NYCEM 2nd-Year Technical Report: December 31, 2000.
Earth Science Tasks: October 1, 1999 - September 30, 2000
NEHRP Site Classes for Census Tracts in Manhattan,
New York City
by
Klaus H. Jacob, Noah Edelblum and Jonathan Arnold
Lamont-Doherty Earth Observatory (LDEO) of Columbia University
P.O. Box 1000 (61 Route 9W, Seismology Building Rm 225)
Palisades NY 10964-8000
Phone: 845-365-8440
Fax: 845-365-8150
Email: jacob@ldeo.columbia.edu
Table of Contents
List of Figures
List of Tables
Summary
Technical Report
1. Introduction
2. Results
3. Borings and Other Data
4. Interpreting Boring Logs
5. Site Classes Using Depth to Bedrock Information
6. Data
Processing
7. Discussion
8. Conclusions
References
Acknowledgements
(Figures will open in a new window)
Fig. 1: Map
of NEHRP Site Classes in Manhattan
Fig. 2: Density
of Borehole Locations
Fig. 3a: Site Classes
for Uptown Manhattan
Fig. 3b: Site Classes
for Midtown Manhattan
Fig. 3c: Site Classes
for Downtown Manhattan
Fig. 3d: Geologic
Cross Sections
Fig. 4a: Decision
Tree Flow Chart, Part 1
Fig. 4b: Decision
Tree Flow Chart, Part 2
Fig. 5: Generic
V100 as a Function of Depth to Bedrock
Fig. 6: Sample
Boring, Data Fields
Fig. 7: Access
Data Base Sample Page
Table 1: Shear wave velocities assigned to different materials
other than soils
Table 2: Definition of site classes A through E
The purpose of work during the second year of this planned 3-year
project is to produce a census-tract-based map of the NEHRP geotechnical site
classes for the entire Island of Manhattan. Geotechnical site conditions affect
the shaking levels, and hence the amount of damage for a given earthquake, with
softer soils generally producing higher shaking levels than stiffer site
classes. The obtained map for Manhattan replaces the default site conditions
which assume a relatively soft site class (Class D) for all census tracts, with
what is considered a representative site condition for each census tract based
on geological information which is largely based on construction-related
borings. The new map is to be used by the NYCEM Consortium to test how results
from the HAZUS algorithm for computing earthquake losses depend on the quality
of locally derived geological and other input data for New York City.
The
results indicate that most of the higher elevations in Manhattan have stiff site
conditions. They belong to either the hard rock class A, or the stiff rock/soil
sites of Class B that dominates the map at high elevations in Uptown Manhattan.
Class B generally tends to indicate a veneer of at least 5 feet of firm soils
over rock. In Midtown Manhattan the intermediate site class C is dominant. In
the Lowlands of Manhattan, most but not all of them along fault zones in Upper
Manhattan and in Downtown (Lower) Manhattan and at the outer fringes of the
Island where man-made fills are very common, soft soil conditions of NEHRP class
D are dominant. Only 2 census tracts, located on the Upper Eastside of
Manhattan, have been assigned to the softest site class E. This scarcity of E
sites exists despite the fact that several individual borings in other census
tracts indicated E class conditions. But these conditions were not sufficiently
pervasive throughout the census tract to bring the entire tract into class E.
This fact points to the nagging problem that census-tract based site class
categorization is inherently problematic since the demographic constructs of
census tracts do not neatly coincide with geological boundaries. We expect some
additional improvements for the Manhattan site class map during the 3rd-year
effort using recently uncovered data sources, although during that final project
period the focus will be on the Metropolitan region at large rather than
Manhattan.
Technical Report: Site Categories for Manhattan Island, New York
City
This project attempts to provide census-tract-based
geotechnical site classes for usage in the computer program HAZUS (NIBS, 1997)
as part of the NYCEM mission to improve earthquake loss estimations for New York
City. The task is to replace for each census tract the HAZUS default-value site
class D with site classes A - E based on actual geologic site conditions. The
work performed in the 2nd year of this intended 3-year project uses the basic
techniques developed during the 1st year (Jacob, 1999) in accordance with the
NEHRP Seismic Provisions (FEMA, 1998a and b). The 1st - Year Report and work was
limited to a test area in Lower Manhattan, below 59th Street. The 2nd- Year
Report extends this technique to the entire Island of Manhattan. In addition
various improvements were included. They are:
- The number of boreholes was
increased in lower Manhattan by filling in data gaps largely using borings
obtained from the MTA / NY Transit Authority geotechnical boring files.
-
Interpretation and implementation of the so-called 10-ft rule of the NEHRP
Seismic Provisions which assigns the site category E to any soil profile that
contains a continuous 10-ft thick layer of very soft soils. This rule had not
been implemented during the 1st - Year pilot project.
- The shear wave velocity
of basement rock was raised to 8,000 ft/sec to reflect more realistically the
prevailing rock types in Manhattan.
- Additional geologic information,
including depth to bedrock and mapped geologic features, was more extensively
used to fill in data gaps in census tracts with few or no geotechnical borings
available.
2. Results: Map of Site Classes for Manhattan Census
Tracts
The following
map (Figure 1) is the main result of this 2nd-Year project and is based on data
taken from available boreholes, depth to bedrock, and existing geological
surveys in Manhattan. Boring logs and depth to bedrock were obtained from MTA
and NY City -DDC records (see Jacob, 1999). Geological surveys by Baskerville
(1990, 1992, and 1994) and Fluhr and Murphy (1944) helped to establish
underlying bedrock contours, types of rock formations, and surface topography at the grade level.
3. Boring and Other Data Locations and Density
Figure 2 shows the locations of all borehole and depth to bedrock records.
Data density was greatest in lower/midtown Manhattan areas and was sparse for
areas north of 137th Street. This reflected availability of data by the MTA
(Metropolitan Transportation Authority / NY Transit Authority), DDC (NY City
Department of Design and Construction, see Jacob 1999) and DBR (Depth to Bedrock
Records, Fluhr and Murphy, 1944). MTA records in particular were restricted by
the amount of construction or renovation projects for large bus terminal and
subway sites. The numbers in brackets [ ] used in the text boxes in Figure 2
represent: [1] Baskerville (1990), Sheet 3/3; [2] Baskerville, (1994), Sheet
2/2; [3] Fluhr and Murphy, (1944); [4] DDC Boreholes; [5] Depth to Bedrock; [6]
MTA Boreholes.
Figures 3a-d provide more detailed views for the site-class maps and give
explanations for some census tracts regarding the data sources that were used in
determining the dominant site class. For census tracts that did not have one or
more representative boring records, site class determinations relied on the
evidence of geological surveys (Baskerville, 1990, 1992, 1994) and bedrock
elevation (Fluhr and Murphy, 1944) which together with topography yields soil
depth. Figure 3a demonstrates the details of this technique in census tracts
above 137th Street. Commentary boxes explain for some of the census tracts how
the assignment to the various site categories was made.
In Northern Manhattan there is a preponderance of Manhattan Schist and Inwood
Marble bedrock formations. During Pleistocene time, from about 40,000 to 10,000
years ago, much of Manhattan was covered by ice. The glaciers cut away the
softer Inwood marble, leaving elevated outcrops and rocky sites made primarily
of Manhattan Schist. These sites are generally indicated by the firmest site
class designations A and B (the latter when covered by a veneer of soils) in the
schist formations extending to the Upper North West of Manhattan. They are
contrasted by softer D-class sites in the Inwood Lowlands (see Figure 3a). This
latter area has considerable amounts of fill and sediment; for this reason it
was designated to the soft site class D.
The 125th Street Fault Zone runs in a southeasterly strike direction from
125th Street near the Hudson River on the Westside of Manhattan, through the NE
corner of Central Park near 110th Street and 5th Avenue, to East
96th Street
near the East River. It is a glacially excavated depression covered with silts,
clays, swamp deposits and fills (Figure
3b). Near the southeastern end of this
fault zone, just east of the NE corner of Central Park, we find the only two
census tracts assigned to the softest site class E. Figure
3c shows the
preponderance of soft soils of site class D in the Lowlands of Lower Manhattan. Figure
3d reproduces schematic geologic cross sections at various locations in
Manhattan (Baskerville, 1994). They give the correct impression that the
foundation of Manhattan is largely very firm rock. But these structural sections
do not serve full justice to the fact that there are many surficial pockets of
soft soils in Manhattan that can amplify the shaking during earthquakes of the
buildings erected in or on these soils overlying very firm rock.
4. Interpreting MTA and DDC Geotechnical Boring
Logs
Figure 4a and 4b
show flow diagrams explaining the methodology for
determining the site classes based on geotechnical boring logs (primarily SPT
blow counts). They depict in graphic form the same scenarios (boring depth hb in
relation to total soil thickness hs, or depth to bedrock) discussed on pages 13
and 14 of the 1st Year NYCEM Report (Jacob, 1999). Two assumptions were changed
or added for the 2ndYear work, however. Shear wave velocity for bedrock at the
bedrock/soil interface was increased as discussed below to better reflect the
prevailing rock formations in the Manhattan region. The 10-ft Rule (for details,
see text below) was added to ensure that soft soil layers were taken into
consideration in close accordance with to NEHRP Provisions (FEMA, 1998a &
b).
The decision trees of Figure
4a and 4b should be used in conjunction with
Tables 1 and 2. The latter are identical to those in Jacob (1999), except for
changes noted, and reflect the values assigned to various materials and site
classes. Two changes were made:
(1) Vs for bedrock found in Manhattan was
adjusted to higher values using the Handbook of Physical Constants (1966) as a
guidance for the type of generic bedrocks typical for Manhattan; accordingly Vs
for Manhattan Schist was increased to 8000 ft/sec (up from 5,000 ft/sec for
bedrock used in the Year 1 report). Site classes for all DDC Borings from the
first year report as well as new MTA borings were recalculated using these
assumptions.
(2) The 1997 NEHRP Provisions (FEMA 1998a; Chap. 4, Sec. 4.1.2.1)
do not specify SPT bore counts or shear velocity when determining whether sites
qualify for the 10-ft Rule. We therefore had to redefine the Rule as applicable
whenever a 10-ft layer of soils or soft clays is present with bore counts N less
than ten (N<10) and shear wave velocities Vs<400 ft/s.
Table 1: Assigned
velocities for materials differing from regular soils, and for which blow count
information was not available.
Material |
Vs
(ft/s) |
Any materials where drill sank
under own weight (Blow count too low to measure) |
100 |
Primary Silt |
300 |
Top Soil (in upper 10 feet) |
300 - 400
|
Miscellaneous Fill |
400 |
Steel Layer with Void
underneath* |
500* |
Brick |
500
|
Brick and Concrete (Rubble) |
500 - 700 |
Asphalt |
1,000 |
Rip Rap |
1,000
|
Concrete |
3,000 |
Weathered Rock |
3,000 |
Boulders |
5,000 |
Hard Rock |
8,000** |
(*) eliminate if not pervasive, (**) new value chosen
for Year 2 work and Report
The averages V100 (or N100 ) for the upper 100ft below grade were computed
using Equations 6 and 7 of Jacob (1999), and the results were then categorized
into site classes as follows (Table 2) :
Table 2: Definition of NEHRP Site Classes by Velocity V100 and Blow Count
N100,
Site Class |
Velocity Range |
Blow Count Range |
A: |
V100 ³
5000 ft/s |
not applicable |
B: |
2500 £ V100 < 5000 ft/s |
N100 ³
100 |
C: |
1200 £ V100 < 2500 ft/s |
50 £ N100<100 |
D: |
600 £ V100 < 1200 ft/s |
15 £ N100<50 |
E: |
V100 < 600 ft/s |
N100<15 |
5. Soil Class Determinations Using Depth to Bedrock (DBR)
Only
For DBR soil borings without geotechnical data (no SPT blow counts), or for
areas in which the general soil thickness was inferred from difference between
topographic elevation of the grade level minus elevation of the bedrock, we use
the same generic shear wave velocity function versus depth h in soils
established in Jacob (1999);
|
Vs (ft/s) = 435.11 + 11.08 h (ft) |
(1) |
Vs extends to depth h' and the remainder between h' and h = 100 ft is made of
rock with shearwave velocity, Vr = 8,000 ft/s. For this combined soil-rock
profile we can calculate analytically the depth-averaged shear wave velocity by
dividing total distance (depth) traveled by the total travel time using the
following formulae:
|
V100 = 100 ft / S
ti |
(2) |
where |
S
ti = tsoil + trock |
(3) |
with |
tsoil = ò [1/
Vs (h)] dh |
(4) |
for the soil depth range 0<h< h', which yields, after integration with
equ. (1) for Vs (h)
|
tsoil = [0.0903 sec] x | ln (435 + 11.08h)
| |
(5) |
whereby the term between |…..| in Equ. (5) needs to be evaluated at the
upper and lower boundaries h= h' and h=0, respectively (i.e. upper minus lower
value), and
|
trock = (100 - h' (ft)) / 8,000 ft/s |
(6) |
Figure 6 shows the relationship between V100 and depth to bedrock using the
above-mentioned formulae. The limitations of this model were evidenced by the
fact that soils could not be classified as E-type because V100 could never be
less than 600 ft/s . Site classes are associated with the following depth ranges
for h', the depth of the bedrock/soil interface (rounded to the nearest 5-ft
interval): Class A: 0£h'£5ft; Class B: 5<h'£15 ft; Class C: 15<h'£50 ft;
Class D: 50<h'£100 ft.
6. Data-Processing of Boring Logs
The boreholes were chosen by location where information was needed and
availability. Generally, borehole logs were documented following the format of Figure
6.
A database created in Microsoft Access was used to organize all data from the
boring logs. Each borehole log used the cross streets of Manhattan to show the
exact location of where the borehole was taken. Street corners were matched to a
geocoded map (using MapInfo Professional) to find a fairly accurate latitude and
longitude of each borehole.
The format of the Access Database can be seen below in Figure
7.
After all of the information from the borehole logs were entered into the
database, an Access report was created and analyzed in MS Excel to obtain the
site class for each boring following the principles outlined in the decision
trees (Figure 4a and 4b).
Many of the problems outlined in the first-year report (Jacob, 1999)
persisted during the extension of work to entire Manhattan Island, and some new
problems were encountered. The most obvious problems were:
(1) Scarcity of
borings with geotechnical information.
(2) In some census tracts the available
borings with localized geotechnical information seemed unrepresentative of the
overall site conditions as inferred from borings to bedrock or bedrock elevation
(i.e. depth of soil column). This especially applies where the 10-ft rule
required assignment of site class E, while the remainder of borings or depth to
bedrock information suggested a much stiffer site class.
(3) The 10-ft rule for
soft-soil site class E cannot be directly applied since definitions in the NEHRP
Seismic Provisions (FEMA 1998a and b) rely on geotechnical parameters other than
SPT blow counts N and shear wave velocity Vs . For this reason we had to
introduce a new definition for the 10-ft rule based on Vs and N that was not
included in the original NEHRP Provisions, and thus is not formally recognized
by the geotechnical community. More details on this issue are discussed below.
(4) The size and shapes of census tracts impose a rather course spatial
resolution and their geometries do not conform well to natural boundaries
between soil classes. Hence prior production of a geologically based map of site
classes would be preferable before the assignment of census tracts to site
classes. This assignment could then use some weighting scheme. Such a scheme may
involve weighting the site classes proportional to area, or by the exposed asset
values built on respective site classes within a given census tract.
(5) We used
in Upper Manhattan (above 59th Street) the generic relation of shear velocity
vs. depth of Equation (1) that had been established for Manhattan soil profiles
below 59th Street during the Year-1 work (Jacob, 1999). We did not perform any
tests to see whether this relation applies to the soil profiles of Upper
Manhattan, largely because of lack of calibration data in this area.
Given the geotechnical input data available at this time, we believe that the
resulting census-tract-based site class map (Figure
1) is a substantial
improvement over using a uniform default site class D in all of Manhattan for
loss estimates with HAZUS. We believe, however, that further improvements are
possible. In fact, in late 2000, while cooperating with the MTA / NY City
Transit Authority (courtesy of NYCTA Chief Geologist A.N. Shah), we became aware
of a new data source. It is a map that had been prepared during pre-WW-II times
and is known as "Rock Data Map of Manhattan" (Scale: 1 inch in 600
feet; size about 3 by 8 feet). This map has been produced for the Borough
President of Manhattan (S. Levy), by the Department of Public Works and the
Civil Works Administration. The map shows the elevation of the rock floor, based
on core borings and excavations at several thousand locations in Manhattan. We
secured a hardcopy of this precious and rare map and have it since digitized. It
is a here to fore untapped resource not included in any known digital database
or scheme for determining NEHRP site categories in NY City. When its information
will be included during the Year-3 phase of this project, we expect further
refinements to the site class map now presented here as Figure
1. Whether or not
these refinements will be producing substantially differing HAZUS loss estimates
for the census-tract-based calculations for building-related losses, remains to
be seen. However, once the loss estimations will be extended to lifelines and
essential facilities, then such a higher-resolution, and geologically more
meaningful map of site classes will probably contribute to much more accurate
loss estimates for these non-building-related categories of assets.
The current results for Year 2 (largely represented by Figure
1, and the corresponding HAZUS files) have been forwarded to the Princeton
University HAZUS
engineering team (see Nordenson et al. 1999, for the equivalent 1st-year
effort). This team carries out the HAZUS loss computations for NYCEM for Year 2
for all of Manhattan using both improved building inventories and the improved
site classes reported here.
Related efforts of HAZUS-based earthquake loss estimations in the larger New
York City Metropolitan region and adjacent areas, including Westchester County (McGinty
and Wear, 2000) and in New Jersey (largely near Newark), were coordinated by
NYCEM during this 2nd-year effort. One major focus of this coordination was on
establishing a uniform methodology for assigning NEHRP geotechnical site
classes. For this purpose a "NYCEM Geotechnical Workshop" was convened
at the Lamont-Doherty Earth Observatory in Palisades, NY, on March 3, 2000. It
was attended by 20 practitioners of the HAZUS algorithm and by geotechnical
experts from the following locations and organizations in and near the NYCEM
area of interest:
States: New York, New Jersey, Delaware;
Organizations: NYSEMO,
NJ EMO, New York State Geological Survey, New Jersey State Geological Survey,
Delaware State Geological Survey, Columbia University, CUNY, Princeton
University, University of Delaware, Mueser Rutledge Geotechnical Engineers in NY
City; and from the Department of Information Technology of Westchester County,
NY. This workshop was preceded by extensive correspondence between the NYCEM
Geotechnical Expert Group and the Geotechnical Subcommittee of the NEHRP Seismic
Provisions operating under the FEMA-funded guidance of NIBS (National Institute
of Building Sciences) and its BSSC (Building Seismic Safety Council). The
purpose of this exchange was to fully coordinate NYCEM's interpretations of the
NEHRP methodology of determining site classes with those intended by the
national code committees that had created the site class definitions in the
first place. During the NYCEM geotechnical workshop and the prior correspondence
between NYCEM and the NEHRP Geotechnical Subcommittee it became very apparent
that there will always be local conditions that will be difficult to interpret
in strict terms of NEHRP site classes. This is especially so in the presence of
widely differing types of input data that may not in all instances conform to
NEHRP standards (i.e. may lack information on standard blow counts or shear wave
velocities or even composition of soils. But this coordination effort also lead
to a virtually unanimous consensus among the geotechnical experts involved. It
was concluded that the remaining differences stem largely from consideration and
interpretation of geotechnical input data that by their date of origin and
nature do not always conform to NEHRP standards. Such auxiliary data may
contribute valuable information to increase the spatial resolution. The fact
that such auxiliary data may not conform to NEHRP standards will in most cases
not introduce undue large uncertainties into the HAZUS loss estimates. At least
they are not expected to do so in comparison to all the other sources of
uncertainties inherent in the HAZUS methodology and related input data issues,
for instance those concerned with the valuation and fragility of the built
assets.
Baskerville, C.A., (1990), Bedrock and Engineering Geology Maps of New York
County and Parts of Kings and Queens Counties. 3 sheets. USGS Open File Report
89-462.
Baskerville, C.A. (1992), Bedrock and Engineering Geology Maps of Bronx
County and Parts of New York and Queens Counties, New York. USGS Miscellaneous
Investigation Series MAPI-2003, Scale 1:24,000, 2 sheets, US Geological Survey,
1992.
Baskerville, C.A. (1994), Bedrock and Engineering Geology Maps of New York
County and Parts of Kings and Queens Counties, New York, and Parts of Bergen and
Hudson Counties, New Jersey. USGS Miscellaneous Investigation Series MAPI-2306,
Scale 1:24,000, 2 sheets, US Geological Survey, 1994.
FEMA (1998a). 1997 Edition: NEHRP Recommended Provisions for Seismic
Regulation for New Buildings, Part 1 - Provisions. Published by the Federal
Emergency Management Agency (FEMA), as FEMA # 302, Washington DC
FEMA (1998b). 1997 Edition: NEHRP Recommended Provisions for Seismic
Regulation for New Buildings, Part 2 - Commentary. Published by the Federal
Emergency Management Agency (FEMA), as FEMA # 303, Washington DC
Fluhr, T., W. and J.J. Murphy (1944), Map showing 25-foot Contours of Bedrock
of the Boro of Manhattan, City of New York, Map # 120-A. Dated 3/17/1944.
Handbook of Physical Constants (1966). Revised Edition; Sydney P. Clark Jr.,
Editor. The Geological Society of America. Memoir 97. Section 9: Seismic
Velocities (by Frank Press) pp. 195-218;
Jacob, K.H. (1999). Site Conditions Effecting Earthquake Loss Estimates for
New York City. Lamont-Doherty Earth Observatory of Columbia University,
Palisades NY. May 6, 1999. 35 pages. http://www.nycem.org/techdocs/siteCondsYr1/default.asp.
NIBS (1997), Earthquake Loss Estimation Technology -HAZUS, User's Manual;
prepared by the National Institute of Building Sciences (NIBS) for FEMA.
Washington D.C. (1997).
McGinty, L. and S. Wear (2000). Initial Earthquake Loss Estimation Analysis
for Westchester County, New York. Draft Report prepared by the Department of
Information Technology, County of Westchester. http://giswww.co.westchester.ny.us
Nordenson, G., G. Deodatis, M. Tantala and A. Kampf (1999). NYCEM (New York
City Area Consortium for Earthquake Loss Mitigation), 1st Year Technical Report,
May 1, 1998 to April 30, 1999: EARTHQUAKE LOSS ESTIMATION STUDY FOR THE NEW YORK
CITY AREA. Department of Civil Engineering and Operations Research; PRINCETON
UNIVERSITY; April 1999. http://www.nycem.org/techdocs/lossEstYr1/default.asp
We are grateful to the MTA / NYC Transit Authority to allow us generous
access to their boring files. NYSEMO contributed additional funding for
internship salary while MCEER administered the FEMA funding crucial for the
success of this project. We would like to acknowledge especially the roles that
Andrea Dargush (MCEER), Dan O'Brien (NYSEMO), Bruce Swiren (FEMA), Guy Nordenson,
George Deodatis and Michael Tantala (all Princeton Univ.), and especially Dr. A.
N. Shah (MTA / NYCTA) played to facilitate various aspects of this study.