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Arizona Geological Survey

Abstract
A series of small earth movements occurred along the slopes of State Route 87 at about mile
marker 224 (between the Bush Highway and Route 188) throughout the winter of 2007‐2008,
culminating with a landslide on Friday, 21 March 2008. This landslide buckled the southbound
lanes, displaced the northbound lanes, and closed the highway for nearly a week. The mass
movements occurred on slopes that were constructed with re‐vegetated, laid back slopes; soil
nail walls; and rip rap‐lined channels. However, our reconnaissance mapping indicates that
most, if not all, of the slope movements are located within a much larger, older landslide
adjacent to, and cut by, SR‐87. No specific trigger for the landslides was immediately apparent,
although a combination of factors (e.g., precipitation, groundwater levels, etc.) may have
contributed to conditions for the slope failure. Headscarps of various types of landslides are
present along both sides of the highway. Results from our preliminary investigation indicate
that up to three other paleo‐landslides may be present in the immediate vicinity. The full extent
and nature of these landslides are unknown and require further investigation to evaluate their
potential to be reactivated and risk they pose to the highway.
Introduction
On 21 March 2008, a landslide occurred at about mile marker 224 on State Route 87 (SR‐87)
(northeast of Iron Dike mountain), in Gila County, causing sufficient damage to require the
highway to be closed for six days while cleanup and mitigation efforts were underway. Several
other slope failures in the vicinity have occurred at least since 2005, and most recently during
winter 2007‐2008.
In the mid‐1990s, the Arizona Department of Transportation (ADOT) contracted with the United
States Geological Survey (USGS) to map a portion of the Kitty Joe Basin as part of the
geotechnical investigation of the new alignment for SR‐87. The USGS (Conway, 1995) produced
a map and report characterizing the geology of the area. Conway indicated the presence of two
landslides adjacent to the area of the 21 March slope failure, but based on his mapping neither
landslide crossed SR‐87 (Figure 1). Conway addressed the landslides in his report, however,
stating that further investigation should be made of the landslides prior to building a road
through the area. ADOT completed the new route in 2000.
In accordance with Arizona Revised Statute (ARS) 27‐52, the Arizona Geological Survey (AZGS)
began a preliminary reconnaissance of the landslide events on 23 March 2008. The statute
directs AZGS to “map, describe, and monitor known and potential geologic hazards and
limitations to land and resource management,” and to “provide technical advice and assistance
in geology to other state and local governmental agencies engaged in projects in which the
geologic setting, character, or mineral resources of the state are involved.”
We summarize our findings below. In this report, the term “Easter weekend landslide” refers to
the landslide that buckled the highway on 21 March; the term “winter 2007‐2008 landslides”
refers to slope failures occurring along both sides of the highway in this area; and the term
“paleo‐landslide” refers to the older, larger landslide mass in which the Easter weekend
landslide and the winter 2007‐2008 landslides occurred. We labeled this group of landslides the
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Arizona Geological Survey

“Iron Dike landslides” after Iron Dike Mountain, a prominent peak located three‐quarters of
mile southwest of the Easter weekend landslide (Figure 1).

Figure 1 Regional aerial photographic map showing probable extent of landslide deposits as mapped by Conway (1995).

Within the limits of the constricted right of way for the SR‐87 alignment, ADOT constructed a
new highway alignment with re‐vegetated, laid‐back slopes; soil nail walls; and rip rap‐lined
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Arizona Geological Survey

drainage channels. ADOT has had to perform continual maintenance on the slopes, and in 2005,
two of the soil nail walls failed. Prior to the Easter weekend landslide, ADOT had initiated a
geotechnical investigation of the site, which they escalated after the landslide occurred.

B

A

Figure 2. Slope failures that occurred during winter 2007‐2008 near mile marker 224. A. The southern‐most landslide on the
west side of the highway; note the broken shotcrete (white arrows). The metal fence in the foreground is the median of the
highway; the Jersey barriers behind the fence are on the shoulder of the southbound lane, separating the highway from debris.
B. Slope failure (inside yellow ellipse) in material from an older landslide on the east side of SR‐87. Scarps are present above the
highest tier of soil nail walls; material is moving down slope over both walls. Photos taken on 23 March 2008 (A) and 25 March
2008 (B) by M. Diaz.

Data from the Maricopa County Flood Control District’s ALERT precipitation gage on Mt. Ord
(5960), approximately 2.5 miles east‐southeast of the landslide, provides an estimate of
precipitation in the landslide area during the winter of 2007‐08. This gage recorded 14 inches of
precipitation between 1 October 2007 and 21 March 2008, when the main landslide occurred
(Appendix A). Smaller mass movements occurred on both the western (Figure 2A) and eastern
(Figure 2B) slopes throughout the winter of 2007‐2008, prompting ADOT to place protective
barriers (k‐bars or Jersey barriers) between the western slope and the highway. Ironically, the
gage recorded only 0.24 inches of rain between 1 March and 21 March; this fell in about a 75‐
minute period on 17 March. This rain event may have caused the pore pressure to exceed
stable limits, thereby increasing the pressure against the soil nail walls and decreasing the shear
strength of the materials within the paleo‐landslide mass.

B

A

Figure 3 A. Close‐up of the lower part of the landslide buckling the highway. B. The landslide displaced the road and divider
about 1 foot to the east. Both views looking north. Photos taken on 23 March 2008 by B.F. Gootee.

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Arizona Geological Survey

On 21 March, sections of the western slope failed in a larger landslide. Based on displaced
material and the buckling of SR‐87 southbound lanes, it appeared that some of the northern
part of the slide plane extended under the highway (Figure 3A). The southbound lanes of the
highway were buckled and broken, for approximately 50 feet along the length of the highway.
One section of the freeway was buckled up to approximately 3 feet in height (Figure 4). The
median was displaced about 1 foot eastward (Figure 3B), and the northbound lanes buckled to a
lesser extent than the southbound lanes. Contractors doing emergency repairs reported that
the buckled section fluctuated vertically for at least five days afterward.

Figure 4 The Easter weekend landslide pushed up the highway, deforming both south‐ and northbound lanes. The southbound
lanes (shown here) buckled approximately 3 feet high and 50 feet along the length of the highway. View is to northwest. Photo
taken on 23 March 2008 by M. Diaz.

Geology and Geomorphology
The geology of the area surrounding the Iron Dike landslides (Cottonwood Basin, central
Mazatzal Mountains) consists of Cenozoic volcanic and basin fill deposits overlying Proterozoic
metamorphic and igneous rocks (Conway, 1995), with limited Quaternary surficial deposits. The
Tertiary basin‐fill sequence consists primarily of clay‐rich, sandy gravel. Basalt flows cap the
topographic highs and are intercalated with basin fill deposits. Surficial deposits include basalt
colluvium and alluvium. Conway (1995) infers a west‐downward fault immediately to the south

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Arizona Geological Survey

of Iron Dike with basalt and basin‐fill deposits offset, although there is no clear evidence for
Quaternary fault activity in the area (Pearthree, 1998; Pearthree and Bausch, 1999).

Figure 5 Unsorted, matrix‐supported clastic material that is suggestive of a debris flow deposit (top) above moderately
indurated, bedded basin‐fill deposits along one of the drainages at the downslope exposure of the landslide. Note the irregular
erosional surface below the deposits in the wall of the outcrop. Photo taken on 28 March 2008 by M. Diaz.

A large, complex paleo‐landslide developed on the northeast flank of Iron Dike Mountain
sometime during the Quaternary. The head of this paleo‐landslide is approximately 1500 feet
wide, with a possible runout of nearly one mile (this investigation). The scarps (head and minor)
are steep but scarp crests are rounded, and the main body is heavily vegetated. Drainages
dissect the probable toe and interior, and may define the lateral extent. Old debris flow levees
and snouts, which are the termini of debris flows, (with clasts ranging from cobbles to boulders,
some of which are nearly 3 feet in diameter) are covered with moss and lichens; the original
matrix has been scoured away and the resultant cavities are being refilled with sediment from
overland flow. These debris flows are present at the inferred toe of a paleo‐landslide deposit on
the east side of the highway (Figure 5). They may have contributed to headward erosion and
the formation of drainages that further dissect the slide(s). Some of the drainages show
evidence for two to three pulses of debris flows. All of this evidence suggests that the large
landslide complex is fairly old, although parts of the complex may have been active fairly
recently.

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Arizona Geological Survey

The main body of the paleo‐landslide
(and thus the Easter weekend
landslide) is composed of clayey sand
and sandy clay; however, areas
within the body of the paleo‐
landslide contain mostly clay with
lenses and beds of sand. The volume
of clay present varies from nearly
100% at the probable failure plane of
the Easter weekend landslide (Figure
6) to <5% near the surface. The
interior of the landslide is deformed Figure 6 Portion of the Easter weekend landslide that damaged the
in places, with sheared surfaces highway. Note pavement, gray artificial fill under the pavement, and red,
(Figure 7A) and slickensides (Figure 7B) clay‐rich, moist sediment under the fill. Photo by B.F. Gootee, 23 March
2008.
present, and undeformed in other
places (with the bedding preserved and only the dip direction changed). It is unclear whether
the Easter weekend landslide, the paleo‐landslide, or some combination of the two caused the
slickensides. At the time of our investigation, the entire slide mass was moist below depths of
approximately 2 inches. Soil and colluvium are present on the surface of the slide; it is unclear
whether the soil developed since the paleo‐event or if the soil has moved with the landslide.

A

B

Figure 7 A. Example of internal shearing within the slide mass (head scarp on the east side of the highway). White dashed lines
indicate sheared features; white arrows indicate direction of relative motion. Similar structures are also visible in the Easter
weekend slide mass on the west side of the highway. B. Slickensides observed in the interior of the landslide (on the west side
of the highway) indicate shearing in the clayey gravel. Photos taken on 28 March 2008 (A) and 23 March 2008 (B) by B.F.
Gootee.

Preliminary investigations indicate that several other paleo‐landslides exist in the area of the
Easter weekend landslide. Examination of topographic maps, digital elevation models (DEMs),
and aerial photography revealed that SR 87 may cross at least three other older landslides in
the immediate vicinity (Figure 9). Questions also remain as to how many landslide events have
occurred within the paleo‐landslide deposits: did they form in fairly discrete events, or did
movement occur in stages? The extent and exact nature of all four of these landslides are
unknown and require further investigation.

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Arizona Geological Survey

B

A

Figure 8 A. View NE from headscarp of paleo‐landslide, looking down the direction of sliding. It appears that the paleo‐landslide
mass extrends across SR‐87. . Yellow lines indicate sharp slope breaks; dashed where inferred or out of the photo’s viewshed. B.
Aerial view to the south Inferred extent of paleo‐landslide in red; dashed black line represents Easter weekend landslide.
Photos taken on 23 March 2008 (A) and 7 April 2008 (B) by B.F. Gootee.

A

B

C

Figure 9 A. Geologic map from Conway (1995); the unit labeled Ql is the main body of the paleo‐landslide (inside the red circle).
The map has been rotated (north to bottom of figure) so that the view is the same as in the digital elevation models (DEMs) to
the right (B and C). Geomorphic features suggest at least four paleo‐landslides may have occurred in this area (red ellipses and
white dashes on C showing approximate locations) and cross SR‐87. Iron Dike Mountain is the highest elevation point shown in
the DEM.

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Arizona Geological Survey

Conclusions and Recommendations
The landslides that occurred during the winter of 2007‐2008 and culminated with the closure of
SR‐87 after the Easter weekend landslide on 21 March 2008 were part of a larger, older
complex of landslides radiating from Iron Dike Mountain and its adjacent ridges. It is possible
that the highway cuts through the deposits of up to four old landslides that had not previously
been recognized. There are several knowledge gaps that need to be addressed in order to
facilitate the development of mitigation strategies, including:
•
•
•
•
•
•
•
•

extent and nature of the Easter weekend landslide;
the depth of the slide plane(s);
additional types of motion that can be expected;
time since last movement of the Iron Dike landslide complex;
verification of other paleo‐landslides in the area;
the number of landslides that may impact the highway;
how many landslide events occurred and what their temporal relationships are; and
whether there are portions of the basin fill sequence that are particularly susceptible to mass
wasting that should be evaluated with respect to other sections of the highway.

Acknowledgements
We would like to acknowledge the following people and agencies for lending support and
information to this investigation: Lee Allison and Phil Pearthree, Arizona Geological Survey; Jim
Wilson, Tom Donithan, and the Arizona Department of Transportation (ADOT); Wayne Harrison,
DMJM Harris; and the Arizona Department of Public Safety. We would also like to acknowledge
Jim Delton, John Lawson, and Nick Priznar, all from ADOT, for their reviews.
Cover photo by Keith Dahlen.
References
Conway, C.M., 1995. Geology of the Kitty Joe Canyon area, Mazatzal Mountains, Arizona (report
prepared for Arizona Department of Transportation), US Geological Survey.
Pearthree, P.A., 1998. Quaternary fault data and map for Arizona: AZ Geological Survey Open‐
File Report 98‐24, 122 p. scale 1:750,000.
Pearthree, P.A., and Bausch, D.B., 1999. Earthquake hazards map: AZ Geological Survey Map 34,
scale 1:1,000,000.

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Arizona Geological Survey

Appendix A:
Precipitation data from the Maricopa County Flood Control District Mt. Ord ALERT rain gage
approximately 2.5 miles east‐southeast of study area (http://156.42.96.39/showrain.html).

FCDMC ALERT System Data Display
5960 Mt. Ord Precipitation Gage
Date
Time
3/21/2008
9:20:19
3/21/2008
6:20:16
3/21/2008
3:20:16
3/21/2008
0:20:16
3/20/2008
21:20:17
3/20/2008
18:20:17
3/20/2008
15:20:17
3/20/2008
12:20:17
3/20/2008
9:20:17
3/20/2008
6:20:18
3/20/2008
3:20:18
3/20/2008
0:20:18
3/19/2008
21:20:21
3/19/2008
18:20:18
3/19/2008
15:20:19
3/19/2008
12:20:19
3/19/2008
9:20:19
3/19/2008
6:20:19
3/19/2008
3:20:20
3/19/2008
0:20:20
3/18/2008
21:20:20
3/18/2008
18:20:27
3/18/2008
15:20:20
3/18/2008
12:20:21
3/18/2008
9:20:21
3/18/2008
6:20:21
3/18/2008
3:20:21
3/18/2008
0:20:22
3/17/2008
21:20:22
3/17/2008
18:20:22
3/17/2008
15:20:22
3/17/2008
12:20:23
3/17/2008
11:49:15
3/17/2008
11:29:12

Cumulative Precip (in)
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.33
14.29
14.25
9

Storm total (in)
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.24
0.20
0.16

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Arizona Geological Survey

Date
3/17/2008
3/17/2008
3/17/2008
3/17/2008
3/17/2008
3/17/2008
3/17/2008
3/16/2008
3/16/2008
3/16/2008
3/16/2008
3/16/2008
3/16/2008
3/16/2008
3/16/2008
3/15/2008
3/15/2008
3/15/2008
3/15/2008
3/15/2008
3/15/2008
3/15/2008
3/15/2008
3/14/2008
3/14/2008
3/14/2008
3/14/2008
3/14/2008
3/14/2008
3/14/2008
3/14/2008

Time
11:20:55
11:12:28
11:03:22
9:20:23
6:20:23
3:20:23
0:20:24
21:20:24
18:20:24
15:20:24
12:20:24
9:20:24
6:20:25
3:20:25
0:20:25
21:20:25
18:20:26
15:20:26
12:20:26
9:20:26
6:20:26
3:20:27
0:20:27
21:20:30
18:20:30
15:20:27
12:20:28
9:20:30
6:20:28
3:20:28
0:20:28

Cumulative Precip (in)
14.21
14.17
14.13
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09
14.09

10

Storm total (in)
0.12
0.08
0.04
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00