Vibration prediction from controlled blast tests Rogério Mota, Laboratório Nacional de Engenharia Civil, I.P., Av. do Brasil, 101, 1700-066 Lisboa, Portugal, [email protected], & Jorge Neves, EDP - Gestão da Produção de Energia, S.A., R. do Bolhão, 36, 4000-111 Porto, Portugal, [email protected], & Nadir Plasencia, EDP - Gestão da Produção de Energia, S.A., R. do Bolhão, 36, 4000-111 Porto, Portugal, [email protected]. Abstract The evaluation of vibrations induced to build structures is one of the main concerns when dealing with blast works. EDP, the main Portuguese producer of electricity is presently incrementing energy production in some old hydropower dams by installing new production plants. The need for not disrupting production during the upgrading works causes some concerns related with vibrations induced to the production engines. Vibration estimation was performed in two of these dams, constructed in a granite environment, by performing controlled blasts. For each dam site, some vibration transmissivity laws were developed with the fitting process. Vibration control during the construction work is expected to be performed and the results from real charges are to be incorporated in order to have a better database for such geological environment.
Transcript
Vibration prediction from controlled blast tests
Rogério Mota, Laboratório Nacional de Engenharia Civil, I.P., Av. do Brasil, 101, 1700-066 Lisboa, Portugal, [email protected],
& Jorge Neves, EDP - Gestão da Produção de Energia, S.A., R. do Bolhão, 36, 4000-111 Porto,
Nadir Plasencia, EDP - Gestão da Produção de Energia, S.A., R. do Bolhão, 36, 4000-111 Porto, Portugal, [email protected].
Abstract
The evaluation of vibrations induced to build structures is one of the main concerns when dealing with blast works. EDP, the main Portuguese producer of electricity is presently incrementing energy production in some old hydropower dams by installing new production plants. The need for not disrupting production during the upgrading works causes some concerns related with vibrations induced to the production engines. Vibration estimation was performed in two of these dams, constructed in a granite environment, by performing controlled blasts. For each dam site, some vibration transmissivity laws were developed with the fitting process. Vibration control during the construction work is expected to be performed and the results from real charges are to be incorporated in order to have a better database for such geological environment.
Introduction EDP is responsible for developing the hydroelectric potential of the Portuguese stretch of the international river Douro. This is achieved in three power plants in cascade (Miranda, Picote and Bemposta), built in the 50s and 60s of the last century. In this region, the river Douro valley is considerably narrow and deep, and so, the reservoirs have small capacities (6 millions of cubic meters (214.3 ft3), in Miranda, 13.3 (475 ft3), in Picote and 21 (750 ft3)), in Bemposta), making them unable to regulate the incoming flows. Picote re-powering scheme (left in figure 1), already under construction on the right (Portuguese) bank of river Douro, is close by and surrounds the existing hydropower plant. The main elements are the hydraulic circuit (a 300 m (984.3 ft) long headrace gallery and a tailrace tunnel with a 150 m (492.15 ft) extension), an underground powerhouse cavern and several access galleries. The new powerhouse is to be installed in a 68x23 m (223.1x75.5 ft) and 58 m (190.3 ft) high cavern with a 150 m (492.15 ft) overburden and is to be built at only 80 m (262.48 ft) away from the existing one (EDP, 2005). Indeed, this is considered to be one of the greatest challenges in the project, since rock excavation will be done by blasting. Bemposta re-powering scheme (right in figure 1) will also be located on the right bank of river Douro, encircling the existing powerhouse and will mainly consist of a headrace circular tunnel with a 12 m (39.37 ft) diameter and a 350.8 m (1150.98 ft) length, a circular shaft (~60 m (196.86 ft) deep and having a 22 m (72.18 ft) diameter), a powerhouse and a 41.6 m (136.49 ft) long tailrace tunnel (EDP, 2007). The excavation of new access tunnels will be of limited extension because the existing access tunnels will be partially used for this new project. In both cases, the proximity of the planned excavations to the pre-existing hydraulic circuits and powerhouse structures makes blast induced vibration monitoring and control important factors for developing these projects.
Figure 1 – General layouts of the re-powering schemes of Picote and Bemposta dams (in red (colour print) or dark grey (B&W print).
Geology and Geotechnics
Picote The studies developed for this project include the analysis of geological and geotechnical information related with the design and construction of the existing Picote hydropower plant and also with the studies of several hypothetical re-powering schemes for the site. Specifically for the current re-powering scheme, mainly to study rock mass cavern location, several boreholes were performed and an investigation programme, comprising both in situ and laboratory tests, was established by Laboratório Nacional de Engenharia Civil (LNEC) (Mota, 2005 and Muralha, 2006). Picote hydropower plant is located in a granitic region of the Miranda plateau. At the hydropower site, river Douro flows in a valley with a canyon configuration having almost vertical banks. The river is aligned with a WNW-ESE fault. Porphyritic granite is the main local lithology. The principal joint sets are: a) N70°-90°W, 80°-90º NNE, b) N5°-30°W, 70°ENE-90°-70ºWSW and c) sub horizontal. The main characteristics of joint sets are represented in Table 1 and figure 2. The existing power plant excavations were carried out in geological-geotechnical zones with good parameters determined by mechanical tests on the granite mass and by geological survey.
Table 1 – Characteristics of Picote joint sets.
Figure 2 - Isodensity diagram: CA – Powerhouse cavern axis. Aerial photography was analysed in order to better understand geological structures. Geological survey was difficult, not only due to pre-existing construction and deposit mask but also due to the water reservoir. The observation of existing unlined galleries was very useful. In order to characterize the rock mass of the main cavern, boreholes and blast vibration tests were performed. In these boreholes permeability tests and geomechanical tests were done. The latter consisted of both stress tensor tube and borehole dilatometric tests, but laboratory tests, such as, uniaxial compressive strength (UCS), “ultra-sound”, brazilian, triaxial and shear tests were also carried out. The results corresponding to the cavern rock mass are summarised in Table 2. The permeability tests showed an impervious rock mass.
CA
SET F1 F2 F3 F4 F5
Avg.
directionN78ºW N24ºW N32ºW N14ºE N16ºW
Avg. dip 88ºNE 78ºNE 12ºSW 89ºSE 71ºSW
Persistence 3 - 10 m > 10 m 3 - 10 m 3 - 10 m 3 - 10 m
Weathering W2 W2 W3 - W4 W2 W2
Aperture closed closedclosed to
>10mmclosed closed
Infilling none none none none none
JRC 11 11 11-14 3-7 3-7
JCS (MPa) 38 37 18 46 31
Seepagedry to
damp
dry to
dampdry dry dry
Table 2 – More relevant results from the investigation program (Picote).
Weathering/ Fracturation
Er (GPa)
Ed (GPa)
σσσσr (MPa)
VP (m/s)
VS (m/s)
W1 (W2) / F1-2 37 10 77 4500 2600 Er - Elasticity modulus; Ed – Dilatometric deformation modulus; σr – Uniaxial compressive strength; VP and VS - Ultrasound longitudinal and transversal wave velocities
Bemposta Geotechnical site investigation and characterization benefited from data obtained in previous exploration and from in situ and laboratory test programs included in the characterization of both the dam and the powerhouse sites for the project of these structures. The re-powering exploration program included: detailed geological and geotechnical mapping on the surface and in old unlined tunnels, exploration diamond drill-holes with permeability tests, in situ tests (stress tensor tube, small flat jack (SFJ), as well as borehole dilatometer (BHD)) and laboratory tests (UCS, ultra sound, triaxial, brazilian and joint shear tests). The rock mass consists of Hercynian high-grade metamorphic rocks originated from the partial melting and injection of pre-existing Cambric metaturbiditic rocks. The water intake and most part of the headrace tunnel will be excavated in anisotropic schistose migmatitic rocks (figure 3) and both the powerhouse and the tailrace tunnel in gneissic (anatexis) granite. Figure 3 is an interpretative vertical section developed through the headrace tunnel axis, where the geotechnical zones are represented in different shades of grey. This tunnel will be excavated mostly in the geomechanical zone 1 (GZ1), corresponding to the darker greyish colour in figure 3 (EDP, 2007). The exceptions are at the beginning of the tunnel (water intake) and at the places where fault zones or pegmatite lodes intersect the excavation. Some of the more relevant geomechanical characteristics (Muralha, 2007) of GZ1 are presented in Table 3. In the case of migmatite test results (Table 3), range values are presented instead of average values due to the strong anisotropy of this rock.
Ed - In situ deformation modulus, determined by BHD tests (in granite) and SFJ tests (in migmatite); Er - Elasticity modulus; σr - Uniaxial compressive strength; σt - Tensile strength; VP and VS - Ultrasound longitudinal and transversal wave velocities. The permeability of the rock mass is, generally, less than 2 Lugeon units, except in the first 10 to 20 m (32.81 to 65.62 ft) near the terrain surface or, locally, in depth, associated to faults or more densely fractured zones (EDP, 2007).
Figure 3 – Interpretative section through the headrace tunnel axis (horizontal scale = vertical scale). The rock mass presents two main sub vertical joint sets plus one sub horizontal (figure 4 – equal area projection, lower hemisphere), the latter one being more developed in the gneissic granite. Table 4 includes typical values for joint set characteristics as observed for the migmatite (J1 and J2), at the existing old tunnels, and for the gneissic granite (J3) that outcrops at the new powerhouse location. The J1 set is sub parallel to migmatite schistose. The average values for joint friction angle and apparent cohesion are 32.8º and 0.102 MPa (1.02 bar) (Muralha, 2007), respectively. The existing faults are, mostly, steeply inclined and sub-parallel to J1 or J2 sets or to the E-W direction. Fault thickness is variable, tends to decrease with depth and, generally, is less than 1 m (3.28 ft), with fault gouge infilling.
Predicting Blast vibration The use of explosives is one of the main methods used in mining and civil works in hard rock environments. Usually, there are buildings and other man-made structures nearby, which must be preserved and not disturbed. The ground vibration effects of blasting operations transmitted to such structures are the major environmental concern involving this method. Several studies have been done in the search for the best way to predict the damage produced by the vibrations induced by explosive blasts (e.g., Vuolio, 1990 and Singh, 1993). Recently, Toraño et al. (2006) proposed a Finite Element Method procedure, which accounts for some rock parameters, such as, the Young modulus of the rock mass, its density and the Poisson coefficient. For the model calibration, these authors used a general vibration transmissivity law for limestone, based on the scaled distance concept (Singh, 1993). This law was obtained from several field measurements (Balsa, 1989 and Figueroa and González, 1989) that takes account of the explosive charge (W, in kg) and the distance (D, in m) between the shot point and the measurement location:
651.1757.03085 −= DWv , (represented in mm/s). Other authors have developed similar laws for several types of mass rocks (e.g., Medveddev, 1968, Vuolio, 1990, Hagimori et al., 1993). Moura Esteves (1993) reported a series of historical measurements done by LNEC in different geological environments, in Portugal. Medvedev’s vibration transmissivity law is within the area delimited by these values, so it is usually used in Portugal for calculating charges at the beginning of works:
5.1
31
1900
−
=
W
Dv , (represented in mm/s).
In both works subsequently presented, different vibration transmissivity laws were developed and compared with Medvedev’s equation.
Practical blast induced vibration tests In the scope of the studies about re-powering of Picote and Bemposta dams, blast induced vibration tests were performed in order to have a vibration transmissivity law for each site, which could act as a tool in planning the excavation works for installing the new powerhouses. In Picote, due to the impossibility of drilling holes in the old galleries used for the dam construction, old shallow boreholes – around 0.5 m (≈1.64 ft) -, which remained from the opening works of a road tunnel near the current production plant, were used to perform the blasts. The possibility of a rock mass decompression due to the use of these boreholes was a high risk that had to be taken in order to perform the work. In this site, the explosive charges varied between 0.0175 and 0.07 kg (0.039 to 0.154lb), for a range of distances between 6.35 to 52 m (20.83 to 170.61 ft). As the test site was a road tunnel, only a fairly small amount of explosives could be used in order to avoid the destruction of its walls. In Bemposta, both the presence of a set of galleries near the present underground powerhouse, which remained from the construction works, and the simultaneous execution of rock mass mechanical tests inside them, made it possible to perform a series of boreholes in the migmatitic rock mass. These
Scale 1:1500
Legend
Measurement point Shot location
Old powerhouse
Gallery I
Gallery II Gallery III
boreholes were executed with 1 to 2.5 m (≈3.28 to 8.20 ft) length in three interconnected galleries, which go up and down in the rock mass from its interconnection point, allowing a higher number of different distances between shot and measurement points (figure 5). The explosive charges used in this site varied between 0.036 and 0.28 kg (0.079 to 0.617 lb) and the distances between boreholes and measurement points ranged from 14.7 to 72.1 m (48.23 and 236.56 ft). Several distances and charges were used - a higher number in Bemposta – in order to have the best possible representation in a D/W1/3 versus vibration velocity bi-logarithm graph. Figure 5 – Location plant of works performed in Bemposta dam. From the interconnection, Gallery III goes to higher topographic levels and Galleries I and II to lower levels – Gallery I is near water intake to powerhouse, and Gallery II near water outtake. The measured data was compared with historical data collected by LNEC (Moura Esteves, 1993) and with Medvedved’s equation (see last section), and mention must be made of the fact almost all the data are within the historical data area (figure 6). In spite of the fact that the rock mass present in Picote is of better quality (compare geotechnical results presented in Tables 1 to 4), the resulting vibration level is very low. This seems to be due to the shallow depth of boreholes - some of them with surrounding decompressed material - and to the low explosive charges, resulting in vibration transmissivity laws different from those typical of granite (Vuoli, 1990).
- Topographic mark
In Bemposta, the different positions of the three galleries, relatively to migmatite schistosity, influence the travel path of the wave motion as it spreads concentrically from the rock hole. This could be the reason for the data spread noticed. Such a phenomenon cannot be noticed in Picote, as the granite mass is very homogeneous. For each dam site, a few vibration transmissivity laws were developed with the fitting process. For a better comparison between both locations some of them are presented in figure 6. Figure 6 - Vibration velocity (mm/s) versus D/W1/3; with best-fit lines, delimited area of LNEC historical data and Medvedev’s vibration law (thick blue line). Left – Picote dam. Right – Bemposta dam (D – distance; W explosive charge).
Discussion and Conclusions The good quality rock at Picote site was not necessarily represented by good vibration transmissivity laws, probably due to a conjugation of a set of factors, namely, few shot points, shallow boreholes and the fact of some of the latter being surrounded by decompressed material and low explosive charges. At Bemposta dam site, in spite of a more complex geologic environment, good vibration transmissivity laws were achieved. With Medvedev’s equation, for the same distances used in these tests, usually higher charges provide higher vibration values. Therefore, this vibration transmissivity law can be used as an upper limit for blast design for the distances far from the old powerhouses. However, as the production turbines cannot be subject to high vibration levels, for safety reasons, the vibration transmissivity laws defined on site should be used to determine the upper bounds of vibration induced in the near field, namely, equation a) for Picote site and equation b) for Bemposta site.
1 10 100 1000D/W1/3
0.1
1
10
100
1000
v (m
m/s
)
Delimited area of LNEC'shistorical data
Recorded data
v=1900 (D/W1/3)-1,5
(Medvedev's equation)a) v=241,257 (D/W1/3)-1,5
b) v=14,25 (D/W1/3)-0,65
c) v=386,26 (D/W1/3)-1,65
d) v=28,56 (D/W1/3)-0,85
1 10 100 1000D/W1/3
0.1
1
10
100
1000
v (m
m/s
)
Delimited area of LNEC'shistorical data
Recorded data
v=1900 (D/W1/3)-1,5
(Medvedev's equation)a) v=2280 (D/W1/3)-1,65
b) v=1252 (D/W1/3)-1,5
c) v=1006 (D/W1/3)-1,65
d) v=548 (D/W1/3)-1,5
It is necessary to control vibration during the construction work. Furthermore, the results from real charges are to be incorporated in order to have a better vibration transmissivity law for each geological environment, as the vibration levels are expected to rise with the higher explosive charges applied to the undisturbed rock mass.
References Balsa, J., 1989, Leyes estadísticas de transmisividad en distintos tipos de roca, Canteras y Explotaciones, nº 272, pp 61-73. EDP - Gestão da Produção de Energia, S.A., 2005, Aproveitamento Hidroeléctrico do Douro Internacional - Picote - Reforço de Potência – Projecto. Unpublished Report. (In Portuguese) EDP - Gestão da Produção de Energia, S.A., 2007, Aproveitamento Hidroeléctrico do Douro Internacional - Bemposta - Reforço de Potência – Projecto. Unpublished Report. (In Portuguese) Figueroa, A., González, E., 1989, Resultados del tratamiento estadístico de los estudios de vibraciones producidos por voladuras, Canteras y Explotaciones, nº 263, pp 104-115. Hagimori, K, Terada, M., Ouchterlony, F., Furukawa, K. E Nakagawa, K., 1993. Using slot drilling to reduce vibrations and damage from tunnel blasting in urban areas. Proc. 4th Int. Symposium on Rock Fragmentation by Blasting - Fragblast-4. Vienna, Austria, pp 97-104. Medvedev, S. V., 1968, Evaluation of seismic safety during blasting operation in mines. Bulletin of the Earthquake Research Institute, Vol. 46, pp 687-696. Mota, R., 2005, Geophysical investigation for the upgrading works of Picote Dam. Report LNEC nº 225/2005 NGE (in Portuguese). Mota, R., 2007, Study of the vibrations induced into the rock by controlled explosions for the upgrading works of Bemposta Dam, Report LNEC nº 28/2007 NGE (in Portuguese). Moura Esteves, J., 1993, Vibration control induced by explosions in the construction industry. LNEC, pp108. (in Portuguese). Muralha, J., 2006, Rock mass characterization tests for the new powerhouse of Picote Dam. Report LNEC nº 71/2006 NFOS (in Portuguese). Muralha, J., 2007, Rock mass characterization tests for the new powerhouse of Bemposta Dam. Report in preparation. Singh, S.P., 1993, Prediction and determination of explosive induced damage. Proc. 4th Int. Symposium on Rock Fragmentation by Blasting - Fragblast-4. Vienna, Austria, pp 183-192. Toraño, J., Rodríguez, R., Diego, I., Rivas, J.M. and Casal M.D., 2006, FEM models including randomness and its application to the blasting vibrations prediction, Computers and Geotechnics, 33, pp 15-28.
Vuolio, R., 1990, Blast vibration: Threshold Values and Vibration Control, Acta Polytechnica Scandinavica - Civil Eng. and Building Construction Series nº 95, pp 146.
