upon a pile foundation. Their per- formance during the earthquake was night and day. This example shows how micropiles or other deep foundation techniques can signifi- cantly improve the performance of structures during strong ground shaking. (See Deep Foundations July/Aug 2012, page 77) Seismic Design Considerations Micropiles can be designed to provide tensile, compressive and torsional resistance, overturning and uplift capacity, settlement control and stabilization of slopes. All the benefits of micropiles can help control structural damage during an earthquake. When designing the micropiles to control earthquake damage several basic steps should be followed: 1. Def ine the seismic load: magnitude, epicentral distance, amplification, acceleration, vel- ocity, displacement, duration 2. Define the building charac- teristics: distribution of weight, structural configuration, predominant periods of resonance 3. Define the ground conditions: lique- fiable or sensitive materials, slopes, groundwater, rock, layering 4. Develop a suitable layout and sizing based on geotechnical and structural capacities The design analysis should verify that the micropiles have sufficient tensile, compressive and bending capacity for the particular application. Readily available procedural manuals and numerical methods can be used to refine the design including: 1. FHWA Micropile Design and Construc- tion Reference Manual 2. Simplified equivalent lateral analysis 3. Dynamic nonlinear analysis 4. Deformation/performance analysis using finite element or finite difference solutions 52 • DEEP FOUNDATIONS • JULY/AUG 2013 magnitude (Mw) 5.8 event, occurred on August 23, 2011 at 1:51 p.m. One of the largest felt earthquakes in our lifetime, the earthquake was felt over most of the Eastern U.S. and in Canada. Moderate damage was observed in the epicentral region near Mineral, Va., with light to moderate damage throughout the Washington Metropolitan Region, approximately 132 km (82 mi) northeast of the epicenter. Recon- naissance after the earthquake found damaged chimneys in Mineral and Alexandria, Va. The United States Geological Sur- Close-up of Micropile Installation The most important component to a successful seismic design is communi- cation within the team throughout the project, from start to finish. This is especially true when designing for rare earthquakes where considerable uncer- tainty and consequences of failure are a reality. Any changes to the project before and during construction need to be discussed to make sure the designs are appropriate and implemented correctly. The team should consist of the architect, structural engineer, geotechnical engineer, seismologist/geologist, and general and specialty contractors. The 2011 Virginia Earthquake Large earthquakes in the Central and Eastern U.S. are rare. However, they present a substantial risk where high population densities and an aging, “un- tested” infrastructure exist. The general lack of experience and preparedness in an area of infrequent seismicity magnifies the consequences of strong ground shaking. The 2011 Virginia Earthquake, a moment vey ShakeMap shows the perceived shaking intensity during the earthquake. The map shows that all of Virginia felt the earthquake, with the strongest motions observed in the epicentral region up through the nation’s Capital. The areas in yellow- orange show where the highest levels of shaking were experienced. Unique geologic and seismological factors led to increased shaking intensity towards the northeast in the direction of the nation’s Capital. The majority of the damage was con- centrated to brittle, unreinforced masonry and precast structures including two recently constructed public schools in Louisa County. Hundreds of structures in the Washington Metropolitan Region suffered damage, notably the Washington Cathe- dral, the Washington Monument and the Smithsonian Castle. Fortunately, no casual- ties resulted from the earthquake, however estimates of damage approach $500 million. Implications with Current Codes Current seismic codes, such as the International Building Code, are based on 2500-year ground motions. Analysis of recordings from the 2011 Virginia Earthquake indicates that the event was representative of 500-year ground motions; much lower than current design levels used in the codes. Given the level of damage in this moderate event shows that the destruction would have been more substantial had the earthquake been larger or closer to concentrated population centers around Washington.
