Acquista:
Il problema del controllo vibrazionale durante la realizzazione di gallerie in ambito urbano rappresenta un tema di particolare interesse. Tipologia delle lavorazioni, condizioni geologico-geotecniche, caratteristiche degli edifici e dei fabbricati interessati sono aspetti che influenzano il problema e definiscono l’impatto delle attività sulle cose e sulle persone. In particolare, il fenomeno può essere causa diretta o indiretta di danni più o meno gravi a costruzioni, infrastrutture, sottoservizi e di disagio per le persone che ne abbiano percezione. Aspetti generali, riferimenti normativi e risultati di misurazioni in corso d’opera nell’ambito di gallerie metropolitane sono qui trattati in relazione a scavi di tipo meccanizzato, condotti mediante l’impiego di TBM.
Vibrations during TBM excavation. Details and analyses
of real cases
Due to its non-invasive approach and considerable
speed, excavation with TBM is the
preferred choice for building tunnels in urban
contexts where disturbances must be
contained as much as possible.
Despite these advantages, the rotating
head that performs the excavation generates
periodic stress on the face, which
propagates underground causing vibrations.
The effects of those vibrations can
be environmental (people perception) or
structural (building integrity) on which we
will focus in this paper.
Depending on their importance, the damages
on buildings can be classified in cosmetic
(merely aesthetic alteration), minor
or major (damage of structural elements).
In order to correctly evaluate and predict the effects, it’s important to characterise
the examined vibratory phenomenon. In
particular, an excavating TBM excitates
surrounding buildings with what can be
represented as a motion induced at their
base: the elastic waves generated by the
machine (source) pass through the ground
which transmits them to the foundations by
modifying their spectral content.
In terms of duration, the phenomenon can be
considered continuous. The distinction between
a continuous response (long duration
excitation) and a transitory response (short
duration) of a certain structure is based on
its intrinsic characteristics. The discriminating
reference period is, in fact, 5 times the
“time constant of the building” (0), which
depends on the first period of vibration, on
the structure geometry, on the dampening
factor and on the structure type. For normal
constructions, it assumes a value of a few
seconds, clearly much lower than the average
duration of underground work cycles.
For the measurements, it is necessary to
establish the nature of the phenomenon.
In the case of excavations with a TBM, although
the presence of sources other than
the main one (such as street traffic) can create
an inevitable aleatory “disturbance”, the
acting force presents a mostly deterministic
nature with a periodic tendency over time;
this makes it possible to carry out a study in
terms of frequency with a Fourier analysis.
Analysing the spectre of the vibratory phenomena
associated to the TBM excavation,
the frequency field in which it is concentrated
is between 10 and 70 Hz. Comparing
the highest harmonic components of the
acting force to the frequencies of the construction
(both estimated and measured),
the last appear lower scattering any critical
condition (resonance).
The methods used to measure vibrations
are generally chosen on the basis of distinctive
elements of excitation and in function
of the purpose of measurement. In general,
data can be collected in terms of displacements
(d), velocity (v) or acceleration (a)
vs time, depending on the frequency range:
the representation in terms of displacements
accentuates the low frequencies
components rather than the high frequency
ones that are accentuated in the representation
in terms of acceleration.
Considering the field of frequencies typically
associated with the excavation process
of a TBM (>100Hz) and the availability of
efficient accelerometers on the market, it is
common practice to measure acceleration.
Data collection must be followed up by the
processing necessary to obtain parameters
which can be compared with normative values,
that normally refer to threshold velocities:
PPV (peak particle velocity): defined as
the maximum value of the velocity vector
modulus.
PCPV (peak component particle velocity):
defined as the maximum value of a modulus
of one of three orthogonal components of
the velocity vector.
Some of the regulations on this subject differentiate
the limit values on the dominant
frequency range that can be evaluated by
means of the Fourier analysis.
The main documents, which indicate threshold
values under which it isn’t expected for
constructions to suffer even aesthetic damage,
are the following:
[1] DIN 4150 – Erschütterungenim Bauwesen
- Einwirkungen aufbaulichen Anlagen;
[2] BS 7385-1 - Evaluation and measurement
for vibration in buildings - Guide
for measurement of vibrations and
evaluation of their effects on buildings;
[3] SN 640312a - Effet des ébranlements
sur les constructions.
Granting that:
Measurements must be taken according to
three mutually orthogonal axes.
The reference values only take into account
the direct effect of vibrations.
Excess of the indicated limits does not necessarily
imply damage, but indicates the
need for deeper analysis.
The vibrations caused by a TBM are classifiable
as a continuous phenomenon.
Tables 3, 4 and 5 report a summary of the
reference values of PPV or PCPV for the
vibrations induced on different types of
constructions.
The complex of experimental surveys carried
out during TBM mechanical excavation
for different metro lines in different geographic
contexts, has pointed out very different
vibrational level values. They depend
on the type of grounds present in the sites,
and on the type of examined receptor (in
terms of structure, geometry and mass) as
well as on the altimetric plane distance between
the source of the vibrations and the
receptor point.
In general, it was observed that the levels
are quite contained in the presence of alluvial
grounds with gravels and poorly or
averagely gathered sands and in absence of
cementing elements on the superficial layer.
Whilst in the presence of conglomerates or
more gathered/compact formations, there
is usually an increase of the vibrational levels,
which is also in function of the consistency
of the excavated material.
The presence of constructions in the immediate
vicinity, which reshapes the superficial
portion of the ground by compacting
it (e.g. the surveys carried out for the Milan
Metro), as well as medium-large boulders,
generally intensifies the phenomenon.
Where the grounds are sufficiently homogeneous
and the distance between the source
of the vibrations and the receptor point is
limited, the presence of a building with its
own geometry and mass causes a decrease
of the vibrational levels compared to those
observed on the ground (due to the cushioning
effect of the mass). In the absence
of other parameters, it’s expected that an
increase of the mass of the construction will
cause an indicatively proportional decrease
of the recorded vibrational levels.
In absence of other ground characteristics,
the depth of excavation significantly
influences the vibrations. Indeed, a deeper
source reaches the surface engaging a
wider portion of ground and therefore a
greater mass; this factor, as mentioned before,
contributes to cushion partially the
vibrational effects.
Quantitatively, with reference to the examples
given, the values summarised in Figure
13 show a PCPV value equal to around 1.5
mm/s at a frequency of 40 Hz. If interpreted
in the light of the BS 7385-1, this data
should be compared to the threshold value
of 25 mm/s (see Table 4), and be therefore
acceptable. The maximum values deduced
from the Brescia Metro show PPV values
equal to 4.00 mm/s at the foundations and
1.90 mm/s at the elevations.
The first factor, if interpreted according to
BS 7385-1 (foundation values), is acceptable;
the second, if interpreted according
to SN 640312a (the elevation areas being
considered the most vulnerable), is acceptable
regardless of the associated frequency value.
Se vuoi essere informato sulle nostre novità editoriali, registrati alla nostra newsletter!