1975_12_106

106 CAN. GEOTECH. J. VOL. 12, 1975

Urban Fringe Development and Slope Instability in Southern Saskatchewan

Deportment of Civil Engineering, University of Saskatcllewan Saskatoon, Saskatchewan S7N OW0 Received April 22, 1974

Accepted October 7, 1974

The development of the areas surrounding urban centers in Saskatchewan has become of increasing concern because of slope instability problems. Proglacial meltwater channels and spillways provide seemingly attractive sites for rural estates. An investigation revealed that most of the slopes along these channels are unstable where Regina lacustrine clay or Bearpaw shale are exposed. A large building constructed near Lumsden is experiencing severe damage because it was constructed on landslide debris. South of Saskatoon, a similar problem exists along the east bank of the Saskatchewan River where a high water table is present in a deltaic sand overlying a soft lacustrine clay. An old barn at the Riverside Golf Course is spectacular evidence of the history of the movement of a large slide block in this terrain.

Certain problems exist in convincing the public that a stability problem exists in these areas. Some form of landuse zoning based on risk of failure is needed. However, local municipal governments do not have the resources to conduct the necessary research to establish effective controls.

Le diveloppement des zones limitrophes des centres urbains de la Saskatchewan est devenu le sujet d'attention croissante a cause de problemes d'instabilitk de pentes. Les chenaux et les deversoirs formes par 1'ecoulement des eaux de fonte des glaciers constituent apparemment des sites attrayant pour I'implantation de proprietes rurales. Une Ctude a r6v61e que la plupart des pentes le long de ces chenaux 6taient instables lorsque I'argile lacustre de Regina ou le schiste Bearpaw sont exposes. Un batiment important construit pres de Lumsden est en train de subir des dommages importants par suite de sa localisation sur des debris de glissement de terrain. Au sud de Saskatoon un probleme similaire existe le long de la rive est de la rivitre Saskatchewan, oh une nappe phreatique elevCe est presente dans un sable deltaique surmontant une argile lacustre molle. Une vieille grange sur le terrain de golf Riverside est un timoignage spectaculaire de I'histoire du mouvement d'un important glissement de masse sur ce terrain.

I1 y a certaines difficultks a convaincre le public de I'existence d'un probleme de stabilite dans ces zones. Les gouvernements municipaux locaux n'ont pas les ressources pour faire les re- cherche~ necessaires pour Ctablir des contr6les effectifs. On suggere qu'un programme de forage et de reconnaissance geophysique permettrait de definir suffisamment les dimensions du probleme pour Ctablir des contr6les realistes en un an, alors qu'une Ctude detaillee prendrait

[Traduit par la Revue] plusieurs annees.

Introduction Population trends in most parts of southern

Canada have followed a consistent pattern towards centralization from rural to urban areas. This has caused rapid growth in urban development which among other things has aggravated problems associated with air pollution, noise and transportation congestion; along with these physical problems, numerous social problems have developed as well. To escape these un- desirable features or urban life and the crushing tax burden associated with them, the more affluent urban dweller is looking to the rural environment of urban fringe areas to establish a country estate. Thus, he hopes to have the best of two worlds in that he escapes the un- desirable attributes of urban life, but retains the advantage of ready access to the cultural Can. Grotech. J., 12, 106(1975)

and commercial advantages of the city. Along with this residential dcvelopmcnt, various in- stitutions and services inevitably follow. Un- fortunately in the Regina and Saskatoon areas, the most sought after locations arc along valley slopes which unknown to the owners are often extrcmely unstable.

This urban fringe development in southern Saskatchcwan has created a steadily increasing pressurc on the rural environment which pre- sents a dilemma to rural municipal governments. The complcxity of political jurisdictions makes the problem even more difficult to re- solve. However, what is probably most important is that development is taking place with inadequate controls with respect to terrain instability. Furthermore, the small municipalities do not have the manpower nor financial re-

NOTES 107

sources to conduct proper terrain studies on which to base control measures and land-use planning.

A preliminary investigation was initiated by the Civil Engineering Department at the Uni- versity of Saskatchewan at Saskatoon with the assistance of the Saskatchewan Research Coun- cil in an attempt to identify the scope of the problem, probable causes, and to establish the re- quirements for a detailed investigation and research program. The results of the investigation indicated the problem was much more serious than at first realized.

Urban Fringe Development There are many criteria that could be used

to select a building site in rural areas. Certainly, this will vary depending on the kind of development. However, there arc four basic require- ments that are fairly obvious. They are:

1. Shelter from fierce winter storms which are part of the prairie environment.

2. A scenic view. 3. A water supply. 4. Suitable foundation conditions. Unfortunately, it was found that the fourth

criterion has not been considered in most in- stances. Furthermore, the first three require- ments can be factors associated with slope instability. For example, the only topographic relief and tree growth in the Regina area are found in proglacial meltwater channels that have been cut into the drift and in places into the bedrock; most of these slopes are unstable. Similarly, the Saskatoon area has the most scenic view along the banks of the South Sas- katchewan River where a high water table in deltaic and lacustrine sediments provides an excellent domestic water supply, it so happens that this combination of water and sediments results in slope instability. Slope instability problems also exist in Estevan, Prince Albert, and North Battleford.

Slope Instability in the Regina Area North of Regina, the valleys of the Qu'Ap-

pelle River system and its tributaries offer the only major relief in the area providing attractive sites for development. The main valleys are those of the Qu'Appelle River, Last Moun- tain Lake, and Boggy, Wascana, and Flying Creeks (Fig. 1 ) . All of these valleys presently

have some degree of residential development ranging in density from very low to very high, most of which is centered around the Lumsden area.

Geology of Landslides in the Regina-Lurnsden Area

Regina is situated in a former proglacial lake basin (Christiansen 1961 ). The overlying sediments are highly plastic clays which are fissured and slickensided below 2 m from the surface. Underlying the surface clays north of Regina is till of varying thickness. Below the till is clay shale of the Upper Cretaceous Bear- paw Formation (Fig. 2 ) . The bottom elevation of the valleys in this area range from 1840 to 1610 ft (561 to 491 m) above sea level.

The properties of the clay shale of the Bear- paw Formation have been studied in considerable detail by Peterson (1954) and Peterson et al. (1960). They found that it is very difficult to obtain laboratory shear strength values that will agree with field measurements. So far the most reliable data has been from field measurements of existing slopes. Natural slopes measured in the Lumsden area were found to range from 4: l1 to 7: 1 with a mean value of 5.5: 1 where the drift-shale contact was above floodplain elevation. It should be noted, however, that these slopes have failed, but are presently inactive where they have not been disturbed. Highway engineers in Saskatchewan have been using 7: 1 as the steepest safe slope in clay shale based mainly on experience. How- ever, slopes as flat as 8: l have failed.

Till is usually a material of high stability and will stand at slopes of 2.5: 1 or steeper in some locations (Sauer 1974). On the other hand, the overlying clay is generally of low stability.

Natural slopes in Regina clay have not been analyzed in detail. However, slopes cut at 2: 1 on the Regina Ring Road failed during the summer of 1974. The index properties of Re- gina clay are given in Table 1. Peterson et al. ( 1 960) reported the liquid limit in clay shales of the Bearpaw Formation as ranging from 80 to 150 and the plastic limit from 18 to 27 which is higher than Regina clay. Therefore, natural safe slopes in Regina clay can be expected to

'Slope ratios shown are horizontal distance to vertical distance. Thus 8: 1 represents 8 ft horizontally for every 1 ft rise.

CAN. GEOTECH. J. VOL. 12. 1975

FIG. 1. Meltwater channels north of Regina where urban fringe development is taking place.

NOTES

ELEVATION

FEET METERS

S R C 7 2 - 1 / I 0 1973 WASCANA CREEK SE 16-29-18-21 W 2

TESTHOLE

ml C02 / g SAMPLE

k FLORAL a

FORMATION

SUTHERLANO

BEARPAW FORMATION

SAND

SILTY

(i'_._l CLAY

GEOLOGY

Et GRAVEL

CLAY

BY

FIG. 2. Stratigraphy north of Regina. The bottom levels of meltwater channels in this area range from elevation 560 to 490 m, which means that the deeper channels are eroded well into the clay shale of the Bearpaw Formation.


FIG. 3. Slumping in Regina clay. An oversteepened slope along Wascana Creek.

TABLE 1. Statistical analysis of index properties of Regina clay (results are from samples taken from

13 test pits)

X s K v % Sand 0.67 - 2.0 - % Silt 24.3 5.5 18.5 22.7 % Clay 75.0 5.4 18.5 7.2 WL 76.8 2.6 7.8 3.4 WP 29.7 2.2 7.6 7.5 IP 47.2 2.9 8.6 6.2

After Mickleborough (1970); X = Arithmetic mean, S = Standard dewation, R = Range, and V = Coefficient of variation.

be steeper than those for clay shale. Regina clays are highly fissured with distinct slicken- sides indicating that they are highly overcon- solidated and probably exhibit residual shear strength behavior. The Ring Road failures occurred 5 years after construction which tends to support this conclusion.

Therefore, even though till in the area is rela- tively stable, the sediments above and below it are extremely unstable. As a result, unstable slopes north of Regina are of two basic forms and almost continuous:

1. Oversteepened slopes in Regina clay. 2. The disturbance of slopes along valleys

which are cut into the underlying clay shales of the Bearpaw Formation where previous landslides have taken place.

With the exception of actively eroding river banks in Regina clay, where failures were found to be active (Fig. 3 ) , most of the slopes in the Regina-Lumsden area are quasi-stable. In other words, they are not perceptibly moving at the present time, but they have moved in the past. This is true particularly in valleys cut into the shale underlying the drift.2 An excellent example can be found in the Last Mountain Lake Valley where high density development in the Last Mountain channel (Fig. 4) has taken place. Perhaps it is misleading to term these slopes as quasi-stable because it is not known for certain if they are in fact moving, but at a very slow rate. This question is probably aca- demic because disturbance by construction ac- tivity will almost always reactivate slope movement unless carefully planned.

It is interesting to note that there are many homes located on old slide blocks in the town of Lurnsden that have experienced very little difficulty. Some of the older homes in the Lumsden area have existed for 50 years or more without any signs of distress. Close ex-

'In glacial deposition, there are two geomorphic agents involved, namely ice and meltwater. Both of these agents deposit a variety of sediments referred to collectively as drift. The two basic kinds of drift are till (ice deposition) and stratified drift (meltwater deposition).

FIG. 4. Slopes cut through the drift into Bearpaw shale in the Last Mountain Lake Valley north of Regina have failed in the past. Severe ground cracking has been observed in some of the developed areas. On the other hand, high density urban and resort development has been established at A where no movement has taken place. Lower density developments have taken place across the lake. Note the actively unstable areas at B and C. The area at D is sand and gravel outwash overlying till whereas at E the surface material is till. (Courtesy of the National Air-photo Library).

CAN. GEOTECH. J. VOL. 12, 1975

FIG. 5. St. Michael's Retreat on Boggy Creek near its entry into the Qu'Appelle. The wing on the left aligned towards the photograph is undergo in^ severe distress. Note the sag in the roof of the building indicated bv the arrow.

., property development.

~onsaeraTble landscaping was involvedu in this

amination, however, revealed that the grounds associated with these homes have not been landscaped and services are minimal. On the othcr hand, St. Michael's Retreat on the west slope of Boggy Creek has experienced severe distrcss (Fig. 5) . Slope indicators show that movement is taking place at a depth of 40 ft (12 m). Excavation and fill of up to 6 m was involved in the development of this property which likely disturbed the gravitational equi- librium of the slope; the weight of a small structure relative to earth mass is very small. Unfortunately most modern developments in- volve considerable landscaping.

Urban development results in significant changes in the soil microclimate (Hamilton 1969). Lawn watering, leakage from water mains and sewers can create large changes in the elevation of the water table. In the dry prairie environment where natural evapotrani- piration rates are high, the maintenance of lush green lawns requires intensive irrigation. The presence of buildings and pavements further decreases evapotranspiration. All of these factors cause increases in soil moisture or a rise in the water table, which in turn, cause instability along valley slopes.

Most laymen, and some professionals do not

appreciate the relative inflexibility of building structures in relation to small earth movements. At St. Michael's Retreat, severe structural damage has resulted from a total earth movement of about 18 cm (Figs. 6 and 7) . Damage is so severe that one wing of the building can no longer be used. The movement is retrogressive and other parts of the building may be torn apart, however, corrective measures are under study.

Slope Instability in the Saskatoon Area Landslides along the South Saskatchewan

River have been a problem for many years in Saskatoon. Major slides along Saskatchewan Crescent, near the University have been a problem to city engineers which apparently they have under control. However, continuous sliding along 13 km of the east river bank south of the city has become of increasing concern because similar to the situation in Regina, urban fringe development is taking place in this area. So far, only low density residential development has taken place (Fig. 8) . No doubt pressure for higher density development will increase. City engineers in Saskatoon are aware of the problem, but have no control over the area south of the city.

NOTES 113

FIG. 6 . St. Michael's Retreat at Lumsden. Severe structural damage to one wing of this structure is shown in this photograph. Note the wall cracking and deformed door frames. This part of the building has been closed.

Geology of Landslides in the Saskatoon Area The area south of Saskatoon is covered by

lake basin clays and deltaic silts and sands (Christiansen 1968). The deltaic sands overlie the lacustrine clays. These deposits were found to be as much as 33 m thick overlying a soft highly compressible till. At a depth of 110 rn the bedrock which is the Bearpaw Formation (Fig. 9 ) was found. Therefore, landslides in this area are related to drift deposits rather than bedrock clay shales.

Slope instability along the east bank of the river south of the city appears to be primarily related to soft lacustrine clays and groundwater in overlying deltaic sand (Christiansen and Meneley 1970). Two deep testholes confirmed this hypothesis.

Preliminary observations indicated very large landslides have occurred in the past and some are active today along the 13 km section of the river bank. The history of the slides has not been studied. It is known, however, that the slides have been taking place for a long time as indicated by the old barn at the Riverside Country Club Golf Course (Figs. 10 and 11). The Gardiner Dam upstream may have a stabilizing effect, however, slides are still active in the area. A recent slide (April, 1973) de- stroyed the pump house at a Golf course (Fig. 12) .

FIG. 7. St. Michael's Retreat. Fracture and vertical displacement in the floor slab are shown here.

FIG. 8. Stereogram of Moon Lake and the River south of Saskatoon. Very large landslides can be seen at Beaver Creek (A) and all along the east bank up to D. A terrace at B appears stable although the east backslope appears unstable. Note the sand dunes at E formed in surficial deltaic sands. Underlying the sand is a soft compressible clay. Above C is the Riverside Country Club Golf Course. (Courtesy of the National Air-photo Library).

ELEVATION FEE'

1625

1525

1425

1325

1225

T METERS

NOTES

U OF S 7 2 - 0 / 15 1973 MOON LAKE

NW 16- 1 - 35-6-W3 TESTHOLE

r n l C 0 2 / g SAMPLE

0 10 20 30 4 0 50

R = I0 OHMS LL 0

a

THOLOGY

OXIDIZED TILL

UNOXlDlZED T I L L

SAND 8 GRAVEL

SILTY CLAY

CLAY

FIG. 9. Stratigraphy along the east river bank south of Saskatoon.

CAN. GEOTECH. J. VOL. 12, 1975

FIG. 10. The history of landslides is indicated in this photograph. The building which is an old barn (Fig. l l ) , is sitting on a shelf that appears to be a slide block at ground level B. Elderly residents in the area state that this barn was at one time at ground level A which is fairway on the Riverside Golf Course.

FIG. 11. Evidence of rotational sliding is shown in this photograph of the old barn shown in Fig. 10. The original ground elevation is indicated by the letter A, the present ground level by the letter B. Note the amount of deformation in the wooden structure.

NOTES

FIG. 12. Recent (April, 1973) slope movements resulted in the destruction of the irrigation pump house at the golf course. The foundation of the pump house is shown here.

Conclusions A preliminary investigation indicated that

scvcre slope instability problems exist along valley slopes near Regina and Saskatoon. The causes of the slides are related to oversteepened slopes in surficial lacustrine clay and exposed clay shale of the Bearpaw Formation in the Regina area, and groundwater in deltaic sands overlying a lacustrine clay in the Saskatoon area.

Development is taking place in both of these areas without any effective control with respect to slope stability. One large structure has already been seriously damaged by slope move- mcnt. Furthermore, it was found that very few individuals involved in the development are aware of the problem and of those interviewed, most were difficult to convince that there is indeed a potential problem. There are probably scveral reasons for this attitude, two of them

most slopes are quasi-stable or are moving so slowly that natural erosion and vegeta- tion obscure any evidence of movement. when slope failures are activated, the movements are very slow and not particularly spectacular and, therefore, often diagnosed as settlements or some other

phenomenon rather than massive earth movements.

Some form of effective landuse zoning based on risk of failure is urgently needed. However, before this can be done, a detailed study of the geology in the area and an analysis of existing slopes is required. So far very little information is available for this purpose, mainly because of the limited resources of the local political authorities and a lack of recognition of the problem.

Acknowledgments The author wishes to acknowledge the assis-

tance of Dr. E. A. Christiansen of the Saskatch- ewan Research Council for supervising test drilling, interpretation of the stratigraphy, and analysis of drill cuttings. The continuing co- operation between Dr. Christiansen and his staff and the staff in the Department of Civil Engineering at the University of Saskatchewan at Saskatoon is resulting in significant advances toward understanding geotechnical problems in Saskatchewan.

CHRISTIANSEN, E. A. 1961. Geology and groundwater resources of the Regina area, Saskatchewan. Saskatch- ewan Research Council, Geol. Div., Rep. No. 2.

1968. Pleistocene stratigraphy of the Saskatoon


area, Saskatchewan, Canada. Can. J. Earth Sci. 5, pp. 1167-1173.

CHRISTIANSEN, E. A., and MENELEY, W. A. 1970. Slope instability. I n Physical environment of Saskatoon, Canada. Edited by Christiansen, E . A., National Re- search Council ofCanadaPubl. No. 11378, pp. 51-52.

HAMILTON, J. J. 1969. Effects of environment on the per- formance of shallow foundations. Can. Geotech. J. 6(1), pp. 65-80.

MICKLEBOROUGH, B. W. 1970. An experimental study of the effects of freezing on clay subgrades. Unpubl. M.Sc. thesis, Univ. Saskatchewan Library, Saska- toon, Sask.

PETERSON, R. 1954. Studies of the Bearpaw shale at a damsite in Saskatchewan. Proc. A.S.C.E., Soil Mech. Found. Div. 80, Separate 476.

PETERSON, R., JASPAR, J. L., RIVARD, P. J., and IVERSON, N. L. 1960. Limitations of laboratory shear strength in evaluating stability of highly plastic clays. A.S.C.E. Conf. Shear Strength Cohesive Soils, Boulder, Col- orado, pp. 765-791.

SAUER, E . K. 1974. Geotechnical implications of Pleis- tocene deposits in southern Saskatchewan. Can. Geotech. J. 11(3), pp. 359-373.

Failure along Planes of Weakness

E. 2. LAJTAI Department of Geology, UniversiQ of New Br~mswick, Fredericton, New Br~mswick

Received August 13, 1974 Accepted October 7, 1974

The importance of the micro stress field in the failure of discontinuous rocks is emphasized. The factors controlling failure are identified as, the micro stress set up at the ends and at irregularities ofjoint segments, the macro stress which controls the growth of the microfracture to macroscopic dimensions, and the external constraint which may arrest the propagating failure surface. By causing microfracture, the micro stress is largely responsible for the destruction of the rock bridges and the asperities occurring along the plane of weakness. In the initial stages of failure, the various types of tensile and shear microfractures form a zigzag pattern which on continued deformation becomes incorporated in a wide shear zone running along the original plane of weakness.

L'importance des champs de contraintes locaux dans la rupture des roches discontinues est mise en evidence. Les facteurs controllant la rupture sont identifies comme Btant les concentrations de contraintes aux extremites et k proximite des irr6gularit6s des joints, les contraintes globales qui controlent la croissance de microfractures jusqu'k des dimensions macroscopiques, et les conditions aux limites qui peuvent arrster la propagation de la surface de rupture. En provoquant des microfractures, les concentrations de contraintes sont largement responsables de la destruction des dalles rocheuses jouant le rBle de ponts et des asperites presentes dans les plans de glissement. Dans les phases initiales de la rupture, les differents types de micro fractures de tension et de cisaillement foment un reseau en zig-zag qui, avec le developpement des dCformations, est progressivement incorpore B une large zone de cisaillement suivant le plan de faiblesse initial. [Traduit par la Revue]

Introduction The course of a failure surface across a mass

of discontinuous rock depends chiefly on the pattern of preexisting planes of weakness. In a simple situation, a single plane of weakness itself may lead to failure. In more complex rock structures, however, a developing failure surface shifts from one set of discontinuities to another, cutting across solid rock bridges in the process. The shear strength along the eventual failure surface therefore consists of a combination of solid rock strength in rock bridges and Can. Geotech. J., 12,118 (1975)

discontinuity strength along the initially frac- tured or already sheared sections.

The determination of the contribution of solid rock to the total shear strength is a major and usually unsolvable problem. The difficulty of its evaluation is sometimes so overwhelming that the practical step is to neglect it altogether. This, however, has the same effect as assuming that the strength of a rock mass is no more than that of sand. Consequently one could not allow slopes steeper than a few degrees over 30, despite the fact that most rock slopes, even

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