Geologic Hazards - Geotechnical Engineering Investigation - page 732

Sta. 235 Sta. 240

Sta. 245 Sta. 250 Sta. 255

Sta. 260 Sta. 265 Sta. 270

Sta. 275

1900

GA

1800

Lower
block

1700

Fill

Bridge
pier

Till

Weathered
shale

Residual soil

1600

Weathered shale

Fresh
shale

1500

1400

FIGURE 9.33
Geologic section, Forest City Landslide, South Dakota (From Hunt, R.E. et al., 3rd International Conference, Case
Histories in Geotechnical Engineering , St. Louis, 1993. With permission.)

Marine Shale: Panama Canal Slides
Event : Massive slides occurred during 1907 and 1915 in the excavation for the Panama
Canal in the Culebra cut (Binger, 1948; Banks, 1972).
Geology : On the plan view of the slide areas (Figure 9.34), the irregular to gentle topog-
raphy of the Cucaracha formation (Tertiary) is apparent. The Cucaracha is a montmoril-
lonitic shale with minor interbedding of sandstone and siltstone more or less horizontally
bedded but occasionally dipping and emerging from natural slopes. It is heavily jointed
and slickensided, and some fractures show secondary mineral fillings. Natural slopes in
the valley were relatively gentle, as shown on the geologic section given in Figure 9.35,
generally about 20° or less. Laboratory consolidation tests gave values for preconsolida-
tion pressure as high as 200 tsf.
Slide history : Excavations of the order of 300 ft in depth were required in the Cucaracha
formation. Some minor sliding occurred as the initial excavations were made on slopes of
1:1 through the upper weathered zones to depths of about 50 ft. The famous slides began
to occur when excavations reached about 100 ft. They were characterized by a buckling
and heaving of the excavation floor, at times as great as 50 ft; a lowering of the adjacent
ground surface upslope; and substantial slope movements. Continued excavation resulted
in progressive sliding on a failure surface extending back from the cut as far as 1000 ft. The
causes of the sliding are believed to be stress relief in the horizontal direction, followed by
the expansion of the shale, and finally rupture along a shallow arc surface (Binger, 1948).
Analysis : Banks (1972) found that at initial failure conditions, the effective strength enve-
lope yielded 0. For the case of an infinite slope (see Section 9.3.2) without

slope seepage these values would produce a stable slope angle of 19°, or for the case of
seepage parallel and coincident with the slope face, 1/2

φ 19° and c ' ≈

, or 9.5°. Since movements had
occurred, the value 9.5° is considered to be the residual strength.
Solution: The slides were finally arrested by massive excavation and cutting the slopes
back to 9.5° (1/2

φ

r ), which is flatter than the natural slopes. Banks reported that measure-
ments with slope inclinometers indicated that movement was still occurring in 1969, and
that the depth of sliding was at an elevation near the canal bottom.

φ