BFC34402
CHAPTER 4.0
SLOPE STABILITY
4.1
TYPES & CAUSES OF SLOPE FAILURE
Slope failures major categories
1. Fall. This is the detachment of soil
and/or rock fragments that fall down a
slope.
2. Topple. This is a forward rotation of
soil and/or rock mass about an axis
below the center of gravity of mass
being displaced.
3. Slide. This is the downward
movement of a soil mass occurring on a
surface of rupture.
4. Spread. This is a form of slide by
translation. It occurs by “sudden
movement of water-bearing seams of
sands or silts overlain by clays or loaded
by fills”.
5. Flow. This is a downward movement
of soil mass similar to a viscous fluid.
Topple
Fall
Slide
Spread
Flow
4.1
TYPES & CAUSES OF SLOPE FAILURE
4.1.1
Types of slope failure
a) Translational slide:
i.
ii.
Failure along a weak zone of soil.
Sliding mass can travel long distances before
coming to rest.
iii. Common in course-grained soil.
b) Flow slide:
i.
Occurs when internal & external condition force
a soil to behave like a viscous fluid & flow down.
ii. Multiple failure surfaces usually occur & change
continuously as flow proceeds.
iii. Can occur in wet & dry soils.
4.1
TYPES & CAUSES OF SLOPE FAILURE
4.1.1
Types of slope failure
c) Rotational slide:
i.
ii.
Common in homogenous fine-grained soil.
3 types of rotational slides;
1. Base slide:
✓ By an arch engulfing the whole slope.
✓ A soft soil resting on stiff layer.
2. Toe slide:
✓ Failure surface passes through the toe
of slope
3. Slope slide:
✓ Failure surface passes through the
slope
4.1
TYPES & CAUSES OF SLOPE FAILURE
4.1.2
1)
2)
3)
4)
5)
6)
7)
Causes of slope failure
Erosion (a & b)
Rainfall (c)
Earthquake (d)
Geological features (e)
External loading (f)
Construction activities (g)
Rapid drawdown (h)
4.2
4.2.1
STABILITY OF INFINITE SLOPES
Factor of safety
Generally:
When Fs is equal to 1, the slope is in a state of impending failure. Generally, a value of
1.5 for the factor of safety with respect to strength is acceptable for the design of a stable slope.
4.2
4.2.2
STABILITY OF INFINITE SLOPES
Infinite slope- Without seepage
4.2
4.2.2
STABILITY OF INFINITE SLOPES
Infinite slope- Without seepage
OK
4.2
4.2.2
STABILITY OF INFINITE SLOPES
Infinite slope- With steady seepage
4.2
4.2.2
STABILITY OF INFINITE SLOPES
Infinite slope
Examples
4.3
METHOD OF SLICES
4.3.1
Finite slope
What the different between finite and infinite slope?
a) When the value of the critical height of the slope (Hcr) approaches the
height of the slope (H), the slope generally may be considered finite.
b) The surface of potential failure can be considered to be curved or plane
failure surfaces.
4.3
4.3.1
METHOD OF SLICES
Finite slope- Circular Failure Surfaces Analysis
Modes of failure
4.3
4.3.1
METHOD OF SLICES
Finite slope- Circular Failure Surfaces Analysis
Analysis method 1: Mass procedure
For the case of critical circles, the developed
cohesion can be expressed by the relationship
4.3
4.3.1
METHOD OF SLICES
Finite slope- Circular Failure Surfaces Analysis
Analysis method 1: Mass procedureExamples
A slope is to be cut to construct an embankment
as shown in Figure 1. The mass of W1 and W2 are
294.4 kN and 435.2 kN respectively. The location
of these masses are l1 = 3.5m and l2 = 5.3m . By
using the mass procedure method or Swedish slip
circle method:
(i) Calculate the total moment of the driving force
about point O.
(ii) Determine the factor of safety of the trial circle.
(iii) Comment what happens to the calculated factor
of safety if the height and the gradient of the slope
increases.
Figure 1
4.3
4.3.1
METHOD OF SLICES
Finite slope- Circular Failure Surfaces Analysis
Analysis method 2: Method of slices
4.3
4.3.1
METHOD OF SLICES
Finite slope- Circular Failure Surfaces Analysis
Analysis method 2: Method of slicesExamples 1
4.3
4.3.1
METHOD OF SLICES
Finite slope- Circular Failure Surfaces Analysis
Analysis method 2: Method of slicesExamples 2
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
4.4.1
Cut Stability
➢ Cuttings are excavated, whereas embankments are built.
➢ A decrease in total stresses and also a decreasing in pore pressure when excavation of the
removal of soils.
➢ Seepage regime develops in the slope after construction which it depends on the drainage
condition and the permeability of the soil.
➢ Therefore, an increment in pore pressure, constant in total stress but falls steadily in effective
stress.
➢ Besides, effective normal stress decreases too since it’s a part of proportional of soil shear
strength. So that, the long term stability of a cutting is therefore more critical than its stability
at the end of construction.
4.4
4.4.2
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Embankment stability
➢ Built by rolling or otherwise compacting layers of selected soil.
➢ Compaction process squeezes out air, but as the built up height increases, the lower layers experience
an increase in pore pressure. In coarse grained soils, the excess pore pressures dissipate quickly.
➢ In fine grained soils, the excess pore pressure is slow to dissipate and consolidation may continue for
several years.
➢ The installation of horizontal or vertical drainage blankets is used to speed up this process.
➢ In the course of time, the pore pressure decreases and the effective stresses increase, therefore the
shear strength increase.
➢ Thus, the most critical stability condition for an embankment occurs at the end of construction, or
sometimes during construction.
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Cut in slope designs
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Cut in slope designs
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Cut in slope designs
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Fill in slope designs
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Fill in slope designs
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Fill in slope designs
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Stability of cut and fill in slope designs
4.4
CUT STABILITY, EMBANKMENT & NATURAL SLOPE
Stability of cut and fill in slope designs
4.5
SLOPE STABILIZATION METHOD
4.5.1 Drainage
➢ Drainage is one of the most widely used methods for improving stability.
➢ Clearly surface water must be removed and build-up of water pressures in tension cracks
prevented. Subsurface drainage must be designed to reduce the water pressures acting on
actual or potential slip surfaces; in this way, the value of the pore pressure, u, is reduced,
thereby producing an increase in the factor of safety.
4.5
SLOPE STABILIZATION METHOD
4.5.1 Drainage
➢ Several methods exist for drainage system, including horizontal drains and vertical drains.
4.5
SLOPE STABILIZATION METHOD
4.5.1 Drainage
➢ Drainage may also be achieved by the use of electro-osmosis and by planting suitable
vegetation.
4.5
4.5.2
SLOPE STABILIZATION METHOD
Restraining Structure
➢ Restraining structures such as piles, retaining walls and anchors may be used to improve
stability. It must be appreciated that the forces and moments to which these structures are
subjected may be very large and hence careful design is essential.
4.5
SLOPE STABILIZATION METHOD
4.5.3 Modification of Slope Geometry
➢ Changing the geometry of a slope to improve stability can involve the: excavation to unload
the slope; filling to load the slope; and reducing the overall height of the slope.
4.5
6.5.4
SLOPE STABILIZATION METHOD
Replacement
➢ Where the slip surface is not unduly deep, removal of all (or part) of the slipped material and
replacement provides a relatively simple and straightforward remedial measure. The removed
soil may be replaced by free-draining material (in which case some additional benefit may be
achieved by drainage) or by light structure such as geofoam.
4.5
4.5.5
SLOPE STABILIZATION METHOD
Geotextiles
Geo-textiles can be used for:
➢ Segregation of layers: Rock-fill lay on soft ground to form a road or embankment base can be prevented
from punching into the soil below using a geotextile underlay.
➢ Tensile strength: Horizontal membranes can be used to provide tensile reinforcement and reduce
settlement.
➢ A drainage layer: Either as a water-conductor or as a filter to reduce the migration of fine particles into a
granular soil drains.
➢ An impermeable barrier: To prevent or control the flow of contaminated groundwater from or in landfill sites.
4.5
SLOPE STABILIZATION METHOD
The End