Diaphragm Walls as Permanent Basement Walls in Regions of High Seismicity

Reinforced concrete structural slurry walls have been used in the United States since the early 1960s. The typical practice, and one that makes the economics of slurry walls particularly attractive, is to design the walls to act as both temporary excavation support and permanent basement walls. They often serve as multi-story basements and below grade parking for buildings, for tunnels, subway stations, and other buried structures. One of the early applications was for a foundation for a subway station in San Francisco, but for the most part they have been used more extensively in regions of low seismicity. The purpose of this report is to investigate the requirements for extension of this practice to more common use in regions of high seismicity. Structural slurry walls are concrete walls constructed below the ground surface. In slurry wall construction, a trench is excavated using a rectangular clamshell bucket or other specialized equipment. During excavation, the trench is held open by introduction of a bentonite or polymer slurry. Steel reinforcement, if required, is lowered into the slurry-filled trench, and concrete is subsequently deposited by tremie, displacing the slurry. The length of trench open at any one time is limited to a typical maximum of about 20 to 24 feet by excavation stability and concrete placement volume considerations. Each individual concrete placement is referred to as a “panel,” and vertical construction joints separate the panels. Temporary “end-stops” are used as formwork to control the geometry of the panel joints, and horizontal reinforcement is discontinuous at the joints. Structural slurry panels range from 1.5 to 5.0 feet thick, 7 to 24 feet long, and up to 300 feet deep. In the United States, panels that are 2.0 to 3.5 feet thick and depths of 40 to 150 feet are commonplace. Structural basement walls support earth pressures acting laterally against the wall, dead and live loads acting vertically, and in-plane shear and flexure from wind and earthquake loads. The design of permanent slurry walls in regions of low or moderate seismicity is often limited to providing the strength necessary to resist out-of-plane soil pressures and vertical dead and live loads from the superstructure and basement framing. Although these walls also transfer in-plane lateral forces from the superstructure into the soils, the walls are often not specifically designed for these in-plane forces because their inherent strength is usually much greater than the forces being transferred. If resistance to in-plane forces acting on a wall required an increase in vertical reinforcement at the ends of a wall segment, an increase in the cap beam strength, or an increase in the horizontal reinforcement for shear strength, the overall design and construction approach would not vary significantly from current practice. Structural slurry walls have been used to a limited extent for buildings designed for high seismic risk, but there is reluctance on the part of design engineers to use them more often because of concern for how to design these walls to resist in-plane lateral forces, lack of code provisions for reinforcement detailing, and damage that may occur at panel joints. For buildings designed for high seismic risk, such as those assigned to Seismic Design Categories (SDC) D, E, and F as defined in Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10), in-plane shear and flexural actions may likely require modifications of a structural slurry wall only designed for out-of-plane soil pressures and vertical live and dead loads. Design would need to address in-plane lateral forces acting on structural slurry walls and the interaction of the in-plane actions with the out-of-plane and vertical actions. These issues are discussed in this report, and approaches to design for high seismic risk are presented.