| Author | Shivashankar, Ramaiah |
| Call Number | AIT Diss. no. GT-90-03 |
| Subject(s) | Embankments
|
| Note | A dissertation submitted in partial fulfillment of the
requirements for the degree of Doctor of Engineering, School of Engineering and Technology |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. GT-90-03 |
| Abstract | The general objectives of this research were as follows:
( 1 ) To study the interaction mechanisms and the factors
affecting the pullout resistances, as well as to develop
prediction equations for the pullout resistances of welded-wire reinforcements with low-quality, cohesive-frictional
backfill materials.
( 2 ) To study and evaluate the performance of a welded-wire
mechanically stabilized earth (MSE) wall and embankment
system which utilized locally available, low quality soils
as backfill materials on soft ground conditions.
( 3 ) Comparison of the laboratory pullout resistances with the
predicted pullout resistances using the finite element
program REA at the end of 1 in. (25.4 mm) pull; and the
analysis of the MSE wall/embankment system to predict its
behavior immediately after construction, also by using the
finite element program REA.
A total of 544 pullout tests in 253 set-ups were conducted
in the laboratory using three different locally available, poor
quality, and cohesive-frictional backfill soils comprising of
clayey sand, lateritic soil and weathered clay. Prediction
equations and design curves for the total pullout resistances in
terms of both the soil and the mat parameters were developed from
the pullout test data, by a multiple regression procedur'2. The
laboratory pullout tests with normal stresses up to 13 T/m (130
kPa) proved that even with such poor to marginal quality backfill
materials, the pullout resistances increased with the increase in
the confining vertical normal stresses. It was also confirmed
that with the compacted cohesive-frictional soils on the dry side
of optimum, pullout resistances comparable to that of the good
quality granular backfill materials can be generated. The
parameters (or coefficients) of the prediction equations showed
distinct relationships with the compaction moisture contents for
all the three backfill soils.
The type of bearing capacity failure mechanism in front of
the transverse members of the grid reinforcement was found to
occur as general bearing failure mechanism with increasing
spacing to diameter or (S/D) ratios of the transverse bars;
increasing compaction moisture contents; increasing confining
normal stresses and increasing displacement of the transverse
members through the soil, or in other words, with increasing
stiffness of the backfill soils compared to the stiffness of the
transverse members. Otherwise, the failure mechanism corresponds
to punching shear failure mechanism. Further, it was found that
for (S/D) ratios greater than about 50, the degree of
interference of the passive resistant zone of one of the
transverse members with that of the adjacent transverse members
becomes less significant.
(iv)
Fifteen constant-strain field pullout tests were conducted
on dummy reinforcements left in place in the test embankment at
different elevations. The two outer sections comprising of clayey
sand and weathered clay backfills generally gave higher pullout
resistances from the field pullout tests, while the corresponding
values for the middle lateritic section were found to be very
much lower. These phenomena can be blamed on the arching effects
caused by the excessive subsoil movements and due to the
presence of the inextensible reinforcements. The laboratory
pullout tests generally yielded conservative values.
A full scale experimental and an extensively instrumented
welded-wire wall and embankment system (AIT MSE Wall/Embankment)
of 5.7 m height with one vertical face was constructed on soft
Bangkok clay at the A.I.T. campus. The test embankment used three
different types of locally available, poor quality backfill
soils namely: clayey sand, lateritic soil, and weathered clay, in
the three sections along its length, respectively. The soft clay
in the subsoil is about 6 m thick overlain by a surficial 2 m
thick layer of weathered clay crust and underlain by a layer of
stiff clay. The behavior of the AIT wall was monitored both
during the construction and in the post-construction phases, and
the data were analyzed.
It was observed that the large settlements and the lateral
movements of the soft clay subsoil influenced very much the
variations in the vertical pressures beneath the embankment and
the tensile stresses in the reinforcements. The presence of the
inextensible steel grid reinforcements and the interconnection at
the facing caused arching effects, that affected the behavior of
the test embankment. The maximum tension line did not agree well
with either the Rankine or the coherent gravity or the
logarithmic spiral failure planes. Compaction induced stresses
increased the lateral earth pressures considerably and thereby
also increased the tensile stresses in the reinforcements. An
overall assessment of the wall behavior suggests a significant
deviation from that currently established for mechanically
stabilized earth walls resting on comparatively good foundation
subsoils.
The Reinforced Earth Analysis (REA) finite element computer
program was used with the concept of equivalent friction
coefficient for the grid reinforcements, to predict both the
laboratory pullout test results and the wall behavior. The
laboratory pullout tests were treated as plane strain problems,
similar to that of an externally loaded sheet pile, with the
reinforcements being treated as discrete bending elements. The
results were also compared with the corresponding values obtained
by using another finite element computer program NONLIN 1. The
FEM predictions of the pullout resistances were found to lie
between the upper and the lower bound envelopes for the pullout
resistances of the grid reinforcements, verifying the laboratory
pullout results. The FEM predictions of the wall behavior agreed
approximately well with the actual observations.
( v)
Finally, it can be concluded that the welded wire or steel
grids can be effectively used to reinforce poor quality backfill
materials on soft clay foundations. The AIT MSE Wall/Embankment
showed no signs of instability either during construction or in
the post-construction phases and continues to perform
satisfactorily after more than two and a half years since its
construction. |
| Year | 1991 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. GT-90-03 |
| Type | Dissertation |
| School | School of Engineering and Technology (SET) |
| Department | Other Field of Studies (No Department) |
| Academic Program/FoS | Geotechnical and Transportation Engineering (GT) |
| Chairperson(s) | Bergado, Dennes T. ;Noppadol Phien-wej |
| Examination Committee(s) | Balasubramaniam, A.S. ;Karasudhi, Pisidhi ;Honjo, Yusuke
|
| Scholarship Donor(s) | The Government of Japan; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1991 |