The Nicolet slide

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The Nicolet slide

Ref

Ser

TA701

N28t2

no.

48

BLDG

NATIONAL RESEARCH COUNCIL

CANADA

IRe PUB

\TE COMMITTEE ON SOIL AND SNOW MECHANICS

THE NICOLET SLIDE

by

J.

E.

HURTVHISE AXD

P. A.

ROCHETTE

AN

A.L YIED

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1

1957

NATIONAL R"CE • , _...I Ar",CH ｃｏｕｊ｜ＺｃｾｊＧ Reprinted from

Proceedings of the Canadian Good Roads Association 1956

Technical Memorandum No. 48

OTTAWA May. 1957

The Associate Committee on Soil and Snow Mechanics is one of twenty-five such committees appointed by the National Re-search Council. It consists of about twenty individuals appointed for three-year terms. Its function is to stimulate research work in the fields of soil and snow mechanics and to

promote the use of the results of research 111

these and associated fields throughout Canada. The Committee has available funds for re-search grants. All inquiries should be address-ed to the Secretary, Associate Committee on Soil and Snow Mechanics, c/o Division of Building Research, National Research Council, Ottawa, Ontario, Canada.

The Nicolet Slide

By JACQUES E. HURTUBISE, P.Eng.,

Prolcssar of Civil Engineering,

and

P.

A.

ROCHETTE,

Assistant Professor,

Ecole Polytechnique - Montreal

Eastern Canada has been the scene

of periodical landslides. "'\umerous scars, some of them of very large proportions,

can be distinguished on aerial

photo-graphs. Early historical records mention that settlements had to be moved awav

from the shores of Lake Sr. Jean

0;'

account of such landslides. ;\los't of the recent ones have been reported but only

a few have been accurately described

and mention is seldom m;de of the

properties of the soils involved. The geographical distribution of the principal recorded landslides in the Sr.

Lawrence Valley and a map of

sur-face deposits are shown on figures 1 and 2, All of them took place in the lowlands of the valley and in the Lake Sr. Jean area. Along the river, the deposits are [rom the glacial-marine envirorunent fol-lowing the last retreat (north-east

south-west) of the glaciers while, at J .ake Sr.

Jean, the sediments are said to be chiefly

of lacustrine origin without "Led'a

glacialis" fossils.

The investigation of the Nicolet catas-trophe forms part of a general investi-gation of landslides which was initiated a few years ago by the study of the

Des-biens and Rimouski slides, In order to

further this work, a grant was requested and obtained by l'Ecole Polvtechniquc from the :"ational Research Council

(As-sociate Committee on Soil and Snow

;\lechanies-Subcommittee on Canadian Landslides) .

After arrompring to arrive at some

form of correlation bv the study of

aerial photos, the geo'graphical

(Iistri-but ion of slides, the irregularities of a

geological nature, it appeared that a

bettor understanding of this problem

would be attained hv soil investigations

in situ of a number ｾｦ slides. L

When the :" icolet slide occurred,

attention was narurallv concentrated on

that region. Borings' with continuous

sampling and vane tests ill suu were

therefore made last summer to

supple-ment previous studies by engineers and

geologists. It is the intent of this

luper to present a summ'Hy of sonic of

the data collected so far 011 this

pro-blem: \Vhat was the process of sliding and what are the properties of the sub-soil?

The Site

On "'\m'ember 12, ]Y55, at 11.40 a.m.,

a slide occurred at ｾｩ｣ｯｬ･ｴ near the

highway bridge, as illustrated by the

aerial photo taken the following day

(figure No.3). :\ crater of 20 to 30 ft.

in depth, 400 fro in width and 600 fro in length was formed in less than seven minutes with a loss of three lives and

millions of dollars. Fortunarclv, the

school children had left the ａｾ｡､･ｭｩ･

Commcrcialc 40 minutes before and

were due back one hour after the dis-aster.

Fig. 1 and 2

Sketches of the critical. district before

'111d after the slide are ginn in figures '" os. of and 5.

The study of aerial phoros, the con-tours, lines und the view from the

op-posite shore indicate that the area

which flowed was a mound which has

been left standing up by itself by

ancient slides on each side and which eventually, on a geological time scale, was bound to level. The hridge aburrucnt seems to have prevented a lateral spread-ing of the slide.

Information gathered on conditions

previous to the slide reveals that the

; ratural flow of surface water was

eon-ccntrared in an upper sand layer in a general direction towards the bridge and

tint the main sewer (dotted line on the

figures 4 and 5) was inadequate. In the summer preceding the landslide, during

and shortlv after a torrential rain, it

discharged' an abnormally large quantity

of water and suddenly stopped

func-tioning.

In 19-+6 and 1950, small scale slides occurred in this very location and left a berm at half-height of the hank. Re-pair works were then undertaken in the form of a partial fill which was being completed prior to the slide, in order

to flatten the slopes and in an attempt

to protect the toe against the

enlarge-mcnr of a fissure.

From reports of eye witnesses, tire

earth flow can he described as

rerro-grcssive. A first slice was observed to fail on a front of 30 fr., carrying out

Fig. 3

directly ahead of it part of the fill and

the garage (see figure 5). The second

side enlarged the breach on the sourh

side and the clay material was forced

to rotate dO\\<nstream by the fill

pre-viously abutted against the shallows

of the river; it pushed the garage ahead uf it and carried the bulldozer along. A fter the failure of these two slides which took place rclative lv slowly, the movement accelerated with slices falling one after the other in rapid succession.

The school crashed down and its

materials pushed the garage and

bull-dozer further downstream, dispersed the

trees, stopped under the bridge and

caught fire. Sccondary movements took place inside the crater until equilibrium

of the slopes had been re-esta blished.

One house remained practically intact

in its slid while another une was badly damaged.

The sketch in figure 6 illustrates rhc

deepening of the crater around the

bridge abutment (owing to the

rotation-al movcrncnt of the mass of soil around it). the position of the sewer and the

assumed bed of an old brook in the

upper sand layer; the rclativc thicknesses

of clay removed and rcmouldcd clay

arc als;' shown. .

Geological Conditions

The subsoil consists of a thick

de-posit of stratified clay and silt size

material underlaid by a fairly

homoge-neous stratum; these were 'formedCin

the estuary of the Champlain sea after

the retreat of the glaciers and their

properties vary from cplace to pia cc ac-cording to irregularities of the bed and inflowCof fresh \v8ter.

The recent isostatic uplift of these

deposits above sea level has been

fol-lowed by active erosion in the uncon-solidated sediments. This geoLogical

his-tory explains the numerous landslides

as a normal cyclic process of erosion.

The increasing recessiun and flarrcning

of the banks ｾ｜Ｇｩｴｨ each of the slides of

---

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LONGI TL/LJINAL S,sC7/0NA-A

ＨｄｉｓｲｯＢＧＷｾｌ＾ .sc-'JLE) 10 PLAN VI€w aeeO&E 54IDE

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Fig. 4

2('

Fig. 5

PLAN OFｾｌｉｏｅＺＮ ｎｏｖｅｍｅｕＡｾ IZ I!?JSS

5

7. In addition to signs of ancient slides,

many recent superficial scars can be

observed within a few miles in the

Nicolet region.

Such a concentration of slides of all sizes is not uncommon and it is believed that any study of a landslide should not

be limited to the search of immediate

so-called causes or sparks, as the

in-herent weakness of the soil amplifies

the relative importance of exterior cir-cumstances. It is believed that the sta-hility uf these clays decreases with time at a geological scale, the general trend

being accelerated or retarded by

ac-cidental phenomena until a factor of

safety of one is reached (see figure 8). Soil Properties

A continuous sampling with the

Swedish soil sampler was made in two

boreholes, outside (borehole No. 1) and

inside (borehole No.2) of the crater.

Results of water content, liquid and

plastic limits determinations are given in figure 9. It is seen that, underneath the sand, samples have revealed three dif-ferent types of clay. In all three strata, the water content is substantially higher than the liquid limit and indicates a high

sensitivity to remoulding, while the

plasticity decreases from one stratum tu

the other. The visual examination of

freshly cut samples indicates also

dif-ferent types of deposition, noricea bIe

stratification in the brackish water-type light grey upper stratum, homogeneous flocculated grey transition zone, organic typically urar ine (with numerous shells) and dark grey lower stratum.

On account of the high sensitivity of

the clay, vane tests in situ were

sub-sequently made at the location of each borehole and near the bridge abutment.

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At each location, the vane tests were

repeated in two adjacent holes, without casing, and the shear strength measured at intervals of 1.5 feet.

Figure 10 illustrates the good agree-ment found between the results in situ in adjacent holes and the classical

dis-persion occurring with the laboratory

tests.

The variation of shear strength with depth (in the undisturbed and remoulded states) is given in figure 11 for the two locations outside and inside the scar. It

seems that the presence of a water

bearing sand layer has prevented the

formation of a crust at the surface of

the upper clay stratum. The sudden

increase in strength in the transition

zone is similar to that which could have

been produced when this stratum was a superficial one.

The regain of strength with time is illustrated by the results of vane tests

found in the rernouldcd zone above

elevation 20 shown in figure 11, borehole No.2. The thixotropic hardening after a

few months already produces a shear

strength higher than the original one. The sensitivity to remoulding of the deposits decreases noticeably with depth,

thus emphasizing the critical

condi-tion of the upper stratum (figure 12). It increases at each level with the proxi-mity to the river. Near the bridge (hole No.3), the material is quite sensitive and the role of the pile foundation is underlined.

Future Studies

A concise description of the Nicolet slide and some of the data assembled on the soil properties have been pre-sented.

The descriptive study provides the

nature of exterior factors of sliding and suggests an explanation of the extent of

the scar but the relative importance

of the causes and of the mechanism of

sliding require respectively the study

of. soil properties and a stability

anal-ySIs.

Further work is needed in the study of the soil characteristics such as sen-sitivity, plastic activity and peculiarities of deposition of the sediments, and in the study of the factors of decreasing

stability such as electrolyte content and weathering. Any progress in such

in-vestigations will assist in the ultimate

clarification of the problem. Acknow ledgcments

The vane tests were made through

the generous cooperation of ;\fr.

J.

Harris of the consulting firm of Uhl, Hall and Rich of Boston, ;\lass., and

Massena, N.Y., and the authors wish to

express their gratitude.

Permission to reproduce the ae-rial

photo of the slide was kindly granted by

Photo-Air Laurentides of Quebec City.

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Discussion

Following Dr. Vlcverhof's paper, .\lr. Walter questioned rhe author concerning

the criteria for the acceptance or

re-jection of soils for road building pur-poses. Dr. .\lcColough wanted to know

whether soil surveys provide data of

sufficient scope to warrenr imrucdi.itc

work, or whether long term testing had to be performed.

Dr. \ leyerholf suggested that his

in-vestigation did not lead to a satisfacrory

answer to \Ir. Walter's question but

that the provinces did appear to have

general acceptance standards haxc.l on

gradings. Relative to the data from s0;1

suneys it was the author's contention

that the survey at least provided

in-formation for general soil problem evalua-tions and this would lead to more

ex-tensive testing where the survey

m-dicared such need.

Following Mr. LaRochelle's paper, _'.1r. ]. A. Knight questioned the author con-cerning the freezing temperature of the soil with various percentages of calcium chloride additive. Mr. LaRochelle replied

that at 0.5 per cent CaCl., the freezing

temperature was 26°F. and- at 1 per cent

CaCl₂the freezing temperature was 11°F.

Dean Hardy questioned the author

concerning icc segregation. \1r.

La-Rochelle stated that this factor was no, investigated but he had not noticed ,'ny ice segregation during his work. Dean Hardy then suggested that pcrhms 'he experimental work established conditions somewhat more severe than would occur

in practice. He felt that this racror

should be gi\-en addirional cnnsidcrat ion. After Professor Townsend's paper, Dr.

\Ieyerhof questioned the author

con-cerning the problems associated with

cylinder moulds and materials containing

relatively large grain sizes. ProfesiO:'

Townsend replied that the AST;\l have

approved the usc of 6-inch di uncrcr

moulds with the standard comp.rcrive

effort.

.\Ir. Lueder asked whether the author had any information concerning the use

of radioactive isotopes in Canada in

the measurement of soil densities.

Pro-fessor Townsend said that his survev

had not revealed the application (;f

this procedure by any of the highway building agencies; however, he under-stood that some investigations along these

lines were being performed by the

:"ational Research Council and the

Uni-vcrsitv of Saskatchewan. Dean Hardy

confirmed the latter statement and said

that the work was in its preliminary

stages.

In the discussion following i\Ir.

Lueder's paper, Professor Edwards asked the author what might be considered an

ideal scale for soil interpretation work,

Vlr. Lueder replied that scales of 1: 15,000

to l:30,000 were usually adequate. It was

further suggested that larger scales

tended to produce an excess of

informa-tion often resulting in confusion since the primary purpose of airphoro inter-pretation is to delimit boundaries.

Dr. \Ieyerholf suggested that colour photography might be of great value in air photo interpretation. The author re-plied that there was little doubt about

the value of colour photography but

that the cost of this procedure was pro-hibitive.

\Ir. Rochette's paper concerning the

Nicolet slide aroused considerable

dis-cussion with regard to the contribution of the sewer toward the slide by causing

the clay to swell; whether a theoretical

analysis of slope stability had been made; and what reliability might be expected from the sampling procedure.

In reply, .vlr, Rochette said that he did not feel that the sewer was a con-tributing factor. He said that NRC had performed a slope stability analysis and that he really could not comment on the reliability of the sampling of the slide, although he felt it was adequate.

The session ended with Dean Har dv expressing the committee's thanks to the

speakers for their papers and to the

audience for their participation in the discussions.