Liquefaction hazard assessment and ground failure probability analysis in the Kathmandu Valley of Nepal

During the 2015 Gorkha Earthquake (Mw7.8), extensive soil liquefaction was observed across the Kathmandu Valley. As a densely populated urban settlement, the assessment of liquefaction potential of the valley is crucial especially for ensuring the safety of engineering structures. In this study, we use borehole data including SPT-N values of 410 locations in the valley to assess the susceptibility, hazard, and risk of liquefaction of the valley soil considering three likely-to-recur scenario earthquakes. Some of the existing and frequently used analysis and computation methods are employed for the assessments, and the obtained results are presented in the form of liquefaction hazard maps indicating factor of safety, liquefaction potential index, and probability of ground failure (PG). The assessment results reveal that most of the areas have medium to very high liquefaction susceptibility, and that the central and southern parts of the valley are more susceptible to liquefaction and are at greater risk of liquefaction damage than the northern parts. The assessment outcomes are validated with the field manifestations during the 2015 Gorkha Earthquake. The target SPT-N values (Nimproved) at potentially liquefiable areas are determined using back analysis to ascertain no liquefaction during the aforesaid three scenario earthquakes.

Shear strength behaviour of liquefiable sand of petobo on treated by agarose under direct shear test

Liquefaction process is associated with the loss of the shear strength of the saturated loose sands caused by strong earthquakes. Due to mitigitation of liquefaction hazard, an appropriate mitigation of liquefaction using environmentally friendly methods is critical and becoming increasingly important and unavoidable. The laboratory investigation was carried out to study the shear strength behaviour of liquefiable sand of Petobo treated by agarose on different concentration 1%,3% 5%. A series of direct shear test were conducted under three level of vertical stress 10 kPa, 20 kPa, and 30 kPa on the specimen. It was found that the optimum content of agarose which can be considered is at 1%-3%, using stress ratio (τ/σv) analysis shows that stress ratio decreases with increasing the vertical stress on the same agar content. The implication this result that the application of this method must consider variation of material source and characteristic, and the suitable level of vertical stresses.

Earthquake Induced Liquefaction Analysis and Ground Improvement as a Remedial Measure: A Review

Liquefaction is the phenomenon in which partially or fully saturated, loose sandy soils behave like a liquid due to loss of strength and rigidity owing to sudden increase in the pore water pressure as a result of dynamic loading such as earthquake. Liquefaction induced by dynamic loading as a result of earthquake is the most destructive feature of earthquake that may results in settlements and collapse of structures. The severity of this phenomenon can be predetermined by the geological and hydro-geological setup of the soil in the study area. The aim of this study is to present a review of various aspects of earthquake induced liquefaction analysis, case evidences from field studies and some of the liquefaction hazards from past earthquakes. Remedial measures using ground improvement techniques to prevent liquefaction hazard is also studied in this paper. Further, investigating the performance of remedial methods against liquefaction is also presented in this paper.

Hazard characterization for alternative intensity measures using the total probability theorem

Since their inception in the 1980s, simplified procedures for the analysis of liquefaction hazards have typically characterized seismic loading using a combination of peak ground acceleration and earthquake magnitude. However, more recent studies suggest that certain evolutionary intensity measures (IMs) such as Arias intensity or cumulative absolute velocity may be more efficient and sufficient predictors of liquefaction triggering and its consequences. Despite this advantage, widespread hazard characterizations for evolutionary IMs are not yet feasible due to a relatively incomplete representation of the ground motion models (GMMs) needed for probabilistic seismic hazard analysis (PSHA). Without widely available hazard curves for evolutionary IMs, current design codes often rely on spectral targets for ground motion selection and scaling, which are shown in this study to indirectly result in low precision of evolutionary IMs often associated with liquefaction hazards. This study presents a method to calculate hazard curves for arbitrary intensity measures, such as evolutionary IMs for liquefaction hazard analyses, without requiring an existing GMM. The method involves the conversion of a known IM hazard curve into an alternative IM hazard curve using the total probability theorem. The effectiveness of the method is illustrated by comparing hazard curves calculated using the total probability theorem to the results of a PSHA to demonstrate that the proposed method does not result in additional uncertainty under idealized conditions and provides a range of possible hazard values under most practical conditions. The total probability theorem method can be utilized by practitioners and researchers to select ground motion time series that target alternative IMs for liquefaction hazard analyses or other geotechnical applications. This method also allows researchers to investigate the efficiency, sufficiency, and predictability of new, alternative IMs without necessarily requiring GMMs.

The dynamic response of coseismic liquefaction-induced ruptures associated with the 2019 Mw 5.8 Mirpur, Pakistan, earthquake using HVSR measurements

The Mirpur area of Pakistan was severely damaged by extensive coseismic liquefaction following an earthquake of Mw 5.8 on 24 September 2019. Villages within 6 km of the epicenter were adversely affected due to extensive coseismic liquefaction-induced surface and shallow subsurface deformations. The earthquake affected all types of buildings and key infrastructure (e.g., the Upper Jhelum Canal and the main Jhelum–Jatlan road). Field observations and associated effects are presented, including horizontal-to-vertical spectral ratio (HVSR) data sets acquired from three sites to evaluate the site response characteristics of the liquefaction-affected soil profiles. As a result, rupture events strongly influenced spectral features (amplitude and frequency) and site-specific 1D shear-wave velocity profiles at sites S1 and S2. The dynamic behavior of HVSRs across ruptures at sites S1 and S2 corresponds to varied levels of seismic amplification, demonstrating the impact of liquefaction-induced ruptures of seismic origin on the site response that have not been reported previously in the literature. The consistent HVSR pattern of well-established high-frequency peaks at site S3 adjacent to partially damaged to completely collapsed buildings of different types further indicates the susceptibility of potential liquefaction hazard. These results agree with the surface liquefaction signatures in the field, revealed by inverted electrical resistivity tomography models in terms of liquified sand plugs, clay lenses and associated fractures, and increasing trends of radon concentration in the soil with decrease in the distance toward ruptures. Additionally, the successful application of HVSR as a cost-effective and speedy tool attests to the potential of the proposed approach in furnishing complementary information for better assessment of liquefaction hazards in the developing world, where financial constraints are a major issue. This can help with seismic hazard analysis and mitigation in the Mirpur area and may have applications in other seismically active regions of the world.