Martin B. Hudson

Monitoring of a Los Angeles Metro Red Line Subway Deflection Due to Adjacent Deep Excavation

The Wilshire Grand Redevelopment project in downtown Los Angeles includes the demolition of a 16-story hotel built in the early 1950s and the construction of a new 73-story tower that will be the tallest building in the western United States. The construction of the basement and foundation of the new tower required excavation up to 93 feet (28 m) deep. One side of the basement excavation, with an excavation height along that side of up to 57 feet (17 m) required placement of shoring within about 6 to 10 feet (1.8 to 3.0 m) horizontally from a 400-foot-long (122 m) section of the Los Angeles Metro Red Line subway tunnel. A monitoring and contingency plan was established for the shoring itself and the subway tunnel. The shoring monitoring included periodic surveying, slope inclinometers, load cells on tie-back anchors, and strain gauges on raker braces. Deflection and vibration monitoring was performed on the interior of the Red Line tunnel utilizing automated total survey machines installed in the tunnel; the monitoring system provided real time data and automated alerts were provided when threshold readings were exceeded. Presented herein is a description of: (1) deflection and earth loading criteria for monitoring; (2) response levels utilized during monitoring; (3) data transmission methodologies; (4) communication protocols; (5) installation of instruments; (6) evaluation of monitoring data; and (7) modifications to monitoring protocols during project.

Design of a Deep Tied-Back Excavation Adjacent to the Los Angeles Metro Red Line Subway

The Wilshire Grand Redevelopment project in downtown Los Angeles includes the demolition of a 16-story hotel built in the early 1950s and the construction of a new 73-story tower that will be the tallest building in the western United States. The construction of the basement and foundation of the new tower required excavation up to 93 feet (28 m) deep. One side of the basement excavation, with an excavation height along that side of up to 57 feet (17 m) required placement of shoring within about 6 feet (1.8 m) to 10 feet (3.0 m) horizontally from a 400-foot-long (122 m) section of the Los Angeles Metro Red Line subway tunnel. The temporary shoring support system for the Wilshire Grand deep excavation consists of soldier piles spaced generally at 8 feet on center with multiple levels of tieback anchors which extended above the subway tunnels. The bottom-most level of bracing consisted of rakers supporting walers attached to the soldier beams, because a bottom-most level of tie-back anchors could not be installed due to the presence of the subway tunnels. The shoring monitoring included periodic surveying, slope inclinometers, load cells on tie-back anchors, and strain gauges on raker braces (raking struts). Deflection monitoring was also performed on the interior of the Red Line tunnel. The paper presents a description of: (1) deflection and earth loading criteria for design; (2) geometric constraints of shoring design; (3) limitations on tie-back shoring due to limited easement width; (4) comparison of deflection and load from design criteria using results obtained from geotechnical instrumentation; (5) raker preloading methodology; (6) temperature dependence on raker loading; and (7) deflection distribution as a function of distance from shoring wall. Lessons learned regarding preloading of rakers and deflection of shoring and retained earth will be described.

The Right Mix

An innovative combination of deep soil mixing and spread footings will support a community hospital in California, preventing liquefaction of the deep and varied soils in the face of a strong earthquake.

Sustainable Foundation Support of Community Memorial Hospital against Liquefaction Hazards

Community Memorial Hospital of Ventura, California, is constructing a six-story replacement hospital building on a site with significant liquefaction potential induced by ground motions in the event of a design earthquake with peak ground acceleration of 0.6g. The historic high groundwater level at the site was relatively shallow and geotechnical information indicated that the fine-grain soils below the groundwater level at a depth of 5.2 m to as deep as 24 m could liquefy and result in settlement up to 27 cm. Various deep foundation alternatives were considered and several ground improvement methods. In consideration of the effectiveness of controlling the development of excess pore pressures and limiting the ground settlements, deep-soil mixing was selected as the preferred method of ground improvement. The use of deep-soil mixing of the potentially liquefiable soils allowed the hospital building to be supported on conventional shallow spread footings with acceptable total and differential settlements after the maximum considered earthquake. An Alternate Method of Compliance was required by state building officials to provide justification for use of deep-soil mixing, construction procedures, and measures for quality control and quality assurance. The Alternate Method of Compliance was approved by the regulatory agency, and deep-soil mixing of the site has been completed and the hospital structure is currently being constructed.

Seismic Resilience Design for a Concrete Box Reservoir

The proposed 110-million-gallon reinforced concrete Headworks Reservoir structures are planned by the Los Angeles Department of Water and Power (LADWP) to be part of the water supply system of the City of Los Angeles, and resilience of the water supply system is crucial for continued water supply in the event of a disaster, such as an earthquake. The seismic resilience of the reservoir structure is a function of the cracking and associated leakage that would be expected due to static loading and during the design level earthquake. Evaluation of the seismic deformation of the structure was accomplished utilizing a soil-structure interaction (SSI) model to evaluate performance of the reservoir structure in an earthquake, after the initial design based on standard code-based design procedures. SSI was used to provide information on the structural behavior of the reservoir, and to understand relative movement of inlet and outlet pipelines. In addition, a leak detection system was incorporated into the design. PROJECT DESCRIPTION AND GEOLOGY The proposed Silver Lake Complex Replacement Project consists of the Headworks Reservoir Complex (East and West Reservoirs), a Hydropower Plant to be constructed on approximately 12 acres within the Headworks Spreading Grounds, and a series of new water conveyance pipelines to and around the existing Silver Lake Reservoir (not at the location of the proposed reservoir site). The current proposed location of the reservoir complex including major project components planned are

A Comparison of SASW Survey Results with In-Situ Field Investigation Methods

A geotechnical investigation of a site for two large buried water storage reservoirs was recently conducted. The geotechnical investigation used multiple insitu field methods to characterize the recent alluvial and fluvial deposits overlying bedrock that underlie the reservoir site. The recent deposits had a gravel, cobble and boulder content which made characterization of these deposits by the usual in-situ methods, such as the standard penetration test (SPT), more difficult. Because of the coarser grained materials, Becker Penetration Tests (BPTs) were performed to provide continuous penetration resistance at several locations throughout the large site. To provide additional information, Spectral Analysis of Surface Waves (SASW) testing was performed to characterize the thicknesses and shear wave velocity (Vs) profiles of the alluvial and fluvial deposits and the underlying bedrock. The SASW results were compared with the results of shear wave velocity profiling using the suspension logging technique performed in several rotary wash borings at the site. The SASW shear wave velocity profiles, in combination with the other investigation methods, were useful in determining the nature of the subsurface materials and refinement of the soil and bedrock profiles for liquefaction and soil-structure interaction analyses required in the design of the reservoir structures.

Development of new Los Angeles seismic analysis criteria for tall buildings—site-specific considerations

Many tall buildings are planned and constructed in the city of Los Angeles. Significant building code changes as a result of recent earthquakes will complicate the design and plan checking for new buildings, not only in Los Angeles, but throughout all seismic regions in the United States and around the world. This paper reports on new efforts to develop seismic analysis criteria for Los Angeles. The initial efforts have been in the areas of the design basis ground motions, near-source effects on ground motions and building design, and the first yield limit state. A study of the design basis ground motions in the Los Angeles area indicates that the present code-defined level of ground motion may underestimate the seismic forces in design. Near-source effects would affect practically all of the heavily populated and built-up portions of Los Angeles due to the many known and suspected active seismic sources in the region. Studies of the group motions suggest that a return period of about 40 years may be reasonable to estimate the earthquake demand for the first yield limit state.