Michael F. Riemer

Mechanisms of Seismically Induced Settlement of Buildings with Shallow Foundations on Liquefiable Soil

Seismically induced settlement of buildings with shallow foundations on liquefiable soils has resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate postliquefaction reconsolidation settlement in the free-field. A series of centrifuge experiments involving buildings situated atop a layered soil deposit have been performed to identify the mechanisms involved in liquefaction-induced building settlement. Previous studies of this problem have identified important factors including shaking intensity, the liquefiable soils relative density and thickness, and the buildings weight and width. Centrifuge test results indicate that building settlement is not proportional to the thickness of the liquefiable layer and that most of this settlement occurs during earthquake strong shaking. Building-induced shear deformations combined with localized volumetric strains during partially drained cyclic loading are the dominant mechanisms. The development of high excess pore pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced ratcheting of the buildings into the softened soil are important effects that should be captured in design procedures that estimate liquefaction-induced building settlement.

Shear Strength of Municipal Solid Waste

A comprehensive large-scale laboratory testing program using direct shear (DS), triaxial (TX), and simple shear tests was performed on municipal solid waste (MSW) retrieved from a landfill in the San Francisco Bay area to develop insights about and a framework for interpretation of the shear strength of MSW. Stability analyses of MSW landfills require characterization of the shear strength of MSW. Although MSW is variable and a difficult material to test, its shear strength can be evaluated rationally to develop reasonable estimates. The effects of waste composition, fibrous particle orientation, confining stress, rate of loading, stress path, stress-strain compatibility, and unit weight on the shear strength of MSW were evaluated in the testing program described herein. The results of this testing program indicate that the DS test is appropriate to evaluate the shear strength of MSW along its weakest orientation (i.e., on a plane parallel to the preferred orientation of the larger fibrous particles within MSW). These laboratory results and the results of more than 100 large-scale laboratory tests from other studies indicate that the DS static shear strength of MSW is best characterized by a cohesion of 15 kPa and a friction angle of 36° at normal stress of 1 atm with the friction angle decreasing by 5° for every log cycle increase in normal stress. Other shearing modes that engage the fibrous materials within MSW (e.g., TX) produce higher friction angles. The dynamic shear strength of MSW can be estimated conservatively to be 20% greater than its static strength. These recommendations are based on tests of MSW with a moisture content below its field capacity; therefore, cyclic degradation due to pore pressure generation has not been considered in its development.

Centrifuge Testing to Evaluate and Mitigate Liquefaction-Induced Building Settlement Mechanisms

The effective application of liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. Centrifuge experiments were performed to study the dominant mechanisms of seismically induced settle- ment of buildings with rigid mat foundations on thin deposits of liquefiable sand. The relative importance of key settlement mechanisms was evaluated by using mitigation techniques to minimize some of their respective contributions. The relative importance of settlement mechanisms was shown to depend on the characteristics of the earthquake motion, liquefiable soil, and building. The initiation, rate, and amount of liquefaction-induced building settlement depended greatly on the rate of ground shaking. Engineering design procedures should incorporate this important feature of earthquake shaking, which may be represented by the time rate of Arias intensity i.e., the shaking intensity rate. In these experiments, installation of an independent, in-ground, perimetrical, stiff structural wall minimized deviatoric soil deformations under the building and reduced total building settlements by approximately 50%. Use of a flexible impermeable barrier that inhibited horizontal water flow without preventing shear deformation also reduced permanent building settlements but less significantly.

Shear modulus and material damping of municipal solid waste based on large-scale cyclic triaxial testing

Representative dynamic properties of municipal solid waste (MSW) are required to perform reliable seismic analyses of MSW landfills. A comprehensive large-scale cyclic triaxial laboratory testing program was performed on MSW retrieved from a landfill in the San Francisco Bay area to evaluate the small-strain shear modulus, and strain-dependent normalized shear modulus reduction and material damping ratio relationships of MSW. The effects of waste composi- tion, confining stress, unit weight, time under confinement, and loading frequency on these dynamic properties were evaluated. The small-strain shear modulus depends primarily on waste composition, confining stress, unit weight, and time under confinement. The normalized shear modulus reduction and material damping curves for MSW depend on waste composition and confining stress. Based on the results of this study and a review of literature, strain-dependent shear modulus reduction and material damping relationships are recommended for use in landfill design.

Physical Characterization of Municipal Solid Waste for Geotechnical Purposes

A procedure to characterize municipal solid waste (MSW) for geotechnical engineering purposes is developed based on experience with waste characterization and testing. Existing MSW classification systems are reviewed briefly, and the field and laboratory waste characterization programs of two important projects are presented. Findings on the influence of the wastes physical composition on its mechanical response from these projects and recent studies of MSW are integrated to develop a waste characterization procedure for efficient collection of the relevant information on landfill operation and waste physical characteristics that are most likely to affect the geotechnical properties of MSW. A phased approach to implementation of this procedure is proposed as a best practice for the physical characterization of MSW for geotechnical purposes. The scope of the phased procedure can be adjusted to optimize the effort required to collect relevant information on a project-specific basis. The procedure includes a systematic evaluation of the moisture and organic content of MSW, because they are important factors in the geotechnical characterization of MSW.

Drained response of municipal solid waste in large-scale triaxial shear testing

A comprehensive laboratory investigation was performed on municipal solid waste (MSW) from a landfill located in northern California using a large-scale triaxial (TX) apparatus. An improved, standardized waste specimen preparation method was developed and used to prepare 27 large-scale TX specimens (d=300 mm, h=600-630 mm). The effects of waste composition, confining stress, unit weight, loading rate, and stress path on the drained stress-strain response of MSW were investigated. Waste composition has a significant effect on its stress-strain response. The commonly observed upward curvature of the stress-strain response of specimens composed of larger-sized waste materials results from the fibrous constituents (primarily paper, plastic and wood) reinforcing the waste matrix. This effect is greatest when the MSW specimen is sheared across the long axis of the fibrous particles. Due to this significant strain hardening effect and wastes in situ stress state, a limiting strain failure criterion of 5% axial strain from the K(o) field consolidation state is judged to be most appropriate. Results from this test program and data from the literature indicate that the TX compression secant friction angle of MSW varies from 34° to 44°, with 39° as a best estimate, at a confining stress of one atmosphere (assuming c=0). The friction angle decreases as confining stress increases. The friction angles measured in this testing program are representative of failure surfaces that are oriented at an angle to the predominant orientation of the long axis of the fibrous waste particles. These friction angles are higher than those obtained in direct shear tests where shearing typically occurs parallel to the orientation of the fibrous waste particles.

Dynamic Properties of Geosynthetic Interfaces

An experimental study of geosynthetics was carried out on a shaking table to investigate the relationship between dynamic friction resistances and shear displacement rate and to examine other frictional characteristics of geosynthetic interfaces. A cyclic, displacement rate-controlled experimental setup was developed. The subsequent multiple rate tests showed that interfaces that involve geotextiles have unique shearing characteristics that can be differentiated from the interfaces not involving geotextiles. It was found that shear strengths of geosynthetic interfaces tend to increase with increasing displacement rate for combinations of geosynthetics that involve geotextiles; whereas, shear strengths of interfaces not involving geotextiles do not.

Recent findings on the static and dynamic properties of municipal solid waste

The design of Municipal Solid Waste (MSW) landfills requires the selection of representative MSW material properties. The profession’s understanding of the mechanical response of MSW has evolved significantly in recent years, but many aspects of the response of MSW remain uncertain. A research project involving five organizations was conducted to systematically evaluate the factors that affect the static and dynamic properties of MSW. A summary of major findings from the work performed to date by this team of investigators is briefly described in this paper. INTRODUCTION The selection of representative MSW properties is important for the reliable performance of landfill analyses such as the static and seismic slope stability, the seismic response of landfills, the design of the containment system, and the design of facilities for post-closure development. The profession’s understanding of the response of MSW has evolved significantly since the early work performed by Sowers (1973) and Landva and Clark (1986). However, despite many advances in our understanding of MSW properties, many of the factors affecting the response of MSW remain largely uncertain. In 2002, the US National Science Foundation funded a collaborative research project that involved the University of California at Berkeley (UCB), the University of Texas at Austin (UT), and Geosyntec Consultants to evaluate systematically factors that affect the static and dynamic properties of MSW by means of in situ investigations and an extensive large-scale laboratory testing program. Subsequently, Arizona State University (ASU) and the University of Patras, Greece GEOCONGRESS 2008: GEOTECHNICS OF WASTE MANAGEMENT AND REMEDIATION 177 (UP), joined the project team. The fieldwork and laboratory testing performed by this team of investigators are briefly described in this paper. Relevant findings are presented that provide insight regarding the mechanical response of MSW. FIELD INVESTIGATION AND WASTE CHARACTERIZATION Waste sampling was coordinated by Geosyntec Consultants. Two large-diameter (760 mm) borings were augered to depths of 10 m and 32 m using a bucket auger at the Tri-Cities landfill, located in the San Francisco Bay Area. Shallow and deep bulk samples of waste were retrieved and stored separately in 39 sealed 55-gallon drums. Bulk samples of similar visual composition from the same depth interval within a borehole were designated as sample groups. Sample groups included relatively young waste (waste placed within the past 2 years) and relatively old waste (waste placed for approximately 15 years) as well as waste of varying composition. In situ unit weight tests were performed using a gravel replacement procedure developed by Geosyntec Consultants (Matasovic and Kavazanjian, 1998) and described in Zekkos et al. (2006a). The MSW unit weight was found to range from 10 kN/m near the surface to 16 kN/m at depth. Shear wave velocity soundings were performed by UT at the boring locations using the Spectral Analyses of Surface Waves (SASW) method. The shear wave velocity was found to vary from 75 to 210 m/sec at the surface, reaching 250 m/sec at a depth of 25 m (Lin et al., 2004). Waste material was characterized using a procedure that was developed to efficiently collect relevant information about the waste material. The procedure is described in Zekkos (2005) and included segregating the waste into material larger and smaller than 20 mm (0.75 in). This segregation is useful because: a) material <20 mm is composed predominantly of equidimensional particles, including soil from daily cover, organic materials, and some fine waste inclusions, whereas material >20 mm consists mostly of fibrous constituents; b) material <20 mm can be characterized using conventional soil mechanics tests and can be tested using conventional size geotechnical testing equipment. Evaluation of the influence of the fibrous >20 mm material on MSW was an important part of this investigation. About 50-75% by weight of each waste sample was <20 mm material. This material contains a significant amount of soil (and soil-like) particles, but also contains organic material so that it is lighter, and softer than many inorganic soils. The remaining coarser material consisted primarily of paper, plastic, wood, and gravel. Constituents such as metals, glass, stiff plastics, and textiles, comprised a significantly lower percentage of the material by weight and by volume. Laboratory tests were performed on the three sample groups summarized in Table 1. Group A3 is older material sampled from a relatively large depth. Group C6 is younger material sampled from a relatively shallow depth. Group C3 was selected for testing as the most different sample group from the previously tested sample groups A3 and C6. An extensive large-scale and small-scale laboratory testing program on material from these groups was performed at the various institutions through a coordinated testing program. A brief summary of the laboratory testing performed on the waste and some of the primary findings are presented in this paper. 178 GEOCONGRESS 2008: GEOTECHNICS OF WASTE MANAGEMENT AND REMEDIATION Table 1. MSW sample groups tested Α3 C6 C3 Borehole BH-2 BH-1 BH-2 Depth, m 25.6-26.2 7.6-9.6 3.5-4.5 % moisture content 12 13 23 % organic 15-30 10-16 20-36 Age (years) 15 <1 2 As measured on the smaller than 20 mm material at 55 C. LABORATORY TESTING AT THE UNIVERSITY OF CALIFORNIA AT BERKELEY AND THE UNIVERSITY OF PATRAS Large-scale cyclic triaxial (CTX) equipment (d=300 mm, h=600-630 mm upon specimen preparation) was used at UCB to perform cyclic triaxial, triaxial compression, extension and lateral-extension tests. Additional conventional-scale cyclic triaxial tests were performed on waste that was processed to make all particles less than 20 mm in size. A direct shear (DS) box, 300 mm by 300 mm by 180 mm, was used at UP to perform shear testing. 1-D compression tests were performed prior to shearing. More information about the devices is presented in Zekkos (2005). The laboratory testing program evaluated the effects of unit weight, compaction effort, confining stress, waste composition, and loading rate on the monotonic stressstrain response and shear strength, and the dynamic properties, i.e. the small-strain shear modulus, strain-dependent shear modulus and material damping, of MSW. Poisson’s ratio was also measured during some of the triaxial tests. A total of 23 large-scale direct shear and 27 large-scale monotonic triaxial tests were performed. As shown in Figure 1, direct shear test specimens prepared with the same compaction effort and with 100%, 62% and 12% <20 mm material, were found to have a conventional concave (e.g. hyperbolic) stress-displacement response and similar shear resistances. The similarity in shear resistance is attributed to the fact that shearing occurred parallel to the sub-horizontal orientation of fibrous materials within the waste, and thus, the contribution of the fibrous materials is minimal. The tendency of the fibrous materials to be oriented horizontal was confirmed by sample inspection after testing. This tendency was also observed in triaxial testing of this investigation, and has also been observed in the field by Matasovic and Kavazanjian (1998). Subsequent direct shear tests performed with waste fibers oriented perpendicular to the shear failure surface yielded a convex, upward curvature in the stress-displacement response, which is attributed to the progressive mobilization of the fibrous materials within the waste matrix. Figure 2 illustrates this anisotropic aspect of MSW behavior, comparing two specimens with the same composition, prepared with the same compaction effort, and tested at the same normal stress. The only difference between the two specimens is the orientation of the >20 mm, fibrous materials. The stressdisplacement response is different. Details are provided in Zekkos et al. (2007a) Consistently with the direct shear tests, the stress-strain response in triaxial compression was found to be strongly dependent on fibrous waste content. An upward curvature of the stress-strain curve was observed for all the triaxial compression tests, with the exception being specimens with 100% <20 mm material. This observed behavior may also be attributed to the progressive mobilization of fibrous materials during shearing, as shearing in triaxial compression occurs at an angle from the horizontal orientation of the fibers. This explanation is supported by the observation GEOCONGRESS 2008: GEOTECHNICS OF WASTE MANAGEMENT AND REMEDIATION 179 that the shear resistance in triaxial compression was higher than that observed in direct shear. The secant friction angle was found to decrease with confining stress both in direct shear and triaxial testing, indicating that a nonlinear strength envelope may be most appropriate for MSW. Shear resistance was found to increase with unit weight, compaction effort, and strain rate (for axial strain rates between 0.5%/min and 50%/min). Test results are presented in Zekkos et al. (2007a, 2007b) in greater detail. 0 200 400 600 80

Investigation of the performance of the New Orleans regional flood protection systems during Hurricane Katrina: Lessons learned

The recent flooding and devastation of the greater New Orleans region during hurricane Katrina represented the most costly peace-time failure of an engineered system in North American history. Extensive investigations and analyses have been performed by several major teams in the wake of this disaster, and some very important lessons have been learned. Many of these have very direct and urgent applications to levee systems in other regions throughout the U.S., and the world. Lessons include the importance of proper evaluation of risk and hazard; so that appropriate decisions can be made regarding the levels of expense and effort that should be directed towards prevention of catastrophe, and the levels of post-disaster response capability that should be maintained as well. The making of appropriate decisions, given this information regarding risk levels, is then also important. Also of vital importance are numerous “engineering” lessons regarding analysis, design, construction and maintenance; hard-won lessons with applications to flood protection systems everywhere. We must now do everything possible to capitalize upon these; and to prevent a recurrence of this type of catastrophe in the future. 1 Professor, Dept. of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, California 94720. Email: seed@ce.berkeley.edu GSP 161 Embankments, Dams, and Slopes Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org

Effect of High Confining Stresses on Static and Cyclic Strengths of Mine Tailing Materials

This paper presents the results of static and cyclic undrained triaxial testing on underflow (coarse fraction) tailing materials for a very high proposed tailing storage facility in seismically active southern Peru. The results of these tests, ranging in initial effective confining pressures from about 0.1 to 5.8 MPa, showed contractive but generally ductile behavior at higher initial effective confining pressures likely caused by grain breakage. These test results formed an important part of the design of the tailing storage facility. As part of expanded mining operations, a large tailing storage facility (TSF) has been designed and was placed into operation in November 2006 at the Cerro Verde Mine in southern Peru. The mine is located in a region of high seismicity dominated by earthquakes occurring along the Peru-Chile Subduction Zone, which has a history of producing very large earthquakes that could cause significant shaking at the TSF site. The maximum credible earthquake (MCE) for design is a moment magnitude 9.0 (Mw) megathrust event estimated to be capable of producing a peak horizontal acceleration at the top of bedrock of 0.47g. The TSF includes a starter dam that will be raised over a 22-year period to its ultimate height by the “centerline construction method”. This method involves separating the whole tailing materials produced by the milling process into a coarse fraction (tailing underflow) and a fine fraction (tailing overflow) by a cycloning process. The underflow materials are then placed in relatively thin lifts and compacted to high density to construct an embankment that provides drainage and strength characteristics to enhance static and dynamic stability. The overflow tailing materials are hydraulically deposited upstream of the compacted underflow embankment. The design envisioned a maximum fines content of 15% and a compaction specification of 98% of Standard Proctor Density (ASTM D698) for the underflow material used to construct the embankment. The large anticipated stresses toward the base of the very tall ultimate embankment and the high seismicity environment underscore the need to understand the likely undrained response of these materials to both static and cyclic loading conditions. This paper describes static and cyclic undrained triaxial tests performed during design of the TSF on reconstituted specimens of the tailing underflow materials, presents results of the study, and summarizes outcomes from testing that were important to the design.