CONSTRUCTION OF A MOTORWAY EMBANKMENT TASKS 1. From the experimental soil data provided, determine the most adequate shear strength parameters of the soil. The data provided includes stress conditions...

1. From the experimental soil data provided, determine the most adequate shea
Strength parameters of the soil. The data provided includes stress conditions
at failure from drained triaxial compression tests on initially un-sheared clay
as well as tests from samples taken from the slip surface. In addition, a shear
Stress - shear displacement data from a direct shear test on the initially un-sheared test
is also performed.
When the soil sample has failed, the shear stress on the failure plane defines the shear
strength of the soil. Thus, it is necessary to identify the failure plane.
As per the graph plotted from the Table 3: stress displacement data for a direct shear
ox test on an intact sample of clay the maximum shear strength of soil is around
96.5kPa.
From tri axial test σ' [kPa] co
esponding to 96.5kPa is 206.8 kPa
We know putting C= 0 for CD test ; shear stress = 96.5 and σ'= 206.8
Or96.5=206.8* tanφ
Or φ= 25.01 degree
The soil grains are in an interlocked state. As the sample is subjected to shearstress,
the stress has to overcome the resistance offered by the interlocked a
angement of
theparticles. Experimental evidence indicates that a significant percent of the peak
strength is due tothe interlocking of the grains. In the process of shearing one grain tries
to slide over the other andthe void ratio of the sample which is the lowest at the
commencement of the test reaches themaximum value at point 96.5kPa
The shear stress also reaches the maximum value at this level. Any further increase of
strain beyond this point is associated with a progressive disintegration of the structure of
the soil resulting in a decrease in the shear stress. Experience shows that the change in
void ratio due to shear depends on both the vertical load and the relative density of the
soil. At very low vertical pressure, the void ratio at failure is larger and at very high
pressure it is smaller than the initial void ratio.

2. By back analyzing the slip surface determine the shear strength acting on the
slip surface. It is recommended to use the conventional method of slices, i.e.
Swedish method or Bishop solution, to perform the calculation. Assume that
drained conditions operate on the slip surface and that the factor of safety
(FS) is unity. The pore water pressures can be determined from the
piezometric line shown in the handout. Note that additional laboratory have
indicated that the unit weight of the clay is 19 kN/m^3 How does your result
Compare with that one you derived in (1) above?




FOS=1
U= pore water pressure
P= total normal force acting on the base of slice
L= length of arch ab on slice
Unit weight = 19kn/m^3
Total depth = 100ft =30m
U=10 Kn/m^3* z
Sin i= 30/200=.15
Cos i= .988
Condition given:
1.The landslide had a width of approximately 300 m and extended about 200 m in the
up slope direction.
2.Below ground surface of 10 m = z
3.Soil behaves in a drained manner.

Shear strength of soil
Ʈf= σ-u=( ɣsat z cos^2 i - ɣw z cos^2i)* tan φ
= 19 x 10 x .988^2 x tan 25.01=86.87Kn/m2
The shear strength obtained from stability analysis is (86.87Kn/m2) less
than that of value we are obtaining fromtriaxialtest, which is 96.5 Kn/m2.
Factor of safety = 2.013
3. By assuming that the motorway embankment can be modelled as a vertical
line load of 5000 kN/m, acting perpendicular to the cross section of the slide,
investigate how the factor of safety changes with the position of the embankment
centroid for the following two conditions:
a) The embankment is built quickly such that excess pore water pressures
are generated on the slip surface. For the purposes of this calculation you
may assume that the increase in normal total stress and pore water
pressure on the slip surface due to the embankment load are such that
there is no change in the normal effective stress.
= 302.98Kn/m2
σ =302.98*.988=299.35 Kn/m2
U=ɣw z cos^2 i= 10*10*.988^2=97.61 Kn/m2
Shear strength of soil
Ʈf= (σ-u)*tan φ =201.74*tan 25.01=94.12 Kn/m2
Ʈ= 302.98 sin i= 303.186*.15=45.45 Kn/m2
FOS= 2.07
) The embankment is constructed slowly, such that no excess pore wate
Pressures are generated on the slip surface.
These calculations can be performed as a sequence of slice analyses in
which the line loads, representing the weight of the embankment and the
pore water pressures generated on the slip surface are applied to each slice
in turn. Plot the factor of safety against position of the centre of the slice.
= 302.98Kn/m2
σ =302.98*.988=299.35 Kn/m2
Shear strength of soil
Ʈf= σ*tan φ =299.35*tan 25.01=139.65 Kn/m2
Ʈ= 302.98 sin i= 303.186*.15=45.45 Kn/m2
FOS= 139.65/45.45= 3.07
4. Describe the advice you would give to the motorway designers as to the best
location for their embankment, assuming that it must cross the landslide at
some point. What remedial measures would you suggest to stabilise the
landslide?
A road, railway line or canal is normally raised onto an embankment made
of compacted soil (typically clay or rock-based) to avoid a change in level
equired by the te
ain, the alternatives being either to have an
unacceptable change in level or detour to follow a contour. A cutting is
used for the same purpose where the land is originally higher than
equired.
Embankments need to be constructed using non-aerated and
waterproofed, compacted (or entirely non-porous) material to provide
adequate support to the formation and a long-term level surface with
stability....