Soft Soil Sites Research Articles

Atypical soil behavior during the 2011 Tohoku earthquake (М w = 9)

To understand physical mechanisms of generation of abnormally high peak ground acceleration (PGA; >1g) during the Tohoku earthquake, models of nonlinear soil behavior in the strong motion were constructed for 27 KiK-net stations located in the near-fault zones to the south of FKSH17. The method of data processing used was developed by Pavlenko and Irikura, Pure Appl Geophys 160:2365–2379, 2003 and previously applied for studying soil behavior at vertical array sites during the 1995 Kobe (М w = 6.8) and 2000 Tottori (М w = 6.7) earthquakes. During the Tohoku earthquake, we did not observe a widespread nonlinearity of soft soils and reduction at the beginning of strong motion and recovery at the end of strong motion of shear moduli in soil layers, as usually observed during strong earthquakes. Manifestations of soil nonlinearity and reduction of shear moduli during strong motion were observed at sites located close to the source, in coastal areas. At remote sites, where abnormally high PGAs were recorded, shear moduli in soil layers increased and reached their maxima at the moments of the highest intensity of the strong motion, indicating soil hardening. Then, shear moduli reduced with decreasing the intensity of the strong motion. At soft-soil sites, the reduction of shear moduli was accompanied by a step-like decrease of the predominant frequencies of motion. Evidently, the observed soil hardening at the moments of the highest intensity of the strong motion contributed to the occurrence of abnormally high PGA, recorded during the Tohoku earthquake.

Estimation of inelastic displacement demands of flexible-based structures on soft soils

This study aims to develop efficient tools for performance-based seismic design of soil-structure interaction (SSI) systems on soft soils. To simulate the SSI effects, linear and nonlinear 'equivalent fixed-base single-degree-of-freedom' (EFSDOF) oscillators as well as a sway-rocking SSI model were adopted. The nonlinear dynamic response of around 10,000 SSI models and EFSDOF oscillators having a wide range of fundamental periods, target ductility demands, and damping ratios were obtained under a total of 20 seismic records on soft soil sites. Based on the results of this study, a practical method is developed for estimating the base shear and maximum displacement demands of a nonlinear single-degree-of-freedom structure on soft soil deposits. In the proposed procedure, the effect of frequency content of ground motions is considered by normalising the period of vibration by the spectral predominant periods, while the nonlinear EFSDOF models are used to improve the computational efficiency.

Estimation of inelastic displacement demands of flexible-based structures on soft soils

This study aims to develop efficient tools for performance-based seismic design of soil-structure interaction (SSI) systems on soft soils. To simulate the SSI effects, linear and nonlinear 'equivalent fixed-base single-degree-of-freedom' (EFSDOF) oscillators as well as a sway-rocking SSI model were adopted. The nonlinear dynamic response of around 10,000 SSI models and EFSDOF oscillators having a wide range of fundamental periods, target ductility demands, and damping ratios were obtained under a total of 20 seismic records on soft soil sites. Based on the results of this study, a practical method is developed for estimating the base shear and maximum displacement demands of a nonlinear single-degree-of-freedom structure on soft soil deposits. In the proposed procedure, the effect of frequency content of ground motions is considered by normalising the period of vibration by the spectral predominant periods, while the nonlinear EFSDOF models are used to improve the computational efficiency.

Mitigation of railway-induced vibration by using subgrade stiffening

Railway-induced ground vibration is often associated with sites with soft ground. Stiffening of the subgrade beneath the railway track is one particular measure that has potential to reduce the vibration level at such sites. However, the mechanisms behind this reduction are not well understood. Here, the effects are examined in the context of two alternative approaches: (i) subgrade stiffening, where the soil directly under the track is stiffened, and (ii) stiff inclusions introduced at some depth beneath the track, sometimes known as ‘wave impeding blocks’. The efficacy of the measures is considered for different ground types in a parametric study carried out using a 2.5D coupled finite-element/boundary-element methodology. The soil is considered to consist of a soft upper layer over a stiffer substratum; corresponding homogeneous grounds are also considered. With a 6m wide, 1m thick, concrete block directly under the track, the vibration between 16 and 50Hz was found to be reduced by between 4 and 10dB for ground with a 3m deep soft upper layer. For a deeper soft layer the reductions were greater whereas, for a stiffer ground without the soft upper layer, the reductions in vibration from this block were negligible. Slightly smaller reductions in a similar frequency region were observed when the block was positioned 1m below the surface, suggesting that, as with stiffening directly under the track, the reduction in vibration was primarily due to the increase of the effective stiffness of the soil beneath the track rather than the effective creation of a new, thinner soil layer. Jet grouting is considered as an alternative to concrete and, although it is found to be less effective due to its comparatively low stiffness, it may still be considered as a practical measure for existing tracks on soft soil sites. The reduction in vibration from this form of soil improvement with a depth of 3m is similar to that for a 1m thick concrete block. Finally, results are presented for three example sites with different soil properties which show similar trends.

Seismic isolation of buildings for power stations considering soil-structure interaction effects

Nowadays, the power industry in Mexico is building encapsulated power stations, because they are a more efficient, safer and cheaper technology for power transformation, and they also require of smaller spaces. Encapsulated power stations have been already built in cities of Mexico where the seismic risk and hazard is negligible. Given all the advantages described above, Comisión Federal de Electricidad (CFE), the Mexican power company, is studying the possibility to build this type of station in the Valley of Mexico, where the earthquake risk and hazard are high. Encapsulated power stations are composed of a series of high diameter pressured steel pipes that contain a toxic gas as an insulation media for the electric cables that host inside. Therefore, it is required that for severe earthquake shaking, encapsulated power stations remain fully operational and that there will be no risk of a gas leaks, particularly in the soft soil sites of the Valley of Mexico. The use of base isolation in soft soils such as those found in the lakebed region of Mexico City is atypical and must be seriously evaluated. In soft soils, there is a higher risk of dynamic instability of base isolated structures for severe earthquake shaking due to the likeliness of resonant response with the ground, global rocking effects or sudden differential settlements of the ground. Research studies devoted to assess the feasibility of employing pendular isolation systems for the benchmark architectural model of SF6 power stations for six soft soil sites in the Valley of Mexico are summarized in this paper. Soil-structure interaction (SSI) effects have been included. The results of the study allowed concluding that pendular isolation systems can be used for an effective seismic isolation of SF6 power stations in relatively soft soils.

Expected seismic performance of irregular medium-span simply supported bridges on soft and hard soils

This paper presents the parametric study of irregular RC bridge structures subjected to strong seismic records. We determine the expected damages and the concentration demands on short piers of bridges with columns of unequal height located on soft and rigid soil sites. Medium length span bridges are analyzed using the most common structural configurations built in many countries with several height pier configurations. The structures were subjected to strong seismic ground motions recorded on soft and hard soils of earthquakes generated at subduction seismic sources. The parameters of interest in the study are the strength and stiffness characteristics of the substructure and the influence of the dynamic characteristics of the seismic records. The parameter combinations produced more than three hundred 3D non-linear time history analyses conducted with the Perform-3D software. Based on the evaluation of damages indexes, we determine the pier expected damages and the importance of the soil type, on the behavior of irregular pier bridge substructures. It is also quantified the impact of the pier configuration in the global behavior of the bridges and the influence of the shortest pier in the expected damages of the other piers. In most of the cases, the piers adjacent to the tallest piers of the bridge were the elements more affected by the irregularity.

Safety Assessment of Nuclear Power Plants for Liquefaction Consequences

In case of some nuclear power plants constructed at the soft soil sites, liquefaction should be analysed as beyond design basis hazard. The aim of the analysis is to define the postevent condition of the plant, definition of plant vulnerabilities, and identification of the necessary measures for accident management. In the paper, the methodology of the analysis of liquefaction effects for nuclear power plants is outlined. The procedure includes identification of the scope of the safety analysis and the acceptable limit cases for plant structures having different role from accident management point of view. Considerations are made for identification of dominating effects of liquefaction. The possibility of the decoupling of the analysis of liquefaction effects from the analysis of vibratory ground motion is discussed. It is shown in the paper that the practicable empirical methods for definition of liquefaction susceptibility provide rather controversial results. Selection of method for assessment of soil behaviour that affects the integrity of structures requires specific considerations. The case of nuclear power plant at Paks, Hungary, is used as an example for demonstration of practical importance of the presented results and considerations.

Assessment of seismic hazard in the Erzincan (Turkey) region: construction of local velocity models and evaluation of potential ground motions

Abstract: The fundamental step in estimation of seismic damage and losses in urban areas is identification of regional potential seismic hazard. The accuracy of seismic analyses depends on the reliability of the local input parameters used in the corresponding hazard and loss models. This paper presents detailed seismic hazard analyses for an earthquake-prone region using locally derived source and site parameters. Main components of this study are construction of local seismic velocity models, probabilistic and deterministic seismic hazard analyses, and estimation of corresponding potential ground motions. The study area is Erzincan, which is a city on the eastern part of the North Anatolian Fault Zone. Located at a triple junction of major fault systems within a basin structure, Erzincan experienced major events (Ms ~8.0) in 1939 and (Mw = 6.6) in 1992. This study presents the first discussion in the literature on site-specific probabilistic and deterministic hazard analyses for Erzincan. Using locally derived input parameters in site response modeling and hazard analyses, the earthquake potential of Erzincan is investigated in detail. Probabilistic seismic hazard analyses with a hybrid source model composed mainly of line sources show that for a return period of 475 years, the maximum peak ground acceleration value in the Erzincan city center is computed to be almost 1 g. On the other hand, probabilistic hazard analyses with only area sources yield ground motion amplitudes that are almost half of those obtained by the hybrid model. The deterministic hazard analyses also show that peak ground acceleration in the city center for a scenario event of Mw = 7.0 reaches 1.25 g at a soft soil site located at 2 km in distance from the fault plane. In summary, numerical results obtained with locally derived input parameters indicate that Erzincan has significant potential for hazard in terms of both local earthquake occurrence and site amplifications.

Response of structures to seismic sequences corresponding to Mexican soft soils

This is paper presents the results of an analytical study aimed at evaluating the effect of narrow-banded mainshock/aftershock seismic sequences on the response of structures built on very soft soil sites. Due to the scarce availability of recorded seismic sequences in accelerographic stations located in the lake-bed of Mexico City, artificial narrow-banded sequences were employed. In the first part of this study, a parametric investigation was carried out to identify the mainshock/aftershock ground motion features that have detrimental effects in the seismic performance of equivalent single-degree-of-freedom systems representative of framed-buildings that house standard and essential facilities. In the second part of this work, the seismic response of two (8- and 18-story) steel-moment resisting frames that house essential facilities is examined. It is concluded that buildings with fundamental periods of vibration longer than the dominant period of the mainshock can experience a significant increment in their inter-story drift demands due to the occurrence of an aftershock.

Site-dependent organization structure of seismic microtremors

The time dynamics of seismic microtremors recorded in several sites in Chisinau City (Moldova) are analysed by means of the informational approaches of the Fisher Information Measure (FIM) and the Shannon entropy. The sites show different soil characteristics. One of the investigated sites, Minar Calcar (MC), is located on hard rock, while the others on soft soil. Our analysis has shown that MC, which is very different from the other sites, is featured by the highest FIM and the lowest Shannon entropy. This indicates that the properties of order and organization of seismic microtremor time series depend on the soil characteristics of the sites, suggesting a higher organization structure and lower disorder in those microtremors recorded in hard rock sites with respect to those recorded in soft soil sites.

Empirical correlations for the compression index of Irish soft soils

Numerous correlations have been developed in the literature relating the compression index Cc of soft soils to simple index properties that serve as a useful reality check on oedometer test results. However, many of these empirical correlations are specific to soils of a certain geographic region and/or geological origin and therefore may not be applicable in other contexts. Compression index data and corresponding index properties, specific to a good geographic spread of Irish soft soil sites, have been compiled in this paper. Of all the forms of correlation considered, the relationship between compression ratio Cc and the natural water content is the most fruitful in terms of allowing preliminary predictions of compression index to be made.

Effects of Thixotropy and Cement Content on the Sensitivity of Soft Remolded Clay

Abstract Strength loss of sensitive clay slopes is a common concern for soft soil sites. A useful method for studying this problem is through physical model tests. However, a problem that deters more widespread use of such tests is the difficulty of obtaining the large samples of natural sensitive soil that would be required. The work described in this paper was motivated by the need to develop practical procedures to produce artificial sensitive clay for use in model tests. To reach this goal, some fundamental experiments were performed to explore the effects of cement content, water content, aging, and remolding after aging on the undrained shear strength. Two soils [San Francisco Bay mud (SFBM) and Yolo loam (YL)] with several different cement and water contents were studied. After mixing and aging and measurement of the aged strength, the soils were again remolded to determine the aged-remolded undrained strength. The sensitivity of the aged clay could then be calculated by dividing the aged strength ...

The Response Analysis of the Train-Bridge System under Seismic Action

This article based on bridge structural dynamics, vehicle dynamics and soil dynamics theory, set up the practical space mode, with an example of a dual-line 32 m single box section beam on Beijing-Shanghai Railway Line. According to the typical soft soil sites of Shanghai, the dynamic response of the bridge was analyzed by the large finite element software ANSYS under seismic action. Moreover the impact about the dynamic interaction of pile-soil-structure affecting on the design of high-speed railway bridges was got which will provide reference and basis for the construction and design similar project.

Effect of seismic sequences in reinforced concrete frame buildings located in soft-soil sites

On September 19 and 20, 1985, two strong subduction interface earthquakes struck Mexico City leading to a large stock of damaged, or even collapsed, reinforced concrete (RC) building structures located in soft soil sites of the former lake-bed zone. The aim of this study is to gain further understanding on the effects of soft-soil seismic sequences on the seismic response of RC framed-buildings. This investigation employed artificial sequences since only two real sequences were gathered during the 1985 earthquakes. The nonlinear response, in terms of peak and residual lateral inter-story drift demands of four RC buildings having 4, 8, 12, and 16 stories, was evaluated. Results show that the relationship of the damaged period of the building (i.e. period of the building after the mainshock) to the predominant period of the aftershock, which is a measure of its frequency content, has a significant impact in the building response.

KIESSI-3D 프로그램을 이용한 대형 3차원 SSI 해석

Abstract The soil-structure interaction(SSI) effect should be considered to accurately assess the seismic response of structure constructed on soft soil site other than the hard bedrock. Recently, the demand of SSI analysis has increased due to strengthening of the regulatory guidelines of nuclear power plant such as the USNRC SRP 3.7.2. In this study an accuracy and running time of the KIESSI-3D program for large-scale 3D SSI analysis were investigated. The seismic SSI analysis using the KIESSI-3D program was performed for several examples of large-scale three-dimensional soil-structure interaction system. The analysis results were compared with those of the ACS/SASSI program. Good agreements in transfer functions at selected locations showd that KIESSI-3D yields accurate solution for large-scale SSI problem. Moreover, it was found that running speed of the KIESSI-3D for large-scale 3D SSI analysis is much faster than that of the ACS/SASSI about 30～2000 times. Keywords :KIESSI-3D, soil-structure interaction, dynamic infinite element, PARDISO, seismic design

Implementation of Displacement Coefficient method for seismic assessment of buildings built on soft soil sites

This paper presents the results of an investigation aimed at extending the Coefficient Method for the seismic assessment of existing buildings built on very soft soil conditions. In the first part of this investigation, the lateral displacement response of four steel frames and six reinforced concrete frames under a set of 20 earthquake ground motions recorded on very soft soil sites of the old bed-lake of Mexico City is investigated. It is shown that the seismic response of the buildings strongly depends on the ratio of the first-mode period of vibration of the structure to the predominant period of the ground motion (T/Tg). In the second part of this study, a Displacement Coefficient method approach is employed for obtaining estimates of maximum inelastic roof displacement demands. Error statistics indicates that the Coefficient Method provides reasonably good estimates.

Efectos torsionales en estructuras sobre suelo blando

In this work the torsional vibrations of structures on soft soil caused by the combined effects of structural asymmetry and base rotation are evaluated. A simplified soil-structure system with five degrees of freedom is analyzed. The foundation is considered to be embedded into a uniform layer resting on an elastic half-space, under the incidence of inclined SH waves. Instead of matching the maximum torsional moments due to the independent effects of structural asymmetry and base rotation, a new approach for computing the design eccentricity is applied. Specifically, torsional eccentricity coefficients compatible with the peak coupled lateral-torsional response are obtained, using an ensemble of narrow-band earthquake motions typical of soft-soil sites in Mexico City. It is shown that these coefficients are not constant along the principal axes, but they depend on the position of the resistant element. Hence, it is necessary to compute them without assuming a linear variation, as is done nowadays. The torsional eccentricity coefficients can significantly differ from the currently codified values, particularly for torsionally flexible structures for which both coefficients may be negative. This implies that the lateral displacement is reduced by the effects of torsion.

Dynamic response characteristics of super high-rise buildings subjected to long-period ground motions

Spectrum characteristics of different types of seismic waves and dynamic response characteristics of super high-rise building structures under long-period ground motions were comparatively analyzed. First, the ground response wave (named LS-R wave) of a soft soil site with deep deposit, taking long-period bedrock seismic record as input, was calculated by wave propagation method. After that, a TOMAKOMAI station long-period seismic record from the Tokachi-Oki earthquake and conventional El-Centro wave were also chosen. Spectrum characteristics of these waves were analyzed and compared. Then, a series of shaking table tests were performed on a 1:50 scale super high-rise structural model under these seismic waves. Furthermore, numerical simulation of the prototype structure under these excitations was conducted, and structure damages under different intensive ground motions were discussed. The results show that: 1) Spectrum characteristics of ground response wave are significantly influenced by soft soil site with deep deposit, and the predominant period has an increasing trend. 2) The maximum acceleration amplification factor of the structure under the TOM wave is two times that under the El-Centro wave; while the maximum displacement response of the structure under the TOM wave is 4.4 times that under the El-Centro wave. Long-period ground motions show greater influences on displacement responses than acceleration responses for super high-rise building structures. 3) Most inelastic damage occurs at the upper 1/3 part of the super high-rise building when subjected to long-period ground motions.

Inelastic displacement ratios for evaluation of stiffness degrading structures with soil structure interaction built on soft soil sites

In this study, inelastic displacement ratios are investigated for existing systems with known lateral strength considering soil structure interaction. For this purpose, SDOF systems for period range of 0.1-3.0 s with different hysteretic behaviors are considered for a number of 18 earthquake motions recorded on soft soil. The effect of stiffness degradation on inelastic displacement ratios is investigated. The Modified Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading and the elastoplastic model is used to represent non-degrading structures. Soil structure interaction analyses are conducted by means of equivalent fixed base model effective period, effective damping and effective ductility values differing from fixed-base case. For inelastic time history analyses, Newmark method for step by step time integration was adapted in an in-house computer program. A new equation is proposed for inelastic displacement ratio of system with SSI with elastoplastic or degrading behavior as a function of structural period (<TEX>$\tilde{T}$</TEX>), strength reduction factor (R) and period lengthening ratio (<TEX>$\tilde{T}$</TEX>/T). The proposed equation for <TEX>$\tilde{C}_R$</TEX> which takes the soil-structure interaction into account should be useful in estimating the inelastic deformation of existing structures with known lateral strength.

Soil structure interaction effects on structural parameters for stiffness degrading systems built on soft soil sites

In this study, strength reduction factors and inelastic displacement ratios are investigated for SDOF systems with period range of 0.1-3.0 s considering soil structure interaction for earthquake motions recorded on soft soil. The effect of stiffness degradation on strength reduction factors and inelastic displacement ratios is investigated. The modified-Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading and the elastoplastic model is used to represent non-degrading structures. The effect of negative strain - hardening on the inelastic displacement and strength of structures is also investigated. Soil structure interacting systems are modeled and analyzed with effective period, effective damping and effective ductility values differing from fixed-base case. For inelastic time history analyses, Newmark method for step by step time integration was adapted in an in-house computer program. New equations are proposed for strength reduction factor and inelastic displacement ratio of interacting system as a function of structural period(<TEX>$\tilde{T}$</TEX>, T) ductility (<TEX>${\mu}$</TEX>) and period lengthening ratio (<TEX>$\tilde{T}$</TEX>/T).