June 28, 2013
Gerald Green
Director
Jackson County Planning Department
401 Grindstaff Cove Road, Suite A-258
Sylva, NC 28779
Dear Gerald,
Please find included here our report to the Jackson County Planning Board regarding the
association of slope angle and development practices to the occurrence of landslides. In this
report we have strived to include data from the NC Geological Survey’s landslide database and
Appalachian Landslide Consultants’ landslide database, as well as information from a variety of
outside references and sources.
We have also included items that we found during research that you didn’t ask for, but we
thought would be a positive addition to the report. There is a “Useful References” list that the
Planning Board members can use for more detailed or comprehensive discussions relating to
development on slopes. Appendix 1 shows an example of setback requirements. Appendix 2
discusses site investigation and design guidelines.
We appreciate this opportunity to provide this report to the Planning Board so that they may
make informed decisions when developing guidelines for development in Jackson County.
Sincerely,
Appalachian Landslide Consultants, PLLC
Jennifer Bauer, L.G.
Principal Geologist/Co-owner
Stephen Fuemmeler, L.G.
Principal Geologist/Co-owner
Table of Contents
1. Introduction …………………………………………………………………………………….
1.1 Importance of a Geotechnical Investigation
1.2 Slope Stability – The Basics
1.3 Stability Analysis
2. Development/disturbance of slopes vs. slope failure ………………………
3. Compaction………………………………………………………………………………………
4. Height of artificial slopes..…………….…………………………………………….……
5. Constructed Slope Gradient …………………………………………………………….
6. Cuts and Fills……………………………………………………………………………………..
7. Benching of artificial slopes………………………………………………………………
7.1 Benching in soil
7.2 Benching in rock
8. Setbacks for structures …………………………………………………………………….
8.1 Length of areas affected by landslides
8.2 Other setback references and guidelines
9. Other useful references of good design and building practices………..
10. Cited References …………………………………………………………………………….
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Appendix 1 - Clark County, Washington state setback requirements…….
Appendix 2 - Site Investigation and Design guidelines……………………..…….
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1. Introduction
The purpose of this report is to help answer some of the questions that the Jackson County Planning
Board has about the relationship between development and slope disturbance and landslide potential.
We have organized this report according to specific topics related to the modification of slopes, so that
it may become a quick reference guide. Each part may be read as a separate entity, although
information in the Introduction will apply to all sections of the report.
1.1 Importance of a Geotechnical Investigation
Although this report may give examples of guidelines for development used in other various parts of the
country or world, we simply include these as examples. These examples can be used to show when
there is a need for a more detailed, site-specific slope stability evaluation by a qualified licensed
geotechnical engineer and engineering geologist. Due to variability of the site and the project (including
gradient of slopes, soil and rock material, hydrology, intended design of development, desired
construction practices, project budget), each case may need its own stability analysis and design to meet
stability and project owner needs.
Guidelines for information that should be included in a geotechnical investigation can be found in the
International Building Code, Chapter 18, Section 1803.6 (ICC 2011). See also Appendix 2.
1.2 Slope Stability – The Basics
Simply stated, the stability of a slope depends on the forces resistant to sliding being greater than the
forces driving the sliding.
Resistant forces > Driving forces = Stability
Resistant forces < Driving forces = Instability
The two factors that have the greatest effect on slope stability are Slope gradient and Groundwater (US
Army, 1992).
Slope Gradient
Slope gradient, or the steepness of the slope, is affected by Normal force (perpendicular to the slope),
Frictional resistance to sliding, and Downslope force (driving force pulling object down) (Figure 1.1) (US
Army, 1992).
Normal
Frictional
resistance
Figure 1.1
Downslope
3
“A block of uniform soil fails, or slides, by shearing. That is, one portion of the block moves past another
portion in a parallel direction. The surface along which this shearing action takes place is called the shear
plane, or the plane of failure [Fig 1.2]. The resistance to shearing is often referred to as shear strength.
Pure sand develops shear strength by frictional resistance to sliding; however, pure clay is a sticky
substance that develops shear strength because the individual particles are cohesive. The presence of
clay in soils increases the shear strength of the soil over that of a pure sand because of the cohesive
nature of the clay” (US Army, 1992). The shear strength of a soil and the angle at which it will start
sliding along its grains (friction angle) are largely dependent upon the soil type, which is determined by
the size and the types of grains that make up that soil.
The stability of a soil block depends on the resisting forces of shear strength (frictional resistance to
sliding and cohesion of the particles) relative to the driving forces of the downslope component of the
weight of the soil. The steeper the slope gradient, the stronger the resisting forces must be to keep the
soil in place to overcome the driving forces.
Intact Block
Resisting force > Driving force
Shear strength
 Frictional
resistance
 Cohesion
Downslope
weight of soil
Failed Block
Shear Plane/Plane of Failure
Resisting forces < Driving forces
Figure 1.2
Shear strength
 Frictional
resistance
 Cohesion
Downslope
weight of soil
Groundwater
In addition to slope gradient, groundwater plays a large role in slope stability. There are two force
components of groundwater, the uplift force, and the seepage force. The uplift force reduces the
interlocking force on the soil particles, which reduces the frictional resistance to sliding. A thinner soil
mantle has a greater potential for sliding under the same groundwater height as a thicker soil mantle,
due to the uplift force being applied along more of the thickness of a thinner soil. However, adding soil
in the hopes of decreasing the uplift force may lead to increasing other factors that decrease frictional
resistance. All factors must be considered before modifying the slope (US Army, 1992).
4
Seepage force is the drag that water exerts on soil particles as it moves downslope due to gravity.
Seepage force is on the driving force side of the stability equation, since the water tends to carry soil
downslope with it. The best way to decrease the uplift force and seepage force of water is to properly
design a groundwater control system (US Army, 1992).
Permeability of the soil measures how well air and water can flow within the soil. Permeability
determines how soil will be affected by the uplift and seepage forces of water. Different soil types,
different compaction levels, and different soil origins can have vastly different permeabilities. In
general, sands and gravels are much more permeable than soils with more silt and clay (US Army, 1992).
This is due to the way the grains interlock causing friction, as well as the cohesion of the soil particles.
Sands and gravels tend to have more pore space, or space between the grains, therefore allowing water
or air to more easily pass through them.
Often times during heavy rain events, these pores can fill up with water to the point that the soil
becomes saturated and pressure begins to build on the soil grains. This increase in pore water pressure
can push the grains apart so they can no longer stay together, leading to a slope failure.
1.3 Stability Analysis
There are many methods to determine the stability of a slope. We will not go into these in detail, as the
appropriate method should be determined by the geotechnical engineer during evaluation of a specific
site. However, in this report you will see discussions of Factors of Safety (FS). The Factor of Safety is a
ratio of the resisting forces to the driving forces, or the resisting forces divided by the driving forces.
Slopes that have a FS greater than 1 are considered stable.
FS =
Resisting Forces
Driving Forces
We give you this refresher because the following sections have to do with either increasing the resisting
forces or decreasing the driving forces in order to stabilize a slope.
2. Development/disturbance of slopes vs. slope failure
What is the relationship between development/disturbance of slopes and slope failure or landslide
activity?
If not designed and constructed properly, development and disturbance of mountain slopes can
oftentimes lead to slope failure, especially in areas where the FS is naturally approaching 1 before
modification.
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Bromhead (1992) stated that the critical slope-destabilizing factors, in order of importance for most
cases, are
● Increase in pore-water pressures
● Removal of support at the toe either by erosion or by excavation
● Changes in soil or rock shear-strength properties
● Seismic loading
● Loading at the head of the slope by placement of fill or structures
When evaluating the data in the North Carolina Geological Survey’s (NCGS) and Appalachian Landslide
Consultants’ (ALC) databases, we see that 26.8% of landslides have occurred on slopes that have been
modified by human activity (See Table 2.1). Since 1940, 64% of all known landslides in western NC
started on slopes that people have modified. This post-1940 number better represents landslide activity
that has taken place during more recent development activities. It also excludes a large dataset
collected on over 2000 landslides that occurred in Watauga County during the August, 1940 storms.
In Jackson County, 60.8% of currently mapped landslides have been on modified slopes (Table 2.2). (The
Jackson County dataset is an incomplete one, as field verification of many landslides has not been
completed).
Types of Slope
Movements
Creep/Subsidence
Debris/Earth Slide
Debris/Earth Slide/Flow
Debris/Earth Blowout
Debris/Earth Flow
Modified
Unmodified
unknown
Total
4
3
7
307
78
2
387
48
86
1
135
5
298
303
221
1971
1
2193
Rock Fall
27
3
1
31
Rock Slide
74
4
78
Weathered Rock Slide
102
12
114
Unknown
131
12
32
175
Total for All Counties
919
2467
37
3423
% Total for All Counties
26.8%
72.1%
1.1%
Table 2.1: Statistics as of 06/25/2013 from the NCGS and ALC databases by
landslide type. Landslide type in general accordance with Cruden, D.M and D.J.
Varnes (1996).
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Modified
Jackson County
79
Unmodified
Unknown
45
Total
6
% of All
Counties
130
3.8
% of total - Jackson
County
60.8%
34.6%
4.6%
Table 2.2: Statistics as of 06/25/2013 from the NCGS and ALC databases for Jackson County
County
All Counties
% of total – All Counties
Jackson County
Cut Slope
Fill
Other/
Total
% of Total
Embankment
Unknown
26.8
430
471
18
919
46.8%
51.3%
2.0%
100.0
38
39
2
79
60.8
% of total – Jackson
48.1%
49.4%
2.5%
100.0
County
Table 2.3: Statistics as of 06/25/2013 from the NCGS and ALC databases on failures on different types
of modified slopes.
Table 2.3 shows statistics for the number of failures that have occurred on cut slopes and embankment
fills in the entire database, and in the incomplete Jackson County data. This shows that the occurrence
of cut failures and fill slope failures is very close to equal, indicating that both need attention during
design and construction.
When building in mountain terrain, cuts and fills are inevitable. If unstable slopes are identified before
design and construction, the expense to avoid the problem areas can be minimized. If avoidance is not
possible, then efforts to increase the resisting forces and reduce the driving forces must be taken to
ensure stabilization (Holtz and Shuster, 1996).
In the case of unstable terrain, the US Army Field Manual, Chapter 10, (1992) says that “radical changes
in road grade and road width may be required to minimize site disturbance. Excavated material may
need to be hauled away to keep overloading of unstable slopes to an absolute minimum. The location of
safe disposal sites for this material may be a serious problem in steep terrain with sharp ridges. Site
selection [for excavation material] will require just as much attention to the principles of slope stability
as to the location and construction of the remainder of the road.” This statement serves as a good
reminder that excavated and waste material must be disposed of in a stability conscious manner.
3. Compaction
Would compaction of disturbed areas (road beds, building pads, etc) reduce the likelihood of slope
failure/landslide activity?
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Compaction of the soil in the disturbed areas increases the forces resistant to failure. Compaction
increases the density of the material, reduces the pore space, and thereby reduces the adverse effect of
ground water (US Army, 1992). Compaction generally increases the soil’s shear strength and decreases
its compressibility (Gresser)
In one study by Iverson et al. (2000), compacting the soil led to lower hydraulic conductivity (the ease at
which a fluid moves through pore spaces or fractures in the soil or rock), increasing the amount of water
needed for failure. They showed that failure in denser compacted sandy soil tends to be more slowmoving and episodic than in the less compacted materials. In some cases, the movement in the more
dense soil was 300 times slower than the motion of a landslide in the loose soil. Friction angles at
failure, the angle at which the soil starts sliding, in the more compacted soil were higher than those in
the loose soil (see Table 3.1). In this study, “all compaction loads were applied normal [perpendicular] to
the slope. The longest compaction periods produced the lowest porosities, and vibratory compaction
produced more uniform porosities than did foot traffic.” This indicates that compaction over time, leads
to fewer pore spaces.
Table 3.1 from Iverson et al. (2000)
The US Army Field Manual, Chapter 10, (1992) points out that it is important to not only compact the fill
to reduce the risk of road failure when crossing small drainages, but to also evaluate the density of the
material that the fill will be placed upon to see if it will support the fill without failure. This evaluation
should take place during the design process. As indicated in a report by Lamb (1975), fill placed on
residual soils are common sites for particularly dangerous landslides in Hong Kong, although the concept
also applies to western North Carolina. Lamb (1975, 1996) says, “Generally residual soils have higher in8
place permeabilities than the same materials once they have been transported, mixed, and compacted.
Therefore, compacted fills often act as low-permeability caps, disrupting the natural seepage pattern
and increasing pore pressures.” It is important that the properties of both the material that fill is being
placed on, as well as the fill itself be taken into consideration when evaluating the stability of a slope.
Along the same line of thinking, the NCDOT Subdivision Manual (2010) notes that no base course for a
roadway (soil or aggregate) shall be placed on muck, pipe clay, organic matter or other unsuitable
material. We see many fill slopes with logging slash, tree roots, and branches in the embankment. This
material should not be placed in the soil, as it deteriorates over time, leading to voids and pathways for
water to further erode the slope.
The US Army Field Manual, Chapter 8, (1992) is entirely dedicated to information about Soil
Compaction. Table 3.2 indicates their minimum compaction requirements for different soils and
applications. In this table, compaction is based on the US Army Corps of Engineers (MIL-STD-631A) CE55
compact test given in TM 5-530, which is similar to the ASTM D-1557 Modified Proctor in the 6 inch
mold, as well as the AASHTO T-180 Modified in the 6 inch mold (US Army, 1992). It is important that the
compaction method in the field be correlated to the compaction test used in the lab in order to achieve
the best results.
Table 3.2 Minimum compaction requirements from US Army Field Manual Chapter 8 (1992).
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Table 3.3 from the US Army Field Manual (1992) compares different types of compaction tests. Different
methods are used depending upon the equipment that will be compacting the soil in the field. Many
guidelines suggest using the ASTM D-1557 or other modified compaction test, because equipment today
is heavier, and more closely correlates with the modified test compaction effort.
Table 3.3 Compaction test comparisons from the US Army Field Manual Chapter 8 (1992).
The US Army Field Manual discusses compaction of embankments and has recommendations for lift
thicknesses based on the type of soil and method of compaction. For silty sands and gravel-sand
mixtures (often found in WNC), the lift thicknesses vary from 6 to 12, depending upon the type of
compaction method (US Army, 1992, Chapter 8, Table 8-3).
The NCDOT recommends compacting each layer for its full width to a density equal to at least 95% of
that obtained by compacting a sample of the material in accordance AASHTO T-99 Standard compaction
test as modified by the Department (2012).
The USFS Low Volume Roads BMPs states that retaining wall backfill is typically compacted to 95% of the
AASHTO T-99 maximum density (Keller and Sherar, 2003).
For shallow foundations of buildings on compacted fill, the International Building Code recommends
Compacted fill material 12 inches (305 mm) in depth or less need not comply with an approved report,
provided the in-place dry density is not less than 90 percent of the maximum dry density at optimum
moisture content determined in accordance with ASTM D 1557. For fill greater than 12 inches in depth,
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a geotechnical report must include the minimum acceptable in-place dry density as a percentage of the
maximum dry density (ICC, 2011, section 1083.5.8)
4. Height of artificial slopes
Would limiting the height of artificial slopes reduce the likelihood that slope failure/landslide activity
will occur?
We could not find many studies directly linking height of a slope to the stability of it. However, height of
the modified slope should be taken into consideration when doing the stability analysis because each
slope will be different depending upon materials being used, and the steepness of the slope. Steepness
of the slope seems to be more of a factor than height with respect to stability, or the driving forces
overcoming the resisting forces
A Brand and Hudson (1982) study of 177 cut slopes in Hong Kong in residual soils indicates that cut
slopes with higher angles fail more often than lower angle cut slopes of the same height. This leads one
to believe that the slope angle may play a larger role in slope stability than slope height does. This also
indicates that increases in cut height and cut angle are inversely related to the stability of cut slopes.
(Lambe, P.C. 1996)
5. Constructed Slope Gradient
Because gradient, or steepness, of constructed slopes seems to be such an important factor to slope
stability, we have added a section on it here.
Table 5.1 from the USFS Low-Volume Roads Engineering Best Management Practices Field Guide (Keller
and Sherar, 2003) presents a range of commonly used cut and fill slope ratios appropriate for the soil
and rock types described for low-volume (less than 400 Average Daily Traffic) roads.
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Table 5.1 Common stable slope ratios for varying soil/rock conditions (Keller and Sharer,
2003)
In the NCDOT Roadway Design Manual (2013, Section 1-2 Figure F-1), they recommend using the
steepest practical slopes as determined by the NCDOT Geotechnical Unit in order to minimize cost and
rights of way issues. They recommend that “interstate slopes should not be steeper than 2:1 except in
rock excavation and Freeways and expressways should not be steeper than 1 ½:1 or 2:1.” The NCDOT
manual says theses slope ratios do not apply to subdivision roads, but these ratios match closely with
other references we have seen (Keller and Sherar, 2003). The NCDOT Subdivision Roads, Minimum
Construction Standards does not include a recommended slope ratio for cuts or fills (2010).
Most landslides in WNC start on natural slopes greater than 40% (~22o) or 2 ½:1, as shown in Figure 5.1.
In Figure 5.2, we take a closer look at the failures that started on slopes modified by human activity. We
compare these failures to the post-construction steepness of the slope. More than half of the failed
embankment slopes that have been mapped by ALC and the NCGS have modified slopes of over 82%
(40o) which is close to 1 1/4:1 (Fig 5.2).
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Natural Slope %
Figure 5.1. The number of landslides vs. the steepness of pre-construction, natural slopes. The
modified failures occurred on slopes disturbed by human activity. The unmodified failures occurred
on natural slopes( ALC and NCGS database as of 2/2013).
Figure 5.2. From the ALC and NCGS database as of 6/25/2013 Comparing the number
of cut slope failures and fill slope failures to the modified steepness of the slope in %.
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6. Cuts and Fills
When designing cuts and fills, it is important to consider the resisting forces and driving forces of the
slope before and after it is modified. Below is a figure from the US Army (1992) that shows how the
center of gravity of slopes can be changed by either loading the head or unloading the toe of the slope
(Fig 6.1). See how unloading the head and loading the toe, in the right example, lowers the center of
gravity and decreases potential for failure. It appears that some subdivision roads in western N.C. are
cut in without taking this careful balance into consideration.
Figure 6.1 Road construction across short slopes (US Government, Dept. of Army 1992)
Figure 6.2 and Figure 6.3 from the USFS Low-Volume Roads Engineering Best Management Practices
Field Guide show typical cut slope and fill slope design options, respectively, for varying slope and site
conditions. These are included as examples since on-site conditions can vary greatly. Slope stability and
design should be evaluated by a qualified professional on a site-specific basis. (Keller and Sherar, 2003).
They recommend full bench cuts for roads when the natural ground slope exceeds ~60% (see Figure
6.2b). Many of the slopes in Jackson County exceed 60%, yet full bench slopes may be uneconomical. In
these cases, proper design and construction of the fill is critical to avoiding failures, particularly in
drainages or stream crossing areas.
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Figure 6.2 Cut Slope Design Options (Keller and Sherar, 2003, Fig 11.1)
15
Figure 6.3 Fill Slope Design Options (Keller and Sherar, 2003, Fig 11.2)
16
Fill slope failures from the shoulders of the road (Fig 6.4) are the most common type of road
embankment failure we see in WNC. These areas are often not compacted as well, or are finished
without the same care as the road bed. Sometimes utilities are buried in the outside shoulders and
refilled without compacting the soil. This practice leaves uncompacted soil and provides a pathway for
water to flow along the utilities or between the more compacted soil of the road bed and the less
compacted soil of the shoulder. Even though shoulders are not as important to the function of the road
as the roadbed itself, they still must be constructed to adequate standards to prevent fill slope failures.
We often see an analogous situation when loose fill is placed on the outside of otherwise stable house
pads.
Figure 6.4 ALC photo of embankment failure from the shoulder of the roadway.
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7. Benching of artificial slopes
Would requiring benching of artificial slopes reduce the likelihood of slope failure/landslide activity?
7.1. Benching in soil
There are benefits and drawbacks to benching in soil. Benches help to reduce the height of a continuous
slope, but including them in soil slopes, may lead to over steeping of the fill slope. As in the height
discussion, steepness of the slope is more of a factor in stability than height. Terraces or benches can be
used in soil slopes to provide a way to move water off of a large slope and away from the fill (Keller and
Sharer, 2003). They may also catch any failing material from upper portions of the slope. Drainage on
these benches must be maintained to ensure that ponding of water does not occur. Ponding can lead to
saturation of the fill material downslope, leading to failure.
Benching native soil before adding fill can lead to an easier work surface for compacting fill. Benching
beneath fill better ties the fill into the natural slope, especially for long, steep fill slopes. This is a better
alternative to benching the fill material or to placing fill directly on top of the native material of steep
slopes, which may be close to the tipping point of failure already (Fig 6.3b)
According to Holtz and Schuster (1996), “....Benching has been used to reduce the driving forces on a
potential or existing landslide [by reducing the weight of material]... Benches also serve to help control
surface runoff and provide work areas for placement of horizontal dams.” They emphasize that the
effectiveness of the benches will be determined by the characteristics of the site and the design.
7.2 Benching in rock
Benching of rock slopes is more common. These benches provide a place for rock fall to collect.
However, according to Wyllie and Norrish (1996), “Excavation of intermediate benches on rock cuts
usually increases the rock-fall hazard.” Any rocks collected on these benches must be removed
periodically, which could be a maintenance issue. If rocks are not removed, the bench could fill with
rocks, and then be the cause of further rock fall downslope when rocks above bounce off the full bench,
or when rocks on the full bench fail.
8. Setbacks for structures
The Board is considering requiring that disturbed areas on properties with a slope in excess of 35% be
setback a minimum distance from property lines. Is there a certain setback that is recommended in
order to reduce the likelihood of impacts on the adjacent property? Should the setbacks increase as
the slope of the property becomes greater?
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Setbacks for disturbed areas can be derived in many ways. Perhaps the best way to address this
question is to look at a few established practices in use elsewhere. Before looking at those examples, we
will discuss the variables involved in determining the size of the area that might be affected by a
landslide and then look at the extent of areas affected by landslides in WNC by examining the data in
the ALC and NCGS databases.
8.1 Extent of areas affected by landslides
In typical residential developments, cut slope failures are generally stopped by the bench (i.e. road or
house pad) below the cut. They usually do not affect a large area, but can block roads or damage
houses. Cut slope failures tend to encroach 3 to 10 feet on the ground upslope at the time of failure.
Unstable cuts can also propagate instability upslope by undermining the toe of marginally stable slopes.
The area that might be affected in this scenario is influenced by many variables including rock type and
weathering, rock structure, hydrology, and slope. This information can only be determined by a detailed
site investigation.
Fill slope failures have the potential to travel over a thousand feet downslope, though the majority
travels less than 200 feet downslope. The main variables that affect the distance fill slope failures travel
is the amount of failed material, the presence of water, and the steepness of the ground below. Fill
slope failures that travel long distances are invariably classified as debris flows and are quickly routed
into topographic drainages. While it is unrealistic to have a setback of several hundred feet downslope
from all embankments, a setback of a couple dozen feet from drainages or areas identified as potential
debris flow pathways would be beneficial. Similar to cut failures, fill slope failures typically encroach 3 to
10 feet on the ground at the top of the embankment. This is usually confined to the shoulder of road
(Figure 6.4), but might affect the outer portion of the roadway, or any section built on fill. The inner
portion of the roadway is often on in-place rock or soil and may likely be spared.
Figure 8.1 shows the relationship between slope configuration (fill slope, cut slope, unmodified slope),
the length affected by the landslide, and the natural, pre-construction ground slope of the area. There is
a weak trend toward longer affected areas by landslides that originate on steeper ground, particularly
for fill slope failures. The most obvious pattern develops by looking at the lengths traveled vs slope
configuration. Cut slope failures, as expected, cluster at the bottom of the graph with short lengths. Fill
slope failures also cluster at the bottom of the graph but several traveled a considerable distance. The
unmodified slope failures exhibit a much more random distribution pattern with no obvious clusters.
This graph shows that the majority of the cut and fill slope failures in the databases travelled less than
600 feet downslope.
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Figure 8.1. From ALC and NCGS data as of 6/26/2013 showing length of areas affected by landslides vs.
the natural ground slope for both fill and cut slope failures. Note: Two data points were excluded to
better see the majority of the data (slope 43, length 5300; slope 45, length 5300)
Figure 8.2 shows the length of affected areas vs. the modified ground slope (or constructed slope angle)
for both fill and cut slope. We include this graph as a comparison to Figure 8.1 to illustrate the
differences between the steepness of the modified slopes and the natural slopes. This graph also shows
the constructed slope angles at which potential instability becomes a concern. Comparing the two
graphs also indicates that the slope of the original ground might have more to do with the distance the
failure traveled than the steepness of the constructed slope.
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Figure 8.2. From ALC and NCGS data showing length of areas affected by landslides vs. the modified
ground slope for both fill and cut slope failures. Note: Two data points were excluded to better see the
majority of the data (slope 43, length 5300; slope 45, length 5300)
8.2 Other setback references and guidelines
We have summarized three references that can be used as examples for determining setbacks from the
top and bottom of slopes. Keep in mind that all of these examples determine setback based on the
proximity to the top or bottom of the slope, not the proximity of property lines. These examples would
likely have implications on what is considered a buildable lot. The slopes in these examples are assumed
to be natural, but the same guidelines may apply to constructed slopes as well.
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8.2.1
Clark County, Washington state setback requirements (see
http://www.codepublishing.com/wa/clarkcounty/clarkco40/clarkco40430/clarkco40430020
.html and Appendix 1).
Clark County, Washington breaks their requirements up for two different areas, those on Steep Slope
Hazard Areas and those on Landslide Hazard Areas. Steep Slope Hazard Areas have development activity
on or within 100 feet of slopes steeper than 40% that do not have a mapped landslide hazard. On these
areas, they have buffers for development at both the bottom and the top of steep slopes. The buffers
are broken up for slopes between 40% and 100% and those greater than 100%. These buffers are
determined by a complex set of rules based on projected angles and slope height (see Appendix 1 and
Fig 8.3). Building setbacks then extend 8 feet beyond the buffers unless the project does not require a
landslide protection area.
These setbacks can be modified with a technical report demonstrating an alternative buffer provides the
same protection. Vegetation removal beyond approved areas is not permitted without a geologic hazard
area study.
For areas designated as Landslide Hazard Areas (Amended: Ord. 2005-04-15), the requirements are
slightly different. Landslide Hazard Areas are defined by a variety of criteria including past landslide
activity, having certain slope, geology, and hydrology indicators, and areas mapped by the Washington
Department of Natural Resources as having potential instability (see Appendix 1 for a full list). The
minimum buffer from all edges of the landslide hazard area is 50 feet, and can be extended or reduced
in certain cases.
22
Figure 8.3a
Figure 8.3b
Figure 8.3c
Figure 8.3. From Clark County, Washington Unified Development Code Title 40 Section 40.430.020
23
8.2.2
Moorpark, California; California Building Code
The city of Moorpark, CA has put together a summary of the California Building Code setbacks from the
top of slopes and toe of slopes to help the public understand the requirements (Young, R. 2011)
http://ci.moorpark.ca.us/moorparkcity/img/hc.pdf.
“3. Setback from top of slope: The bottom of the house foundation must be setback from the face of the
slope (measured horizontally) a minimum of ⅓ the vertical height of the hillside with a maximum
required setback of 40 feet. Note that the location of property lines, fences, etc. does not matter. The
height of the hill matters” (Fig 8.4)
“4. Setback from the toe of the slope: The face of the structures must be set back from the toe of the
slope a minimum of ½ the height of the slope with a maximum required setback of 15 feet. Note that
the location of property lines, fences, etc. does not matter. The height of the hill matters.” (Fig 8.4)
“5. Alternative setbacks may be provided where a report by a registered engineering geologist or soils
engineer indicates that the structure will be stable.”
“6. Protection: Setbacks from the toe of slopes can often be reduced by installing a protective retaining
wall either as a part of the structure or separate from it. Such walls must be designed by an engineer to
withstand the impact of mudslides.”
Figure 8.4. From Moorpark, CA Building and Safety “Hillside Construction, Help for the Homeowner”
(Young, 2011)
24
8.2.3 International Building Code (ICC 2011)
The International Building Code recommendations are similar to California’s. The relevant sections are
included below.
1808.7.1 Building clearance from ascending slopes.
“In general, buildings below slopes shall be set a sufficient distance from the slope to provide protection
from slope drainage, erosion and shallow failures. Except as provided in Section 1808.7.5 and Figure
1808.7.1 [Fig 8.5], the following criteria will be assumed to provide this protection. Where the existing
slope is steeper than one unit vertical in one unit horizontal (100-percent slope), the toe of the slope
shall be assumed to be at the intersection of a horizontal plane drawn from the top of the foundation
and a plane drawn tangent to the slope at an angle of 45 degrees (0.79 rad) to the horizontal. Where a
retaining wall is constructed at the toe of the slope, the height of the slope shall be measured from the
top of the wall to the top of the slope.”
1808.7.2 Foundation setback from descending slope surface.
“Foundations on or adjacent to slope surfaces shall be founded in firm material with an embedment and
set back from the slope surface sufficient to provide vertical and lateral support for the foundation
without detrimental settlement. Except as provided for in Section 1808.7.5 and Figure 1808.7.1 [Fig 8.5],
the following setback is deemed adequate to meet the criteria. Where the slope is steeper than 1 unit
vertical in 1 unit horizontal (100-percent slope), the required setback shall be measured from an
imaginary plane 45 degrees (0.79 rad) to the horizontal, projected upward from the toe of the slope.”
1808.7.5 Alternate setback and clearance.
“Alternate setbacks and clearances are permitted, subject to the approval of the building official. The
building official shall be permitted to require a geotechnical investigation as set forth in Section
1803.5.10”
Figure 8.5 Foundation Clearances from Slopes (ICC 2011)
25
9. Other useful references of good design and building practices
Military Soils Engineering, US Army Field Manual 5-410 Chapter 10
http://www.itc.nl/~rossiter/Docs/FM5-410/FM5-410_Ch10.pdf
Chapter 10 focuses on Slope Stabilization. It discusses the geology/slope interaction, resisting forces and
driving forces of slope stability, slope failure mechanisms, unstable slope indicators, road location
techniques and construction techniques for different soil rock types.
Military Soils Engineering, US Army Field Manual 5-410 Chapter 8
http://www.itc.nl/~rossiter/docs/fm5-410/fm5-410_ch8.pdf
Chapter 8 discusses Soil Compaction, moisture-density relationships, different testing methods, different
soil types, soil compaction recommendations, compaction of embankments, field density determination,
compaction equipment.
USDA Forest Service/USAID Low-Volume Roads Engineering Best Management Practices Field Guide, by
Gordon Keller & James Sherar
http://ntl.bts.gov/lib/24000/24600/24650/Index_BMP_Field_Guide.htm
This document has lots of information on all aspects of designing and constructing roads with low traffic
volume (less than 400 cars a day).
“A quick primer on Low-Volume Roads” Dr. Joseph Roise, Dept of Forestry and Environmental
Resources, NCSU.
http://www.ces.ncsu.edu/nreos/forest/feop/Agenda2008/forum/RoiseRoadsBasics.pdf
PowerPoint Presentation on the USAID/USDA Low-Volume Roads Engineering document.
Clark County, Washington Unified Development Code, Critical Areas and Shorelines, Geologic Hazards,
Chapter 40.430. http://www.codepublishing.com/wa/clarkcounty/.
Discusses how geologic hazards are defined, standards relating to developing in these areas, and how
these standards are administered.
International Building Code 2012, Chapter 18 Soils and Foundations
http://publicecodes.cyberregs.com/icod/ibc/2012/icod_ibc_2012_18_sec001.htm.
Discusses general guidelines for geotechnical investigation requirements, grading and fill, shallow and
deep foundations, retaining walls, and setbacks
Website with a table comparing slope degrees, gradient, and percent.
http://www.engineeringtoolbox.com/slope-degrees-gradient-grade-d_1562.html
26
10. Cited References:
American Association of State Highway and Transportation Officials (AASHTO) T-99-10-UL. 2010.
Standard Method of Test for Moisture-Density Relations of Soils Using a 2.5-kg (5.5-lb) Rammer and a
305-mm (12-in.) Drop. Standard Specifications for Transportation Materials and Methods of Sampling
and Testing, 33rd Edition and AASHTO Provisional Standards, 2013 Edition. Washington, DC
American Association of State Highway and Transportation Officials (AASHTO) T180-10-UL . 2010
Standard Method of Test for Moisture-Density Relations of Soils Using a 4.54-kg (10-lb) Rammer and a
457-mm (18-in.) Drop. Standard Specifications for Transportation Materials and Methods of Sampling
and Testing, 33rd Edition and AASHTO Provisional Standards, 2013 Edition. Washington, DC
American Association of State Highway and Transportation Officials (AASHTO). 1990. A Policy on
Geometric Design of Highways and Streets. Washington, D.C., 1044pp.
American Society for Testing and Materials (ASTM) Standard D1557, (2009), Standard Test Methods for
Laboratory Compaction Characteristics of Soil Using Modified Effort, ASTM International, West
Conshohocken, PA, DOI: 10.1520/D1557-09
Anderson, L.R., J.R. Keaton, T.F. Saarinen, and W.G. Wells II. 1984. The Utah Landslides, Debris Flows and
Floods of May and June 1983. National Academy Press, National Research Council, Washington, D.C., 96
pp.
Brand, E.W., and R.R. Hudson. 1982. Chase - An Empirical Approach to the Design of Cut Slopes in Hong
Kong Soils. In Proc., Seventh Southeast Asian Geotechnical Conference, Hong Kong, Vol.1, pp. 1-16.
Bromhead, E.N. 1992. The Stability of Slopes, 2nd ed. Blackie Academic and Professional Publishers,
Glasgow (imprint of Chapman & Hall, London and New York), 411 pp.
http://www.scribd.com/doc/93754887/Bromhead-1992-the-Stability-of-Slopes-2nd-Ed
Casagrande, A., and S.D. Wilson. 1951. Effect of Rate of Loading on the Strength of Clays and Shales at
Constant Water Content. Geotechnique, Vol. 2, No. 3, pp. 251-263.
Cruden, D.M. and D.J. Varnes. 1996. Landslide types and processes, In: Landslides:
Investigation and Mitigation, (Turner, K.A., Shuster,R.L., ed.)Transportation Research Board Special
Report 247, National Research Council, Washington, DC, 36-75.
Gresser, Charles S., Soil Compaction and Stability. Giles Engineering and Associates.
http://www.gilesengr.com/Literature/soil_compaction_and_stability_long.pdf
Holtz, R.D. and R.L. Schuster. 1996. Stabilization of Soil Slopes. In Landslides Investigation and Mitigation
(Turner, K.A., Shuster, R.L., ed.), Transportation Research Board Special Report 247, National Research
Council, Washington, D.C. p.445
International Code Council (ICC). 2011. 2012 International Building Code. International Code Council
Publications, Country Club Hills, IL. http://publicecodes.cyberregs.com/icod/ibc/2012/index.htm
27
Iverson, R.M., M.E. Reid, N.R. Iverson, R.G. LaHusen, M. Logan, J.E. Mann, and D.L. Brien. 2000. Acute
Sensitivity of Landslide Rates to Initial Soil Porosity. Science. Vol 290. pp. 513-516.
http://www.geo.oregonstate.edu/classes/geo582/week_4_1_hillslope_processes/iverson_science_00.p
df
Keaton, J.R., and G.H. Beckwith. 1996. Important Considerations in Slope Design. In Landslides
Investigation and Mitigation (Turner, K.A., Shuster,R.L., ed.), Transportation Research Board Special
Report 247, National Research Council, Washington, D.C. p.429-438.
Keller, G. and J. Sherar (USFS). 2003. Low-Volume Roads Engineering Best Management Practices Field
Guide. USDA Forest Service; US Agency for International Development; Conservation Management
Institute, Virginia Polytechnic Institute and State University.
http://ntl.bts.gov/lib/24000/24600/24650/Index_BMP_Field_Guide.htm
Lambe, P.C. 1996. Residual Soil. In Landslides Investigation and Mitigation (Turner, K.A., Shuster, R.L.,
ed.), Transportation Research Boar Special Report 247, National Research Council, Washington, D.C.
p.512
Lambe, P. 1975. Slope Failures in Hong Kong. Quarterly Journal of Engineering Geology, Geological
Society of London, Bol. 8, pp. 31-65.
Naval Facilities Engineering Command (NAVFAC). 1986. Soil Mechanics. Naval Facilities Engineering
Command Design Manual 7.01 (revalidated by change September 1986), US Navy, Alexandria, VA.
North Carolina Department of Transportation (NCDOT). 2013. Complete revision no. 7 for Roadway
Design Manual 04-03-2013
https://connect.ncdot.gov/projects/Roadway/Roadway%20Design%20Manual/01.%20General%20Desig
n.pdf
North Carolina Department of Transportation (NCDOT). 2012. Standard Specifications
for Roads and Structures. Contract Standards and Development Unit, Raleigh, NC.
https://connect.ncdot.gov/resources/Specifications/Specification%20Resources/2012%20Standard%20S
pecifications.pdf
(NCDOT) Conti, G., T.R. Gibson, J. Nance. 2010. Subdivision Roads, Minimum Construction Standards.
North Carolina Department of Transportation. Raleigh, NC. 56p.
https://connect.ncdot.gov/resources/stateroads/Subdivisions/Subdivision%20Manual%20January%202
010.pdf
Skempton, A.W., and J.N.Hutchinson. 1969. Stability of Natural Slopes and Embankment Foundations. In
Proc., Seventh International Conference on Soil Mechanics and Fundation Engineering, Mexico City, pp.
291-340.
US Bureau of Reclamation. 1980. Earth Manual. Bureau of Reclamation, U.S. Department of the Interior,
810 pp.
28
US Government, Department of the Army (US Army). 1992. Military Soils Engineering Field Manual 5410. Washington, DC . 427 pp. http://www.itc.nl/~rossiter/Docs/FM5-410/FM5-410_Ch10.pdf
US Government, Dept. of the Army, the Navy and the Air Force Corps of Engineers. 1971. TM 5-530
Materials Testing. 1 v
Wu, T.H. 1996. Soil Strength Properties and Their Measurement. In Landslides Investigation and
Mitigation (Turner, K.A., Shuster,R.L., ed.), Transportation Research Board Special Report 247, National
Research Council, Washington, D.C. p.322
Wyllie, D.C. and N.I. Norrish. 1996. Stabilization of Rock Slopes. In Landslides Investigation and
Mitigation (Turner, K.A., Shuster,R.L., ed.), Transportation Research Board Special Report 247, National
Research Council, Washington, D.C. p.494
Young, R. 2011. Hillside Construction, Help for the Homeowner. Moorpark Building and Safety. 1 pp.
http://ci.moorpark.ca.us/moorparkcity/img/hc.pdf
29
Appendix 1
Clark County, Washington state setback requirements:
http://www.codepublishing.com/wa/clarkcounty/clarkco40/clarkco40430/clarkco40430020
.html
Steep Slope Hazard Areas.
1. Except for mineral extraction practices, development activity on or within one hundred (100)
feet of slopes steeper than forty percent (40%) that do not have a mapped or designated
landslide hazard shall comply with the requirements of this section.
2. Buffer and Setback Distances.
a. Activities at the base of ascending slopes (building at the bottom of a steep slope):
(1) For slopes greater than or equal to forty percent (40%) and less than one hundred
percent (100%), buffers shall extend a distance away from the toe of the slope that is
equal to the vertical height of the slope divided by two, but not to exceed fifteen (15)
feet (Figure 8.3a). For slopes less than one hundred percent (100%), the toe of the
slope is defined as a distinct break in slope at the base of a steep slope.
(2)
For slopes greater than one hundred percent (100%), the buffer shall extend a
distance back from the toe of the slope equal to the height of the slope divided by two,
not to exceed fifteen (15) feet. The buffer shall be measured horizontally from a plane,
drawn tangent to the top of the slope at an angle of forty-five (45) degrees to the
proposed structure (Figure 8.3c).
(3) The setback shall be eight (8) feet beyond the buffer.
b. Activities at the tops of descending slopes (building at the top of a steep slope):
(1) For slopes greater than or equal to forty percent (40%) and less than one hundred
percent (100%), buffers shall extend a distance back from the top of the slope equal to
the vertical height of the slope divided by three (3), but not to exceed forty (40) feet.
The top of the slope is defined as a distinct break in slope at the top of a steep slope
(Figure 8.3a).
(2)
For slopes greater than one hundred percent (100%), the buffer shall extend a
distance back from the top of the slope equal to the height of the slope divided by
three (3), but not to exceed forty (40) feet. The buffer shall be measured horizontally
from a plain drawn at forty-five (45) degrees (one hundred percent (100%) slope) from
the toe of the slope to the proposed structure (8.3b).
30
(3) The setback shall be eight (8) feet beyond the buffer.
c. For projects not required to have a landslide protection area under Section 40.430.030(B), the
setback from the steep slope shall be equal to the buffer distance set in this subsection.
3.
The responsible official may approve buffers and setbacks which differ from those required by
Section 40.430.020(D)(1) if the applicant submits a geologic hazard area study described in
Section 40.430.030(C), which technically demonstrates and illustrates that the alternative
buffer provides protection which is greater than or equal to that provided by the buffer
required in Section 40.430.020(D)(1).
4. The responsible official may increase buffers or setbacks where necessary to meet requirements
of the International Building Code.
5. All portions of steep slope hazard areas and steep slope buffers on the site which are planned to
be undisturbed by permitted development activities shall be designated as landslide
protection areas in accordance with Section 40.430.030(B).
6.
Other than for exemptions listed in Sections 40.430.010(B)(3) and 40.430.030(B), vegetation
removal is not allowed on slopes over forty percent (40%) without an approved geologic
hazard area study described in Section 40.430.030(C)(5).
7.
Buffers, landslide protection areas and setbacks for steep slopes on projects having approved
grading shall be based on regulated steep slopes that remain after that grading.
Definitions (from Title 40 Section 40.430.010 http://www.codepublishing.com/WA/clarkcounty/)
For purposes of this chapter, the following definitions shall apply:
1. “Steep slope hazard area” means an area where there is not a mapped or designated landslide
hazard, but where there are steep slopes equal to or greater than forty percent (40%) slope.
Steep slopes which are less than ten (10) feet in vertical height and not part of a larger steep
slope system, and steep slopes created through previous legal grading activity are not
regulated steep slope hazard areas. The presence of steep slope suggests that slope stability
problems are possible.
2. “Landslide hazard area” means an area that, due to a combination of slope inclination, soil type
and presence of water is susceptible to landsliding in accordance with the following criteria:
a. Areas of previous slope failures including areas of unstable old or recent landslides;
b. Areas with all three (3) of the following characteristics:
(1) Slopes steeper than fifteen percent (15%),
31
(2) Hillsides intersecting geologic contacts with permeable sediment overlying a low
permeability sediment or bedrock, and
(3) Any springs or groundwater seepage;
c. Slopes that are parallel or sub-parallel to planes of weakness, such as bedding planes, joint
systems and fault planes in subsurface materials;
d. Areas mapped by:
(1) Washington Department of Natural Resources Open File Report: Slope Stability of
Clark County, 1975, as having potential instability, historical or active landslides, or as
older landslide debris, and
(2) The Washington Department of Natural Resources Open File Report Geologic Map
of the Vancouver Quadrangle, Washington and Oregon, 1987, as landslides;
e. Slopes greater than eighty percent (80%), subject to rock fall during earthquake shaking;
f. Areas potentially unstable as a result of rapid stream incision, stream bank erosion, and
stream undercutting the toe of a slope;
g. Areas located in a canyon or on an active alluvial fan, presently or potentially subject to
inundation by debris flows, debris torrents or catastrophic flooding;
h. Areas within one hundred (100) feet of an open-pit mine sites subject to steep slope hazard
or landslide hazard.
3. “Seismic hazard area” means an area subject to severe risk of damage as a result of earthquakeinduced soil liquefaction, ground shaking amplification, slope failure, settlement, or surface
faulting. Relative seismic hazard is mapped on the NEHRP Site Class Map of Clark County,
published by the Washington Department of Natural Resources.
4. “Volcanic hazard area” means an area subject to possible low and high density pyroclastic flows
as shown on the Volcanic Hazard Map of Clark County.
(Amended: Ord. 2005-04-15)
Landslide Hazard Areas.
1.
A development proposal on a site containing a landslide hazard area shall meet the following
requirements:
32
a.
A minimum buffer of fifty (50) feet shall be established from all edges of the landslide hazard
area. The buffer shall be extended as required to mitigate a steep slope or erosion hazard or
as otherwise necessary to protect the public health, safety and welfare. In cases where the
diameter of the landslide area is smaller than fifty (50) feet, the buffer width may be
reduced to less than fifty (50) feet at the discretion of the department;
b.
All portions of landslide hazard areas and buffers shall be designated as landslide protection
areas in accordance with Section 40.430.030(B).
2.
Other than exempt activities, clearing or alteration of a landslide is allowed only if the following
are met:
a. A development proposal does not decrease slope stability on contiguous properties;
3.
b.
Mitigation is based on best available engineering and geological practice and is described in
an approved geologic hazard area study as specified in Section 40.430.030(C)(5);
c.
Such clearing or alteration of a landslide is certified safe as designed and under anticipated
conditions by a registered geotechnical engineer or geologist licensed in the state of
Washington.
Neither buffers nor a landslide protection area will be required if the activity meets the
requirements of Section 40.430.020(E)(2).
33
Appendix 2 - Site Investigation and Design guidelines
When looking over recommendations or guidelines for site investigations and design, we found many
studies and references. The most comprehensive was included in Chapter 16 of Landslides Investigation
and Mitigation, “Important Considerations in Slope Design” (Keaton and Beckwith, 1996).
This chapter discusses the importance of careful selection of slope design parameters, which must
reflect:
1. “Site-specific environmental conditions potentially affecting slope stability
2. Strength properties of the materials forming the slope
3. Economic considerations
4. Other facility design constraints”
1. Environmental Conditions:
1.1 External loads
Human activity in the vicinity of a slope, particularly a marginally stable one, may increase the forces
tending to cause failure. In some circumstances, these activities may improve the stability of some
slopes (rock bolts, buttress fills). In other circumstances, they might adversely affect stability (grading of
nearby slopes, adjacent construction, blast damage, vibrations from passing vehicles).
1.2. Climate Conditions
Precipitation and runoff estimates should be used to design the optimum size of drainage collection
system on slopes. Assume a shallow depth to groundwater and a storm recurrence interval that
corresponds to the economic importance of preventing a slope failure.
1.3. Urban storm-water discharge
Urban storm water, leaking water mains, and sewer lines can cause soil to become saturated, build up
subsurface hydrostatic pressures, lead to erosion on slopes, and create erosion gullies along a buried
conduit (culvert or trenches backfilled with free-draining material) during heavy rains. “Backfill with
hydraulic properties similar to those of the surrounding soil can alleviate the potential for trenches to
act as conduits. Storm-water culverts can become plugged with sediment, causing backup of water that
can overtop and erode embankments. (Anderson et al. 1984)”
2. Material Properties and Site Characteristics
Complete characterization of the site is necessary to understand the materials and defects in those
materials for design.
Material properties include:
● soil and rock texture (grain size), mineralogy (including organic matter), degree of weathering,
porosity, water content, specific gravity, unit weight, strength of intact material (cohesion
intercept and friction angle)
34
Mass properties include:
● permeability, slaking, drill-core rock-quality designation, defect condition (orientation, spacing,
roughness, openness, and infilling), mass strength, groundwater conditions
Not all may be necessary for a specific analysis. This will be determined by the site, the desired
modification, the design, and construction
Engineering analyses usually include information relating to topography, subsurface conditions and
external loads. “The importance of an accurate geologic model on which to base engineering analyses
and slope designs cannot be overstated.” Groundwater levels should represent the extreme case a slope
will have to withstand. “The design mass properties of geologic units should be reasonable, but not
overly conservative” (Keaton and Beckwith, 1996).
3. Economic Considerations
“Design parameters must be selected to reflect the characteristics of the site under so-called “design
conditions” (cut slope, fill slope, earth reinforcement, full saturation of the slope). Mass properties of
the earth materials at the site must be based on geologic descriptions of the site and results of field and
lab tests. Engineering analyses must reflect the inherent geologic uncertainties, acceptable levels of risk,
and appropriate conservatism in design” (Keaton and Beckwith, 1996). Acceptable risk includes
determining design life, determining recurrence intervals for events that may impact the facility, and
deciding on the tolerance level for these impacts.
4. Other design constraints include (Keaton and Beckwith, 1996):
Location: rural or urban, aesthetics, urgency of repair, public safety
Dimensions
Design Standards (AASHTO 1990, NAVFAC 1986, USBR 1980)
Costs and source of funding
Maintenance coordination with design
Balancing cut and fill
Disposal of waste rock and soil
Roadside hazards to traffic including traffic patterns and tolerable delays
35