Riverbank instability and development control in Saskatoon

Riverbank instability and development control in Saskatoon
A. W. CLIFTON
Clifrorl Associcrtes, Regitla, Sask., Carlaclcr S7L 6A8
AND
J. KRAHNAND D. G. FREDLUND
Dep.pcrrtmetlt of Civil Etrgirreeri~lg,Utliversi~j~
of Saskntcl~e~vnr~,
Snskatoorl, Scrsk., Catlacla S7N 0 WO
Received May 2, 1980
Accepted October 15, 1980
Slope instability along the banks of the South Saskatchewan River Valley has been a continual problem for Saskatoon's City Engineering Department sitice 1913. It has affected building
and briclge foundations, damaged municipal roads and services, and interfered with park and
playground development. Where the glacial till in the banks is overlain by lacustrine clays,
silts, and sands, the lower portion of the slip surface invariably follows the stratigraphic
contact between the till anti the overlying materials. Piping of intertill stratified drift is the
major cause of slope instability where the till is exposed at the su~face.Gro~tndwaterfrom the
natural infiltration and from lawn watering greatly affects the slope stability. Remedial techniques have cot-tsisted primarily of subsurface drainage systems.
Increased pressure for developme~italong the riverbank has led to the recent establishment
of the Meewasin Valley Authority which has complete control of development within and
along the river valley. Opposition to the Authority's establishment has been strong. Its success
in controlling development near unstable slopes will only be proven with time.
L'instabilitC des pentes le long des berges de la rivikre Saskatchewan sud a CtC un problkme
permanent pour le service ctu GCnie de la ville cle Saskatoon depuis 1913. Cette instabilitk a
affect6 les fonclations de b2tinients et de ponts, endommagC les rues et les services, et nuit au
dCveloppement des parcs et terrains de jeu. Lorsque la moraine glaciaire est recouverte d'argiles
lacustres, de sables et de silts, datis les talus, la partie infkrieure de la s ~ ~ r f a de
c e glissement s~tit
toujours le contact stratigraphique entre la moraine et le rnateria~tsus-jacent. L'erosion interne
cle couches intercalaires dans la moraine est la cause principale tl'i~lstabilitC18 oil la moraine
aftleure. L'infiltration, naturelle ou res~~ltant
de I'arrosage des jardins, affecte considCrablenlent
la stabilitC des pentes. Les mesures correctives ont comportkes essentiellement des systkmes de
drainage enterrCs.
L'intensification clu dCveloppement ~ ~ r b a le
i nlong des berges a conduit B la creation rkcente
de la Meewasin Valley Authority qui contrijle L'ensemble dtt dCveloppement le long de la vallCe.
L'opposition B la crCatioil de ce corps publique a CtC forte. Son succks dans le contr6le tlu
dCveloppernent B proximitC des talus instables sera dCmontrC avec le temps.
[Traduit par la r e v ~ ~ e ]
Can. Geotech. J., 18,95-105 (1981)
Introduction
Riverbank instability has been a problem in the
city of Saskatoon for a long time. The first slope
stability problem the city's engineering department
had to cope with dates back to 1913. Since that time,
slope movements have been a continual problem.
They have affected the foundations of buildings and
bridges, damaged municipal roads and services, and
interfered with the development of parks and playgrounds. Furthermore, there is an ever increasing
need to identify potentially unstable areas because of
increased pressure for both recreational and residential development along the riverbank.
This increased pressure for development along the
riverbank has led to the recent establishment of a
river valley authority. This authority has total con-
trol over the development within and along the river
valley. Acceptance of this authority has met with a
great deal of opposition, and this is of significance in
evaluating the entire concept of development control
along such areas as riverbanks and slopes.
This paper presents a history and description of the
major slope failures which have occurred in the
Saskatoon urban area, a discussion of the causes of
instability, a review of stabilization techniques used,
a summary of the responsibility and powers of the
newly established river edge authority, and a discussion of public reaction to the authority.
Instability History and Description
Locations of rnajor areas of riverbank instability
in Saskatoon are marked on a vertical air photograph
0008-3678/81/010095-11$01.00/0
@ 1981 National Research Council of Canada/Conseil national de recherches du Canada
96
CAN. GEOTECH. J. VOL. 18, 1981
FIG. 1. Slope instability locations in Saskatoon.
in Fig. 1. Discussions of the slides and slope movements which follow are given by moving generally
from the south to the north along the river in the
direction of the flow.
The first major slide within the southern part of
the city occurred in Diefenbaker Park. This slide
interfered with the development of the park in that a
relatively steep and sharp head scarp developed. It
CLIFTON ET AL.
FIG.2. The Queen's House of Retreat Slide.
was considered unsafe for the park users to stand too
close to the crest edge. A substantial amount of
material slid relatively quickly, with a resulting lobe
of debris extending into the river.
The next major slide towards the north is the
Queen's House of Retreat Slide shown in Fig. 2. This
slide is actively moving at the present time with the
head scarp encroaching on the private property. The
material below the slide toe is landfill placed, in part,
to prevent riverbank erosion. The base of the slip
surface, however, appears to be above the landfill
level. This is of significance in selecting remedial
techniques as will be discussed later.
Between the 19th Street Bridge (just north of the
Idylwyld Bridge) and the Broadway Bridge there
have been several landslides. The first one occurred
along this section in 1913. Construction of a streetcar
railway followed a route along the riverbank from
the present east approach to the Broadway Bridge,
down an incline, and then onto the 19th Street Bridge.
This was the only traffic bridge across the river at
that time. The long incline was known as the Long
Hill. Shortly after the start of operation of the streetcar system, a slide occurred along Long Hill, interfering with the operation of this system. This situation
apparently created considerable concern because the
damage was severe enough to halt the rail traffic.
Figure 3 shows the present condition of this riverbank
section. Later, another slide occurred along this
section just below Nutana Collegiate. This slide
encroached upon the Collegiate's playgrounds and
presented a problem with the realignment and con-
struction of the approaches to the 19th Street Bridge.
In May of 1929 the first of a series of slides took
place along the east riverbank between the Broadway
and 25th Street Bridges. This slide, known as the
McCraney Slide, was about 10 m wide with the toe of
the slide being approximately half way up the slope.
Fifty metres of walkway and curb along the crest
dropped 1 metre.
The next slide along this section did not occur until
1954. At this time a slide occurred about a city block
south of the McCraney Slide opposite 15th Street.
This slide was 45 m wide at the crest and 60 m wide
at the toe. It differed from the McCraney Slide in
that the sliding mass extended from the crest of the
bank down to the river level and a significant amount
of debris moved into the river.
In the early 1960's two more slides occurred along
this section of the bank; one just south of the 25th
Street Bridge opposite 18th Street and one just north
of the Broadway Bridge opposite 13th Street. Saskatchewan Crescent running along the crest of the
18th Street Slide was seriously damaged. The 13th
Street Slide created considerable difficulty with maintaining the city's natural gas lines. Both the 18th and
13th Street Slides were similar to the McCraney Slide
in that sliding took place only in the upper portion
of the slope. Slope movements associated with the
13th Street Slide affected the stability of the east
Broadway Bridge abutment. These movements were
measured using slope indicators, the results of which
are discussed below.
The present condition of this riverbank section is
CAN. GEOTECH. J. VOL. 18, 1981
FIG.3. The riverbank between the Broadway and 19th Street Bridges.
FIG.4. The riverbank north of the Broadway Bridge.
shown in Fig. 4. The arched bridge is the Broadway
Bridge and the view is looking downstream. The
"benched" appearance has come about because of
remedial landscaping and bernling,
There are also areas of slope instability north of
the 25th Street Bridge, but they arise from piping as
opposed to the sliding failures south of the 25th
Street Bridge. Petursson's Ravine and the Penitentiary piping failures, shown in Figs. 5 and 6, are the
most spectacular. Significant riverbank seepage is
also taking place at the Ski Jump Ravine and Devil's
Dip, but the piping is not nearly as severe as at the
former two locations.
There are numerous other slide areas south (upstream) of the city, and they are usually located
where the banks are relatively steep and high. Because the banks are steep and high they provide a
panoramic view, and for this reason these sites have
been prime locations for residential acreage developments. Slides in these areas have been of particular
CLIFTON ET AL.
FIG.5. Petursson's Ravine piping failure.
FIG.6. The Penitentiary piping failure.
concern to the developments identified in Fig. 7. The
Riverside Golf Club, for example, has had great
difficulty in maintaining a stable foundation for
their pump house situated at the river's edge. No
acreage homes have been damaged so far due to
slope instability, but the potential of this happening
is very real because some homes are situated dangerously close to landslides as illustrated in Fig. 8.
The largest and perhaps most active slope presently
is the Beaver Creek Slide shown in Fig. 9. Historical
records and successive air photographs have provided
evidence that the head scarp has retrogressed about
40 m in the last 35 years (Haug el al. 1977). Further
understanding of the slide's movements is being
obtained presently using surface survey and slope
indicator techniques.
Geological Effects
A major factor in the behavior of the riverbank
slopes is the stratigraphy. South of the 25th Street
Bridge the materials can generally be divided into
surficial stratified drift and glacial till. The surficial
stratified drift consists of lacustrine clays, silts, and
sands. Generally, the sediments consist of a thin layer
of lacustrine clay just above the glacial till and
gradually grade into the silt and then into sand towards the surface. These sediments were deposited
during the advance of the delta leading into glacial
Lake Saskatoon as the lake retreated and eventually
drained to the north (Christiansen 1970). A photograph illustrating the stratified drift overlying the till
is given in Fig. 10.
The silts and sands have been eroded north of the
25th Street Bridge and only a thin layer of lacustrine
clay exists near the surface close to the riverbank.
Further north in the area of Petursson's Ravine the
clay has also been eroded and the till is exposed at
ground surface.
100
CAN. GEOTECH. J. VOL. 18, 1981
P
RIVERSIDE
RIVERSIDE
SLIDE-
GOLF COURSE
1
ESTATES
FIG.7. Development and slide locations south of Saskatoon.
South of the 25th Street Bridge the lower portion
of the slip surface invariably follows the stratigraphic
boundary between the surficial stratified drift and
the underlying glacial till. Where this contact is above
river level the sliding mass moves out and falls over
the edge of the original till bank. The photograph in
Fig. 11 illustrates this for a fairly small slide. Measurements of slope movements at the 13th Street
Slide, just north of the Broadway Bridge, have provided conclusive evidence of this. The measurements
are shown in Fig. 12 on a cross section of the slide.
This is further illustrated in Figs. 13 and 14 by the
sections through the Nutana Collegiate and 18th
Street Slides.
At Beaver Creek (Fig. 7) the stratigraphic boundary between the stratified drift and the till is located
well below river level, but the sliding also occurs at
the contact as shown in Fig. 15. The failure mode,
however, differs greatly because of the deep-seated
slip surface. The sliding mass is very large and
consists of several distinct blocks moving at slightly
different rates.
Three distinct till-stratigraphic units have been
identified in the Saskatoon area (Christiansen 1968).
They are known as tills of the Sutherland Group, the
Floral Formation, and the Battleford Formation (the
reference section is at the Ski Jump Ravine). The
Battleford Formation is the youngest unit and the
Sutherland Group the oldest.
The tills of these units are frequently separated by,
or include within them, layers of stratified drift (clay,
silt, and sand layers). A stratum of stratified drift
within the Floral Formation outcrops along the
riverbank in the area of Petursson's Ravine as shown
in Fig. 16. At this point the sediments consist
primarily of silt and it is the piping erosion of this
silt which is responsible for the unstable slopes.
Groundwater Effects
The intertill stratified drift in the area of Petursson's
Ravine contains the Forestry Farm Aquifer. Groundwater enters this aquifer by downward infiltration
and lateral movement along the interface between the
tills from the Strawberry Hills about 3 km to the east
(Meneley 1970). The hydraulic head in the aquifer is
at or below the overlying till and its discharge occurs
FIG.8. An acreage residence south of Saskatoon
CLIFTON ET AL.
FIG.9. The Beaver Creek Slide looking downstream.
FIG.10. An illustration of the surficial lacustrine stratified drift overlying the glacial till.
CAN. GEOTECH. J. VOL. 18, 1981
FIG. 11. Slide debris that has fallen over the glacial till bank.
-
%
E
500
SLOPE
INDICATOR
STRATIFIED
460
460
DRIFT,
1
440;
DISTANCE -metres
TlLL
20
I
40
I
I
60
80
I
100
I
120
I
140
--
160
180
- metres
FIG. 15. The Beaver Creek section.
DISTANCE
FIG. 12. The 13th Street section just north of the Broadway
Bridge.
FLORAL
BATTLEFORD
-INFERRED
SLIP
zI
490
0
+
W
-f
10
20
30
40
50
60
70
80
90
,
SURFACE
STRATIFIED
DRIFT
I
20
SUTHERLAND
VERTICAL
485
4650
1
TlLL
I0 X
HORIZONTAL
FIG.16. The Stratigraphy at Petursson's Ravine (after
Christiansen 1970).
SASK
475
480
100
DISTANCE - metres
FIG. 13. The Nutana Collegiate cross section.
V)
TlLL
E"
SURFACE
I
40
DISTANCE
TILL
I
60
80
- metres
FIG.14. The 18th Street cross section.
140
by gravity drainage from springs along the riverbank.
It is the discharge from this aquifer that causes the
piping failures.
Vertical infiltration from precipitation recharges
the surficial aquifer situated within the surficial stratified drift. It is the hydraulic head within this aquifer
which is, in large part, responsible for the bank
instability south of the 25th Street Bridge. Evidence
103
CLIFTON ET AL.
DEFLECTION
- cm
/ $? I(/p16'" STREET
~ ~ " " 7 < 9 / ~ 1 5 ~ ~ STREET
BROADWAY
YEAR
FIG. 17. Slope indicator movements at the Broadway
Bridge.
e'd,;y\g
/
1913 BRICK
CONDUIT
'+
,---___,
W
[L
a
1640
COLLEGE
STREET
FIG. 18. Static water levels on the University of Saskatchewan campus (after Fredlund 1970).
of the relationship between the infiltration and the
failures is provided by the fact that the slides usually
occur in late spring and early summer when infiltration rates are the highest. The slope movement
measurements made at the Broadway Bridge have
also provided some evidence of this. These measurements, shown in Fig. 17, indicated that the movement
rates are highest in late spring and early summer,
especially during the first 2 years for which measurements were made.
A study of water levels on the University of
Saskatchewan campus has provided interesting data
on the seepage gradient towards the river. Over the
years a large number of test holes have been drilled
on the campus and the practice has been to simply
record the static water levels. Fredlund (1970) examined these data and found that there is a wellestablished gradient towards the riverbank as shown
by the water level contours in Fig. 18. These contours
do not differentiate between groundwater from the
surficial aquifer and seepage from the sand layers
within the till.
Urbanization Effects
Urbanization has had an effect on the riverbank
stability. Groundwater levels within the surficial
aquifer tend to rise due to irrigation of lawns and
gardens (Hamilton and Tao 1977) and possibly due
to leakage from water mains. Watering of lawns and
gardens at the President's Residence on the University Campus, situated prominently on the riverbank,
caused several hundred cubic metres of material to
slide down the slope, with the head scarp only 15 m
away from the residence. Lawn watering was halted
with the result that the water levels dropped nearly
2 m (Hamilton and Tao 1977). After the situation
had been remedied, the water levels began to rise
again when irrigation was resumed.
Remedial Measures
The most prominent technique of stabilizing slides
has been the installation of subsurface drainage
systems. It is of interest that this method was used
for the 1913 Long Hill Slide, even though the principle of effective stress was not presented until 1925.
The system consisted of a section parallel to the river
and a section perpendicular to the bank leading the
seepage to the river. The conduit was constructed
of brick and mortar.
Figure 19 gives a sketch of all the drainage systems
that have been constructed. Most of these were constructed using perforated drainpipe installed in open
trenches, or by tunneling between a series of vertical
boreholes. Most of the systems have been fairly
successful in reducing the water pressures, but difficulties have been encountered because of the variable
nature of the stratified drift. The effectiveness of the
drainage systems has been greatly reduced when they
pass through clayey zones which are not free draining.
An intercepter ditch was constructed at the University President's Residence which has been successful in preventing any further sliding. This was done in
conjunction with expensive underpinning of the
residence.
Berming of the slopes has been used as a remedial
104
CAN. GEOTECH. J. VOL. 18, 1981
FIG.20. Meewasin Valley Authority boundaries in Saskatoon.
measure and the city is continuing to place landfill
along certain sections of the riverbank toe. This is
designed in part to prevent toe erosion. The berming
is effective in eliminating the erosion and possibly
preventing instability, but it is a questionable method
of stabilizing failures because the toes of many slides
are at an elevation above the berm crest, as has been
discussed above.
CLIFTON ET AL.
River Edge Development Control
In spite of the nearly 75 yearsy experience with
unstable slopes, and substantial capital investment in
remedial measures, no significant restrictions on land
usage along the river valley have been enacted. Only
of late have requirements for geotechnical analysis of
building sites been imposed on prospective developers
encroaching on the valley walls.
This situation changed very dran~aticallyin 1979
with the passing of a Saskatchewan Legislature Act
to create a body to oversee all development along the
riverbank and valley. This body is known as the
Meewasin Valley Authority (MVA). The main function of this authority is to oversee the conceptual plan
recommending ways of preserving and developing
the river valley and certain adjacent areas for the use
of present and future generations.
jurisdiction totals
The area under the
416 km2, extending fro111 the south to the north
boundary of the Corman Park Rural Municipality.
Within Saskatoon the boundaries are as shown in
Fig. 20. Much of the area is divided into two zones:
one a control zone and the other a buffer zone. The
control zone covers areas directly affected by the
river such as flood plains, terraces, ravines, creeks,
and unstable banks. The buffer zone is a zone of
influence and covers areas that indirectly affect the
river valley such. as secondary water courses and
springs. In the control zone the Authority has the
power to approve or to refuse proposed improvements. In the buffer zone, the Authority must be
informed before any improvements are initiated. This
applies to both public and private holdings. Inlprovements include buildings, landscaping, road construction, sidewalks, parking lots, and utilities.
The act 'Iso
the Authority to 'lake
regulate such
as
use
of the river, protection of animals, birds, aquatic and
plant life, commercial and industrial activities, poilution of land and water, tramc and parking, and fire
fighting.
The establishnlent of this Authority has met with
tremendous opposition. Outside the City of Saskatoon, the Meewasin Valley boundaries encompass
farm land and many owners of this land believe the
MVA Act greatly limits their property rights. The
main concern is that theresale value of the land will
be greatly reduced because of the MVA's powers. The
same thinking has been prevalent within saskatoon,
Initia1'~9some mortgage conl~anieswere
provide financing for developments within the
MVA boundaries. Various associations have been
105
formed and with the assistance of legal counsel have
opposed the entire concept of the act. The cities of
Prince Albert and Moose Jaw have planned for h i lar authorities, but the plans have been temporarily
shelved because of the opposition that the Meewasin
Valley Authority has received.
There are some land and home owners, however,
who are in favor of the idea and see the Authority as
a means of protecting their Property, especially those
whose Property b ~ r d e r son parkland.
Concluding Remarks
Slope instability has been a problem in Saskatoon
for a long time. Factors affecting the instability are
now well understood; the major factors are the soil
stratigraphy and geology and the discharge of an
urbanization-influenced surficial aquifer or a regional
aquifer. In spite of this understanding, homes and
residential apartments have been built near unstable
dopes. The recent establishment of the Meewasin
Valley Authority, however, could alter this situation.
in doing
be
The success of the
proven with time. At the present it appears that it will
be
the riverbank
and yet at the
time provide the public with
recreational, natural, educational, and research facilities
Acknowledgements
The authors wish to thank the City of Saskatoon's
Engineering Department for making available all
their slope stability records and for granting permission to publish the information.
C H R ~ ~ A N SE.
E NA, 1968. Pleistocene stratigraphy of the
Saskatoon area, Saskatchewan, Canada. Canadian Jo~irnal
of Earth Sciences, 5, pp. 1167-1173.
---- 1970. Physical environment of Saskatoon, Canada.
Saskatchewan Research Coutlcil. Publication No. 11378,
pp. 3-17.
D. G. 1970. Guide to foundation design. UniverFREDLUND,
sity of Saskatchewan Building and Grounds Department.
Internal Report. Saskatoon, Sask.
HAMILTON,
J., and TAO,S. 1977. Impact of urban development
on gro~~ndwaterin glacial deposits. Proceedings, 30th
Canadian Geotechnical Conference, Saskatoon, Sask.,
pp. 2.1-2,35.
HAUG,M., SAUER,
E. K., and FREDLUND,
D. G. 1977. Retrogressive slope f a i l ~ ~ r at
e s Beaver Creek, South of Saskatoon,
Saskatchewan, Canada. Canadian Geotechtiical Journal,
14(3), pp. 288-301.
MENELEY,W. 1970. Physical environment of Saskatoon,
Canada. Saskatchewan Research Council. Publication No.
11378, pp. 39-50.