec North Shore and Labrador Railway. These well-graded tills proved to be very unstable in the construction of the railway along the Ashuanipi Lake and River (M. P. 186 to M. P. 267). The tills existed in nature at very loose densities, and with the prevail-ing Labrador climate were fully saturated. Because they were essent-ially non-cohesive materials, the action of construction equipm.ent tended to densify the tills and in the process expell some of the water. As the tills were rather impermeable (coefficient of permeability in the order of O.1 feet/day) the water could not escape readily.
This process can be seen in Figure 2. Sample 2-20 had a measured water content of 11.5 per cent. Its optimum density in the Proctor test was 128. 91b/cu
it
at 8.5 per cent water content. Inorder for this sample to achieve its optimum Proctor density it must lose 3 per cent water content. Because it is relatively impermeable, the water will not drain quickly and the initial reaction to densification is a high pore water pressure leading to unstable behaviour. Sample 3-05, on the other hand, would be stable when densified from its natural water content to the Proctor optimum. It would behave in an unstable manner if an attempt was made to compact it to the Modified Proctor optimum density.
RECOMMENDATIONS
If the sample supplied on 17 December 1969 is considered representative, then glacial tills from some of the borrow pits must occur in a loose state at relatively high natural water contents. When
..
3
-this material is disturbed by construction equiptnent it tends to densify, but cannot because of high water contents and low perme-ability. This results in an unstable material during construction operations due to high pore water pressures.
It is thought that the best solution lies in the selection of borrow material from pits which have better natural drainage, even if this requires long hauling distances.
A less desirable alternative would be to open up the pit areas with large drainage ditches. Because of the low perme-ability of the till, this would have to be done up to a year in advance of use to be effective.
REFERENCE
(1) Woods, K.B., Pryer, R. W.J. and Eden, W.J. "Soil Engineering Problems on the Quebec North Shore and Labrador Railwayll. American Railway Engineer-ing Association Bulletin, Vol. 60, No. 549, pp. 669-688, Feb. 1959.
TABLE 1
RECORD OF SAMPLES RECEIVED AND TEST RESULTS
Test Water Content Optimum Maximum Density
Hole as Received: Water Content Stand Proctor Test
No. Depth Sample No.
%
%
1b/ cu ft1 5' 175-1-05 11.9 1 10' 175-1-10 8.4 1 15' 175-1-15
*
11.6 9.6 124.9 2 5' 175-2-05 7.0 2 10' -2-10 10.0 2 15' -2-15 9.1 2 20' -2-20*
11.6 8.3 128.8 3 51 175-3-05 8.3 9.0 126.5 3 10· -3-10 8.8 9.9 124.9 3 151 -3-15 10. 5 3 201 -3-20*
10.5e
e
e
MECHANICAL ANALYSIS OF' SOILS
EQUIVAI.ENT GRAIN DIAMETER (MM) NO. DF MESHES PER. IN. (U.S.S, SIEVE SERIES)
I
S 12E OF OPENING (IN:)0.001 40 30 20 16 10 Y1"
.
2.
•
Ｌｾ 0.01 200 100 60 4 I 4 100•
...
--
.
90.-
, 90"iO
.
.
Ｌ＼ｴ［［ｾ ｾ..
80 A ＢＧｾ ; "ｾｾ［ＮＮＧ...
80ＢＧＺｊｾＮｾｾ
_ ...-r,
,
-
ｾ...
c-.,
,
"
"'
6
70,
... J,
,
70""
ｾ 60 I ... 60 I I>- ENVELOPE OF GRAIN SIZE
III
a:5Q . CURVES FROM ONSLR TILLS 50
""
Z-
I lo. I 40 40...
f -Z DESCRIPTION OF SAMPL£ f -'""30 f-,
I--30ｾ AND GRAINS:
fo-""
f0-G. GRAIN SIZE CURVES OF ｾ
-
f-20
,
,
-
20SAMPLES FROM HOLE
-
-NO.3 -10
-
10 -l . - -ｾＮ I 0•
• I I,
" a,
1000 0.001 0.01 0.1 1.0 10 GRAIN SIZE ＨｍｍｾCLAY FINE SILT COARSE FINE
I
SAND COARSE Fl NEI
GRAVELI
COARSE M.I.T. GRAIN SIZE CLASSIFICATIONPROJECT: SAMPLE NO. SOIL MECHANICS LABORATORY
DIVISION OF BUILDING RESEARCH
PLOTTED; DATE: REMARKS: NATIONAL RESEARCH COUNCIL
CHECKED: DATE: OTTAWA CANADA
