1 INTRODUCTION In January 2011, the state of Rio de Janeiro, Brazil, suffered what has been considered as the worst disaster of its history. Thousands of landslides of differing types and proportions reached the Mountainous Region of the State under the action of heavy rain falls, claiming more than 900 casualties and millions of dollars in losses.

Rainfall, by itself, is usually taken as the main triggering factor of landslides. In the case of initially unsaturated soil slopes, water infiltration in the soil profile would promote suction decrease and, consequently, shear strength decrease that would lead to instabilization of the slope. In the case of saturated slopes, increase of water level would also promote decrease in shear strength due to the decrease in effective stress.

In the Mountainous Region of Rio de Janeiro, the slopes comprise profiles of unsaturated residual soils of gneissic origin. In this type of material the saturated permeability is typically of the order of 10-6m/s. The permeability function, however, may vary with a number of factors (e.g. mineralogy, weathering degree).

Failure due to suction loss in gneissic residual soil slopes has been reported by several authors (e.g. de Oliveira al, 2011). In most of these cases, however, preceding precipitation pattern, which affects initial

suction and, together with the permeability function, the rate of rain water infiltration, was much more important than that observed in the case of the 2011 disaster. In other words, it was not clear if only rain water infiltration would explain the large amount of landslides, with variable characteristics, that occurred almost simultaneously in the 2011 event.

According to local people, it was not windy when the heavy rainfall occurred. Thus, any possibility of anticyclone occurrence at the site was discharged. On the other hand, they said that “everything was shacking”. Also, they observed a never before seen number of lightning.

As thunders are associated to lighting, it was considered to be interesting to investigate whether vibrations promoted by thunders could have contributed in some way to the landslides set up.

Bearing the above mentioned on mind, it was started a research program at the Civil Engineering Department of PUC-Rio to investigate whether or not thunder sound waves could induce any disturbance in the soil that could lead to changes in its effective stresses.

This paper presents preliminary results of such investigation, with emphasis on the set up of the experiment, which comprised both the simulation of thunder soundings and the observation of its effects on the instrumented soil block samples.

Response of tensiometers to laboratory thunder sound replicated waves

T.S. Carnavale, T.M.P. de Campos & A.R.M.B. de Oliveira Civil Engineering Department, Pontifícia Universidade Católica do Rio de Janeiro, Brazil

ABSTRACT:

This paper presents part of a research work underway at the Pontifícia Universidade Católica do Rio de Janeiro, Brazil, in which effects of sound waves from thunderstorms are being investigated as a potential source of instability of initially unsaturated soil slopes. Undisturbed soil blocks, retrieved from a site in which hundreds of landslides occurred recently in Brazil, were instrumented with tensiometers, TDRs and accelerometers and subjected to effects of thunder sound replicated waves. This paper presents the general set up of the developed experiment and discusses the response of the tensiometers to the sound waves.

2 SOME THUNDER CHARACTERISTICS The thunder’s sound is directly related to the distance and intensity of lightning. The farther away the observer is from the flash emitted by lightning, the longer will take for the thunder to be heard. (Glassner's, 2000). The location of the thunder points of incidence is important because the phenomenon can lead to an increase of atmospheric pressure near the bright channel (Rakov & Uman, 2003). For Orville (1967), such increase can be of up to 10 atm in the first 5 µs in heated contact channels. Based on the simulation of lightning by rockets launching, Newman (1967) mention that the magnitude of the overpressure varies from 0.3 to 2 atm at a distance of 35cm of the observer. Depasse (1994) says that for an acoustic signal 70 meters far from the listening point the increase is of 0.0000493 atm.

The findings of these authors indicate the occurrence

of a rapid decrease of the overpressure as the

distance between the measurement point and the

sounding source increases. Also, it is evident that the

atmospheric pressure may change a lot as a

consequence of lightning.

In the thunder design, the pertubation incidence

pressure p(t) at the observer in Figure 1 is made by

the overlap of N-waves emited pressure, which

result from the generated pressure at each explosion

point, s.

Figure 1: Lightning quasilinear model where each point of the

channel emits a pressure wave called N. (Adapted from Roy &

Ribner, 1982)

The thunder signature is given by an expression such as that given by Equation 1 (Roy & Ribner, 1982):

(1)

where N represents a emitted perturbation from each emission point s, at the Figure 1; r is the distance from the observer to point s; ct is a function of the sound velocity and time; A is the wave amplitude,

“governed by the energy released per unit length in the lightning discharge” (Roy & Ribner, 1982), and ds is the channel height.

3 MATERIALS Two types of soils were employed in the research. They were sampled from areas located at PUC-Rio and in a Condominium that was destroyed in the 2011 disaster located on the highway RJ 130, Nova Friburgo.

The soil from the PUC-Rio site comprises a yellowish red sandy clay colluvium, classified as CL in the USCS. Its appearance is quite homogeneous, with a microgranular texture. The main minerals present in this soil are quartz and kaolinite. Also, occur oxides of iron and aluminum.

The soil from the Condominium site comprises a brownwish clayey sand colluvium, classified as SC. The main minerals in its coarse fraction are quartz and mica. The main clay minerals are kaolinite and gibbsite. It is also observed the occurrence of iron concretions (Oliveira, 2013).

Block samples with sides of 30cm were collected in each site. Table 1 shows average values of physical indexes of these two materials.

Table 1: Average values of physical indexes

Average Values PUC-Rio Soil Nova Friburgo Soil

Gs 2.65 2,64

ρd (kN/m3) 13.5 11,7

e 0.96 1,26

n 0.49 0,53

S (%) 73,4 50,1

4 METHODS

4.1 Acoustic chamber

An acoustic chamber (Figure 2), made with plywood, was built. Inside it a table, comprising a steel frame with a heavy movable top, set at an angle of 30 degrees, was used to support the block samples.

In front of the table were installed four speakers with amplifiers generating 4.500 watts RMS of power. A computer controlled sound table, positioned outside the chamber, was used to control the amplifiers.

Figure 2: Chamber details: a) table, comprising a steel frame

with a heavy movable top; b) speakers disposal in front of the

soil block; c) acoustic foam installed inside the chamber; d)

control equipment

4.2 Sound replicated waves

The initial step was to replicate the waveforms developed by Lee (2008). For this purpose, the Audacity software was of utmost importance, because through the use of the drawing tool, it was possible to make point-to-point waveforms used in the tests. As shown in Figure 3, clap type of waves, simulating the effect of a unique lightening channel, and rumble type of waves, simulating the effect of multiple lightening channels were replicated. In the case of the rumble type of waves, there were considered different distances of the observer to the bright channel. In Figure 3 it is shown a replicated rumble wave at a distance of 0,5 km from the observer.

Figure 3: Waves used on the tests

4.3 Instrumentation

Soil blocks were instrumented with tensiometers, TDRs, accelerometers and temperature sensors. In

this work, however, mention is made only to the tensiometers.

The T5X UMS tensiometers (Figure 4) were used to measure water tension inside the soil and respectively the matrix potential. These tensiometers work from +100 kPa to -160 kPa (UMS, 2009), with a resolution of 0,06kPa.

Figure 4: Tensiometers T5X

The data acquisition system used for the tensiometers was the DL2e Data Logger, manufactured by Delta-T Instruments. A voltage conditioner working in the range -5 VDC to +5 VDC was coupled to the system. The equipment allows data measurements at time interval of 1 second.

4.3 Testing Methodology

Two sets of testing were performed. In the first set field moulded block samples were confined within a plywood box (Figures 5 and 6) keeping only the top face of the blocks exposed to atmosphere. These testing series were designated as Confined Tests.

Figure 5: Withdrawal of confined blocks located in the

Condominium site

Figure 6: Instrumented confined block ready for testing

The second testing set was performed on unconfined block samples. In these cases, samples were left protected against moisture loss up to the time of the test, when their casings were taken off. These testing series were designated as Unconfined Tests (Figure 7).

Figure 7: Unconfined block ready for testing

Instrumentation was introduced into the blocks with the use of a drilling machine at low speed and drills with the same diameter of the sensors rods and bodies. After drilling and certification of the depth of the holes, the sensors were carefully inserted in the soil blocks for the immediate execution of the tests.

In all cases, the tensiometers were positioned in different parts of the block (Figure 8). Tensiometers C5 and C6 were in the frontal part of the blocks while tensiometer C8 was at the back of the blocks. The other 4 tensiometers were positioned at the laterals of the blocks.

All tests were performed in two stages. In the first stage the samples were either previously left to air

drying for two weeks (Confined Tests) or tested at their natural water content (Unconfined Tests). In the second stage, the samples were moistened up to near saturation using a Mariotte bottle to insert water under a low constant head inside the samples and by direct wetting at their surface. Near saturation was indicated by fairly low suction measurements in the tensiometers and increased water content measurements in the TDR's.

Sound wave emissions were applied to both confined and unconfined blocks at both testing stages. Initially it was emitted a clap thunder replicated wave and then, rumble thunder replicated wave forms, as exemplified in (Figure 3).

Figure 8: Positioning of the tensiometers in the blocks

5 RESULTS AND FINAL CONSIDERATIONS Figure 9 shows typical response of the tensiometers to the sound wave emission considering the testing stage 1 in the soil from PUC-Rio. As indicated in Figure 9a, initial suction varied within circa of 87 and 97 kPa in the dry material. This same magnitude of initial suction was also observed in the dry soil from Nova Friburgo.

Figure 9: Tensiometers results in stage 1 of confined tests in the PUC-Rio soil

a)

b)

The tensiometers installed at the lateral and back sides of the dry blocks from PUC-Rio did not show any variation of suction under the application of the sound waves. This can be better verified in Figure 9b, where the suction scale was amplified. In this Figure, the minor suction variations are within the resolution of the suction measuring system. On the other hand, tensiometers C5 and C6 positioned at the front side of the blocks showed a decrease in suction of the order of 1,5 kPa along time. Such type of suction variation was not observed in the soil from Nova Friburgo and may have occurred due to some moisture flow towards the frontal part of the block that was still happening in this clayey material. Unlike the soil from PUC-Rio, the confined soil from Nova Friburgo in the testing stage 1 (dry soil) did respond to the sound wave application when it was simulated rumble type of waves. As indicated in Figure 10, which shows typical responses of all tensiometers installed in the block, a decrease in suction of the order of 0,5 kPa occurred immediately in response to the rumble 0,5 km replicated sound wave.

Figure 11 shows typical response of the tensiometers in stage 2 (wetted samples) of both confined and unconfined blocks of the PUC-Rio and Nova Friburgo soils. As it can be seen, the tensiometers responded promptly to the emission of both types of wave forms. The magnitude of suction variation ranged from ± 0,1 kPa to ± 0,6 kPa independently on

the position of the tensiometer inside the block. This same type of behavior was also observed in the stage 1 (dry soil) of the unconfined blocks.

In the unconfined blocks the sound waves reached the soil from all sides, being not clear how they may have propagated within the blocks, regardless their initial saturation conditions. In the wetted samples (confined and unconfined) there was no guarantee of having achieved a constant degree of saturation within the blocks as a whole. Indeed, both positive and negative suction variations along time were measured by tensiometers installed in the same sample, indicating the occurrence of moisture flow (wetting and drying) in different parts of the block, regardless the application of the sound waves.

As a result of such variable conditions it has not yet being possible to explain why, under the action of the replicated sound waves, both increase and decrease of suction were monitored in the two studied soils. It is certain, however, that suction variation occurred in the soil blocks and that the tensiometers monitoring system employed was able to respond to the fairly fast applied sound waves (time of application variable from 2 s to 15 s).

Further investigations are presently under way in order to try to explain these first testing results.

.

Figure 30: Typical results of stage 1 in confined tests performed in the Nova Friburgo soil

Figure 11: Typical results obtained in stage 2 of unconfined test on the PUC-Rio soil.

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