2D numerical analysis of the seismic response of a karst rock mass: importance of underground caves and geostructural details

Bruno Giovanni; Carucci Fabrizio

doi:10.2478/sgem-2019-0028

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2D numerical analysis of the seismic response of a karst rock mass: importance of underground caves and geostructural details

Studia Geotechnica et Mechanica

Volume 42 (2020): Issue 1 (April 2020)

By: Bruno Giovanni and Carucci Fabrizio

Open Access

|Mar 2020

Figures & Tables

Amplification factor (a) and spectral ratio (b) values above an underground cave inside a homogeneous and isotropic subsoil crossed by Vs waves, based on the parametric variation of the cave diameter (from [7] modified).

Geological map of the territory in which the study area is located (from [16] modified).

Electric tomography n.1: resistivity values and projection of cores drilling S1 and S2 (from Technical Department of the municipality of Turi (BA) – modified).

Location of geological surveys and section of the geological and geomechanical model.

Stratigraphy obtained from the core destruction borings.

Stratigraphies obtained from rotation drills with continuous coring, RQD trend along verticals and depth of the rock samples for geotechnical laboratory tests.

Geomechanical model section used for the LSR evaluation; the upper karst cave has a height of L= 4.00 m and a width of D= 3.00 m.

Acceleration–period spectrums of the combinations no 8 and accelerogram/velocity time–history of the two earthquakes chosen (red circled) for the LSR analysis: X-component (a) and Z-component (b).

Trend of displacements in the Y-direction (left) and plastic zones (right) in the geomechanical model with two overlapping underground caves: in static (above) and dynamic (below) conditions.

LSR amplification/deamplification, as X-accelerations FA, in each of the control points at the ground level of case study model.

Mechanical parameters of rocks and discontinuities (based on laboratory tests) of the geomechanical model_

VEGETAL SOIL AND “RED SOILS”
r (Kg/m³)	E (Pa)	v	K (Pa)	G (Pa)	φ (grade)	c (Pa)	σ_t (Pa)
1835	20 · 10⁶	0.25	13 · 10⁶	8 · 10⁶	25	50000	0

Main data of the seismic events extracted from the database for the X-acceleration (above) and the Z-acceleration (below)_

Waveform ID	Earthquake ID	Station ID	Earthquake Name	Date	Mw	Fault Mechanism	Epicentral Distance (km)	EC8 Site class
286	146	ST92	Campano Lucano	11/23/1980	6.9	normal	78	A

Geomechanical classification of the altered and the compact calcareous rock mass_

	Parameters	Values	RMR'76		GSI
Rock massAltered and karstic limestone	R₁ (compressive strength)	7	49	φ_b = 35°c_b = 0.15 MPa	49
	R₂ (RQD)	6
	R₃ (spacing of discontinuities)	15
	R₄ (joint conditions)	11
	R₅ (water conditions)	10
Rock massCompact and not altered limestone	R₁ (compressive strength)	7	62	φ_b = 40°c_b = 0.20 MPa	62
	R₂ (RQD)	13
	R₃ (spacing of discontinuities)	15
	R₄ (joint conditions)	17
	R₅ (water conditions)	10

Relevant input data for the accelerograms search with the REXEL 3_5 software_

Target Spectrum	a_g (g)	Geographic Coordinates ED50		Site class	Topographic category	Nominal Life	Functional Type	Limit State	M	R (km)

		Lat. N	Long. E
Italian Building Code 2008	0.074	17.0222	40.9185	A	T1	50 years	III	SLV	6–7	0–100

FA, calculated in the absence and presence of karst caves, as ratio of the maximum X-acceleration and ratio of the Fourier’s amplitudes spectrum in three ranges of frequency_

Output Point ID	AXmax.TopAXmax.Bottom{{A_{Xmax.Top} } \over {A_{Xmax.Bottom} }}		∫0.2Hz2.0HzFourier Ampl.Top (f)∫0.2Hz2.0HzFourier Ampl.Bottom (f){{\int_{0.2Hz}^{2.0Hz} {Fourier\,Ampl.Top\,(f)} } \over {\int_{0.2Hz}^{2.0Hz} {Fourier\,Ampl.Bottom\,(f)} }}		∫2.0Hz3.8HzFourier Ampl.Top (f)∫2.0Hz3.8HzFourier Ampl.Bottom (f){{\int_{2.0Hz}^{3.8Hz} {Fourier\,Ampl.Top\,(f)} } \over {\int_{2.0Hz}^{3.8Hz} {Fourier\,Ampl.Bottom\,(f)} }}		∫3.8Hz5.6HzFourier Ampl.Top (f)∫3.8Hz5.6HzFourier Ampl.Bottom (f){{\int_{3.8Hz}^{5.6Hz} {Fourier\,Ampl.Top\,(f)} } \over {\int_{3.8Hz}^{5.6Hz} {Fourier\,Ampl.Bottom\,(f)} }}

	Without caves	With caves	Without caves	With caves	Without caves	With caves	Without caves	With caves
1	0.060	0.033	0.067	0.039	0.072	0.033	0.065	0.032
2	0.084	0.221	0.120	0.043	0.122	0.097	0.118	0.070
3	0.012	0.129	0.161	0.090	0.176	0.048	0.171	0.040
4	0.730	0.036	0.161	0.090	0.176	0.048	0.171	0.040
5	0.048	0.009	0.098	0.071	0.123	0.040	0.119	0.053
6	0.169	0.057	0.079	0.035	0.113	0.021	0.142	0.024
7	0.093	0.024	0.079	0.035	0.113	0.021	0.142	0.024
8	1.672	0.899	0.259	0.272	0.274	0.172	0.236	0.342
9	0.120	0.019	0.093	0.058	0.143	0.052	0.084	0.113

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DOI: https://doi.org/10.2478/sgem-2019-0028 | Journal eISSN: 2083-831X | Journal ISSN: 0137-6365

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Language: English

Page range: 61 - 73

Submitted on: Apr 12, 2019

Accepted on: Aug 21, 2019

Published on: Mar 19, 2020

Published by: Wroclaw University of Science and Technology

In partnership with: Paradigm Publishing Services

Publication frequency: 4 issues per year

Keywords:

2D local seismic response analysis,

rock mass,

karstic underground caves

Related subjects:

Physics,

Mechanics and fluid dynamics

© 2020 Bruno Giovanni, Carucci Fabrizio, published by Wroclaw University of Science and Technology
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.

Volume 42 (2020): Issue 1 (April 2020)

2D numerical analysis of the seismic response of a karst rock mass: importance of underground caves and geostructural details

Figures & Tables

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Mechanical parameters of rocks and discontinuities (based on laboratory tests) of the geomechanical model_

Main data of the seismic events extracted from the database for the X-acceleration (above) and the Z-acceleration (below)_

Geomechanical classification of the altered and the compact calcareous rock mass_

Relevant input data for the accelerograms search with the REXEL 3_5 software_

FA, calculated in the absence and presence of karst caves, as ratio of the maximum X-acceleration and ratio of the Fourier’s amplitudes spectrum in three ranges of frequency_

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