TUNNEL BORING MACHINE VIBRATION IMPACT PREDICTION METHOD BASED ON SURFACE VIBRATION MEASUREMENTS AND TUNNEL TO SURFACE TRANSFER FUNCTION CALCULATION

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1 TUNNEL BORING MACHINE VIBRATION IMPACT PREDICTION METHOD BASED ON SURFACE VIBRATION MEASUREMENTS AND TUNNEL TO SURFACE TRANSFER FUNCTION CALCULATION Alexis Bigot SOLDATA ACOUSTIC, Nanterre, France Giovanni Farotto SOLDATA ACOUSTIC, Villeurbanne, France Vibration from tunnelling works can be a source of complaint, especially due to ground-borne noise, concerns of damage to structures and potentially adverse effects to vibration-sensitive equipment. For these reasons, prediction and management of vibration levels is not only desirable, but also often a requirement of urban tunnelling projects. Vibration propagation through the ground from a tunnel boring machine (TBM) can be modelled and predicted, but the difficulty is to know the exact vibration emission of the TBM through an equivalent ground. In this paper we present a method that we applied for the construction of the line B metro project in Rennes. This method is based on measurements of vibration levels at the surface at the beginning of the TBM route. Calculations are then carried out to assess the vibration transfer function at the trial area (in mm/s/n) using a 3D Finite Element Modelling (FEM). By coupling these two parameters, TBM force in db per third octave band can be assessed. This force is then used to predict the vibration impact at sensitive buildings along the tunnel path. This method allows to minimize the results uncertainties often caused by the input data and the numerical modelling. It also allows anticipation of vibration risks before the TBM approaches any sensitive area. This method can only be used at the start of the works and will remain valid as long as the ground characteristics stay relatively similar. 1. Introduction Vibration from tunnelling works can be a source of complaint, especially due to ground-borne noise, concerns of damage to structures and potentially adverse effects to vibration-sensitive equipment. For these reasons, prediction and management of vibration levels is not only desirable, but also often a requirement of urban tunnelling projects. Vibration propagation in the ground from a tunnel boring machine (TBM) can be modelled and predicted using engineering tools (FEM/BEM models, or simplified empiric models like BS:5228-2:2009 [1] for example). But whatever the model, the main issue is to know the exact vibration emission of the TBM through an equivalent ground, in terms of amplitude and frequency. This information is not available and must be estimated using vibration measurements. In this paper we present a method based on both vibration measurements and FEM calculations that we applied for the construction of the line B metro project in Rennes so the vibration impact of the TBM can be predicted. 1

2 2. Transfer functions The propagation of vibration from the underground to the buildings can be calculated from the following information: The emission of vibration (1) in terms of Excitation Force Level (EFL). The soil transfer mobility (2). The transfer function between the soil and the foundations (3). The transfer function between the foundation and the floor of the buildings (4). The vibro-acoustic transfer function for ground-borne noise (5). 5. TF Ground-borne noise 4. TF foundations-floors 3. TF soil-foundations 1. Emission of vibration 2. Soil transfer mobility Figure 1: Propagation of vibrations from emission to receivers. 2.1 Excitation Force Level (EFL) of the TBM One of the main issues is the knowledge of the excitation force of the TBM: this input data is not available from the manufacturers. Some examples of excitation force can be found in the literature: Figure 2 presents interesting data for a TBM (cutter head diameter 8,28m, 60 cutters total 17 each, open excavation mode, full face) [2] in different operating conditions. Figure 2: Excitation Force of a TBM - Source [2]. 2 ICSV23, Athens (Greece), July 2016

3 The TBM used for the line B metro project in Rennes is different from the previous TBM: the technology is Earth Pressure Balance (EPB) and the diameter 9.45m. The characteristics in terms of EFL are then expected to be different. Figure 3: Arrival of the TBM at the Gares station on 8 th April Soil transfer mobility The soil transfer mobility can be measured. We present in figure 4 a measurement method based on borehole impact tests. However, this approach is expensive: a specific borehole needs to be drilled for each hammer test site. There are also technical difficulties: the impact hammer energy and the precision of the measurement equipment need to be adjusted for each test site, as a function of the depth of the tunnel and of the distance of the buildings. Figure 4: Bore Hole Impact Test method and picture of an impact hammer. Source: [3]. Another approach for the measurement of the soil transfer mobility would be to make measurement at the bottom of an access well, or from an existing underground tunnel. ICSV23, Athens (Greece), July

4 A less expensive solution is to calculate the soil transfer mobility using FEM or BEM software: it will lead to more uncertainties (calculation uncertainties), but as a first approach it may be a good solution for the tunnelling projects, for which a measurement method would be clearly inadequate. 2.3 Building vibration response The vibration response of a building can be measured (using an impact hammer for example). But a measurement approach must be used for the most sensitive building only. In a first approximation it can be useful to use statistical transfer functions like those published in the RIVAS project and presented in figures 5 and 6. Figure 5: Ground to foundation vibration statistical transfer function in db (mean value ± standard deviation); SBB data for houses and multifamily small buildings, Source RIVAS [4]. Figure 6: Building foundation to floor statistical transfer function in db (mean value and mean value + standard deviation) for 3 ranges of floor resonant frequencies: 10-20, and Hz; the number of samples is given in parenthesis; SBB data for wood floors, Source RIVAS [4]. 4 ICSV23, Athens (Greece), July 2016

5 3. A new approach for TBM vibration prediction Some vibration prediction methods already exist [5], but they are based on measurements of the soil-transfer mobility. A new approach is proposed, and based on 3D FEM calculation of the soil-transfer mobility. This method is based on the following steps: Step1: Choice of a test site at the beginning of the TBM route (for example before the TBM reaches the dense inhabited areas). Step 2: Measurement of the vibration levels caused by the TBM on the soil surface. It s not recommended to measure the vibration inside the dwellings, because the measurements would be disturbed by the vibration response of the building. Figure 7: Location of vibration measurements during tunnelling works. Step 3: 3D FEM modelling of the test site in order to calculate the soil transfer mobility. The input parameters of the model are the characteristics of each soil layer (density, Dynamic Young modulus, Poisson coefficient, etc.). Figure 8: Example of 3D FEM modelling of the point source response of the ground (CODE_ASTER), in order to calculate the soil transfer mobility. The boundary condition must be non-reflecting. The Excitation Force Level (EFL, in db) of the TBM is then calculated with following formula: EFL Lv TF TBM Test _ site 2Test _ site Step 4: 3D FEM modelling of the other sites, where the vibration wants to be predicted. ICSV23, Athens (Greece), July

6 EFL, db ref 1N 1,2 1,6 2,0 2,5 3,1 4,0 5 6, dbv ref m/s The 23 rd International Congress on Sound and Vibration The vibration level on the soil surface is calculated using the following formula: Lv EFL TF New_ site TBM 2New_ site For the assessment of the vibration and ground-borne noise inside the building, we can use in a first approximation the statistical transfer functions of the RIVAS project. Step 5: In case of sensitive areas along the tunnel path, a measurement of the vibration response of the building can be carried out to improve the reliability of the prediction. 4. Results An example of vibration measurement on the soil surface during the tunnelling works is presented below. The vibration levels are low, but the dominant frequencies [40Hz-200Hz] may cause generation of ground-borne noise. Spectres vibratoires (x, y et z) pour Niveau vitesse z Niveau vitesse y Niveau vitesse x -10 Hz Figure 9: Example of vibration measurements during tunnelling works, measured on the soil surface. The 3D FEM modelling of the test site allows the calculation of the EFL of the TBM, as shown in figure 10. In our case study, the EFL calculated is lower than the data available in the literature for typical rock (see Fig. 2). An explanation of this difference can be attributed to the TBM itself, but also from the soil layers in the Rennes underground which are made up of friable or fragmented shales and sandstones. 120 Excitation Force Level of the TBM , Hz Measured on test site Example on typical rock Figure 10: Excitation Force Level of the TBM. The vibration level is then calculated for another area along the route of the tunnel (step 4 of the methodology). 6 ICSV23, Athens (Greece), July 2016

7 dbv ref m/s The 23 rd International Congress on Sound and Vibration 70 Vibration levels on the soil surface , Hz Lv,Test_site Lv,New_site Figure 11: The reference vibration level Lv Test_site is compared to the calculated vibration level Lv New_site. The difference between the curves comes from the difference between the soil transfer mobility of both sites. 5. Discussion The advantage of this method is that it is calibrated by measurements on site. The use of a FEM model can lead to calculation uncertainties, but some of those uncertainties can be corrected, because the soil-transfer mobility is subtracted and then added to the measurements results (see formulas in step 3 and step 4). Other uncertainties are related to the input data, like the soil layers and characteristics. In our case, the dynamic Young s Modulus was not available, and the surveys gave us information about the Pressiometric Modulus (Em, Module de Ménard) only. Thus, some assumptions had to be made in order to estimate the Shear modulus G, using a relation like G = k.em, where k is a constant which was estimated from empiric knowledge (k was in the range 5 to 10). Another unknown parameter of the ground is the damping ratio. This parameter controls the shape of the soil transfer mobility at high frequencies. Some assumption was necessary (ζ= 0,1), because very little information is available in the literature about damping ratio of the ground. 6. Conclusion In this paper we present a method that we applied for the construction of the line B metro project in Rennes. This method is based on measurements of vibration levels at the surface at the beginning of the TBM route. Calculations are then carried out to assess the vibration transfer function at the trial area (in mm/s/n) using a 3D Finite Element Modelling (FEM). By coupling these two parameters, TBM force in db per third octave band can be assessed. This force is then used to predict the vibration impact at sensitive buildings along the tunnel path. This method allows to minimize the results uncertainties often caused by the input data and the numerical modelling. It also allows anticipation of vibration risks before the TBM encounters any sensitive area. This method can only be used at the start of the works and will remain valid as long as the ground characteristics stay relatively similar. ICSV23, Athens (Greece), July

8 REFERENCES 1 BS :2009, Code of practice for noise and vibration control on construction and open site Part 2: vibration (2009). 2 Ho, W., Wong, B., Groundborne Noise & Vibration Impact From Rock Tunnel Boring Machines, Hong Kong Tunnelling Conference (2009). 3 Hanson, C., Towers, D., Meister, L., Transit Noise and Vibration Impact Assessment, US Federal Transit Administration (2006). 4 RIVAS Collaborative project, Definition of appropriate procedures to predict exposure in buildings and estimate annoyance, Deliverable D1.6 (2012). 5 Ho, W., Crockett, A., Raine, A., Measurement and prediction of groundborne noise from a tunnel boring machine, Technical Acoustics (2006). 8 ICSV23, Athens (Greece), July 2016