Nicholas Sitar

Evaluation of factors controlling earthquake-induced landslides caused by Chi-Chi earthquake and comparison with the Northridge and Loma Prieta events

Over 10,000 landslides were triggered by the September 21, 1999, Chi-Chi Earthquake. A large number of these landslides have been mapped from SPOT images and a smaller number were described in a detailed field investigation. Geographic information systems (GIS) was used to conduct a spatial characterization of the slope failures, including distribution of type, size, slope angle, bedrock geology, ground motion, and distance from earthquake source. The most abundant landslides were shallow, disaggregated rock and soil slides. Landslides occurred primarily in Tertiary sedimentary rocks, which are well known for their susceptibility to landsliding in many parts of the world. Landslide concentration values diminish beyond epicentral distances of 40 and 70 km from the epicenter and the surface projection of the fault plane, respectively. Ground motion was found to be the most significant factor in triggering the shallow landslides in the Chi-Chi earthquake. Overall, 74% of all slope failures occurred in regions with vertical ground motions greater than 0.2g and 81% of all slope failures occurred in the region with mean horizontal peak ground accelerations (PGA) greater than 0.15g. These factors were used to compare landslides generated by the Chi-Chi earthquake to the landslides triggered by the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake. The major difference in distribution of landslides between the Chi-Chi earthquake and the two California earthquakes was the distribution of slope angles. In the Chi-Chi earthquake, 90% of the failures occurred on slopes steeper than 45°, while more than 80% of failures occurred on slopes less than 50° in California, apparently reflecting much steeper and tectonically more active geologic setting in Taiwan.

Wireless sensors for wildfire monitoring

We describe the design of a system for wildfire monitoring incorporating wireless sensors, and report results from field testing during prescribed test burns near San Francisco, California. The system is composed of environmental sensors collecting temperature, relative humidity and barometric pressure with an on-board GPS unit attached to a wireless, networked mote. The motes communicate with a base station, which communicates the collected data to software running on a database server. The data can be accessed using a browser-based web application or any other application capable of communicating with the database server. Performance of the monitoring system during two prescribed burns at Pinole Point Regional Park (Contra Costa County, California, near San Francisco) is promising. Sensors within the burn zone recorded the passage of the flame front before being scorched, with temperature increasing, and barometric pressure and humidity decreasing as the flame front advanced. Temperature gradients up to 5 C per second were recorded. The data also show that the temperature slightly decreases and the relative humidity slightly increases from ambient values immediately preceding the flame front, indicating that locally significant weather conditions develop even during relatively cool, slow moving grass fires. The maximum temperature recorded was 95 C, the minimum relative humidity 9%, and barometric pressure dropped by as much as 25 mbar.

Seismic Earth Pressures on Cantilever Retaining Structures

An experimental and analytical program was designed and conducted to evaluate the magnitude and distribution of seismically induced lateral earth pressures on cantilever retaining structures with dry medium dense sand backfill. Results from two sets of dynamic centrifuge experiments and two-dimensional nonlinear finite-element analyses show that maximum dynamic earth pressures monotonically increase with depth and can be reasonably approximated by a triangular distribution. Moreover, dynamic earth pressures and inertia forces do not act simultaneously on the cantilever retaining walls. As a result, designing cantilever retaining walls for maximum dynamic earth pressure increment and maximum wall inertia, as is the current practice, is overly conservative and does not reflect the true seismic response of the wall-backfill system. The relationship between the seismic earth pressure increment coefficient (ΔK AE ) at the time of maximum overall wall moment and peak ground acceleration obtained from our experiments suggests that seismic earth pressures on cantilever retaining walls can be neglected at accelerations below 0.4 g. This finding is consistent with the observed good seismic performance of conventionally designed cantilever retaining structures.

Reliability analysis of contaminant transport in saturated porous media

An approach to probabilistic modeling of contaminant transport based on the first- and second-order reliability methods (FORM and SORM) is presented. FORM and SORM were initially developed for structural reliability applications to estimate the occurrence of low-probability events. They can be readily used with both analytical and numerical models and do not require restrictive assumptions about the problem geometry or about the properties of the media. Sensitivity information is obtained as an integral part of these analyses and is used to identify the variables or parameters which have a major influence on the estimate of probability. Example reliability analyses of one- and two-dimensional transport are used to illustrate the approach, and the accuracy of the reliability methods is evaluated in comparison with Monte Carlo simulations. The results show that FORM increasingly overestimates the probability of exceedance as the spatial variability of the domain increases. SORM, on the other hand, accounts for the nonlinearity of the limit state surface and gives results consistent with Monte Carlo simulation over a range of coefficient of variation of K from 0.1 to 0.7. In addition, the FORM/SORM analyses are shown to provide a computational advantage over the Monte Carlo simulation for low-probability events, because the computational effort is independent of the probability and the results also include sensitivity information. Finally, an example application of system reliability using FORM and SORM shows that problems with multiple limit state surfaces can be readily analyzed and the computational effort is proportional to the number and complexity of the limit state functions.

EMPLACEMENT OF NONAQUEOUS LIQUIDS IN THE VADOSE ZONE

The results of experimental and analytical investigations of the movement of lighter than water organic liquids in the vadose zone are presented. The experiments consisted of physical model tests simulating nonaqueous phase liquid (NAPL) spills in unsaturated, two-dimensional domain above the water table. The evolution of the plume was observed through the transparent side of a tank containing sand, and the contaminant front was traced at appropriate intervals. Visual observations were supplemented with pressure and saturation measurements suitable for quantitative analysis of the process. The experiments show that during infiltration the front advances at constant speed (area/time). After spreading comes to a halt, the bulk of the contaminant is contained within a pancake-shaped lens situated on top of the capillary fringe. Fluctuations of the phreatic surface result in trapping the NAPL below the water table and in spreading the contaminant over a larger area. In layered samples, the behavior is largely geometry dependent, although heterogeneities invariably result in extensive horizontal spreading. Simplified analytical relationships were developed to estimate the spreading rate of the contaminant during infiltration and the thickness of the final immobilized oil lens observed during the experiments. These relationships are based on capillary pressure considerations, which have to be included in order to model the immobilization of the plume at saturations higher than the residual values, which typically occur in vadose zone, when the NAPL encounters the capillary fringe in its downward migration.

TESTING OF REINFORCED SLOPES IN A GEOTECHNICAL CENTRIFUGE

An evaluation of the use of centrifuge modeling as a tool for analyzing the behavior of reinforced soil slopes is presented in this paper. A review of the state-of-the-art indicates that previous centrifuge studies have focused mainly on the performance of reinforced soil vertical walls and that limit equilibrium approaches (used in the design of reinforced soil slopes) have not been fully validated against the failure of models in a centrifuge. As part of an evaluation of the conditions of similarity governing the behavior of reinforced soil structures at failure, scaling laws are specifically derived by assuming the validity of limit equilibrium. It is demonstrated that an Nthscale reinforced slope model should be built using planar reinforcements having 1/N the strength of the prototype reinforcements in order to satisfy similarity requirements. A description of the experimental testing procedures implemented as part of a recent centrifuge testing program is presented, and an example dataset from this investigation is used to illustrate typical results. These include the g-level at failure, visual observation of failure development, and post-failure analysis of reinforcement breakage. The pattern observed in the geotextile reinforcements retrieved after testing indicates that the boundary effects were negligible.

Seismically Induced Lateral Earth Pressures on Retaining Structures and Basement Walls

The evolution of the methodology for the evaluation of seismic earth pressures on retaining structures and basement walls is reviewed together with observations of field performance. The case history data and data from recent experimental work are used to show that the currently used methods are quite conservative and lead to excessively conservative designs in regions where design PGA exceeds 0.4g. Specifically, the experimental data from seismic centrifuge tests shows that the seismic earth pressure distribution for moderate size retaining structures, on the order of 6-7 m high, is triangular, increasing with depth. Moreover, there is no significant increase in seismic earth pressure between unbraced and braced structures with fixed base, while the loads on free standing cantilever structures are substantially lower owing to their ability to translate and rotate. The significance of the observed seismic earth pressure distributions is that the dynamic force can be applied at 1/3H, as is done for static loading, which substantially decreases the design level seismic moments on the structures.

Assessment of Seismic Slope Stability Using GIS Modeling

Abstract In recent years there has been a growing interest in developing GIS based models for mapping and identification of seismically-induced landslide hazards. A review of assumptions in the relationships and the data typically used in GIS seismic slope stability models, shows that there are three components which are commonly used together: pseudo-static slope stability analysis, attenuation of ground shaking models, and Newmarks displacement method (Newmark 1965). These relationships are typically combined in a raster or vector data model to produce a map of seismically-induced landslide displacements. To produce a map of ground displacement, a digital coverage of the geology and a high resolution DEM are needed at the very least. Additional data layers such as shear strength parameters and soil depth can be estimated from the geology and they can add to the sophistication and accuracy of the model. A case study was developed which combines the components of site and slope response in a vector data model and the results show that qualitatively acceptable results can be readily obtained. However, the model falls short of capturing the overall performance of any particular slope and at best it only provides a mean estimate of the potential seismically induced slope displacement. Thus, more sophisticated methods of slope and site response will have to be implemented in this platform to increase the robustness of the model. In particular, the resolution and quality of input data sets will have to be improved and rigorous probabilistic procedures will have to be implemented, so that the error bounds and the magnitude and the source of errors can be fully quantified.

Stability of Steep Slopes in Cemented Sands

The analysis of steep slope and cliff stability in variably cemented sands poses a significant practical challenge as routine analyses tend to underestimate the actually observed stability of existing slopes. The presented research evaluates how the degree of cementation controls the evolution of steep sand slopes and shows that the detailed slope geometry is important in determining the characteristics of the failure mode, which in turn, guide the selection of an appropriate stability analysis method. Detailed slope-profile cross sections derived from terrestrial lidar surveying of otherwise inaccessible cemented sand cliffs are used to investigate failure modes in weakly cemented unconfined compressive strength UCS30 kPa and moderately cemented 30UCS400 kPa sands and their role in the evolution of the geometry of the slopes. The results show that high-resolution slope topography, such as can be obtained with terrestrial lidar, is essential for identifying altogether different failure modes in weakly cemented shear-mode and moderately cemented tensile-mode sand slopes. Analyses show that the standard Culmann method for steep slopes is inappropriate for modeling the stability of cemented sand slopes since it tends to overpredict expected crest retreat and underestimate failure plane angle. Instead, a simplified analysis using infinite slope assumptions, but applied to a slope with finite dimensions subject to changing geometric conditions, such as toe erosion and slope steepening, is suggested for analysis of weakly cemented sand slopes. For moderately cemented sand slopes, a limit equilibrium analysis directly comparing the cliff tensile stress and cemented sand tensile strength is shown to reasonably predict failure conditions and timing as a result of either slope steepening or tensile strength loss, presumably from wetting in most cases. DOI: 10.1061/ASCEGT.1943-5606.0000396 CE Database subject headings: Slope stability; Sand, soil type; Limit equilibrium; Tensile strength; Failure modes. Author keywords: Slope stability; Cemented sand; Limiting equilibrium; Tensile strength; Lidar; Cliffs; Failure mode.

Mobilization of trichloroethene (TCE) during ethanol flooding in uniform and layered sand packs under confined conditions

Five horizontal flow experiments were conducted in homogeneous and layered sand packs to evaluate the effectiveness of mobilization of trichloroethene (TCE) by ethanol flooding. A small quantity (5–7 mL) of pure TCE was injected into the center of each sand pack, and then horizontal ethanol floods were conducted at darcy velocities between 4 and 12 m d−1. Once ethanol contacted the region initially affected by TCE in uniform sand packs, rapid downward and upgradient flow of TCE occurred along sloping ethanol-water interfaces. In layered sand packs the bulk mobilization of TCE appeared to be associated more with phenomena resulting from interfacial tension reduction rather than with enhanced dissolution processes. Heterogeneities promoted a variety of bypassing and mixing phenomena that resulted in, but were not limited to, physical displacement of TCE at the edges of discrete fine-grained lenses due to localized confined flow conditions; upward migration of ethanol through finer-grained soils due to buoyancy; rapid downward mobilization of TCE; upgradient mobilization of TCE around heterogeneities; and migration of TCE into finer-grained soils at locations both upgradient and downgradient of the TCE source zone. Enhanced TCE dissolution was also observed.