DFI Magazine May/June 2020 - 83

certified as small UAS pilots were on site the
following morning. The city communicated
with nearby property owners to inform
them of the planned work. One field
member flew the drone while the other
acted as a visual observer. Fieldwork
consisted of distributing 10 GCPs throughout the site. City surveyors also set GCPs to
confirm location data. The points were well
distributed to ensure accuracy, with half the
GCPs being placed on the upper part of the
bluff and half being placed near the bottom
of the site that is by Wabasha Street.
The rock slide area was near Holman
Field (the city's downtown airport), and in
what is considered restricted airspace. The
engineering firm obtained an Federal
Aviation Administration (FAA) airspace
waiver that allowed inspection work to be
performed (FAA Part 107 Small UAS Rule).
The project area was flown from two
locations: one at the top of the bluff and one
near the bottom of the bluff. The site was
challenging due to the large elevation
change involved and the need to map not
only horizontal, but also vertical, surfaces of
the face. Moderate wind at the site was accelerated at the top of the bluff and approached
25 mi per hour (11 m per second).
High-resolution images were collected
using a preprogrammed autonomous
mapping mission, and using interactive
flights where the pilot was manually flying
the drone. The interactive flights took
advantage of the drone's cruise control and
image auto trigger feature to collect images
of the face of the bluff and the slide area.
The drone handled the wind well, but
the vegetation became hard to process
because of its movement during the flights.
Despite these challenges, the data was
successfully collected in less than four
hours (including setup).
Data Processing - The high-resolution
images were post processed using Pix4D
software to create the project deliverables.
The processing was performed locally on a
high-end computer and also on the Pix4D
cloud processing service. The GCPs used by
the firm and by the city surveyor were incorporated into the resulting site model to
ensure relative and absolute accuracy. Delivverables included an orthomosaic map of
the area to determine site construction limits
and a triangular mesh model of the site.

Wabasha Street rock slide Pix4D map and elevation profile example

Results - The field investigation
proved to be an efficient, safe, cost-effective
way to collect and process the large
amounts of data needed. The entire data
collection effort was accomplished without
any staff having to enter areas that were
considered unsafe or unstable. The
resulting maps and models of the impacted
site were useful to engineers in several ways:

click on that point on the model, and all
photos will be displayed of that particular
element being inspected. The cloud
version also includes annotation tools to
communicate results more effectively.
The Wabasha Street rock slide cloud
model is available at https://cloud.pix4d.
com/pro/project/304433/3d?shareToken
=4548e50ce69f494cbf2b7e96d7dad6f6

* To easily determine the profile of the
bluff at various locations without
manual surveying of the slide area

Deep Foundation Construction Site

* For the geotechnical engineer to
determine the geometry of the slide area,
using the 3D model and point cloud
* For city staff to communicate the extent
and the impact of the rock slide to
stakeholders
* To compare the extent of the slide to
property lines to determine if private
property was affected by the slide,
using the 2D orthomosaic map of the
area (which is dimensionally correct
and can be used to measure distances
and to convey the extent of the slide)
Pix4D includes a cloud version that enables
the sharing of the map and models without
the need to transfer large amounts of data
or for the end user to have Pix4D software.
The cloud version has a virtual inspector
tool that serves as a 3D photo log of the
inspection. Once a model is created from
the images, the photos are linked to the
model. As a result, if a particular area of a
project site needs to be reviewed, a user can

The commercial redevelopment of the
O'Gara's site in Saint Paul, Minnesota, is
currently underway. Ryan Companies is
the general contractor and Bolander is
performing the deep foundation work.
The project repurposes the footprint of a
restaurant and parking lot, as well as three
single family homes. The design proposed
for the new Harper Apartments includes
163 apartment units, 205 parking stalls, an
integrated coworking space, apartment
amenity spaces and retail space. The
roughly 2 acre (0.81 hectare) project is
located in an urban area, and deep
foundation shoring is being utilized
during construction.
Fieldwork - Drone fieldwork was
completed January 3, 2019. GCPs were
placed throughout the project site so that
the final deliverables would be survey
quality for both relative and absolute
accuracy. Two autonomous flights were
conducted that collected 298 highresolution images. The flights were
conducted from the job site, with the entire
DEEP FOUNDATIONS * MAY/JUNE 2020 * 83

DFI Magazine May/June 2020

Table of Contents for the Digital Edition of DFI Magazine May/June 2020