Hi David,
Former NYC foundation engineer here. Usually in New York City, Site Class F is triggered for liquefaction susceptibility. The code has a conservative and overly simplified screening process that is sometimes misapplied leading to false positives. Given what I know about Manhattan subsurface conditions, the declining state of geotechnical practice, and the owners' and developers' preference of cheap to good, my first guess would be that you have a false positive. So the first thing, I would check is whether the screening is being done correctly.
If the screening is correct, you next need to evaluate the susceptibility. It appears that the code allows this to be demonstrated by analysis, although I have been involved with projects where a more extensive site-specific analysis was performed, including measurement of shear wave velocity and analysis of site response. This can increase the site class by a couple of steps and I understand pays for itself, at least for mid- and high-rise buildings.
If that doesn't work, you may be looking at drilled shafts, caisson piles and other substantial foundation elements. For a relatively small building, ground improvement might make more sense. These same foundation types might apply if the Site F is due to organics or soft clay. The specific subsurface conditions matters quite a lot.
I am not sure what your preliminary report says versus the "site specific" report you expect to see later, but NYC code is particular about subsurface investigations. Conditions can change significantly over relatively short distances. While I have used phase explorations in the past, I would caution you that when I have seen projects try to get a way with using subsurface data for nearby sites (forget desk studies) it has not ended well. It also violates the code.
I consult on these issues and I am still licensed in New York. If I can be of assistance, please feel free to get in touch.
Good luck.
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Richard J. Driscoll M.ASCE
Lebanon NH
rjd@...www.richardjdriscoll.com------------------------------
Original Message:
Sent: 04-03-2019 10:33
From: David Young
Subject: Seismic Design with Class F Soil
A project in Manhattan (NYC Bldg. Code 2014 - ASCE 7-10) has a Preliminary Soils Report which identifies the site as being underlaid with Type F soils. I've been asked to design a thirteen story concrete building with shear walls for the "worst case" and not wait for the site specific geotechnical report. What is the correct procedure?
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David Young P.E.,M.ASCE
Whitman MA
(781) 447-6988
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