Laboratory investigations for estimation of low amplitude moduli for Bucharest soils

The evaluation of the shear modulus of soils at very small levels of strains was a main concern of researchers. In this paper, we present the relations of shear modulus ratio G/G_max versus shear strain and strain-dependent damping for the Bucharest soils. The improved cyclic triaxial equipment installed at the National Center for Seismic Risk Reduction, NCSRR (now https://ccers.utcb.ro) was used for testing the clayey and sandy soils of Bucharest, the capital city of Romania.

INTRODUCTION

Bucharest city is the most affected urban concentration by Vrancea subcrustal earthquakes, with a high density of building damages, casualties and economic loss due to its relative proximity to the seismic source and the specificity of surface geology. Major historical seismic events generated by the Vrancea source (1802: M_w=7.9; 1940: M_w=7.7 and 1977: M_w=7.4) have indicated the great influence of the soil layers’ characteristics on seismic motion parameters. The surface geological deposits from the Bucharest area are composed of  unconsolidated alluvial layers of cohesive and cohesionless soils with significant variability in thickness and spatial distribution. The relative heterogeneity of young formations in an alluvial basin explains the peculiar site response during Vrancea strong motions.

The equipment for soil testing and investigation, data acquisition and processing systems and the triaxial testing equipment located at the Seismic Risk Assessment Research Center, Technical University of Civil Engineering, formerly installed at the National Center for Seismic Risk Reduction Bucharest, Romania (NCSRR), was made by Seiken and donated by Japan International Cooperation Agency (JICA) through the Technical Cooperation Project on the Reduction of Seismic Risk for Buildings and Structures in Romania.

The NCSRR triaxial equipment can solve the dynamic problems with soils subjected to a strain level as small as 10^-6 (used to evaluate the soil strength in comparison with stresses induced by external loading and the settlement of ground or structures associated with the deformation of soils. The automatic stress strain path control and monitoring (see example in Fig. 1) for triaxial test is as follows:

• For lateral loading a pneumatic pressuring system is used. An air pump and a servo EP transducer make possible the accurate control of the air pressure.
• For axial loading it is important to have a high resolution as well as a large stroke of axial displacement. We use a servo-pneumatic loading system.
• For very small vertical displacements a pair of gap sensors are used.

Figure 1: Example of monitoring of axial pressure and axial cyclic deformation

Though soil deformation under seismic loading is relatively small, its modulus is dependent on dynamic stress or strain level. Soil moduli such as Young’s modulus and shear modulus decrease as the level of stress or strain increases. Therefore, the nonlinearity of dynamic deformation characteristics is significant in seismic response analysis. All the moduli E, ν, G, and h depend on strain range but the dependency of ν is considered rather small.

The present paper is in line with the international practice approach by providing reliable data obtained from detailed surveys performed on different soil types of Bucharest and proposing curves for characterization of shear moduli and damping of the near-surface sediments to be further integrated in seismic response studies.

RESULTS FROM THE DYNAMIC TRIAXIAL TESTS ON BUCHAREST SOILS

1. Clayey soils

During the last years, we conducted a series of dynamic triaxial tests on the clay soils (NEAGU & ARION, 2012, ARION & NEAGU, 2007, ARION & NEAGU, 2013) – Fig. 2. The deepest soil samples were taken at 67 m by using the double-core barrel sampler.

Figure 2: Example of sampling, preparation and testing of the clay samples

Figure 3 presents the relations of shear modulus ratio G/G_max versus shear strain and the strain-dependent damping for the Bucharest clay samples. Also, we represented in Fig. 3 the strain-dependent modulus and damping curves quoted in the literature. VUTECIC and DOBRY (1991) propose a family of curves which are the averaged relations indicating the effect of the plasticity index on the strain-dependent modulus and damping of cohesive soils.

Figure 3: Test results on clayey samples and comparison with analytical model curves

2. Sandy soils

The deepest sandy soils samples were taken at 39 m by using the same double-core barrel sampler.

Figure 4 illustrates the relations of shear modulus ratio G/G_max versus shear strain and the strain-dependent damping for the sandy soil samples. Also, we represented in this figure the strain-dependent modulus and damping curves quoted in the literature; the Electric Power Research Institute (EPRI, 1993) proposed a family of curves for sand layers at different depths.

Figure 4: Test results on sandy samples and comparison with analytical model curves

3. Model curves for Bucharest soils

Using the tests results we computed the mean curves for the shear modulus ratio G/G_max and the hysteretic damping ratio, h (%) versus cyclic shear strain, γ_a (%) and also the corresponding standard deviation values.

For the cohesive soils the proposed values/curves are presented in Fig. 5 (a) and Table 1, and for sandy soils the proposed values/curves in Fig. 5 (b) and Table 2. In Figs. 5 we also presented the m+σ and m-σ curves for the shear modulus ratio G/G_max and the hysteretic damping ratio, h (%) versus cyclic shear strain.

CONCLUSIONS

Soil information can contribute to the development of earthquake disaster mitigation strategies and the continuous improvement of earthquake-resistant design regulations.

Data related to the seismic characterisation/behaviour of ground conditions, the proposed curves/values of dynamic shear modulus and damping will be integrated into a national internet-based platform SETTING (2021-2023) that will provide thematic services in the field of Earth observation, as a contribution to the European Plate Observing System EPOS. The platform will also include the directory of Romanian laboratories and institutions performing geotechnical and geophysical investigations of interest for seismology and earthquake engineering purposes. This data will accompany, on the platform, seismologic and GPS/GNSS data.

Table 1: Statistical indicators (average, m and standard deviation, σ values) for the shear modulus ratio G/Gmax and damping h and corresponding cyclic shear strain for Bucharest cohesive soils

Figure 5: The proposed mean curves (med) and med±σ for the shear modulus ratio G/Gmax versus shear strain and the strain-dependent damping for the Bucharest soils and comparison with analytical model curves

Table 2: Statistical indicators (Average, m and standard deviation, σ values) for the shear modulus ratio G/Gmax and damping h and corresponding cyclic shear strain for Bucharest sandy soils

ACKNOWLEDGEMENTS

The authors would like to acknowledge the cooperation of Japanese specialists during the JICA Project as well as the generous funding provided by the Japan International Cooperation Agency (JICA). We kindly acknowledge the support of the Building Research Institute (BRI), Tokyo Soil Research, and Seiken Inc.

Part of the presented work received support through the SETTING Project Integrated thematic services in the field of Earth observation: a national platform for innovation, No. 108206, cofinanced from the Regional Development European Fund (FEDR) through the Operational Competitivity Programme 2014-2020.

REFERENCES

1. ARION C., NEAGU C., (2007), Laboratory investigation for estimation of seismic response of the ground, ISSRR2007 International Symposium on Seismic Risk Reduction. The JICA Technical Cooperation Project in Romania;
2. ARION, C., NEAGU, C., (2013), Laboratory investigation for estimation the seismic response of ground, Bulletin of the International Institute of Seismology and Earthquake Engineering, 47, pp. 149- 156, Editor: Ministry of Construction, Building Research Institute, Japan;
3. EPOS, the European Plate Observing System, epos-eu.org;
4. EPRI (1993). Guidelines for determining design basis ground motions, Electric Power Research Institute, Palo Alto, California;
5. http://ccers.utcb.ro/;
6. NEAGU, C., ARION, C., (2012), Dynamic laboratory investigation for soil seismic response, paper 2051. 15^th World Conference of Earthquake Engineering, 24-28 September 2012, Lisbon, Portugal. CDROM, 8 pp;
7. NEAGU C., (2015), Local soil condition and nonlinear soil response influence on design seismic action. Ph.D. Thesis, Technical University of Civil Engineering of Bucharest (in Romanian);
8. SEIKEN Inc., (2003). Technical Documentation of triaxial testing apparatus (Pneumatic Cyclic Test, Static Strain Test) Model No. DTC-367;
9. SETTING Project (2021-2023). Integrated thematic services in the field of Earth observation – a national platform for innovation, co-financed from the Regional Development European Fund (FEDR) through the Operational Competitivity Programme 2014-202, https://setting.epos-ro.eu/;
10. The Japanese Geotechnical Society. Standards of Japanese Geotechnical Society for Laboratory Shear Test (English Version). June 2000, 118 pp.;
11. VUCETIC, M., and DOBRY, R., (1991), Effect of soils plasticity on cyclic response, Journal of Geotechnical Engineering, ASCE, 117 (1), 898-907.

Authors:

Cristian ARION − Seismic Risk Assessment Research Center, Technical University of Civil Engineering (UTCB)

Cristian NEAGU − Dublin City Council

Alexandru ALDEA, Radu VACAREANU − Seismic Risk Assessment Research Center, UTCB

[Proceedings of the 17th Danube European Conference on Geotechnical Engineering (17DECGE), June 7-9, 2023, Bucharest, Romania – https://17decge.ro/]

…citeste articolul integral in Revista Constructiilor nr. 211 – martie 2024, pag. 54-56, 58-60