Parc Riverain

Geophysical and geotechnical analysis of the site (2018)

As part of this mandate, it has been planned to carry out a geomorphotogical and

geotechnical analysis on the selected site to evaluate the soil conditions in place.

This technical note presents the methods and main results of this analysis and

makes some recommendations for the future development of the site.

From the study

The geomorphological and geotechnical component of the feasibility study is

essentially the following:

• Topographic curve production at intervals of 1 m and 5 m;

• Geomorphalogic analysis to map the nature and thickness of surface

deposits, the depth of bedrock, identify geomorphological constraints and

characterize the hydrographic network within the study area;

• Preparation and presentation of mapping of topography and hydrography,

surface deposits and geomorphic constraints, and the depth of rock within

the study site;

• Review of the geotechnical study carried out by EXP (2014)

• Summary assessment of soil stability at the study site taking into account

existing drilling results and geomorphological stress analysis.

In general, soil condition analysis is intended to allow for informed planning of

riparian park development taking into account existing physical challenges.

Description of the study site

... located at the end of Longueuil and Montebelo Streets in Cantley Municipality,

ie. site is on the left bank of the Gatineau River just over 200 m upstream from

the Rapides-Farmer dam, which is operated by Hydro-Quebec., the target lot is

42,233m2, 237 m of bank by a depth of 166 m.

The site is composed of glaciomarine deltaic and prodeltaic sediments and is

included in a hypothetical and medium risk sliding zone according to the map.

Methodology

Geomorphologic analysis including hydrographic network mapping, surface

deposits, geomorphological constraints and bedrock depth was carried out by

photointerpretation, using orthophoto and elevation data from LIDA surveys.

Using ArcGIS software and elevation data, one-meter equidistance topography

and a (hillshade) terrain model were generated.

Photo-interpretation and mapping of the physical medium were performed concurrently on the

screen also using ArcGIS software. Homogeneous units (polygons) with respect to the

thickness of the surface deposits, their statigraphy and the profuse bedrock have been

delimited according to the legend of Howes and Kenk (1997), which classifies the terrain

according to different geomorphological characteristics. (surface deposits, surface

expression, stratigraphy, depth of root and geomorphological process).

For example: Nature of the surface material (WG: glaciomarine sediments)

Surface expression (p: plain <5% slope)

Nature of the underlying material (R: Roc)

Expression of the material of the underlying (u: corrugated)

A field visit was conducted on November 30, 2017 by a Stantec geologist. During

this visit, shallow soundings were conducted using a hand auger to validate the

results of the photo-interpretation and preliminary cardography. General

observations have also been made on the presence of rivers and

geomorphological processes. Following this visit, corrections were made, if

necessary, to the different map layers.

Results

Physiography:

The topographic curves (equidistance of 1 m) generated from the LIDAR data

show that the eastern portion of the site forms a flat surface with a fairly regular

surface and a slope generally less than 5%. The western and southwestern

portions of the site consist of an embankment whose slope varies most often

between 20 and 50%. In this sector, the height difference between the level of the

Gatineau River and the slope located at the top of the slope is approximately 12

-13 m.

The site comprises about ten rivers, generally less than one meter in width,

flowing from east to west. Four of these streams appear to have a steady flow

while the others appear intermittent. The largest watercourses correspond to the

drainage ditch extensions located on both sides of Montebello Street and flow to

the Gatineau River.

In general, observations made during the field visit show that the site has

deficient drainage. The presence of a layer of impermeable soft material (silty

clay) close to the surface promotes the accumulation of water on the flat located

at the end of the streets of Longueuil and Montebello and leads to the formation

of temporary pools.

The topography of the site and the location of the streams are shown in Figure 1

(Appendix).

Surface deposits and depth of bedrock:

The surface deposits encountered on the site were put in place during the marine

transgression of the Champlain Sea that followed the last deglaciation. Locally,

stratigraphy of surface deposits varies with rock topography within the boundaries

of the site.

In the northern half of the site, the loose material is thick and based on existing

drill data (exp., 2014), a layer of silt sand of 2.1 to 2.8 m based on a clay deposit.

silt tree about 8.4 m thick (exp., 2014) before reaching rock about 10.7m below

the surface. The concave shape of the riverbank in this area is indicative of the

absence of rock outcrops in the Gatineau River, as the materials show little

resistance to erosion. In this sector, the slope connecting the bank to the top of

the shelf is affected by regressive geomorphological processes that have led to

the establishment of colluvial deposits. This type of deposit results from the

remobilization of existing materials under the effect of the slope process and

gravity (eg landslide).

In the south half of the site, the surface aerials, mainly silty sand, are generally

thin and have several pebbles and blocks on the surface. The rock is generally

shallow (<3m deep) and is outcropping on the high points of the valleys. edge of

the Gatineau River. The convex shape of the bank and the lack of a retrogressive

process also testify to the presence of rock at shallow depths.

According to the available data (sigeom, 2017), the rock lithology in place consists

of paragneiss, a gneiss resulting from the deformation of sedimentary rocks.

Mapping of surface deposits, including surface expression, stratigraphy, and

material thickness, is presented at the scale of 1: 1250 in Figure 2 (Appendix).

The depth of the rock interpreted from the shading field model and the field

observations is presented in Figure 3. The legend used makes it possible to

distinguish the surfaces or the rock is less than one meter of depth, from 1 to 3 m

of depth and is more than 3 m deep. The rocky outcroppings observed during the

field visit are identified by an X.

Geomorphological process:

Two geomorphological processes that can significantly constrain park

management have been identified in the northern half of the site. In this sector,

the presence of a thick clay deposit contributes to the setting up of retrogressive

processes.

The presence of streams flowing from the surface of the shelf to the Gatineau

River led to the incision of ravines through the clay deposits. One of them is

actively eroding and is incised up to two meters deep in the loose deposits, over a

length of nearly 50 m from the bank (photo 1). The zones affected by the gully are

identified by the geomorphological process V on the map of surface deposits

(Figure 2).

The scouring of the foot of the embankment, under the effect of the waves and

ice, also led to the triggering of landslide. These gravitational movements led to

the gradual decline of the riparian slope until a temporary equilibrium slope was

reached. Under the effect of the erosion of the foot of slope, the slope of

equilibrium can again be unbalanced and a new landslide can follow. This

continuous erosion process is probably at the origin of the concave shape of the

bank.

The southern half of the site is not favorable to the implementation of this type of

erosion process leading to the decline of the slope, since the rock is at shallow

depth or is flush.

Revision of the geotechnical report (exp, 2014)

The firm EXP. conducted drilling in the month of October 2014 to assess the risk of

occurrence of highly retrogressive landslides and to provide recommendations for

foundations and load-bearing capabilities, freeze protection, set-up from a floor

slab, to drainages as well as to the pavement structure. The slope stability

assessment was carried out according to the four criteria established by F.Tavenas

in an article entitled (Landslides in Canadian Sensitive Clays - State of the Art) The

results obtained conclude that the sectors of the drilling F1 and F2 (Figure 3)

present a zero risk of highly retrogressive slip and that the F-3 drilling sector could

not present a major risk of large landslide. The study proposes, among other

things, the recommendation to preserve a slope edge protection strip to delimit

the safe zone.

Based on our reading of this study, no conventional stability analysis providing a

safety factor of 1.5 (recommended factor) has been conducted. Only the risk of

highly retrogressive slip has been evaluated. In the event that a bank stabilization

structure is developed to provide public access to the bank, a stability study

should be conducted south of the site.

Conclusion and recommendations

The site targeted by the Municipality of Cantley occupies a relatively flat ground in

its half east whereas its half west is dominated by a slope of 12-13 m height

compared to the level of the Gatineau river below.

The site occupies a deposit of glaciomarine origin, or its northern half presents a

layer of silty sand with an observed thickness ranging from 0.8 to 2.7 m thick,

which rests on a deposit of silty clay more 10 m thick (exp, 2014). The southern

half of the site has a surface expression controlled by the underlying rock

topography at shallow depths of 3.5 and 1.1 m in drill holes F-1 and F-2,

respectively (exp. 2014)

This landscape configuration results in the northwestern portion of the site being occupied by

a 30-40% tilt slope in a silty clay deposit more than 10 m thick. This embankment is affected

by retrogressive geomorphological processes such as gully and landslides.

Summary assessment of soil stability:

In the geomorphological and stratigraphic context, soil stability is an important

issue for park management.

The geotechnical study conducted by the firm EXP. (2014) concluded that there is

no risk of a highly retrogressive slip in the F-1 and F-2 drilling sector (south sector

of the site) and that the F-3 drilling sector (north sector of the site) could present

a major risk of large-scale landslide.

However, in the context of the development of a municipal park providing for

riverine developments, the safety of users and the sustainability of public

investments seem to us to require a more detailed analysis scale than the risk of

highly retrogressive sliding (sliding of large extent). Given the current erosion

processes along the northern half of the riparian slope, it seems that the risk of a

landslide, even modest and involving a small volume of materials can not be

dismissed. The deciding of a mass movement of a few tens of cubic meters would

be enough to threaten the physical integrity of the users and the durability of the

installations.

Recommendation

• Inasmuch as the municipality would like to offer public access to the

shoreline in the northern portion of the site, a coventional stability study

should be conducted to establish a bank stabilization structure. This would

require the completion of new surveys, the extent of which should be

determined according to the planned developments. In addition, the

protection strip at the edge of the concave bank does not seem imperative

to us since the nearness of the rock significantly reduces the risks of

instability associated with the erosion of the bank;

• Site planning should be done in such a way as to minimize tree cutting on

the edge of the concave bank section to limit erosion of waves and floating

ice. The development of a breakthrough or access road to the bank should

be localized in the area where the bank is convex and where the proximity

of the bedrock limits the risk of landslides or landslides. deep ravine

incision.

• The cutting of trees should also be reduced as much as possible along the

edge and along the important slope connecting the bank to the shelf is 12-

13 m higher. The cutting of trees on this slope inclines could favor the

triggering of gravitational process.

• In general, all shoreline settlements (wharf, kiosk, etc.) should be located in

the convex bank (southern portion), rock-controlled, rather than in the

concave bank area (northern portion), which is more favorable to the

triggering of gravitational movements;

• During the site visit, many stacks of waste (building materials, containers,

etc.) were observed. The completion of an Environmental Site Study - Phase

I ESA may be desirable to assess the potential for soil contamination at the

target site to ensure adequate management of the excavated soils in the

presence of contaminated soils;

• A large portion of the flat located at the end of Longueuil and Montebello

streets has a deficient drainage, and at the end, an accumulation of organic

matter 10-15 cm thick resting on silty sand. During the field visit, pools of

water were also present on this flat. This environment could be recognized

as wet (wetland) and should be further characterized by a biologist

specializing in wetland delimitation. If this is the case, a certificate of

authorization from the MDDELCC should be obtained under section 22 of the

EQA to develop it.