soil conditions and site...

International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania

SOIL CONDITIONS AND SITE EFFECTS

Gerhard Huber1, Dieter Hannich2, D. Lungu3,4

ABSTRACT This contribution shall give an overview on the topics of Block III considered in the scope of the CRC 461. It will show the links between the contributions, talks, posters, and papers presented. Additionally some basic effects of non-linear wave propagation referenced in the papers are outlined here.

INTRODUCTION All projects involved in this topic are dealing with different scales of length. For the geophysical group modelling the propagation of earthquakes waves, these scales go up to some decades of kilometres. The projects from geology and hydrogeology concentrate on depths of about 100 m and dimensions in the area up to some kilometres. The soil mechanics part is focused on shallow depths up to 100 m. For soils within these depths the non-linear relations between stresses and strains are governing the response due to earthquake excitation, especially for saturated soils. For shallow depths these effects – e.g. liquefaction or cyclic mobility - are dominating the behaviour under earthquake conditions. The models used, have to be adapted to questions and requirements for the spatial resolution. The links between the projects considered here, follow the paths of waves from the source to the specific location. The wave velocities correspond with the actual scales of length. Due to a variation in wave velocity between rock and soil, some types of matching takes place resulting in a decrease of wave lengths due to decreasing stiffness. For earthquake related surface effects in general only the soil layers 30-100 m below the surface need to be considered in detail. But these depend on the state of soil. Even for homogenous soil the state will change with depth. Many steps have been undertaken to find applicable and acceptable formulations and models. Most of the effects are well known, but a constitutive relation for quantifying these effects is still missing.

CONNECTIONS BETWEEN THE PROJECTS Cooperating projects at Karlsruhe University A1: Deep Seismic Sounding of the Vrancea Zone (Geophysics) A7: Strong Ground Motion Assessment (Geophysics) B3: Seismogenic Potential of the Vrancea Subduction Zone - Quantification of Source and Site Effects from Strong Earthquakes (Geophysics) B4: Non-linear Wave Phenomena in Soft Sediments (Soil mechanics) B6: Geotechnical and Seismic Microzoning of Bucharest (Geology) B7: Hydrogeology and Site Effects by Earthquakes in Bucharest (Hydrogeology)

1 Institute of Soil Mechanics and Rock Mechanics, University of Karlsruhe (TH), Engler-Bunte-Ring 14,

76131 Karlsruhe, Germany, Email: [email protected] 2 Department of Applied Geology, University of Karlsruhe (TH), Kaiserstr. 12, 76128 Karlsruhe, Germany.

3 Technical University of Civil Engineering Bucharest, UTCB; [email protected]

4 National Institute for Historical Monuments, INMI; [email protected]

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The projects are connected by the intermediate results of other projects which are inputs for further projects, Fig.1. The parts B3, A1 and A7 concentrate on geophysical problems. B3 supplies synthesized time histories for earthquake base excitation for different magnitudes for B4, B6 and B7 (Sokolov & Bonjer, 2006; Sokolov, Bonjer & Wenzel, 2004; Sokolov, Loh & Wen, 2000). An urban shake map of Bucharest was developed in B3 based on data from the URS-Project in A1, Fig. 2. Refraction measurements at two parks – Tineretului and Bazilescu –for shear wave velocity estimation were performed by A1 (v. Steht, Jaskolla, & Ritter), Fig. 3. Project B4 is contributing the non-linear wave propagation using constitutive relations from soil mechanics. The developed 1D-model (Osinov, 2003) is also used in B6 and B7. The estimation of material parameters and state of soil for Bucharest based on Seismic-Cone-Penetration-Tests including pore water pressure measurement (SCPTu) is carried out in cooperation with B6 and B7. The non-linear soil/structure interaction to the project C9 related to buildings. The projects B6 and B7 are additionally interconnected by usage of the same database for Bucharest. The equivalent linear and non-linear simulations are carried out in cooperation. A continuous exchange of input data and results has taken place to test and to improve the models for application. The intensive exchange concerns B3, B4, B6, B7 and A1. Seminars together with Romanian partners have been carried out also.

Non-linear soil/structure interaction

Evolution of excess pore pressure, liquefaction potential

Soil response for microzonation

Synthesized data for strong earthquakes in Bucharest,data from weak and medium intensity earthquakes, shake map, refraction measurements in Bucharest

Structural engineering C9

Soil mechanics B4 (subsurface)

Hydrogeology B7 Geology B6

Geophysics B3 / A1 / A7

Figure 1: Connections between projects related to earthquake ground motion

METHODS, LIMITATIONS AND BENEFITS Within the CRC 461 linear, equivalent linear and non-linear models have been applied for ground response analysis. Equivalent linear models (e.g. SHAKE) (Lysmer et al., 1972) and non-linear models based on constitutive relations (here Hypoplasticity and Visco-Hypoplasticity with intergranular strain) (Gudehus & Kolymbas, 1979; Niemunis & Herle, 1997; Niemunis, 2003) have been used.

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Linear equivalent models use stiffness related input values (velocities of shear waves mainly) representing the state of the soil. The mainly considered values are dependent on mean effective stress, shear strain level, void ratio and the degree of overconsolidation for fine grained soils. The soil parameters concerning the dynamic behaviour are usually taken from laboratory tests over a wider strain range up to a moderate strain range (e.g. Resonant-Column-Tests, cyclic shear tests at moderate shear strain levels).

Figure 2: Urban shakemap of Bucharest (B3), Example: Oct. 27, 2004, Mw=5.9, Bartlakowski et al., 2006

Figure 3: Velocity profiles, Park Tineretului, Bucharest, shear-wave-velocities (profiles 2-3), (dashed areas are out of range), contribution by project A1 (v. Steht et al.)

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Even for the case of low strain levels, the measured shear wave velocities in laboratory tests do not agree with carefully measured values in the field. This holds true even if different methods (up-hole, down-hole, cross-hole or refraction) give same results. Usually the results of field tests differ from laboratory tests about a factor of 1.5 - 2 and even more. Influences can be caused by lack of knowledge about the void ratio or density, any kind of cementation and other genesis related facts. A typical result for cementation is a constant shear wave velocity versus depth over a wider range. Especially natural cementation of sands can pretend the soil to be much stiffer (higher wave velocity). The fragile cementation collapses at slightly increased shear strain level and stiffness decreases to values of reconstituted samples of the same material. Therefore the measured field data needs to be interpreted. Similar behaviour is found for partially saturated soils (usually above the groundwater level). The ground response is often calculated by a 1D-wave-propagation model based on the assumption that the refraction process leads to a nearly vertical propagating shear wave near the ground surface or bedrock or outcropping motion (Kramer, 1996). Beside the common assumptions for 1-D-wavepropagation the conditions of symmetry (infinite layers) introduced can overestimate or overemphasise the results. Involving the change of effective stress up to liquefaction, the void ratio or respectively the relative density distribution within the valid range has to be known. However, the relative density or the density of soil in the field can not be determined directly. An interpretation method based on empirical correlations or on a physical model for the sandy layers can be applied (Cudmani et. al., 2001). Seismic-Cone-Penetration-Tests including acquisition of pore water pressure (SCPTu) have been performed to estimate the shear wave velocities and the in situ densities for sites in Bucharest. Nevertheless for an estimation of the ground response parameters and values describing the state of the soil as well as the geometric properties have to be known. Since the non-linear model includes more effects, more state variables and parameters are required for the soil layers.

Figure 4: Compaction of saturated sand column due to earthquake-like base excitation (initial state, after 1st and after 2nd pulse, range of void ratio e: min e= 0.57, max e= 0.83) A simple experiment shows some mechanisms required that implementations of constitutive relations must be able to describe. A column of saturated sand under earthquake-like loading will be compacted or liquefied, Fig. 4.


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Settlement of the surface is found and water is flowing to the top. Repeating this leads to ongoing settlement for every applied loading, but with lower magnitudes of settlement. The volume of water and sand remains constant. Effects to be included are: change of void ratio due to dynamic loading, change of effective stress and pore water pressure and consolidation. For many cases the consolidation needs not to be considered, the approximation with undrained conditions is said to be sufficient for earthquake ground motion calculations. Settlements due to consolidation are considered separately and assumed to occur after the earthquake loading.

Figure 5: Compaction of saturated sand column covered with a silt/sand layer due to earthquake like base excitation, after second pulse (left to right: initial state, water- layer above compacted sand, hydraulic fracturing of silt layer water-layer in between, after collapse of top silt / sand layer. In general fine grained material is less sensitive in changing its state under dynamic loading. However, a sandy layer in between cannot consolidate due to low permeability of the fine grained material (fine silt) surrounding. During an earthquake therefore undrained conditions are assumed. The experiment mentioned before will be repeated but now with layer of silt on top of the saturated sand column. After applying the dynamic loading again, the saturated sand will settle, the silt layer stays at rest and a layer of water can be seen. After a while the silt layer collapses and again the sand column is covered with a low permeable layer and the experiment can be repeated (Fig. 5). If such a water layer exists over a wider area, further wave propagation of shear waves is prohibited (isolation). Kokusho identified this water layer (Kokusho, 1999). Such water layers resulting from earthquakes can explain the occurrence of sand boils after a break-through in a covering soil layer or through fissures in covering soil (Ishihara & Perlea, 1984). The non-linear continuum models used here are not able to give such a solution for the discontinuous problem. An example for the application of non-linear constitutive equations is the Soil-Foundation-Structure-Interaction (SFSI). A 2-D FE-model based on hypoplastic equations for the soil layers has been used. For simulations of non-linear SFSI realistic earthquake excitation has to be used because average magnitude and time dependent order of large amplitudes is important. A soil profile at Treasure Island (groundwater level 2m) has been chosen. A velocity record from Loma Prieta, (Mw = 7.1), 1989, at Yerba Buena Island has been taken as bedrock excitation (Fig. 6). The responses of two types of foundations – a shallow foundation and a pile foundation - are examined.

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Plane strain conditions for the soil elements and the structure elements are assumed. Periodic boundary conditions are used at the vertical boundaries and undrained conditions during the earthquake are presumed.

soil profile

Treasure Island, �

San Fransisco Bay

depth [m] 0.0

2.0

4.0

11.5

8.0

15.0

30.0

sandy fill(hypoplastic)

silty sand(hypoplastic)

young baymud (visco-hypoplastic)

found. depthshallow found.

found. depthpile group

ve

locity [

m/s

]

0 10 20 30

time

[s]

-0,1

0,0

0,1

0,2

velocity time history of 1989 Loma Prieta earthquake�

Yerba Buena Island Island, 90o, rockvelocity record applied on base nodes �

(bedrock signal)

structure and foundation

periodic �

boundary�

condition

40

Figure 6: Seismic SFSI analysis: soil profile, time history for base rock excitation, FE-model boundary conditions

24 cm18 cm12 cm 6 cm

8 cm6 cm4 cm

2 cm

Figure 7: Results of the SFSI analysis for a shallow foundation (top) and pile group (bottom): zones with liquefaction (dark) after 10 seconds (left), displacements vectors after 40s (right). Fig. 7 exhibits displacements and vanishing effective pressures at the end of the earthquake (40s). As mentioned before, when the shear stiffness in the liquefied layer decays, the motion of the soil layer above the liquefied zone decouples from layers beneath, and the upper layer moves almost like a rigid body. This results in a permanent almost uniform displacement at the end of the earthquake in the soil layer above the ground water table. In case of the shallow foundation, reduction of the shear resistance due to the decay of effective stresses leads to a reduction of the bearing capacity of the soil. The development of typical zones indicates clearly a punching of the foundation: large displacements of the soil and the structure take place. The maximal movement of the surrounding soil is the largest


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one for the shallow foundation compared with other foundation types. Liquefaction starts at both sides of the foundation. After 10s, a thin liquefied zone appears under the shallow foundation but the soil at both sides of the foundation does not liquefy and helps to stabilise the foundation. For the pile foundations, the largest total displacements occur in the upper layer. A small shear zone develops at the left side of the base of the piles. Therefore the bearing capacity of the pile group is not strongly affected by the earthquake. The liquefaction does not occur between the piles because the shear deformations are considerably smaller there than around the entire pile group. The results confirm the development and extension of the liquefied zone which strongly depends on the foundation type. These results emphasise the importance of taking into account SFSI for assessing the serviceability of structures founded on soft and liquefiable soils after strong earthquakes. Post-earthquake settlements are not included since drainage is not allowed in the calculation, although it could be done by adding a coupled calculation step enabling filtration. Further aspects concerning vertical displacements, tilting and serviceability are outlined in detail in (Rebstock et. al., 2006). Even with the current limitations the methods developed are still powerful tools. The results do not only dependent on site and soil conditions but also on the global magnitude and the distribution of big amplitudes over the event. This is illustrated by the time history shown in Fig. 6; it contains one strong short pulse (around 10s) leading to an onset of liquefaction.

CONTRIBUTIONS TO BLOCK III FROM THE CRC 461 The following contributions to Block III from the CRC 461 will refer to the mentioned effects in detail. Liquefaction probability in Bucharest and influencing factors Based on data from Bucharest (SCPTu and samples from boreholes) a hydrological model has been developed. Conditions for possible soil liquefaction have been investigated. Using a concept based on results from CPTs the probability of liquefaction, a liquefaction potential index, and a liquefaction severity index have been applied to data from Bucharest. Contour maps for Bucharest concerning the 1977 earthquake have been developed. Numerical modelling of site effects incorporating non-linearity and groundwater level changes For two specific sites in Bucharest numerical simulations with the non-linear 1-D wave propagation model regarding the change of the groundwater level have been performed. Synthesized time histories from A1 have been used for base excitation. It was found that the amplification factor depends on time and magnitude due to the change of state during the earthquake. Isolation effects due to a reduction of effective stress did not occur during the strong shaking but after the event. Field investigations and site response analysis for Bucharest A comparison of 1-D ground response models – equivalent linear and non-linear - for a specific site in Bucharest has been performed. For the non-linear model the hypoplastic parameters have been determined. Since the resulting shear wave velocities from the parameter estimation differ from the values measured in-situ, a comparison nevertheless shows similar results. Using the synthesized base rock signals from A1 the influence of density variations within the sandy layers on the evolution of the pore water pressure and also on the magnitude has been examined.

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Non-linear wave propagation in soil Stationary excitation has an advantage over earthquake-like excitation for testing hypoplastic models. The same hypoplastic relation was used as mentioned above but within a FE-formulation. For saturated undrained conditions the transient response of the soil column (sand) under various excitation frequencies, amplitudes and soil densities was examined. For slowly varying states transient change of the mode of vibration of the column could be identified by a time varying magnification factor. Also asymptotic behaviour of the solution for this strong non-linear problem was found. Shake-box tests These tests were carried out in a shake-box up to 2m height. Advantages of this size are higher stress (benefits for back calculations) and easier realisation of the smooth “flexible” boundaries. In contrast to tests with earthquake-like excitation being performed earlier, stationary sinusoidal excitation was used for these tests. The test results for sand specimens show transient modes of vibration – fast or gradually changing – depending on the magnitude of excitation and the state of soil. This is accompanied by a corresponding variation of pore water pressure. For higher soil densities but also depending on state and excitation cyclic mobility was observed. Within each cycle the soil shows four phases: a phase of contractancy for lower strains followed by a phase of dilatancy for bigger shear strains and then the two phases are repeated.

CONCLUSIONS Incorporating the transient change of effective stress up to liquefaction requires the application of non-linear constitutive relations. Parameters and states for the soil layers have to be determined for the simulation of non-linear wave propagation using the implemented relations. Steps towards a practical application have been made close to conventional methods. Liquefaction has been found to be not only a destructive effect, it can also be an effect that reduces or prohibits further propagation of shear waves.

ACKNOWLEDGEMENTS The research for the presented work was supported by the Deutsche Forschungs-gemeinschaft (DFG) in the scope of the Collaborative Research Centre (CRC 461) “Strong Earthquakes: A Challenge for Geosciences and Civil Engineering”.

REFERENCES Bartlakowski, J., Wenzel, F., Radulian, M. and. Ritter, J., and Wirth, W. (2006). Urban

shakemap methodology for Bucharest. Geophysical Research Letters, 33, L14310, doi:10.1029/2006GL026283.

Cudmani, R., Osinov, V. A., 2001. The cavity expansion problem for the interpretation of cone penetration and pressuremeter tests. Canadian Geotechnical Journal, 38:622-638.

Gudehus, G., Kolymbas, D., 1979. A constitutive law of the rate type soils. 3th Int. Conf. Num. Meth. Geomech., Aachen, ed. Balkema 1979

Kramer, S.L., 1996. Geotechnical Earthquake Engineering, Prentice-Hall International Series in Civil Engineering and Engineering Mechanics, Prentice Hall, New Jersey.

Ishihara, K., Perlea, V., 1984. Liquefaction-associated ground damage during the Vrancea earthquake of March 4, 1977, Soils and Foundations, vol.24, No. 1, pp. 90-112

Kokusho, T., 2000. Water film in liquefied sand and its effect on lateral spread. J. Geotech. Engrg. ASCE, 125(10), pp. 817-826.


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Lysmer, J., Seed, H. B., Schnabel, P. B., 1972. Shake: A computer program for earthquake response analysis of horizontally layered sites. Technical Report UCB/EERC-72/12, Earthquake Engineering Research Center, University of California, Berkeley.

Niemunis, A., 2003. Extended hypoplastic models for soils. Monografia No. 34, Politechnika Gdanska.

Niemunis, A., and Herle, I., 1997. Hypoplastic model for cohesionless soils with elastic strain range. Mech. Cohesive-fictional Mater., Vol. 2, No. 4, pp. 279-299.

Osinov, V. A., 2003. A numerical model for the site response analysis and liquefaction of soil during earthquakes. In: Natau, O., Fecker, E., Pimentel, E. (Eds.): Geotechnical Measurements and Modelling, Swets & Zeitlinger, Lisse, pp. 475-481.

Rebstock, D., Wienbroer, H., Huber, G., Bühler, M., 2006: Fundamental mechanisms and requirements for a seismic soil-foundation-structure interaction approach. ESG 2006, Grenoble, pp. 233-242.

Sokolov, V. Y. and Bonjer, K.-P., 2006. Modeling of distribution of ground motion parameters during strong Vrancea (Romania) earthquakes, Proceedings of First Europ. Conf. on Earthquake Engineering and Seismology, Geneva, Switzerland, paper 363.

Sokolov, V. Y., Bonjer, K.-P. and Wenzel, F., 2004. Accounting for site effect in probabilistic assessment of seismic hazard for Romania and Bucharest: a case of deep seismicity in Vrancea zone. Soil Dynamics and Earthquake Engineering, Vol. 24, pp. 929-947.

Sokolov, V. Y., Loh, C. H., and Wen, K. L., 2000. Empirical model for estimation Fourier amplitude spectra of ground acceleration in Taiwan region. Earthquake Engineering and Structural Dynamics, Vol. 29, No. 3, pp. 339-357.

v. Steht, M., Jaskolla, B., and Ritter, J. R. R. Near surface shear wave velocity in Bucharest, Romania. Geophysics, revised.

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