A geotechnical investigation was completed for the proposed design and construction of two (2)
pedestrian bridges adjacent to the existing CP Rail bridges across the Otonabee River in the City of
Peterborough. The proposed bridge design consisted of one three-span (West Bridge) and one singlespan (East Bridge) structural steel through truss bridge across the river. The West Bridge consists of
three simply supported steel trusses supported by two new concrete abutments and the existing railway
bridge piers. The East Bridge consists of a single span steel truss and will be supported on new concrete
abutments.
A borehole investigation was undertaken at the location of the proposed abutments for the new bridges. A
barge mounted drill-rig was employed to position the boreholes as close to the proposed abutments as
possible. Soil sampling and rock coring was completed at the proposed abutment locations. Based on the
investigation results, foundation options for the abutments were developed and included a combination of
shallow and deep foundation options using spread footings and H-piles to derive support from the
underlying bedrock surface. Given the presence of boulders within the subsurface soils, helical piles were
not considered a viable option.
Geotechnical recommendations were developed for the design and construction of an earth retaiing
structure for the proposed walkway on the island between the two bridges. Recommendations for
groundwater seepage, temporary cofferdam construction, temporary shoring systems, vibration
monitoring and settlement monitoring were developed in accordance with the most recent edition of the
Canadian Highway Bridge Design Code (CHBDC).
Cambium Environmental Inc. (Cambium) was retained by Sanchez Engineering to complete a
geotechnical assessment for the proposed design and construction of one three-span and one single
span structural steel through truss bridges adjacent to the existing CP Rail bridges across the Otonabee
River in the City of Peterborough. It is understood that the three-span West Bridge will consist of three
simply supported steel trusses, and will be supported by two new concrete abutments and the existing
railway bridge piers. The East Bridge will consist of a single span steel truss and will be supported on new
concrete abutments. A borehole investigation was conducted at the location of the proposed abutments
for the new bridges. Our findings and recommendations for design and construction of the bridge
foundations are discussed herein.
Four (4) boreholes, designated as BH101-12 to BH104-12 were advanced under the supervision of
Cambium personnel using a barge mounted drill rig equipped with continuous flight hollow stem augers
and split spoon samplers. The boreholes were located as close to the proposed abutments of the two
bridges as possible, given the access constraints of the barge based drill-rig. The boreholes were
augered to refusal on limestone bedrock at depths of 9.4 m to 10.6 m. Upon auger refusal, limestone
bedrock at the borehole locations was cored to depths of 10.0 m and 11.8 m (boreholes BH101-12 and
BH102-12 respectively) using NQ rock coring equipment. The recovery and Rock Quality Designation
(RQD) of the rock cores was measured in the field and the rock cores were placed in dedicated, labelled
core boxes for review and storage.
It is understood that the existing walkway (closed) will be replaced with one three-span and one single
span structural steel through truss bridges adjacent to the existing CP Rail bridges across the Otonabee
River. It is understood that the three-span West Bridge will consist of three simply supported steel
trusses, and will be supported by two new concrete abutments and the existing railway bridge piers. The
East Bridge will consist of a single span steel truss and will be supported on new concrete abutments. A
borehole investigation was conducted at the location of the proposed abutments for the new bridges.
Foundation options for the pedestrian bridges include deep foundations using driven H-piles which derive
their support from the bedrock surface at depth or, in the case of the east abutment for the East Bridge,
shallow foundations bearing on very dense glacial till soils. Recommendations for these bridge foundation
options are provided below.
Helilcal piles are not considered a viable option for the bridges based on the presence of boulders within
most of the subsurface soils encountered in the boreholes. Helical piles hung-up on boulders within loose
or soft soils would not achieve their design load capacity.
At the location of the proposed east abutment for the East Bridge, subsurface conditions consist of a thin
layer of compact sand and gravel overlying dense to very dense gravelly silty sand glacial till soils which
also contain some clay. Based on the density of the glacial till and the clay content which provides some
cohesion and a low hydraulic conductivity, consideration could be given to using a shallow foundation for
this abutment. Shallow foundations are not considered appropriate for the other three abutments due to
the presence of loose sandy and silty soils beneath the river bed.
It is understood that the proposed walkway on the island between the two bridges will require the
construction of an earth retaining structure, and that a mechanically stabilized earth (MSE) structure,
gravity wall or a cantilever wall are being considered. All of these options should work for the proposed
structure, however an MSE wall or a segmental gravity wall may be more appropriate as they can
generally tolerate minor amounts of differential settlement. In addition, potential excavation depths will
be limited by groundwater seepage from the base and walls of any excavations below the river level and
this may not allow for adequate embedment depth for a cantilever wall unless it is founded on piles and
a grade beam.
Even at this elevation, significant groundwater seepage should be expected into any excavations. In
order to maintain the stability of the subgrade soils, excavations for the granular wall base should be
completed progressively in short sections, with clear crushed stone backfill wrapped in a geotextile filter
fabric placed in the excavation immediately following excavation.
Temporary cofferdams will be required to facilitate the construction of pile caps and abutments, and for
the extension of the two existing piers along the West Bridge. Sheet piling is not a practical option for
the cofferdams due to costs, the depths to which the sheet piles would have to be driven, and the
presence of boulders. Cofferdams constructed of sandbags full of pea gravel sitting on and surrounding
a polyethylene membrane, as specified on the preliminary design drawings, would be an appropriate
design
In general, driven soldier piles and lagging are considered the best option for any necessary temporary
shoring systems for this project as there will likely be pile driving equipment onsite for the abutment piles,
and steel sheet piling would be difficult to drive through the cobbles/boulders and dense glacial till
encountered in some of the boreholes Soldier piles and lagging are suitable where it is necessary to
maintain a vertical excavation wall and where no heavily loaded foundations lie within the influence of the
shoring.
. Vibration monitoring should therefore be considered during pile installation to ensure that the
vibrations are maintained at tolerable levels
A monitoring program using settlement monitors is recommended to ensure there is no settlement in
the area of the CP Rail tracks during or after construction of the pedestrian bridges and the retaining
wall. Settlement monitors could consist of steel nails driven into the railway ties and appropriate survey
markings placed on the railway tracks and existing bridge abutments. T
The geotechnical investigation and report will be completed in accordance with the most recent edition of
the Canadian Highway Bridge Design Code (CHBDC), Guideline for Professional Engineers Providing
Geotechnical Engineering Services published by the Professional Engineers of Ontario, and the
Canadian Foundation Engineering Manual.