Riverdog journal

Thesis (M.S.V.S.)--Massachusetts Institute of Technology, Dept. of Architecture, 1979.by Robb Moss.Thesis (M.S.V.S.)--Massachusetts Institute of Technology, Dept. of Architecture, 1979

Scale Model Shake Table Testing of Seismic Earth Pressures in Soft Clay

There is much uncertainty in seismic earth pressures for basement wall design and retaining wall design in clay soil. The research presented herein uses a scale model testing platform on a 1g shake table to measure the distribution and magnitude of seismic earth pressures for the prototype soil conditions of San Francisco Young Bay Mud (YBM). Similitude scaling of the dynamic soil properties, the wall dimensions, and the input time histories (among other variables) affords accurate modeling of the prototype scale wall. For basement conditions, inertial soil structure interaction (SSI) effects are included in the experiments. Care is taken to properly mimic static lateral stress conditions of the walls to achieve active or at-rest conditions for the respective wall designs prior to shaking. Results for a wall of prototype 3m height experiencing a large ground motion is shown and compared to current earth pressure design recommendations

Advances of cone penetration testing in earthquake engineering applications

The Cone Penetration Test (CPT), because of its precision, accuracy, and utility has been increasingly used in earthquake engineering applications in the last decade. This paper provides a brief survey of recent advances in applying the CPT to; liquefaction triggering, post-liquefaction deformations, cyclic failure of clays, dynamic slope stability, and seismic site response. In granular soils the continuous CPT measurements of tip and sleeve resistance are well correlated with the engineering properties of relative density and friction angle. In clay soils the CPT tip resistance is directly proportional to the undrained shear strength. CPT measurements are ideal for weak or soft soil layers, which are the primary culprits in seismic soil failure. The CPT is commonly instrumented with an accelerometer so that shear wave velocity measurements can be made concurrently with penetration measurements. This allows for the measure of the small strain stiffness of the soil for dynamic modeling and site response analysis. For tailings dams and earth slopes the combination of penetration measurements to estimate soil strength and small strain stiffness to assess the modal response provides a complete set of measurements for assessing the dynamic slope stability. For site response analysis the CPT provides the quickest and most cost effective means of layer-specific shear wave velocity imaging of the foundation conditions. A number or recent methods and projects are described in this paper to demonstrate the utility of the CPT in earthquake engineering applications

Nonparametric liquefaction triggering and postliquefaction deformations

This study evaluates granular liquefaction triggering case-history data using a nonparametric approach. This approach assumes no functional form in the relationship between liquefied and nonliquefied cases as measured using cone penetration test (CPT) data. From a statistical perspective, this allows for an estimate of the threshold of liquefaction triggering unbiased by prior functional forms, and also provides a platform for testing existing published methods for accuracy and precision. The resulting threshold exhibits some unique trends, which are then interpreted based on postliquefaction deformation behavior. The range of postliquefaction deformations are differentiated into three zones: (1) large deformations associated with metastable conditions; (2) medium deformations associated with cyclic strain failure; and (3) small deformations associated with cyclic stress failure. Deformations are further defined based on the absence or presence of static driving shear stresses. This work presents a single simplified framework that provides quantitative guidance on triggering and qualitative guidance on deformation potential for quick assessment of risks associated with seismic soil liquefaction failure

Site response analysis considering strain compatible site period

In practice it is common to estimate site effects using a single proxy, or single variable such as 30 m shear wave velocity (VS30) or site period. Many studies have investigated merits of proposed proxies with contradicting recommendations. Yet, most studies indicate the single proxy approach is less than ideal, resulting in large uncertainty. To provide a better understanding of components that drive site response, we performed a parameterized study on 19 shallow soil profiles with VS ranging from 150 m/s to 400 m/s. We propagated 74 input motions through each soil column using one-dimensional equivalent-linear method to produce 1406 site response analyses. The resulting amplification factors (the ratio of surface to base motion) were then analyzed statistically to identify trends. The mean amplification factor, averaged from 74 records, was used to isolate and quantify the effects of VS on site response. Based on analysis of record-to-record trends, we identified two separate mechanisms through which nonlinearity affects site response including “damping increase” and “site period shift”. The interaction of these two mechanisms makes amplification-shaking intensity models highly depth-dependent. The residual standard deviation of amplification factor based on depth-independent models was found to be up to three times larger than the corresponding standard deviation based on depth-specific models. We found strain compatible site period a promising site parameter that complements the predictive information obtained from VS. Finally, a simplified procedure providing a five-point estimate of site transfer function is outlined. The proposed procedure can fill the gap in current practice for an intermediate solution between the numerically rigorous solution and the single proxy approach. Implementation of this procedure is demonstrated in an example

Teaching consolidation: Case study of preloading with vertical drains

This paper describes a ground improvement case study where preloading and prefabricated vertical drains (PVDs) were used to accelerate foundation settlements. The case study is used in a classroom setting with the learning objective of introducing engineering students to methods for estimating settlement of shallow foundations on compressible soils. The project site was developed for a corporate retail chain planning to open a new facility in San Luis Obispo, California. Up to 2.5 meters of fill were needed across much of the site to raise foundations and improvements above the flood elevation. Loads from the fill and the structure were expected to cause total and differential settlements that exceeded the allowable values established by the retailer. To mitigate settlement, the geotechnical engineer developed a preloading plan. Although the soil conditions were complex (e.g., interlayering, dipping strata, variable compressibility), the preloading plan was successful in achieving the desired settlement within 3 months, and subsequent site performance has been exemplary. This case study has been used for several years within a quarter-long shallow foundation design course to teach settlement performance. Learning outcomes from the assignment are summarized in the paper. Students are given the subsurface information and test results originally acquired by the geotechnical engineer. The students, working in teams, try to estimate how much primary consolidation settlement will occur due to the fill plus the preload, and the PVD spacing needed to achieve 90% of that settlement in 3 months. The assignment and relevant data are included herein along with the grading policy. The project culminates with the geotechnical engineer of record presenting in class the results of site monitoring during preloading and consolidation. These results include settlements across the 16,908 m2 site, which were tracked up to three times a week at 20 locations. This project affords students a case study experience that is rich in the “messy” details of a complex and local (i.e. familiar) geotechnical project. Included is a discussion of lessons learned by the instructors who have taught several iterations of this case study

Estimating the Probability of Failure and Associated Risk of the California Bay Delta Levee System

Recent events in New Orleans have shown the magnitude of life loss and property damage that can occur due to the failure of man-made levees. The California Bay Delta and Sacramento levee systems in California pose a similar or greater level of risk to life and property. In order to effectively mitigate the hazard associated with levee failure a systematic evaluation of risk must be carried out. This paper presents preliminary research into the risk associated with the California Bay Delta. A comprehensive list of failure modes for man-made levees is presented. Preliminary empirical data on the temporal frequency of failure and the consequences of failure in the Bay Delta has been compiled. Also presented is the frequency of high water conditions, frequency of strong ground shaking, and a discussion of possible correlation of failure with these loading conditions. Based on preliminary empirical data, the distribution of the risk function is estimated using Monte Carlo simulations. The general objectives of this paper are to present an approach to levee risk analysis for the California Bay Delta, stimulate discussion, outline the data gaps that exist, and push for continued research on mitigating this hazard

Understanding of a negative bowel screening result and potential impact on future symptom appraisal and help-seeking behaviour: a focus group study.

This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1111/hex.12484BACKGROUND: Colorectal cancer (CRC) screening using a faecal occult blood test (FOBt) has the potential to reduce cancer-related mortality. Symptom vigilance remains crucial as a proportion of cancers will be diagnosed between screening rounds. A negative FOBt has the potential to influence how participants respond to future symptoms of CRC. OBJECTIVE: To explore (i) understanding of a negative FOBt and (ii) the potential impact of a negative FOBt upon future symptom appraisal and help-seeking behaviour. DESIGN: Qualitative methodology utilizing focus groups with participants who received a negative FOBt within the National Bowel Cancer Screening Programme in Coventry and Lothian. Topics explored included: experience of screening participation, interpretation and understanding of a negative result, symptom awareness and attitudes towards help-seeking. RESULTS: Four broad themes were identified: (i) emotional response to a negative FOBt, (ii) understanding the limitations of FOBt screening, (iii) symptom knowledge and interpretation and (iv) over-reassurance from a negative FOBt. Participants were reassured by a negative FOBt, but there was variability in the extent to which the result was interpreted as an "all clear". Some participants acknowledged the residual risk of cancer and the temporal characteristic of the result, while others were surprised that the result was not a guarantee that they did not have cancer. DISCUSSION AND CONCLUSIONS: Participants recognized that reassurance from a negative FOBt could lead to a short-term delay in help-seeking if symptoms developed. Screening programmes should seek to emphasize the importance of the temporal nature of FOBt results with key messages about symptom recognition and prompt help-seeking behaviour.This study was funded by the National Awareness and Early Diagnosis Initiative led by Cancer Research UK, the Department of Health, NHS England and Public Health England. Award number C12357/A12240

Standard Penetration Test-Based Probabilistic and Deterministic Assessment of Seismic Soil Liquefaction Potential

This paper presents new correlations for assessment of the likelihood of initiation (or “triggering”) of soil liquefaction. These new correlations eliminate several sources of bias intrinsic to previous, similar correlations, and provide greatly reduced overall uncertainty and variance. Key elements in the development of these new correlations are (1) accumulation of a significantly expanded database of field performance case histories; (2) use of improved knowledge and understanding of factors affecting interpretation of standard penetration test data; (3) incorporation of improved understanding of factors affecting site-specific earthquake ground motions (including directivity effects, site-specific response, etc.); (4) use of improved methods for assessment of in situ cyclic shear stress ratio; (5) screening of field data case histories on a quality/uncertainty basis; and (6) use of high-order probabilistic tools (Bayesian updating). The resulting relationships not only provide greatly reduced uncertainty, they also help to resolve a number of corollary issues that have long been difficult and controversial including: (1) magnitude-correlated duration weighting factors, (2) adjustments for fines content, and (3) corrections for overburden stress