Seismic Subsidence Research Articles

Seismically induced settlement of ground experiencing undrained shaking and laterally constrained compression

A method is proposed for estimating seismically induced settlement of saturated sands experiencing undrained shaking and laterally constrained compression. An empirical relationship is developed between the seismic coefficient of vertical compression, mvs, and standard penetration test blow count, N60, as a function of factor of safety against liquefaction, Fℓ, based on data interpreted from 18 sets of laboratory cyclic direct simple shear tests and 23 cases of field observations of seismic liquefaction. The proposed method is compared with seismic settlement observed at 78 sites subjected to 7.1–8.0 magnitude earthquakes, and with predictions by the previous well-known methods of settlement analysis for undrained shaking. For silty–clayey sands, the significant effect of the plasticity of fines on seismic settlement is illustrated. The use of pre- or post-earthquake penetration resistance for back-analyses of field seismic settlement observations is examined. A tentative correction factor is suggested for seismic settlement estimated based on the assumption of undrained shaking and laterally constrained compression for liquefied saturated sublayers at small distances from drainage boundaries or under buildings with small breadths, which may experience volumetric compression during ground shaking.

Strain-based assessment of liquefaction and seismic settlement of saturated sand

This study presents results from an extensive experimental research on saturated clean sand deposits subjected to seismic loading. A total of 29 freshly reconstituted soil samples were tested under...

Experimental Study on Mitigations of Seismic Settlement and Tilting of Structures by Adopting Improved Soil Slab and Soil Mixing Walls

Settlement of surface structures due to subsoil liquefaction is a big issue in geotechnical engineering. It has been happening during earthquakes in liquefaction-prone areas for many years. Mitigations have been proposed for this problem. The improved soil slabs and vertical mixing soil walls combined with lowering ground water levels (GWLs) were proposed in this study. Experiments were carried out by adopting a 1-g shaking table test. Two different soil densities with uniform and eccentric loads were included. Combined with lowering GWLs, three different soil slabs with a length of 40, 60 and 80 cm and two different mixing walls with soil and plastic were studied and compared. Results show that the horizontal soil slabs have good performance to reduce the settlement of structures. On the other hand, the vertical soil mixing walls did not reduce the settlement effectively, but its performance could be improved by lowering of GWLs.

Improving the structure and mechanical properties of loess by acid solutions – An experimental study

The structure of loess is the primary cause of loess-related disasters, such as collapse, seismic subsidence, and liquefaction. To improve the pore structure of loess more effectively, chemical methods are preferred over conventional physical methods. It is found that acidic solutions can readily dissolve the salt crystal cement composed of CaCO3 and other soluble salts in loess, thereby easily and thoroughly destroying the macropores of loess and resulting in a new, superior structure with improved mechanical properties. A series of laboratory tests was performed to investigate and compare the structural and mechanical properties of natural collapsible loess and loess treated with acid (LTA). Scanning electron microscopy (SEM) analysis revealed that the well-spaced granules with point contacts of natural loess were transformed into cemented interlocking amorphous grain modes in LTA. Compression and collapsibility tests proved that the pore structure of the loess was easily destroyed and improved by the addition of an acid solution, and the resulting LTA was more resistant to single and multiple collapse events than the natural loess. Static triaxial tests confirmed that the enhancement of the original cohesion force and curing cohesion force among soil particles improved the shear strength of LTA. Dynamic triaxial tests showed that, because of a lower void ratio (e) than the natural loess, LTA had a better seismic performance as well as a lower likelihood of liquefaction. The new acid-addition presoaking method is shown effective in improving mechanical properties of loess. The implication of this finding is significant for the construction in the loess deposits. The results also provide a valuable reference for studies on construction in loess polluted with acidic wastewater.

A regression-based approach to predict crest settlement of embankment dams under earthquake shaking

Settlement and deformation of the embankment dams are among the major damages caused by earthquakes, eventually leading to dam instability. Therefore, accurate assessment of the seismic settlement of embankment dams is of particular concern. This study aims to evaluate the settlement of embankment dams subjected to earthquake loads using regression-based methods. A wide-ranging real data on crest settlement of embankment dams caused by earthquakes was collected and analyzed. Yield acceleration of dam, maximum horizontal earthquake acceleration, fundamental period of dam body, predominant period of earthquake, and earthquake magnitude were considered as the most influential parameters affecting the seismic crest settlement of embankment dams. Using support vector regression method as well as multiple linear regression method, two models were developed to estimate earthquake induced crest settlement of embankment dams. Subsequently, sensitivity analysis was conducted in order to assess the behavior of the proposed models under different conditions. Finally, the accuracy of the proposed models was compared with the existing relationship for estimation of earthquake induced crest settlement of embankment dams. Although both MLR- and SVR-based models have an acceptable accuracy in estimation of the crest settlement of embankment dams under earthquake loading, the SVR-based model has a higher accuracy.

Evaluating 2D numerical simulations of granular columns in level and gently sloping liquefiable sites using centrifuge experiments

The response of a layered liquefiable soil profile, with granular columns as a mitigation strategy, was evaluated via numerical and centrifuge modeling. Comparisons were made for a level site containing a single granular column and for a pair of gentle slopes, one of which was mitigated with a network of dense granular columns. The results reveal the abilities and limitations of two state-of-the-art soil constitutive models. All simulations were performed in 2-dimensions using: 1) the pressure-dependent, multi-yield-surface, plasticity-based soil constitutive model (PDMY02); and 2) the bounding surface, plasticity-based, Manzari-Dafalias (M-D) soil constitutive model, both implemented in OpenSees. Numerical model parameters were previously calibrated via element testing. Both constitutive models under-predicted PGA near the surface at different distances from the granular column, but they better predicted spectral accelerations at periods exceeding 0.5 s (particularly M-D). The M-D model generally predicted seismic settlements well, while PDMY02 notably underestimated soil's volumetric compressibility and strains. Both models accurately predicted the peak value and generation of excess pore pressures during shaking for the unmitigated slope, leading to a successful prediction of lateral deformations. However, lateral movement of the treated slope was poorly predicted by both models due to inaccuracies in predicting the dissipation rate in the presence of drains. Both models came close to predicting the performance of gently sloping, liquefiable sites when untreated. But further advances are required to better predict the rate of excess pore pressure dissipation and seismic performance when the slope is treated with granular columns.

Response characteristics and preventions for seismic subsidence of loess in Northwest China

Seismic subsidence of loess had been verified by microstructure characteristic, dynamic triaxial test and in situ simulation test using blasting vibration. It has gradually become a significant subject in the field of geotechnical earthquake engineering. Loess is widely distributed in China, which typically has a loose honeycomb-type meta-stable structure that is susceptible to a large reduction in total volume or subsidence upon ground motion. Seismic subsidence contributes to various problems to infrastructures that are constructed on loess. This paper provides a review of state-of-the-art work on mechanism, microstructure characteristic and physical mechanics mechanism of the seismic subsidence. Furthermore, the comprehensive explanation, basics and earlier research performed on subsidence amount estimation, seismic subsidence assessment and corresponding preventions of disasters have been presented briefly. The literature review shows that some significant problems, for example, appropriate theoretical basis, multi-variable coupling in assessment, physical processes, mechanical mechanism in estimation, and so on require constant research and development work to overcome the aforementioned factors. Specifically, research on quantitative relation between macro-mechanics and microstructure cannot proceed only from experimental parameters but should establish theoretical connection between them. Further study on seismic subsidence must be developed under the theory of unsaturated soil mechanics. In addition, research on chronological and spatial development law of large-scale seismic subsidence, prediction of subsidence value and anti-seismic analysis of underground structures can be conducted in future.

Amplification of thickness and topography of loess deposit on seismic ground motion and its seismic design methods

The Loess Plateau in China is a region with the biggest thickness and most complicated topography of loess deposit in the world, where is affected by strong earthquakes frequently. The seismic amplification effects were discovered in the Plateau during the Wenchuan Ms8.0 earthquake and some other strong events. The field investigations, observations, and analyses indicated that large number of casualties and tremendous economic losses were caused not only by collapse and damage of houses with poor seismic performance, landslides, liquefaction and seismic subsidence, but also amplification effects of thickness and topography of loess deposit on seismic ground motion. In this paper, the characteristics of strong ground motion recorded in the region were analyzed; shaking table tests were performed and numerical analysis was carried out on amplification of thickness and slope of loess deposit on seismic ground motion. Moreover, we also developed site amplification factors of ground motion for seismic design of engineering projects on loess sites. The results showed that the amplification effects are more predominant with increase of thickness and slope of loess deposit. The ground motion may be amplified in seismic intensity by 1–2°, PGA by 1.1–2.2 times, and predominant period by 1.1–1.6 times respectively.

Mitigation of earthquake-induced damage of breakwater by geogrid-reinforced foundation

ABSTRACTA strong earthquake often precedes a tsunami, and a breakwater may settle during the earthquake. Such seismic subsidence of the breakwater may reduce its ability to block the tsunami, and the tsunami may easily enter coastal areas by overflowing it. This study deals with the instability of a breakwater due to an earthquake. In addition, to protect a breakwater from damage caused by an earthquake, a new concept of using geogrid for reinforcing the foundation of a breakwater is introduced. To determine the behavior of unreinforced foundation and to evaluate the effectiveness of the proposed reinforced foundation under different earthquake loadings, a series of shaking table tests were performed. It was observed that the earthquake generated excess pore water pressures and deformations of foundation ground were main reasons of failure of the breakwater. The reinforced foundation was found effective to reduce the earthquake-induced damage of the breakwater, and finally it makes the breakwater resilient against earthquake-induced forces. Numerical simulations were also performed to elucidate the mechanism of reinforcement–breakwater–soil–water system under different earthquake loadings.

Seismic liquefaction performance of strip foundations: Effect of ground improvement dimensions

Evidence from field case studies, as well as from experimental and theoretical research, suggest that the detrimental effects of seismic liquefaction on the performance of surface foundations on level ground may be mitigated in presence of a non-liquefiable soil crust of adequate dimensions and shear strength. This paper refers to the case where the non-liquefiable crust is not natural, but it has been artificially created by ground improvement, and focuses upon the effect of the size (thickness and width) of the improved area on the seismic settlement and the degraded post-seismic bearing capacity of the foundation. Size effects are evaluated numerically, starting from the reference case of an infinitely extending improved soil crust and progressing to more realistic cases of ground improvement of gradually decreasing lateral extend of improved soil. Guidelines are provided for a cost-effective design of ground improvement, based on the rate of seismic settlement reduction with increasing dimensions of the improved ground.

Mitigating the seismic settlement of foundations on sand by ground improvement techniques

Geotechnical design of shallow foundations on clean fine sands often requires provisions for earthquake-induced settlements. In this regard, there have been many investigations on the volume-change behaviour of fine sands during dynamic and cyclic loading, and different methods have been developed to estimate ground settlement. However, less attention has been given to the role of ground improvement in reducing earthquake-induced settlements. This paper focuses on Anzali sand, which is representative of most sands found in the northern Iran, a region subject to high seismic activity. This sand was studied with the aid of physical modelling in the laboratory, and three different soil stabilisation techniques were applied to improve its stiffness and strength against volume change. Investigations revealed that among these methods, the cement treatment technique has an appreciable effect on the mitigation of seismic settlement.

GENERAL STATEMENTS OF BUILDINGS ANALYSIS AND PROTECTION BY POSSIBLE COMBINED INFLUENCE OF SEISMIC LOADS AND BASE SUBSTANTIAL DIFFERENTIAL SETTLEMENTS

Common results of cycle of investigations and developments for the reinforced concrete and masonry buildings analysis and protection methods in conditions of possible combined influence of seismic loads and base substantial differential settlements are considered in the article.

Numerical Analysis of Ground Motion Effects in the Loess Regions of Western China

Loess is widely distributed in the western part of China and its area comprises 6.6% of Chinese territory. Because of the characteristics of high porosity, low strength, and weak cementation, the loess is characterized by high seismic vulnerability which has been observed and confirmed by many researchers at home and abroad. The postquake field investigation and the laboratory study have shown that the ground motion effects, including the amplification effects and slope effects, were obvious in loess sites. Moreover, the causes of landslides, seismic subsidence, and liquefaction were mainly attributed to this special soil structure and properties. In this paper, based on the data of shear wave velocity of typical loess in Lanzhou region, combining the results of dynamic triaxial tests, the numerical analysis of ground motion effects in the loess regions is analyzed. The results reveal how the ground motion effects are checked and demonstrated the important role of numerical simulations while studying the characteristics of ground motions.

An Experimental Investigation on the Load Settlement and the Bulging Behaviour of Stone Columns Placed on Highly Palstic Clays

Background: Stone columns are used to increase bearing capacity, improve the drainage conditions, reduce foundation settlements, improve slope stability, reduce seismic subsidence, and reduce lateral spreading and liquefaction potential, control the deformation and accelerate consolidation, permit construction on loose/weak soil fills. Statistical Analysis: When a stone column is placed on high plastic clays, the load carrying capacity of the stone column would vary depending upon various parameters. This experimental investigation was performed to check the ultimate load carrying capacity and the bulging failure of stone columns with varying Length to Diameter (L/D) ratio of the stone column, Consistency Index (IC) and the Liquid Limit (LL) of the soil on which the stone column is resting by conducting various tests like CBR test. Findings: After conducting the tests it is found that the ultimate load carrying capacity of the stone column increases up to a certain L/D ratio, beyond that value there would be no further increase in ultimate capacity. The Vertical extent of Bulging (BV) is mainly influenced by L/D ratio and the lateral extent of Bulging (BL) is predominantly influenced by IC. The results thus obtained from the present investigation will be helpful in finding the failures in stone columns by varying the load acting on it and also to check the load carrying capacity of stone columns with other parameters with respect to the L/D ratio. Improvements: As bulging failure is considered to be the predominant type of failure for stone columns especially in clayey soils, an attempt can be made to see that the length of stone column doesn't exceed 2-3 times of its diameter beyond which the bulging would surely happen.

Procedure to Estimate the Seismic Settlement of Partially Saturated Soils

The performance of near-surface geosystems during earthquake shaking depends on the free-field response of the soil layer and soil–structure interaction mechanisms. This paper presents a brief summary of the variables involved in considering the free-field response of partially saturated soil layers, and provides a step-by-step procedure to estimate the seismic settlement of a soil layer with a given initial relative density and degree of saturation. The procedure, which has been validated in a previous study using centrifuge physical model tests, was tabularized and presented in a flow chart. This methodology was followed to estimate the seismic settlement of different sand layers having different initial relative densities. It was observed that the degree of saturation has the greatest impact on loose soil layers, which also show the greatest amount of seismic settlement. However, partially saturated soil layers resulted in lower seismic settlement compared with dry or saturated soil layers.

Effect of Different Groundwater Levels on Seismic Dynamic Response and Failure Mode of Sandy Slope.

Heavy seismic damage tends to occur in slopes when groundwater is present. The main objectives of this paper are to determine the dynamic response and failure mode of sandy slope subjected simultaneously to seismic forces and variable groundwater conditions. This paper applies the finite element method, which is a fast and efficient design tool in modern engineering analysis, to evaluate dynamic response of the slope subjected simultaneously to seismic forces and variable groundwater conditions. Shaking table test is conducted to analyze the failure mode and verify the accuracy of the finite element method results. The research results show that dynamic response values of the slope have different variation rules under near and far field earthquakes. And the damage location and pattern of the slope are different in varying groundwater conditions. The destruction starts at the top of the slope when the slope is in no groundwater, which shows that the slope appears obvious whipping effect under the earthquake. The destruction starts at the toe of the slope when the slope is in the high groundwater levels. Meanwhile, the top of the slope shows obvious seismic subsidence phenomenon after earthquake. Furthermore, the existence of the groundwater has a certain effect of damping.

Short-term pre-2004 seismic subsidence near South Andaman: Is this a precursor slow slip prior to a megathrust earthquake?

We report here on the campaign GPS data from the Andaman Islands just previous to the great 2004 Sumatra–Andaman earthquake. The campaign-mode acquisitions at Port Blair showed that the site started to subside between 2003 and 2004. In addition, during this period, the horizontal displacement of Port Blair with respect to Indian plate, deduced from 1996 to 2000 GPS data, changed its orientation to that obtained during the 26th Dec 2004 co-seismic. This short-term subsidence can be modeled as slip in the up-dip portion of the fault, a slip that is equivalent to an earthquake with moment magnitude of 6.3. Previously, slow slip was thought to appear at intermediate depths roughly 35–55km but simple models of the deformation at this single site suggest slow slip at much shallower depth than this. This observation of subsidence obtained by GPS methods is in rough agreement with subsidence observed from tide gauge data. Campaign-mode GPS data between 1996 and 2000 suggest uplift for Port Blair during the inter-seismic period and so does the reported field observations of interseismic micro-atoll emergence. Lack of exposed land with GPS stations along the southern part of the thrust fault deprive of arriving at any indication of this preseismic subsidence in those areas. Although GPS data is lacking the geological indices reported from some sites on the Alaskan Coast, for example, imply short-term subsidence just previous to the great 1964 earthquake. The pre-earthquake subsidence recorded in Port Blair, therefore, may have global implications as a precursor signal of great earthquakes at least along some of the subduction zones.

Mitigation of seismic settlement of light surface structures by installation of sheet-pile walls around the foundation

Settlement of surface structures, which is particularly a private house, due to subsoil liquefaction is not a new issue in geotechnical engineering. It has been happening during earthquakes in liquefaction-prone areas since many years ago. However, to date no reliable measure against this problem with reasonable cost has been proposed to people. In this paper, results of a series of 1-g shaking table tests which have been conducted to evaluate performance of a possible mitigation against this problem are presented. The proposed mitigation herein is installation of sheet-pile walls around the foundation. In order to reduce the cost of mitigation, sheet-piling with gap and half-length sheet-piling were examined. The experiments were conducted in different ground water levels. It is found out that installing sheet-pile walls in relatively low ground water level can stop settlement of structures completely. Sheet-piling with gaps delays initiation of settlement but it may increase the ultimate settlement of structure. In addition, it is found that formation of a water film under the building׳s foundation is the governing mechanism of post-shaking settlement of structures.

Seismic Parameter Research of Marine Soil

In East China Sea, especially in Zhejiang and Fujian Provinces, classic marine deposit soft soil with characteristics of high compressibility, high sensitivity and low strength is widely distributed. Seismic subsidence occurs under earthquake causing substantial losses. Though a number of effective studies are carried out on the causes, mechanisms and methods on seismic subsidence, there are more other aspects to be studied due to marine soil’s complexity. In this paper, the classic marine soft soil in southern sea area of Taizhou Bay is studied on its seismic parameter based on the tri-axial seismic experiment. The seismic parameters and residual strain formula are obtained, and compared with normal continental deposit soft soil.

Studies on Liquefaction Failure Forms and its Damage to Engineering

The liquefaction failure forms are reviewed, including foundation strength failure, sand boil, seismic settlement, large ground displacement and flow slide. Taking pile foundations and buried pipeline as examples, there suggested some measures to reduce hazards of liquefaction.