M. Asce

TRADITIONAL AND ADVANCED PROBABILISTIC SLOPE STABILITY ANALYSIS

The paper contrasts results obtained by the traditional First Order Reliability Method (FORM) and a more advanced Random Finite Element Method (RFEM) in a benchmark problem of slope stability analysis with random shear strength parameters. The key difference between the methods is that RFEM takes into account spatial correlation in a rigorous way allowing slope failure to occur naturally along the path of least resistance. Both methods lead to predictions of the probability of slope failure as opposed to the more traditional factor of safety measure of slope safety, however they give significant different results depending on the value of the correlation length. For small correlation lengths FORM is generally conservative, however it is shown that there is a “worst case” correlation length for which FORM leads to unconservative predictions of slope reliability.

Characterizing the Liquefaction Resistance of Aged Soils

The occurrence of liquefaction in soils is often evaluated using the simplified procedure originally proposed by Seed and Idriss based on in situ indices. Although numerous studies have been conducted to improve and extend this procedure, the effect of age on liquefaction resistance is still poorly understood and correction factors have not been generally accepted. Nine published studies on the effect of age are reviewed in this paper. A regression line representing the average variation in liquefaction strength grain with time was developed from cases where strength was expressed based on recommended liquefaction resistance curves. This regression line is considered an update of the previously proposed relationship by Arango et al. The results indicate that commonly used liquefaction evaluation procedures can be overly conservative in many natural soil deposits.

Life Cycle Impacts for Concrete Retaining Walls vs. Bioengineered Slopes

A case study at the site of a creek restoration project within the Presidio in San Francisco, California is presented that analyzes the life-cycle environmental impact between a conventional reinforced concrete retaining wall compared with a bioengineered slope. Evaluation criteria were based on overall life-cycle costs (planning, design, construction, operations/maintenance, however decommissioning was not evaluated) and the magnitude of environmental impact. For our study, environmental impact was characterized based on total energy consumption (TJ) and associated Global Warming Potential (GWP, CO2Equivalent). The magnitude of environmental impact was calculated using the online Economic Input Output Life Cycle Analysis (EIO-LCA) tool. We found that biostabilization methods had about one-half the environmental impact as compared with utilization of conventional reinforced concrete retaining walls. However, the total (design) lifetime cost for biostabilization was found to be higher than that of the reinforced concrete wall (due mainly to maintenance costs where we assumed that the bioengineered slope would be actively maintained).

Experimental Simulation of Rainfall and Seismic Effects to Trigger Slope Failures

Two slopes - one at 30 0 and 40 0 were prepared with loose sand in a Plexiglas model container of 1.2 m x1.2 m x 1.2 m size. The slope was poured with a rainfall of 0.5 mm/min for 3 hours. Variation in degree of saturation, suction and apparent cohesion with depth were measured. The numerical calculation showed a yield seismic coefficient of 0.21. However, the slope failed catastrophically at a seismic acceleration of 0.5g. The tensiometer data showed a drop in suction after seismic shaking, which can be a main cause of the catastrophic failure. BACKGROUND Numerous long-run out landslides were observed during the recent Mw 9.0 Tohoku Earthquake (Pradel et al., 2011). Several large scale landslides were observed when strong typhoons (Typhoon Ma-on and Roki) hit the earthquake affected area four and six months, respectively after the devastating earthquake. This caused a property loss of over

Multiple Resistance Factor Methodology for Service Limit State Design of Deep Foundations using a "t-z" Model Approach

50M and killed more than 9 people. This shows that after a slope has been loosened by an earthquake, a significant degradation in stability can occur when the slope is exposed to heavy precipitation. Therefore, it is essential to evaluate the effect of seismic shaking on the slope when the slope is loosened by earthquake or when a partially saturated soil slope loses its strength during seismic shaking. SOIL TESTING METHODOLOGY Dry sand was compacted loosely in a Plexiglas Model (dimension 1.2 m x 1.2 m x 1.2 m) at the void ratio of 0.7 in two different slopes of 30 0 and 40 0 (Fig. 1).

Shear Wave Velocity of Weakly Cemented Silty Sand During Drained and Undrained Triaxial Compression

For the AASHTO Load and Resistance Factor Design (LRFD) of deep foundations, resistance factors must be applied to the calculated nominal load capacity to determine the nominal resistance of the foundation. The deep foundation is adequate if the nominal resistance is greater than the calculated factored load. In most current methods, the side and tip resistance are calculated separately and are added to determine the nominal capacity of the foundation. When using the t-z model, however, the percentage of the design load carried by side resistance and tip resistance is known at any applied load. Therefore, it is beneficial to consider how the uncertainties in the side and tip resistance model parameters each affect the uncertainty in the total load capacity of the foundation. In this paper, resistance factors have been calculated for the design of deep foundation systems based on side and tip resistance. The model parameters are back-calculated using actual load test data and the variability in the side and tip parameters is examined separately in a series of Monte Carlo simulation analyses. Histograms of the deep foundation load capacity, corresponding to an allowable total displacement, are developed for the simulations and analyzed to calculate separate side and tip resistance factors.

Liquefaction Susceptibility of Fine-Grained Soils in Charleston, South Carolina Based on CPT

There is increasing use of small strain shear modulus, calculated from the shear wave velocity (v s ), in geotechnical site characterization, analysis and design. Shear wave velocity is significantly influenced by stress state of the soil among many other parameters. In this paper the effect of shear stress on shear wave velocity of weakly cemented sands during drained and undrained triaxial compression is evaluated. The results showed that shear wave velocity during drained shear is dependent on σ1 and during undrained shear it is dependent on σ3. During drained compression, vs increased with σ1 up to a principal stress ratio of 6-12, depending on the level of cementation and beyond this stress ratio v s decreased significantly even as σ1 continued to increase up to failure. It is hypothesized that the observed behavior of vs during shear could be used as a precursor to failure for projects involving sensitive or structured soils.

Temperature Response in a Pervious Concrete System Designed for Stormwater Treatment

The liquefaction susceptibility of four fine-grained soils in Charleston, South Carolina was examined primarily using cone penetration test (CPT) measurements. Ages of these four soils range from <6,000 years to about 30 million years. The liquefaction susceptibility criteria by Robertson and Wride appear to be adequate for the three younger soils, which are estuarine deposits. However, as noted previously by Li et al., the criteria incorrectly predict liquefaction susceptibility for the oldest soil, called the Cooper Marl. The Cooper Marl is a deep marine deposit that consists of 60-80% calcium carbonate and often classifies as MH to CH, based on the Unified Soil Classification System. The results illustrate the usefulness of also measuring pore water pressure during cone testing in fine-grained soils, for classification and liquefaction evaluation. A new CPT-based liquefaction susceptibility chart for screening out non-susceptible fine-grained soils is proposed.

Probabilistic and Deterministic Slope Stability Analysis by Random Finite Elements

For pervious concrete to function optimally as both a pavement and stormwater treatment solution, both aspects must be considered together as a system. The pavement must possess the required strength and freeze-thaw resistance for surface durability and also an appropriate permeability to convey stormwater to the lower aggregate base retention area. This paper presents data obtained from a fully instrumented pervious concrete parking lot at Iowa State University (ISU). The site contained two 15 cm (6 in.) thick pervious concrete sections overlying 30 cm (12 in.) or 46 cm (18 in.) base configurations, while the control concrete section was placed directly on compacted soil. Temperature sensors monitored the freeze-thaw behavior of the system for both pervious sections and a standard concrete control. Ultimately, water samples will be collected from both the standard concrete control section and from tiling installed in the pervious concrete aggregate base for comparison of stormwater constituents and flow. The freeze-thaw results show a substantial lag in frost penetration of the pervious system and immediate thaw once melt water becomes present. Maximum temperature observed in the pervious concrete layer was always greater than the surrounding air temperature.

One-dimensional Probabilistic Uncoupled Consolidation Analysis by the Random Finite Element Method

Program PES (Probabilistic Engineered Slopes) provides a repeatable methodology allowing the user to perform a slope stability analysis on a one-sided and two-sided sloping structure using a deterministic or probabilistic approach. Program PES, in contrast with other deterministic or probabilistic classical slope stability methodologies, is cable of seeking out the critical failure surface without assigning a pre-defined failure surface geometry. The probabilistic approach of program PES applies the Random Finite Element Method (RFEM) by Griffiths and Fenton (1993) taking into account the soil spatial variability and allowing the use of different random fields to characterize the spatial variation of any material type. The methodology is compared against the probabilistic approach proposed by the program SLOPE/W version 7.14 (Geostudio Group, 2007), and demonstrates its potential for predicting probability of failure ( p f ) in non-homogeneous soil structures characterized by phreatic conditions and potential post-earthquake liquefiable conditions. The p f results obtained by program PES have proved that underestimating the influence that the soil material variability has on the computation of p f will lead to lower results of probability and underestimate of the risk of slope instability. Program PES capabilities could be used by the engineering practice to prioritize intervention activities within a risk context.