Recently in Southern California, geogrids were employed successfully as soil reinforcement to
stabilize a landslide that was subject to large ongoing movements. The slide occurred on the
side-hill of a canyon, which is overlooked by Colony Hill, and upscale residential community.
Area residents were concerned primarily with the project's environmental impact--they wanted a
solution that could restore the slope with no visible sign of intervention.
In its final vegetated state, the reconstructed
slope blends into Colony Hill's attractive environment, just as community residents hoped it
would.
Project background
Colony Hill is situated on Soledad Mountain in La Jolla, California--a suburb that actually is
within the San Diego city limits. On October 30, 1996, a water main on the north side of the
mountain suffered a break. This event, compounded by unusually heavy rainfall, led to a
landslide.
The water-main alignment traverses a western-facing canyon slope with an approximately
1.5:1-1:1 (horizontal to vertical) incline. The slope, like most of San Diego County's
coastline, is underlain by the Point Loma geologic formation. Locally, the bedrock is intensely
sheared and jointed by the inactive Mount Soledad fault. The formation has developed a 20-ft
thick "mobile zone" of overburden soils that are subject to continuous downslope creep.
Heavy rains in late October 1996 likely accelerated the ongoing downslope movement, separating
the pipe near the top of the canyon. The break created erosion gullies up to 20 ft wide and 20
ft deep, causing a loss of lateral support and a massive landslide involving the upper-middle
area of the canyon slope.
Although temporarily inconvenienced by the loss of water service, the area residents were not
immediately affected by landslide instability. However, the slope could have eroded further
and increased the landslide mass. Homes directly above the landslide would have been placed at
serious risk of collapse. This became a concern for the local community and ultimately led to
the city's intervention. Priority was given to the preservation of the pleasing environmental
surroundings, which include a recreational park with a pathway that stretches from the top of
the hill to the bottom of the canyon.
San Diego's Risk Management Department hired locally based Group Delta Consultants to provide
a speedy, cost-effective solution for remediation of the Colony Hill site. The department
cooperated to replace an approximately 300-ft-long segment of 16-in. water main and to repair
the landslide. Later, project responsibility switched to the city's Engineering and Capital
Projects Department.
The Design
The consultants considered such design issues as access limitations, environmental constraints,
landslide-debris removal, graben stability, hillside reconstruction with on-site clayey soils,
and replacement of the high-pressure water line. In view of the significant displacements
within the entire landslide mass, which essentially affected a 20-ft-thick "mobile zone," the
designers concluded that removal and replacement of the entire slide mass would be required.
Such an approach would ensure long-term, underlying, lateral support to the slope-top homes.
An additional concern was the reconstructed slope have sufficient dimensional stability to
preclude future tensile failure of the water main.
Because of the slope geometry, grids were
extended to the back of the excavation to ensure stability.
The slide affected only the upper two-thirds of the canyon side slope, while the canyon base
had more than 25 ft of compressible, saturated alluvium with a healthy stand of riparian
vegetation. For these reasons, it was decided to limit slope repair to the upper failed portion
and incorporate a new engineered fill keyed into the existing canyon sidewall. Because the
mobile zone was located below the landslide's toe, the new engineered fill's toe was placed
behind the lower mobile zone. Slope reconstruction started at this point.
The basal zone of the landslide consisted of a 1-2-ft thick, weathered, remolded and sheared
highly plastic silty clay (CH). The soil in this area possessed unconfined shear strengths
of 3.56-7.11 lb/in.2 (0.5 kg/cm2). The bulk of the slide mass consisted
of clayey sands to sandy clays, with unconfined compressive strengths ranging from 42.66 to
56.88 lb/in.2 (3-4 kg/cm2). Effective shear strengths of the composite
fill were determined to be = 24°, c = 100 psf.
Stability analyses, which included seismic considerations, were performed with SLOPE/W Geoslope
software. Geogrid reinforcement was viewed as the most practical solution for controlling
creep deformation of the recompacted clayey fill. When dealing with expansive clay fills,
geogrid reinforcement helps limit progressive downhill creep. At Colony Hill, the geosynthetics
enabled a fairly steep side-fill to be reconstructed with less than desirable on-site clayey
soil. Despite poor soil quality, only on-site soil was used in order to limit project costs.
The contractor, Vadnais Corp. of San Diego, selected Paragrid 80/15 high-modulus polyester
geogrids, manufactured by Terram and supplied by Maccaferri Gabions Inc., Sacramento. The
product was selected due to cost-effectiveness and ease of installation.
Cross section showing slope-stabilization
design.
Concerns about graben-stability along the headscarp area, particularly after removal of 20 ft
of slide debris, necessitated the use of temporary tieback anchors to facilitate excavation
of the slide mass. Landslide removal commenced from the headscarp and was accomplished as a
series of benches. Grading proceeded only after each row of tiebacks was installed and
post-tensioned.
Primary reinforcement was accomplished by placing the grids at 4-ft vertical spacing. The slope
geometry and relatively weak soil strengths necessitated that the geogrid be extended to the
back of the excavation. Deep construction benches also facilitated sufficient anchorage to
develop the design-tension capacity of the grid behind the critical failure surface. (The
failure's lateral geometry essentially followed that of the original landslide.) With a
resulting minimum required safety factor of 1.5, the actual mobilized design strain
effectively limits future post-construction slope creep.
Topographic map of the slope, after landslide.
A layer of 4-ft-wide Terram-Grid 35/30 was installed as secondary reinforcement between each
primary-grid layer. The material's roughly 1-in.-square open areas encouraged root
development. By helping to establish vegetation, the system promoted surficial stability.
Installation
Slope repair required the removal of the entire active landslide mass, which involved up to
40,000 yd3 of grading. Then, the engineered fill was built in two
sequenced-construction phases, each approximately 60 ft. high.
The lower part of the engineered fill was strengthened by 30-50-ft long geogrid layers,
complemented by intermediate strips of secondary reinforcement. The grids were installed
without any kind of connection.
Geogrid layers reinforce the engineered
fill that replaced the landslide mass.
The approximately 300-ft-long segment of A-C ("Transite") pipe where the watermain break
occurred was replaced with a stronger and more flexible ductile iron pipe. A subsurface
pier-supported trestle was constructed to support the water main in the canyon bottom, which
was composed of 25 ft of loose, saturated alluvium.
The contractor's 10-person crew completed the job in January 1998, about three months after
they began. As shown in the photo, the hillside is now totally covered by
vegetation and blends well with the surrounding area.
Summary
The use of geogrids for improving deep-seated and surficial stability facilitated the economical
repair of a high-visibility landslide within an urban coastal-hillside development. Limited
access and environmental constraints dictated the reuse of existing clayey landslide materials
subject to continuing downslope creep.
For Colony Hill and other projects, geogrids provide a proven and effective method of in-situ
soil reinforcement. The materials' tensile capacity essentially arrests the undesirable,
long-term, downslope creep often observed in natural hillside environments.
Paolo Di Pietro is technical director of Maccaferri Gabions Inc., Williamsport, Md.
Walter F. Crampton was the design engineer on the Colony Hill project for Group Delta
Consultants Inc., San Diego, CA.