David G. Zapata-Medina

Method for Estimating System Stiffness for Excavation Support Walls

AbstractExcessive excavation-induced movements are major concerns for most underground construction projects in urban areas. These movements can lead to significant damage in adjacent structures. When average to good workmanship is employed during the installation process of the excavation support systems, the consequent ground movements are most influenced by the support system stiffness. Therefore, choosing the most appropriate stiffness for an excavation support system is crucial to minimizing excavation-related damage to adjacent buildings and utilities. This paper presents a semiempirical design methodology that facilitates the selecting of the excavation support system stiffness in such a way that limits excavation-related ground movement. As part of the proposed design methodology, a new parameter was developed called the relative stiffness ratio. This new parameter relates the strength and stiffness of the soil with the stiffness of the excavation support system and was developed from a comprehens...

Static Stability Formulas of a Weakened Timoshenko Column: Effects of Shear Deformations

The static stability analysis of two-dimensional Timoshenko columns weakened at an arbitrary section is derived in a classic manner. The effects of shear deformations along the column, influenced by the additional shear force induced by the applied axial load as the member deforms according to the modified shear equation proposed by Haringx, are presented and studied in detail. The proposed model also captures: (1) the influence on the buckling load capacity of the column when an arbitrary weakened section is formed at any location; (2) the tension buckling phenomenon due to the low shear stiffness of columns made of composite materials or elastomeric rubbers; and (3) the beneficial effects of an additional lateral bracing located at the weakened section to alleviate the buckling load reduction of the column. Seven classical and nonclassical cases of columns mostly used in civil and mechanical engineering are summarized in condensed formulas which allow the straightforward determination of buckling loads and shapes.

Effect of Gas on the Mechanical Behavior of Medium-Dense Sands

AbstractThis paper presents the results of monotonic and cyclic triaxial tests performed on gassy, medium-dense sand specimens. These sands are representative of conditions in a loose-sand deposit that was densified using multiple blasting passes at a site in South Carolina. The equipment and laboratory testing procedures used to reproduce the postblast densification conditions observed at a field test are described in detail. Results of undrained and drained compression tests showed that the gassy-specimen responses were bounded by the fully saturated drained and undrained responses. The undrained test results showed that the presence of gas, even in small amounts, made the soil more compressible and restricted the buildup of excess pore-water pressures as compared with those pressures observed in fully saturated specimens. Additionally, the shear strengths of gassy, medium-dense sand specimens were similar to those observed in saturated, loose sands sheared under drained conditions. For a given cyclic s...

Effects of Construction-Induced Stresses on Dynamic Soil Parameters of Bootlegger Cove Clays

This paper presents the influence of construction-induced stress changes on the dynamic soil parameters of Bootlegger Cove Formation (BCF) clays at the site of the Port of Anchorage (POA) expansion project. Results are presented based on numerical simulations of the POA wharf construction and laboratory experimentation where both free-field and postconstruction conditions are reproduced. The postconstruction stability of the foundation clay is considered under both static and dynamic loadings. For the dynamic case, a cyclic triaxial loading equivalent to the contingency-level earthquake (CLE) established for design was used, whereas for the static case, undrained triaxial compression and extension loadings to failure were employed. The results from numerical simulations and laboratory experiments are presented and compared with field measurements and performance data collected during the project construction. The results obtained from specially designed cyclic triaxial probes show the large effects that the construction-induced stresses can have on the dynamic soil parameters and illustrate the importance of adequately considering such effects. Testing protocols and procedures are proposed for quantifying postconstruction soil parameters to better reflect field values and values at small (and otherwise appropriate) strain levels.

Defining Y2 Yielding From Cyclic Triaxial Tests

This paper presents the yielding characteristics of overconsolidated Bootlegger Cove Formation (BCF) clays extracted from the Port of Anchorage construction site. Additionally, it extends the critical shear strain concept to define the point at which irrecoverable deformations begin (Y2 yielding) to the case of fully reversed loadings during undrained cyclic triaxial (CyTX) tests. On-specimen linear variable differential transformers (LVDTs) and an internal load cell with high accuracy were employed to define stress–strain responses and yield points of the specimens. This paper also presents Y2 yield points obtained using standard procedures available in the technical literature based on monotonic triaxial testing to show that the size and location of the Y2 yield surface can be determined from undrained CyTX tests.

Direct Approach for Designing an Excavation Support System to Limit Ground Movements

Traditionally, excavation support systems are designed solely on the basis of satisfying limit equilibrium, using apparent earth pressure diagrams. Using this approach, the support system design becomes a function of the maximum anticipated earth pressure and is governed by overall structural stability as opposed to maximum allowable horizontal or vertical deformation. This approach produces a support system that is adequate with regards to preventing structural failure, but may result in excessive wall deformations and ground movements. This paper presents a design methodology that facilitates the sizing of all components of the excavation support system in such a way that limits the maximum lateral and vertical excavation-induced deformations. Based on the fundamental approach of the presented design methodology, structural and basal stability is

Physical Modeling of Supported Excavations

As a result of the complexities associated with analyzing and evaluating supported excavations, numerical modeling techniques are often employed. Often, there is a great deal of uncertainty associated with the validity of the assumptions and approaches made regarding numerical modeling. Physical models can be used to evaluate complex geotechnical systems whose performances are highly dependent on construction techniques, nonlinear soil-structure interactions, and variable geometries. Physical model tests can be used as alternatives to numerical models or as supplements. Data from physical model tests is often used to verify the validity of the various assumptions and approaches of the numerical models. The data can also be used to calibrate numerical models to the anticipated loading conditions. This paper presents an evaluation of several efforts that utilized physical modeling to investigate the behavior of excavation support systems and the associated ground deformations. Model test results are compared to the state-of-the-practice ground deformation prediction methodologies and to field observations. The information presented herein shows that scale model test data can be reliably extrapolated to equivalent prototype data. Thus, small scale laboratory tests at normal gravity, centrifuge tests, and small scale field tests can all used to evaluate excavation support system behavior and soil response associated with deep excavations. These findings are particularly important in that often full-scale field test cannot be performed.

Blast densification: A proposed methodology to quantify the amount of densification required to prevent liquefaction and flow in sandy soils

This paper presents a methodology to quantify the amount of blast densification or number of passes required for a given project to prevent liquefaction and ...