Soft Alluvium Research Articles

Excess Pore Pressure Migration Analysis Due to High Embankment Construction – Case Study East Kalimantan

A 42-inch pipeline traverses a predominantly flat right-of-way (ROW), running from south to north in East Kalimantan. Adjacent to the ROW, a coal mine concession was located on the western side, while the Mahakam River lies a further 3 km to the east. A mining waste dump has been constructed since 2010, situated in an area underlain by soft alluvium soil (Qa). The waste was stacked, reaching heights of up to 75 meters, with its toe approximately 200 m from the edge of the ROW. In 2016, a failure occurred in the ROW, causing the 42-inch pipeline to shift a maximum of 6.8 m horizontally, and rise by 2.0 m within a 300 m span. A geotechnical investigation was then conducted, consisting of 7 CPTu with dissipation testing. The CPTu results indicated high pore pressure, with a layer of soft clay ranging from 15 to 32 m thickness found in the ROW area. A hypothesis was formulated suggesting that the soft clay was not fully consolidated. Hence, the failure of the pipeline was possibly caused by the migration of excess pore water pressure accumulated during the construction of the waste dump. Results of the investigation indicated that the permeability coefficient was 2.5 times greater in the horizontal direction compared to the vertical ones (kh/kv = 2.5), allowing the pore water pressure to migrate more easily in the horizontal direction. This study aims to elucidate how the migration of excess pore water pressure in the horizontal direction influences ground stability. The analysis was conducted using finite element software MIDAS GTS NX, with the kh/kv varying from 2.5 to 100 times to explore excess pore pressure movement behaviors. The results of this study confirm that excess pore pressure migration can occur horizontally if the horizontal permeability coefficient is larger than its vertical counterpart. Thus, this study highlights that the greater the permeability coefficient and the larger the ratio, the further the excess pore pressure travels. Moreover, the horizontal displacement increases with the permeability coefficient ratio.

Soil dynamic response mapping and vulnerability analysis of earthquake hazard in Banda Aceh using HVSR method

Banda Aceh is a city with a high level of seismicity and earthquake risk. The seismicity level is mainly caused by two Sumatran Fault segments, i.e., the Aceh and Seulimeum Segments. Soft alluvium deposits aggravate the earthquake hazard in Banda Aceh. The presence of soft alluvium layers above the more rigid soil layer will cause earthquake wave amplification. This amplification phenomenon can increase the amplitude and energy of earthquake waves, making the earthquake more destructive.Therefore, dynamic response mapping and subsurface structure modeling are required to determine the soil characteristics of the research area. Such information is needed for the infrastructure and planning development of Banda Aceh City. This study obtained the subsurface model from microtremor data and a sequence of processing and modeling techniques. The process uses the Horizontal to Vertical Spectral Ratio method (HVSR). The resonance frequency and amplification values are identified from the ratio between the horizontal and vertical spectrum components depicted in the HVSR curves. Based on the obtained results, Banda Aceh has a predominant frequency values range of 0.54-11.72 Hz and an amplification range of 1.22-6.26. Low predominant frequency is mainly located in the study area’s southeast and center, which is associated with large sediment thickness. Meanwhile, high amplification values are mainly located on the study area’s southeast, middle, and northwest sides, which are associated with increased sediment depth and geological features in the form of swamp sediment. Then the value of seismic vulnerability (Kg ) can be obtained based on the predominant frequency and amplification value. In this study, the value of seismic vulnerability ranges between 0.72-31.77 area with relatively high vulnerability values in the middle, southeast, south, and southwest of the study area.

Pipejacking clogging detection in soft alluvial deposits using machine learning algorithms

‘Clogging’ is a common issue encountered during tunnelling in clayey soils which can impede tunnel excavation, cause unplanned downtimes and lead to significant additional project costs. Clogging can result in a drastic reduction in performance due to reduced jacking speeds and the time needed for cleaning if it cannot be fully mitigated. The data acquired by modern tunnel boring machines (TBMs) have grown significantly in recent years presenting a substantial opportunity for the application of data-driven artificial intelligence (AI) techniques. In this study, a baseline assessment of clogging in slurry-supported pipejacking is performed using a combination of TBM parameters and semi-empirical diagrams proposed in the literature. The potential for one-class support vector machines (OCSVM), isolation forest (IForest) and robust covariance (Robcov) to assess the tendency for clogging is then explored in this work. The proposed approach is applied to a pipejacking case history in Taipei, Taiwan, involving tunnelling in soft alluvial deposits. The results highlight an exciting potential for the use of AI techniques to detect clogging during slurry-supported pipejacking.

Effects of cement addition on arsenic leaching from soils excavated from projects employing shield-tunneling method

Tunnel construction in soft alluvial deposits is generally done using the shield-tunneling method, a technique that requires the mixing of cementitious materials with excavated soils to reinforce and stabilize the excavated tunnel walls. Although the mechanical properties of soft soils and sediments are improved after introduction of cementitious materials, this process could increase the pH of these geological materials and mobilize naturally occurring arsenic (As). In the present study, the effects of cement addition on As leaching from tunnel-excavated soils naturally contaminated with As were investigated. Sequential extraction experiments were also carried out to determine the chemical forms, solid-phase partitioning and leachability of As in the soils. The results of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed slight changes in the chemical properties of soils due to cement addition but the chemical phases of As remained unchanged. In the absence of cement, sequential extraction showed higher As leaching from the soils when the exchangeable As fraction and total As content increased. In contrast, As leaching increased up to pH 10.3 and then decreased at higher pH values when cement was added. This trend was observed irrespective of the soil samples, which indicates that pH adjustment is an important countermeasure in restricting As leaching from excavated soils.

Impacts of the plane strain ratio on excavations in soft alluvium deposits

Due to tool, time and budget limitations, 2-dimensional analyses of deep excavation are often adopted for engineering practice; thus, the 3-dimensional behaviour is ignored. Because conducting 3-dimensional analyses is very time-consuming and costly, the “plane strain ratio (PSR)” indicator is defined to assist in understanding the 3-dimensional behaviour of deep excavations via the results of 2-dimensional analyses. However, previous works of the impacts on PSR have focused on only the excavation geometry and distance from the excavation corner. In this paper, additional factors that may affect the PSR for excavations in both drained and undrained materials are examined, and it is concluded that not only excavation geometry and distance to the corner but also the ratio of the wall embedded depth to the excavation depth and the wall thickness have comparatively significant influences compared to other factors, whether excavating drained or undrained materials. Lastly, multiple regression is used to modify the value of the PSR from empirical PSR charts for sand and clay considering the impacts of these abovementioned factors.

Modelling liner forces response to very close-proximity tunnelling in soft alluvial deposits

The response of tunnel liner forces to the construction of very close-proximity tunnels is rather complex and depends upon ground deformability, liner joint configuration, intersection angle of new tunnel construction. A consensus with respect to securing the safety of very close-proximity tunnelling has not yet reached. This study characterised the response of tunnel liner forces to the excavation of very close-proximity tunnels, with reference to the field measurements and 2D numerical simulations. The very close-proximity tunnelling was conducted in the soft alluvial deposits at Taipei, Taiwan where two 6.1-m diameter shield tunnels ran in parallel and then stacked each other while entering the reception shaft. The ‘belta’ and the ‘volume loss’ methods were to distribute the earth load to the liner and capture the effect of ground loss respectively and the ‘local strain’ method was to reproduce both the effect of very close-proximity tunnelling and the effect of pass-through by applying a given volumetric strain to soil clusters. The predictions against the three different phases of shield approaching to (66D distance from the Ring H), reaching (1D distance from the Ring H) and passing through (57D distance from the Ring H) the existing tunnel were compared with the field measurements respectively. The principle stress change and rotation were analysed. Parametric studies with respect to the joint number and angle, ground deformability, and rotational stiffness of joint were conducted. The critical angles for the distribution of odd number joint while aligned-inclined tunnelling in the very close proximity of the existing tunnel were suggested. The results are useful in understanding the response of tunnel liner forces to the effect of very close-proximity tunnelling and optimising the joint number and angle towards securing the safety of very close-proximity tunnelling in soft soils.

Settlement of a railway embankment on PVD-improved Karakore soft alluvial soil

This paper presents a case history of the settlement performance of a railway embankment built on a Prefabricated Vertical Drain (PVD) improved soft alluvial soil of Karakore area, Wollo province, north-east of Ethiopia. The embankment was constructed on Awash - Kombolcha - Haragebaya (AKH) railway project for the purpose of preloading so as to facilitate the consolidation settlement in a short period of time; and it was monitored for more than 750 days. Numerical predictions of Class A and Class C (based on back-calculation procedure) were performed using Plaxis 2D. The soft soil creep (SSC) model was employed for the soft soil layers, while the Mohr-Coulomb (MC) model was used for the drainage and fill layers. The predictions were compared with the field monitoring settlement data. Quite a reasonable agreement was achieved from the finite element modelling of Class C prediction when the prediction was compared with the actual field settlement values. Parametric sensitivity studies were carried out to examine the influence of parameters on the rate and magnitude of settlements. The results of the numerical predictions and parametric studies have been discussed in detail. The soil on the flood plain area of Karakore is found to be very soft to soft alluvial deposit and formed over Tertiary Eocene stiff clay. The variable conditions of the soil formation and deposition highly dictated the deformation and excess pore water pressure behaviour of the soft subsoil sediment. The parametric sensitivity study showed that, following the compression index, the initial void ratio, the hydraulic conductivity, and the OCR highly influenced the settlement responses of PVD-improved soft alluvium soil respectively.

Analysis of seismic site characterization of the Isparta basin (southwestern Turkey) using passive surface-wave method (ReMi™) and borehole data

This study presents a site classification of the Isparta basin situated at the upper part of the Isparta Angle, which is one of the most important tectonic components in Turkey. The local conditions of the basin sediments may significantly increase ground motions from earthquakes and cause more structural damage. Vs30 is one of the base parameters used for the determining site classification. In this study, refraction microtremor (ReMi™) method was used to determine the shear-wave velocities structure and hence classifying the site characterization of Isparta basin. The data were collected at 172 locations and converted to shear-wave velocity–depth models and shear velocity (Vs) maps were created. The Vs structures were compared and evaluated with the current borehole records compiled from the area. The site classification map of the Isparta basin was prepared by using the Vs30 map and considering the National Earthquake Hazards Reduction Program (NEHRP) criteria. According to this map, the soft alluvial deposits of the central basin are mostly classified as D, and a few are classified as C. The soil class at the southern part of the basin (old urbanization area of the city) increases to C class due to which the tuff–tuffite content increases. The rock units such as Mesozoic carbonates and Cenozoic flysch located around the basin are classified as A. The soil class B, appears between the class A rocks and the alluvial basin, has relatively thin alluvial slope deposit overlying the rock. The classification obtained from this study may contribute to the studies on future urban planning and seismic risk assessment.

Recent massive incidents for subway construction in soft alluvial deposits of Taiwan: A review

Since the prevention and mitigation of man-made hazards have been deemed to be the key in developing sustainable infrastructure during rapid urbanisation. Recent four massive water leak incidents taken place throughout subway construction in the soft alluvial deposits at Taipei and Kaohsiung, Taiwan, respectively, were reviewed and analysed in this study. The water leak incidents are initiated by the piping either through vertical joints between adjacent wall panels or through the seepage-prone weak zones within soilcrete body. The running leak can be traced using the variation in the piezometric pressure, and similar but smaller variations can also be seen from the soil layers at shallow depths. Lessons learned from this study would be useful in ensuring the highest quality on joints for diaphragm walls and mitigating the effect of ineffective overlapping of jet-grout columns on the watertight effectiveness while digging sump pit or dealing with tunnel-station interface.

Lubrication performance of pipejacking in soft alluvial deposits

Pipejacking technologies has been extensively chosen by many mega cities worldwide to construct their sewer pipelines. Lubrication has been deemed to be the key factor in manipulating the performance of pipejacking works. Inadequate lubrication during pipejacking can lead exaggerated jacking force causing damages to the jacked pipe string and adjoining properties. Due to limited studies, a consensus has not yet reached. This study gives an access of evaluating the lubrication performance via the reduction in the frictional coefficient μ and investigates the impacts of injection type, soil and lubrication natures, and misalignment on the lubrication performance for a pipejacking project in alluvial soil deposits. The effect of misalignment, while ramming through the 8–21 m section of gravel at Drive C, gears up the frictional stress τld to 12.5 kPa (compared to 0.4 kPa of the 2–8 m section of the same gravel) and reduces the reduction to 71%. While traversing through the 24–31 m section of clayey gravel at Drive D, the occasional gravel not being long enough to develop lower face resistance leads the misleading reduction of 60%. These factors have greater influences on the lubrication performance than the injection type. The presented results would be helpful in managing the lubrication performance while pipe ramming for upcoming pipejacking project.

Ground Deformation Analysis of Bolvadin (W. Turkey) by Means of Multi-Temporal InSAR Techniques and Sentinel-1 Data

Surface deformations were observed in Bolvadin, located in the province of Afyon (western Turkey) in the last decade which occurred without any destructive earthquakes. In this study, ground deformation of the Bolvadin region is analyzed by means of multi-temporal interferometric synthetic aperture radar (InSAR) technique with Sentinel-1 synthetic aperture radar (SAR) data. Sentinel-1 data acquired in ascending and descending orbits between October 2014 and October 2018 are processed with the Sentinel Application Platform (SNAP) and Stanford Method for Persistent Scatterers (StaMPS) open source software tools. Deformation velocity maps and line-of-sight (LOS) displacement time series are produced and compared with geology, groundwater level and the water surface area of Eber Lake nearby. Deformation velocity maps reveal significant subsidence in most of the town and surrounding regions, which is confirmed by field observations that show severe damage to the settlements and infrastructure of the town. The most severe subsidence is observed to be in the southern part of Bolvadin with rates up to 35 mm/year, which is characterized by the presence of soft alluvial deposits. Composed of slope debris/talus and conglomerate, the northeastern part of the deforming region experiences a relatively lower rate of subsidence. A strong correlation between LOS displacement time series and groundwater level exists both in the short and long term. Moreover, short term variations in LOS displacement time series also show high similarity with seasonal variations in the water surface area of Eber Lake located a few km southeast of the town. We conclude that the primary cause of subsidence is most probably the overexploitation of groundwater and hydrological changes because of (1) the strong correlation of subsidence with lithological units, (2) the similarity between deformation rate and groundwater level changes, (3) the correspondence of seasonal variations in water surface area and short-term deformation rate oscillations, and (4) the absence of InSAR velocity contrast across the active faults.

Morphotectonic aspects in a part of Naga-Schuppen belt, Assam-Nagaland region, Northeast India

The Belt of Schuppen is an important tectonic element of the Assam-Arakan basin. It is a narrow belt of thrust slices in southeastern boundary of the Assam valley. The Naga Schuppen Belt, consisting of eight or more imbricated thrusts, occurs between the Naga and Disang thrusts. This present study encompasses an area of 4,720 sq km along the Assam-Nagaland border. The Dikhow River, which is a seventh order basin, originates from Naga Hills flowing through a total length of 240 km in the Assam valley and reaches the Brahmaputra. In this study, the morphotectonic analysis of the Dikhow River was carried out to understand the role of active tectonics of the Naga Hills in Nazira-Naginimora areas of Assam and Nagaland respectively. The parameters such as asymmetric factor (AF), transverse topographic symmetric factor (T), and stream length gradient index (SL) were computed. Absolute AF values shows asymmetric to highly asymmetrical shape of the basins (range III-IV) which is also supported by T values. Anomalous SL values were obtained wherein major lineament and tectonic features are present. This indicates that the study area is tectonically active. Further, in the Assam valley it has been observed in the bank stratigraphy of Dikhow River that the Quaternary sediments are deformed. This deformation of the soft older alluvium indicates that the area is undergoing deformation during post Pliocene time. This evidence demands the morphotectonic evaluation of the aforesaid area to demarcate the tectonic activeness of the region in post Pliocene times.

Rethinking the Water Leak Incident of Tunnel LUO09 to Prepare for a Challenging Future

Incident often occurs while deepening excavation pit and/or tunnelling underground pipelines although serious attention from practitioners, engineers, and scientists has received. Prevention and mitigation of incident have thus been deemed to be the key in developing sustainable infrastructure in urban areas. This study analyses and discusses an extensive water leak incident taken place throughout the parallel tunnels LUO09 construction in the soft alluvial deposits in Kaohsiung, Taiwan. Dumping sand bags and quick‐set cement intervened the incident but in vain. The water leak incident is initiated by the piping. The existing vehicle underpass causes the jet‐grout columns installed not exactly in the plumb. Their overlapping is estimated to be less than the design value of 60 cm, and some seepage‐prone weak zones are thus developed. The measured hydraulic gradient being equal to 12.1 and existence of seepage‐prone weak zones are deemed as the main cause initiating the water leak incident. The pinhole test results highlight not only the nonplastic nature of the Kaohsiung silt but also its vulnerability to piping under large hydraulic gradients. Some bullet points that indicate what engineers should do or avoid are learned and summarised.

Controlled heavy-haul traffic loading as a method to remediate liquefiable soft silts

Transporting of extremely large indivisible loads (10 000–30 000 t) is becoming increasingly popular to allow offsite modular construction of infrastructure for oil and gas, mining, and renewable energy projects in remote areas. Such exceptionally large transient loads could encounter unusual geohazards: there is a risk of metastable liquefaction when crossing soft alluvium, causing sudden failure, potential casualties, and severe production delays. Furthermore, temporary roads for these payloads are a large cost to such projects; conventionally designed earthworks and (or) ground improvement are often unaffordable or logistically impossible. This laboratory study indicates the fabric can be strengthened, and the hazard reduced, if the soil is subjected to careful repeated loading that rearranges the initially precarious fabric through gradual accumulation of plastic strains. A novel remediation technique for these temporary haul roads is proposed: managed deployment of increasingly heavy haul vehicles could result in staged fabric rearrangement that strengthens the soil to the point where it would be safe for the heavy vehicles to use it. In so doing, a more economic temporary haul road is open to operations (coupled with observation methods to ensure adequate performance throughout) and production activities are not overly disrupted.

Seismic Vulnerability of Urban Bridges Due to Liquefaction Using Nonlinear Pushover Analysis and Assessing Parameters for Damage Detection

Pre-disaster planning strategies and strengthening to ensure certain degree of performance of deficient bridges can reduce the seismic risk to a great extent. Codes have been steadily moving towards performance based designs. This study aims at investigating the effect of liquefaction during earthquake on bridges founded in the soft alluvium. For this purpose, nonlinear static analysis has been used to derive fragility and vulnerability curves. A two span box girder bridge has been analysed. The bridge foundation soil strata have layers of fine sand to sandy silt deposits. The capacity and vulnerability of the bridge under normal soil and liquefied conditions have been studied. It has been observed that the time period and damping of the structure have increased. Fragility curves have been developed to assess the seismic performance of the bridge. This study will help to ensure the performance criteria/ levels of the bridges and more informed pre-disaster planning strategies to avoid seismic risk and add to the existing knowledge of the concept of performance based design of RC bridges.

Variations of seismic velocities in the Kachchh rift zone, Gujarat, India, during 2001–2013

We herein study variations of seismic velocities in the main rupture zone (MRZ) of the Mw 7.7 2001 Bhuj earthquake for the time periods [2001–05, 2006–08, 2009–10 and 2011–13], by constructing dVp(%), dVs(%) and d(Vp/Vs)(%) tomograms using high-quality arrival times of 28,902 P- and 28,696 S-waves from 4644 precise JHD (joint hypocentral determination) relocations of local events. Differential tomograms for 2001–05 reveal a marked decrease in seismic velocities (low dVp, low dVs and high d(Vp/Vs)) in the MRZ (at 5–35km depths) during 2001–10, which is attributed to an increase in crack/fracture density (higher pore fluid pressure) resulted from the intense fracturing that occurred during the mainshock and post-seismic periods. While we observe a slight recovery or increase in seismic velocities 2011–13, this could be related to the healing process (lower pore fluid pressure due to sealing of cracks) of the causative fault zone of the 2001 Bhuj mainshock. The temporal reduction in seismic velocities is observed to be higher at deeper levels (more fluid enrichment under near-lithostatic pressure) than that at shallower levels. Fluid source for low velocity zone (LVZ) at 0–10km depths (with high d(Vp/Vs)) could be attributed to the presence of meteoric water or soft alluvium sediments with higher water content, while fluid source for LVZ at 10–35km depths could be due to the presence of brine fluids (released from the metamorphic dewatering) and volatile CO2 (emanating from the crystallization of carbonatite melts in the asthenosphere), in fractures and pores. We also imaged two prominent LVZs associated with the Katrol Hill fault zone and Island Belt fault zone, extending from shallow upper-crust to sub-crustal depth, which might be facilitating the deeper circulation of metamorphic fluids/volatile CO2, thereby, the generation of lower crustal earthquakes occurring in the Kachchh rift zone.

Construction Control Chart Developed from Instrumented Trial Embankment on Soft Ground at Tokai of Kedah, Malaysia

In order to develop and successfully implement a cost effective ground treatment design for railway embankment traversing through soft alluvium deposits for high speed train, a fully instrumented trial embankment was carried out at Tokai, State of Kedah, Malaysia for verification of design. Cost effective ground treatment of prefabricated vertical drain (PVD) with temporary surcharging and geotextile basal reinforcement were widely adopted for the 200 km long railway embankment and were designed to meet the stringent performance requirements. From the back analyses using Finite Element Modeling (FEM) of the fully instrumented trial embankment, the performance of the ground treatment is evaluated and verified. Additional FEM analyses were carried out to develop a construction control chart to be used during construction to allow high embankments to be built without compromising on the stability during construction and to meet the tight construction schedule and technical requirements. This paper will present the FEM back analyses results of the fully instrumented trial embankment, the construction control chart developed together with the pre-planned action plan during construction.

Deep excavations in water-bearing gravels in Cork

Historically only shallow basements were included in developments in Cork city owing to the underlying layers of soft alluvium and high-permeability glaciofluvial gravels. In recent times several large schemes, with basements up to 10 m deep, have been completed. In this paper the design and performance of these developments is summarized. Particular emphasis is placed on two schemes at Eglinton St. and Half Moon St. Although dewatering by deep well systems is feasible, flow rates can be substantial. Careful consideration needs to be given to the site-specific properties of the glaciofluvial gravels to permit the level of the cut-off to be efficiently chosen so as to minimize inflows and external groundwater lowering and to mitigate against the possible risk of surface settlements. A ‘soft gel’ grout blanket was used successfully as an alternative at one site. Lateral wall movements were low and it seems possible that more efficient designs are possible, especially if the small strain stiffness and dilation properties of the material are taken into account.

Investigation of the soil amplification factor in the Adapazari region

Turkey is located on the highly active Eurasian plate. A very active strike slip fault, the North Anatolian Fault Zone (NAFZ), crosses Turkey from east to west; earthquakes occurred on this fault on August 17, 1999 (Kocaeli) and November 12, 1999 (Duzce). Regional geology and subsoil conditions can significantly change the characteristics of ground motion. For this reason, determining soil amplification during an earthquake, especially for soft soils, is a very important topic for researchers. In this study, one dimensional ground response analyses were performed for selected Adapazari sites using the August 17, 1999 Kocaeli earthquake strong ground motion record with SHAKE2000 software. Soil characteristics and depth to engineering bedrock at the selected sites are different and the observed level of structural damage at these sites during the Kocaeli earthquake was also different. Calculated soil surface response spectrums at these sites were compared with the recommended design spectra of the Turkish Earthquake Code and the Eurocode 8. According to one dimensional ground response analyses, the calculated response spectra of the selected sites exceed the recommended design spectra of the Turkish Earthquake Code and the Eurocode 8. Calculations show that higher amplification ratios occur at higher periods due to soil behaviour. Results of this study indicate that local geological conditions may amplify ground motion at some periods and, due to this amplification, the calculated response spectra may exceed the recommended design spectra. Therefore, it is clear that local site conditions must be considered for earthquake-resistant engineering designs on soft alluvial soil deposits.

Probabilistic Seismic Hazard Analyses (PSHA) and Liquefaction Susceptibility Evaluation of Kashmir Valley, India

Kashmir valley India is located in seismically active Himalayan-Hindukush belt, which has been witnessed by the number of high magnitude earthquakes during the past centuries. The presence of active seismogenic faults, soft alluvium and shallow groundwater levels in the basin of the Kashmir valley signifies its detrimental effect on the occurrences of future strong ground motions. In view of severe damaging attitudes and frequent occurrences of earthquakes in the region the present paper emphasizes attenuation model for seismic hazards zonation to ensure the safe and comfortable expansion of settlements and industrial establishments. For the purpose detailed study was performed on the seismically deformed beds, water-table conditions and past seismic activities of the area around the valley. The results of study reveal the peak ground acceleration (PGA) of 10, 5 and 2 % probabilities of event in 50-year lifetime with probability of 2 % for PGA = 0.652 (g), 5 % for PGA = 0.460 (g) and 10 % for PGA = 0.372 (g) which varies with the different surface and sub-surface conditions of the area.