The parallel seismic method for foundation depth evaluation: a case study in

Abstract

A case study on the application of the seismic test of recent use in Brazil, called Parallel Seismic (PS), to evaluate the depth of a 250-mm diameter root-pile belonging to the foundations of a telecommunication steel monopole located in Arthur Alvim, São Paulo, Brazil, is presented. The foundation element is inserted in a sandy clay soil. The estimated depth of the foundation element was obtained by analysis of the results from the PS test. It was evaluated as being equal to 11 m, which was later verified to be in agreement with the pile's design drawings. When the possibility to drill boreholes at a site under investigation exists, the method presented in this article becomes a good option for the determination of the unknown depth of foundation elements.

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Fifteenth International Congress of the Brazilian Geophysical Society
The parallel seismic method for foundation depth evaluation: a case study in Arthur
Alvim, São Paulo, Brazil
Otávio Coaracy Brasil Gandolfo* (Institute of Technological Research-IPT); Tiago de Jesus Souza (Aeronautics Institute of
Technology-ITA / American Tower); Paulo Scarano Hemsi (Aeronautics Institute of Technology-ITA); Paulo Cezar Aoki
(American Tower)
Copyright 2017, SBGf - Sociedade Brasileira de Geofísica
This paper was prepared for presentation during the 15th International Congress of the
Brazilian Geophysical Society held in Rio de Janeiro, Brazil, 31 July to 3 August, 2017.
Contents of this paper w ere reviewed by t he Technical Committee of the 15th
International Congress of the Brazilian Geophysical Society and do not necessarily
represent any position of the SBGf, its officers or members. Electronic reproduction or
storage of any part of this paper for commercial purposes without the written consent
of the Brazilian Geophysical Society is prohibited.
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Abstract
A case study on the application of the seismic test of
recent use in Brazil, called Parallel Seismic (PS), to
evaluate the depth of a 250-mm diameter root-pile
belonging to the foundations of a telecommunication
steel monopole located in Arthur Alvim, São Paulo,
Brazil, is presented. The foundation element is
inserted in a sandy clay soil. The estimated depth of
the foundation element was obtained by analysis of
the results from the PS test. It was evaluated as being
equal to 11 m, which was later verified to be in
agreement with the pile’s design drawings. When the
possibility to drill boreholes at a site under
investigation exists, the method presented in this
article becomes a good option for the determination
of the unknown depth of foundation elements.
Introduction
The parallel seismic (PS) test is a geophysical technique
developed in France several decades ago for the
determination of the unknown depth of foundation
elements, which has a methodology similar to that of the
downhole test. Also, studies have been conducted to
develop the PS test for evaluation of pile integrity (Liao et
al., 2006; Huang and Chen, 2007; de Groot, 2014).
Practical advantages of the PS test are the applicability of
the test for different foundation materials (e.g., concrete,
steel, wood or masonry) and the possibility of testing even
when the pile head is not accessible.
Lack of knowledge of the foundation depth is a frequent
problem in Civil Engineering, and arises from the non-
availability of blueprints, the absence of a foundation
project, or existing, but unreliable information. The
geophysical surface methods are generally unsatisfactory
for this type of problem. The PS test, which for its
execution requires a borehole positioned very close to the
foundation element to be investigated, is able to provide
more accurate and reliable results. The length of the
foundation pile is an important parameter for evaluating
bearing capacity, in the case of reusing the structure for
other purposes, for example, when subjected to higher
loads.
This work presents the result of a PS test to evaluate the
unknown depth of a 250-mm diameter root-pile belonging
to the foundations of a telecommunications tower located
in Arthur Alvim, municipality of São Paulo, Brazil.
Parallel Seismic test (PS)
The principle of the PS test is quite simple. A pulse is
generated by the impact of a small sledgehammer hitting
against any exposed part of the structure connected to
the foundation (or on the foundation itself if it is available)
producing elastic waves predominantly of the
compressional type (P-wave) that propagate through the
foundation element. Due to the large contrast in elastic
properties existing between the ground and the material
that constitutes the foundation (concrete, in general),
seismic waves are refracted and captured by three-
component geophones (or hydrophones) placed in a
borehole near to the foundation (around 1 m), and
recorded on a seismograph (Figure 1).
Figure 1 - Parallel Seismic method: a) field test (left) and
typical results (right) (Modified from Niederleithinger,
2012).
The test may be performed on concrete, steel, masonry
or wood foundations. The hole must have a depth which
exceeds by at least 3 to 5 m the maximum expected
depth for the foundation element. The preparation of the
borehole should follow the guidance similar to those used
to perform the downhole or crosshole testing. A PVC-pipe
casing must be installed in the borehole, with its bottom
end capped and the annular space between the pipe and
the wall of the borehole filled with grout to ensure good
contact with the ground.

THE PS METHOD FOR FOUNDATION DEPTH EVALUATION: A CASE STUDY IN ARTHUR ALVIM
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Fifteenth International Congress of the Brazilian Geophysical Society
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If hydrophones are used, the cased borehole should be
filled with water. If geophones are used the borehole must
be dry.
The seismograph records the seismic wave that is
generated on the surface by the impact of the hammer
and captured by the geophones positioned in the
borehole. The procedure is performed starting at the
bottom of the hole to the surface, usually at regular
intervals of 0.5 to 1 m. The test results consist of a
seismogram containing the seismic traces recorded at
different depths tested.
By determining the first arrival time of the wave, a line can
be adjusted, which provides the value of the velocity of
the P-wave (VP) in the material foundation (usually
concrete, which has a high value for VP) and in the
surrounding soil (usually with a VP value lower than the
foundation´s). The depth at which the slope changes is
attributed to the maximum depth of the foundation
element.
The evaluating of the depth of a 230-cm diameter caisson
using the PS test in São Paulo provided good results,
when the obtained depth was compared to the depth
depicted in the caissons documentation (Gandolfo et al.,
2015).
Description of the test site
At the investigated site the existing steel monopole has a
dodecagonal geometry and 42 m in height, at the top, an
outer diameter of 0.493 m, and at the base, 1.481 m. The
structure is located at Pierre Fermat Street, Artur Alvim,
São Paulo. Figure 2 shows the monopole with its
antennas and other details.
Figure 2 - Monopole’s view in Artur Alvim, São Paulo.
The structure is fixed on a foundation block with
dimensions (3.1 x 3.1 x 1.1 m), which comprises 8 root-
piles with 250-mm diameter each (Figure 3).
In order to enhance the signal reception for the seismic
test, the borehole was drilled very close to the foundation
(Figure 4).
The borehole for the PS test, at depth of 19 m, was drilled
0.4 m distance from the block. It was cased with a PVC
pipe of 85 mm in diameter (thick wall) and its lower end
capped. The annular space between the pipe and the wall
of the borehole was filled with cement grout.
The stratigraphic profile at the location of the borehole is
depicted in Figure 5. The soil is predominantly sandy clay
with resistance increasing along depth. It is important to
note that in the first 2 m the soil has a soft consistency.
Beyond 4 m, the soil has a firm consistency. The water
level was found to be at 2 m depth on the date of SPT
test.
Figure 3 - Foundation block detail: a) plan view (above);
b) cross sectional view, showing the elevation of the block
depicting three root-piles and the borehole used for the
PS test (below).

GANDOLFO, O.C.B; SOUZA, T.J; HEMSI, P.S.; AOKI, P.C.
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Fifteenth International Congress of the Brazilian Geophysical Society
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Figure 4 - Borehole for the PS test at 0.4-m distance from
the block.
Figure 5 - Stratigraphic profile at the site.
Field data acquisition
For carrying out the PS test at the site, the following
equipment was used (IPT, 2015): a) seismograph
SmartSeis model of 12 channels (Geometrics), b)
downhole geophones (8 Hz) with three components
(triaxial) and pneumatic clamping mechanism in the wall
of the hole and c) 1.8-kg sledgehammer to generate
seismic waves with a coupled transducer (trigger) (Figure
6).
The impact of the hammer was applied directly against
the central portion of the concrete block (Figure 7). The
signal received by the geophone into the borehole was
recorded at regular intervals of 0.5 m, starting at the
maximum depth reached by the sensor (18 m) to the
vicinity of the ground surface (0.5 m).
Figure 6 - Equipment used for the execution of PS test:
seismograph (above on the left); 1.8-kg sledgehammer
with trigger (above on the right); triaxial geophone hole
with pneumatic system to fix in the hole (below on the
left); equipment on the site (below on the right).
Figure 7 - PS test being performed in the telephone tower
foundation.

THE PS METHOD FOR FOUNDATION DEPTH EVALUATION: A CASE STUDY IN ARTHUR ALVIM
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Fifteenth International Congress of the Brazilian Geophysical Society
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Results
Figure 8 shows the seismogram obtained from the PS
test by recording the traces of the vertical component of
the geophone.
Figure 8 - Seismogram obtained from the PS test.
Picking the first arrival times of the P-wave in the
seismogram, the plot of time versus depth was obtained,
allowing the velocities estimation and interpretation of the
unknow depth of the foundation element (Figure 9).
The point of inflection corresponding to the change in
slope of the fitted straight lines, changing from
V1=3733 m/s (velocity of the seismic P-wave in the
concrete foundation) to V2=1956 m/s (velocity of the P-
wave in the saturated soil, below the foundation) was
interpretaded as being the depth of the foundation
element.
Based on the above criterium, the depth of the root-pile
was estimated as being equal to 11 m (Figure 9).
As shown in Figure 10, the actual depth of this foundation
element is precisely 11 m, as obtained by the PS test
carried-out at the site.
Figure 9 - Time versus depth plot obtained from the first
arrival times of the P-wave read from the field
seismogram of Figure 8.
Figure 10 - Foundation design showing the actual depth
of the root-piles (11 m).

GANDOLFO, O.C.B; SOUZA, T.J; HEMSI, P.S.; AOKI, P.C.
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Fifteenth International Congress of the Brazilian Geophysical Society
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Conclusions
The PS test performed in the case study presented here
with a foundation that consisted of a concrete block with
root-piles of a 250-mm diameter provided a very
satisfactory result. As it is still a recent application in
Brazil, it requires methodological improvement.
The test should be used in other types of foundations with
different characteristics in terms of type, building material,
geometry, diameters, etc., for a more thorough evaluation
of the potential of this technique and improvement of the
operational procedures.
References
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