Monitoring the stability of highway cut slopes utilizing drone photogrammetry

Abstract

Roads have long been an integral asset for civilization. A lot of things have been said, drawn and shown regarding what and what not to do while drafting the routes. No matter how conscientious they are planned and laid out, mishaps occur while placing them on terrain. The demand to tie two points with the shortest possible route, create cut slopes and embankments. They both are bolstered with engineering reinforcements, however the cut slopes, which are always in sight while driving through, gave way in majority of time if those engineering principles are not sufficiently applied. Slope failure occurs when the downward movements of material due to gravity and shear stresses exceeds the shear strength. Therefore, factors exploiting these critical dynamics, increase the chances of slope failures. Six crumbling cut slopes en route to Karabuk from Kastamonu province were investigated in terms of the stability dynamics. Through stereo photogrammetric evaluation, digital surface models, which were sensitive to 0.08 to 0.22 m ground resolutions were constructed. Additionally, soil samples to understand the physical and chemical compositions of the cut slopes were taken from two depths; 0-15 cm and 15-30 cm. Analyses of the models showed that the considerable parts of all of the cut slope areas were graded over 87%, which was the collapse threshold for unprotected or untreated soil surfaces. Lab analyses showed that the binding agents e.g. organic matters, lime, etc. were rather weak because no vegetation to stabilize the already
steep cut slopes was present on any of them.

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* Corresponding Author
Cite this study
(ufukkader@outlook.com) ORCID ID 0000-0002-8020-0031
*(aoaltunel@kastamonu.edu.tr) ORCID ID 0000-0003-2597-5587
Kader U & Altunel A O (2021). Monitoring the stability of highway cut slopes utilizing
drone photogrammetry. 2nd Intercontinental Geoinformation Days (IGD), 257-259,
Mersin, Turkey
2nd Intercontinental Geoinformation Days (IGD) – 5-6 May 2021 – Mersin, Turkey
Intercontinental Geoinformation Days
http://igd.mersin.edu.tr/2020/
Monitoring the stability of highway cut slopes utilizing drone photogrammetry
Ufuk KADER1, Arif Oguz ALTUNEL*2
1Geomatics Engineer, Municipal Infrastructure Affairs, Kastamonu, Turkey
2Kastamonu University, Department of Forest Engineering, Kastamonu, Turkey
Keywords
ABSTRACT
Drone photogrammetry
Digital surface models
Slope stability
Roads have long been an integral asset for civilization. A lot of things have been said, drawn
and shown regarding what and what not to do while drafting the routes. No matter how
conscientious they are planned and laid out, mishaps occur while placing them on terrain. The
demand to tie two points with the shortest possible route, create cut slopes and embankments.
They both are bolstered with engineering reinforcements, however the cut slopes, which are
always in sight while driving through, gave way in majority of time if those engineering
principles are not sufficiently applied. Slope failure occurs when the downward movements
of material due to gravity and shear stresses exceeds the shear strength. Therefore, factors
exploiting these critical dynamics, increase the chances of slope failures. Six crumbling cut
slopes en route to Karabuk from Kastamonu province were investigated in terms of the
stability dynamics. Through stereo photogrammetric evaluation, digital surface models, which
were sensitive to 0.08 to 0.22 m ground resolutions were constructed. Additionally, soil
samples to understand the physical and chemical compositions of the cut slopes were taken
from two depths; 0-15 cm and 15-30 cm. Analyses of the models showed that the considerable
parts of all of the cut slope areas were graded over 87%, which was the collapse threshold for
unprotected or untreated soil surfaces. Lab analyses showed that the binding agents e.g.
organic matters, lime, etc. were rather weak because no vegetation to stabilize the already
steep cut slopes was present on any of them.
1. INTRODUCTION
Roads are the foremost infrastructural need for the
societies to establish secure transportation and
commerce. The principle road building approach is to
connect two points with the shortest possible route
((Umrao et al. 2015), however, geologic and topographic
conditions are not always favorable for this principle to
be laid down, flawlessly. In order to draft and
materialize a road route, the mentioned difficulties must
be eliminated so that a reasonably through right of way
is constructed (Gorcelioglu, 2004). Due in fact to the
geologic and topographic terrain conditions faced in
majority of our country, the routes are planned and
constructed over not so ideal ground, thus many cut-
slopes and the corresponding embankments have to be
devised to finalize the roadbed, effectively. In order to
control the slope stability problems resulting from the
steep slopes during construction, engineering solutions
must be entegrated into the projects (Senturk, 1989).
Six such cut slopes, which have long been eroding
especially during wet seasons, were investigated to
determine the underlying reasons trigerring the mass
movements year after year. Drone photogrammetry was
used to efficiently model the slopes along with the soil
analses for composition and physical characteristics.
2. STUDY AREA
Six cut slopes varying in size on Kastamonu-Karabük
intercity road were investigated for slope failure in this
study.

2nd Intercontinental Geoinformation Days (IGD) – 5-6 May 2021 – Mersin, Turkey
258
Figure 1. Studied cut slope locations on the intercity road en route to Kastamonu from Karabuk
3. METHODOLOGY
Each cut slope was individually flied over utilizing a
preprogrammed drone, DJI Phantom 3 Professional, to
model the surface(s), meticulously. Simultaneously, soil
samples were collected from the upper undisturbed
edges of the slope areas. The samples were taken from
two depths, 0-15 cm and 15-30 cm to analyze the
physical and chemical compositions of the slope
surfaces. In majority of the studies conducted to assess
the slope stability, the term, factor of safety, (FS), is
rather important in better understanding and definition
(Duncan et al. 2014). When FS is explained in terms of
the slope gradient intervals; “stable”, “critical range”
and “fail certain”, can be grouped as followed: < 87%,
87% < x < 148% and 148% < , respectively (Zakaria et
al. 2018) (Figure 2).
Figure 2. Site 2 slope classification resulting from 0.09 m DSM
4. RESULTS
Six digital surface models (DSM) with relatively high
spatial resolutions; 0.22 m for site 1, 0.09 m for site 2,
0.08 m for site 3, 0.1 m for site 4, 0.14 m for site 5 and
0.15 m for site 6, were constructed for slope
classification analyses. Slope classes were generated as
a function of the vertical elevation gain/loss within a
given horizontal distance, thus each slope area was
dissected according to the slope classes specified above,
and the acreages were calculated, accordingly (Table 1).
As clearly visible and suspected from the visual
observations, the cut slopes were unfortunately not
constructed to stay stable in the long run. The acreage
amounts regarding the “critical” and “fail certain” slope
classes were obviously less than the ones aggregated in
“stable” slope class in all cut slope surfaces, however, we
all know how erosion which might innocently start in a
rather small part of a catchment, could trigger large
mass movements when the favorable conditions
materialize. When the acreages were compared
percentagewise, stable parts vs. unstable parts, the
results were nowhere near innocent. The worst was in
site 6 in which exactly one third of the entire slope area
was on the unstable slope classes. The least affected one
still amounting almost one fifth of the entire slope area
on the unstable slope classes was site 5. It was not clear
from the results of this study that the very existence of
these dangerous slope classes was initially there or
were gradually developed due in fact from the relentless

2nd Intercontinental Geoinformation Days (IGD) – 5-6 May 2021 – Mersin, Turkey
259
atmospheric effects hammering on the bare slope
surfaces. However, if this conclusion was the result of
the imperviousness during the construction or
unavoidable circumstances due to some other
constrains e. g. the need to remove more vegetation
beyond the upper reaches of the slopes to lengthen the
slope surfaces, and the excessive amount of surface
stripping needed to compensate the steepness, the
current situation in the studied cut slopes or in many
others alike has not been good in terms of the functions
assigned to them in the first place.
Soil analyses conducted on all of the cut slope
surface areas showed that clay heavy two investigated
depths lacked rather less binding agents, organic matter
and lime. The vegetation removed during the grading
left the slope surfaces vulnerable to precipitation. The
already weak organic matter formed under the
coniferous stands was quickly washed away when the
slope surfaces were prepared, terraced or graded.
Generally, sponge like this top coat on soil surfaces
limited the surface runoff better dissipating the water.
The same organic matter on unstable slopes, on the
other hand, was quickly ravaged by the same water.
Table 1. Cut slope surface area classifications based on the “stable”, “critical” and “fail certain” slope percentages
Site 1 classification
Site 2 classification
Site 3 classification
Slope (%)
Acreage (m2)
Acreage (m2)
Acreage (m2)
x < 87
214144.72
86265.87
53830.06
87 < x <148
21279.15
8250.08
4822.65
148 < x
37574.47
9693.67
7132.64
Total area (m2)
272998
104210
65785
Site 4 classification
Site 5 classification
Site 6 classification
Slope %
Acreage (m2)
Acreage (m2)
Acreage (m2)
x < 87
61416.33
119506.33
70197.90
87 < x <148
6707.54
8687.10
7343.13
148 < x
11507.16
13875.62
15692.90
Total area (m2)
79631
142069
93234
5. DISCUSSION and CONCLUSION
When the cut slopes were left to stand on their own,
they were mostly furnished with engineering
reinforcements to keep them from crumbling if the
gradients were steeper than the proven safe. However,
these have been rather expensive protective measures,
which have been overlooked frequently. When either
the cut slopes or the embankments were left untreated,
they would eventually erode jeopardizing the lifespan of
the main infrastructure that they had actually been
erected to protect. Whenever vegetation was removed
on flat or inclined surfaces, erosion has become a
problem varying in severity depending on the
steepness. Six cut slopes investigated in this study were
just manifesting the types of such adversities. The soil
eroding from the slope surfaces was overflowing the
retaining walls installed at the slopes’ bottom heels.
Although furnished with some terracing, they were all
on bare soil without any protective measure. Such
slopes must first be seeded immediately after
construction to form a perennial shrub cover with long
root systems. A balanced mix of deciduous and
coniferous tree species must also be planted to bolster
the protection in the long run. Drones have been very
powerful and practical in purpose build such case
studies. The precision they provided, was only
surpassed by light detection and ranging (LIDAR). For
periodic status monitoring, there is no better priced
alternative to drone remote sensing today. All
considered, the cut slopes and embankments must be
built with the utmost attention they deserve because
there are means to capture the misdeed, flawlessly.
REFERENCES
Duncan, J. M., Wright, S.G., & Brandon, T.L. (2014). Soil
Strength and Slope Stability. 2nd edition. John Wilet
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Gorcelioglu, E., ( 2004). Orman Yolları-Erozyon İlişkileri,
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