LANDSLIDE
2019.05.29
CHOI JUNGHAE
BASIC CONCEPTS OF
SLOPE EQUILIBRIUM
Slopes in equilibrium with their
environment typically exhibit concave, straight, and convex hillslope profiles shown at left.
Slopes out-of-equilibrium typically exhibit anomalous, hummocky profiles, shown at right
Stepped Topography
• Stepped topography is not necessarily indicative of slope
instability, but can be structurally controlled by underlying
stratigraphy, as shown here. Shales are most often associated with landslides.
Out-of-Equilibrium Slopes
•
Slopes that are
experiencing rampant
erosion are typically
out-of-equilibrium,
and exhibit truncated
concave slope profiles,
shown at left.
• The slope profile is also
influenced by runoff
requirements and the relative position of controlling base level, as sketched here.
• Note change in hill profile as
channel moves away from the ridgeline
Continuation of an erosive cycle, with gradual shift of the hillslope profile, resulting in hummocky topography. Hummocky slopes are typical of slopes experiencing landslippage.
COLLUVIAL
FILLED
Colluvial filled
bedrock hollows
• Bedrock hollows, or
ravines, tend to form in more or less regular
patterns on fractured or regularly jointed bedrock, as sketched here.
• Alluvium is deposited in
•
The most common geomorphic feature capable of
spawning slope stability and erosion problems are
colluvial filled bedrock ravines, or so-called ‘zero order
basins’, such as these.
• Typical pear-shaped bedrock ravine complex, showing recent
debris flows in zero-order basins and accumulated debris in the first-order channel.
• Colluvial filled bedrock ravines are often discernable on aerial
photos as tonal anomalies between smooth colluvial-filled
depressions and bumpy bedrock ribs, bewteen the filled ravines, as shown here. Be dr oc k r ib Recent slumps
•
We can glimpse the character of colluvial filled
depressions by viewing exposures in highway cuts,
such as that shown here along California State Route
1, the Coast Highway.
Three steps in the geomorphic evolution of
colluvial filled bedrock ravines are shown
here: 1) end of erosional cycle, with ravine excavated down to bedrock; 2) beginning of depositional cycle, with infilling by
gravimetric means; and 3) filled ravine during period of geologic quiescence under near-equilibrium conditions
•
Colluvial filled
ravines are
typically
developed upon
regularly jointed
or intensely
fractured strata
Permeability
•
The weathered zone
lying beneath
topsoil is generally
dilated due to slope
creep
•
This zone is
conductive to
shallow seepage,
coalescing in
bedrock hollows
• Colluvium is typified by a heterogeneous assemblage of angular
to subangular blocks with rough stratification, as sketched here.
• These materials can be more pervious than underlying rock and
derive their strength from inter-block friction, with little of no cohesion, as shown at upper right.
•
Tree roots
engender
greater cohesion and
resistance to sliding within
the upper 1 to 2 meters of
the slope.
TYPES OF SLOPE
MOVEMENT
사면거동의 유형
• Materials: rock, soil, debris - soil: sand-sized or finer
particles
- debris: coarser fragments • Type of movement
- internal mechanism of movement - fall, topple, slide, spread, flow • Type of landslide:
material + type of movement Ex) debris flow, rock fall
1) 산사태(landslide)
- 전단강도 초과로 한 개 혹은 복수의 붕괴면을 따라 움직이는 토체나 암체 - 한 덩어리로 넓게 분포, 다양한 이동속도
- 흐름(flow)나 암설(rock avalanche)로 바뀌기도 함
- 포화된 토석(예: 붕적층)은 토석류(debris flow)를 발생시킴 (1) 회전형(rotational)
- 점성토층에서 주로 발생함 - 원호파괴
(2) 전이형 산사태(translational slide) - 기존 평탄면 또는 불연속면 (층리, 단층 등)을 따라 발생 - 횡적 연장성이 좋은 층간 점토층이 붕괴면이 되는 경우가 많음 - 이동블록은 인장절리나 불연속면으로 분리 암석 흙
2) 흐름(flow)
- 흙(점토, 토사), 토석(토석류) 또는 암석블록(암편흐름)의 대규모 이동 - 높은 함수비, 유체 (viscous liquid) 거동
- 수 km의 이동거리 가능, 최대 속도 56 km/h (USGS, 2008) (1) 이류 또는 토사류(mud or earth flow)
- 세립의 균질 물질. 초당 수 m 이동속도 - 발생에 물의 포화가 필요함. - 세립 화산성 퇴적층에서 특징적. (2) 토석류(debris flow) - 세립기질에 암편, 블록, 자갈 등이 혼합 - 느슨하고 압밀되지 않은 퇴적물로 덮여있거나 - 식생이 발달하지 않은 곳에서도 발생 - 25~45°의 경사에서 발생, 5~15°에서 퇴적. - 1~12 or 15m/sec 속도 - 산사태와 함께 발생함 - 포행(creep): 매우 느린 표층이동 - 토양류(solifluction): 동결-융해과정에서 발생 하는 느린 흐름
