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By Timothy B. Carlin, P.E., Earth Engineering Inc. Spend A Little More, Potentially Save A Lot


ulti-story buildings sustain negative cost impacts due to struc-

the costs are per square foot are if the building is designed for a higher seismic rating. What you will find out is that it represents a significant cost to these types of projects. According to the Interna- tional Building Code (IBC) there are six classifications that describe seismic characteristics of the subsurface of a site. The classification ranges from Site class A to F. Site class A is described as “hard rock” and thick layers of soft or loose soil are likely to fall in Site Class F. The six classifications are delineated by how seismic

shear waves travel through the subsurface layers. During the course of a typical geotechnical subsurface inves- tigation test borings are drilled across a site to characterize the soils and rock. An estimation of the density of the soils is in- ferred from the test boring data using the standard penetration test (SPT) rates as measured in hammer blows per foot. The SPT rate, or N-value, can also be used to estimate the seismic Site Class through a procedure described in the IBC. Similarly, other laboratory tests to mea- sure the strength of the soil

can be used to estimate the Site Class. It may seem convenient and cost-effective to use coinci- dental data from the geotech- nical investigation to estimate the seismic Site Class. It is also considered to be an approach which can result in a conserva- tive Site Class estimate - with detrimental cost implications. However, seismic shear wave velocities can be measured, not estimated, using geophysical means for the cost of one day (or even a half day) of testing depending on the size of the site. These methods vary in price and reliability. With the

reliability increasing with the price of the testing. Measuring seismic shear wave velocities using cross-hole seismic techniques between many wells is considered the gold standard, but is also the most expensive. The cross-hole method involves sending equip- ment down one well to transmit waves that are recorded by microphones (geophones) that are sent down separate wells. A second, more cost effective method involves installing a geophone and recorder down a single well while waves are transmitted from the surface. Lastly, the least expensive geophysical method for mea- suring shear wave velocities is completed entirely from the surface. Shear waves are can be actively induced from the sur- face or passively from ambient sources and then recorded by a series of geophones placed on the ground. One day of surface shear wave testing with data crunch- ing and reporting typically costs around $5,000 - $7,000. Depending on the field condi- tions, approximately 5 to 10 acres can be covered in a day. Your geotechnical consultant may employ a geophysicist, or they have a geophysical subconsultant they work with. Although there is no guaran- tee of upgrading the seismic Site Class through shear wave testing, the data is much more reliable, and the cost may only equal a fraction of a percent of the savings to the construction budget. The potential savings may even be more than your geotechnical consultant’s en- tire fee. Earth Engineering Incorpo- rated has successfully recom- mended seismic shear wave analysis to supplement test boring data on numerous proj- ects with the result of upgrad- ing the seismic Site Class. This concept of adding value through intelligent, more comprehen- sive, and technically advanced geotechnical investigation has allowed EEI to add value to several of our larger structure developments. Sometimes it makes a lot of sense to spend a little to potentially save a lot. Timothy B. Carlin, P.E. is a professional engineer with the corporate office of Earth Engineering In- corporated. Mr. Carlin has over ten years of experience managing geotechnical and construction projects. n

tural design c a u s e d b y seismic loads. Although we don’t think about our re- gion as being at a high risk of an earth- quake, build- ings need to

Timothy Carlin

be designed according to the code which protects the struc- ture in just such an event. Ask your structural engineer what


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