2814
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
Coefficient of lateral earth pressure K
Date
South wall, 5.5 m depth below ground level
South wall, 9.1 m depth below ground level
North wall, 9.1 m depth below ground level
Mw 6.5 earthquake,
Dec 10, 2011
Mw 7.4 earthquake,
March 20, 2012
7 CONCLUSIONS
The geotechnical sensors installed in this foundation have had
an excellent response to date. They provide consistent readings,
with well-defined tendencies over time, and will be the reason
for detailed analyses for a more extensive interpretation. The
geotechnical instrumentation attached or installed during
construction of the foundation consists of 18 electronic sensors
of SG type, connected to a digital recorder; and 3 of the VW
type, for monitoring with portable read-out box. The
accelerometers embedded in the footing will provide the time
series of the accelerations suffered by the foundation during
seismic events, and will activate the recording system of the
geotechnical sensors, following a master-slave arrangement.
Figure 6. Time history of the effective coefficient of earth pressure.
In this comparison we are aware that our clay is normally
consolidated, whereas London clay is pre-consolidated.
Undoubtedly, these results are of great relevance to review
assumed hypotheses at the geotechnical design stage of
foundations (Martínez, 2012) consisting of structural cells.
At this first monitoring stage of the foundation we can point
out that vertical pressures under the footing clearly suffered the
increase due to the placement and weight of the footing-column
unit, gravitating on the temporary slab at the bottom of the
excavation. Then, a pressure decrease is appreciated later,
which is interpreted as a transference process of that load
toward the perimeter walls that begin to work as a set by
adherence-friction with the surrounding soil. It seems that in the
structural cell’s behavior, an integral mechanism with the soil
central core predominates, the core moving as a whole together
with the structural cell; an example of this is that pore water
pressure measured at the same depth inside and outside the
walls is practically the same.
6.2
Regarding what was recorded with the first runs of the
Metro trains
During the first trial runs of the Metro-Line 12 trains, we had
the opportunity to record dynamically the different variables
that could potentially be monitored automatically. The result of
these measurements is exemplified in Figure 7, with the
recording of the vertical pressure increase under the footing,
measured with pressure cell 1.
The great sensitivity of the measuring equipment and digital
recording stands out, which allows recording vertical pressure
changes with a resolution of at least 0.05 kPa. The largest
recorded change reached a value of 0.6 kPa, and the average
value was of the order of only 0.32 kPa. If graphically such
vertical pressure increases under the footing seem significant,
their real magnitude must be considered negligible when
compared to acting vertical pressures under sustained load. It is
thus clear that although the measuring systems record them very
clearly, vertical pressure increases under the footing due to Line
12 trains runs are minimal, showing the efficiency of the
foundation system, since the support work is evidently provided
by the adherence-friction on the periphery of the structural cell.
One arrives to those same conclusions observing the variations,
upon the passing of the Metro trains, of the total lateral pressure
on the walls, or the pore pressure in that same contact, results
that are not included in this paper.
On the other hand, the push-in-cells installed at the contact
of the outside walls indicate surprisingly high lateral pressures,
at least soon after being installed. Total horizontal pressures
reach values higher than total vertical pressures, although after
two years, they diminish and reach finally an almost constant
value. These high lateral pressures are clearly beneficial for the
foundation’s overall behavior against lateral actions and a
rotating tendency imposed by seismic rocking moments.
We point out the high values obtained for the coefficient K,
in terms of effective stresses, which was established based on
direct measurements of both total lateral stresses and pore water
pressures. Initial K values of up to 3.4 are reached. Nonetheless,
in a time period of almost two years after the walls were built,
the K value at three points of measurement coincides
asymptotically with an almost unitary constant value.
A first earthquake of low intensity on the foundation caused
a sudden, reduced and transitory horizontal pressure decrease on
the walls, but a rapid recovery of the tendency shown with
sustained loads was observed.
The Metro Line 12 trains runs impose no significant changes
in vertical pressure under the footing, nor on lateral pressures or
pore water pressures on the sides of the perimeter walls.
30.05
30.10
30.15
30.20
30.25
30.30
30.35
30.40
30.45
30.50
30.55
30.60
0
100 200 300 400 500 600 700 800 900 1000
Pressure, kPa
Time, s
Train front approaching ZP16 (50 m)
Train’s front on the ZP16 support
Train on the ZP16
support
Train’s end going far ZP16
Train far from the ZP16
support
8 REFERENCES
Dunnicliff, J. (1988), Geotechnical instrumentation for monitoring field
performance, John Wiley & Sons, 608 pp.
Martínez, S. (2012), Method of simplified analysis for a new foundation
type in soft soils, Doctoral Thesis, UNAM, Mexico (in Spanish).
Mendoza, M.J., Romo, M. P., Orozco, M. y Domínguez, L. (2000).
“Static and seismic behavior of a friction pile-box foundation in
Mexico City clay”,
Soils and Foundations
, Vol. 40, No. 4, Japanese
Geotechnical Society, 143-154.
Mendoza, M. J. (2004) Behavior of a friction pile-box foundation in
Mexico City, under static and seismic loading, Doctoral Thesis,
UNAM, Mexico (in Spanish).
Peck, R. B. (1969) “Advantages and limitations of the observational
method in applied soil mechanics”,
Géotechnique
19, 2, 171-187.
Figure 7. Changes of the total vertical pressure under the footing due to
Metro train transit. Pressure Cell SG1.
Tedd P. y Charles J. A. (1982). “In situ measurement of horizontal
stress in overconsolidated clay using push-in spade-shaped pressure
cells”, Technical Notes.
Géotechnique
31, 4, 554-558.
Terzaghi, K. y Peck, R. B. (1967). Soil Mechanics in Engineering
Practice, John Wiley & Sons, New York.
