DESIGN AND CONSTRUCTION ISSUES … › images › files › geotech › 2010...While any slurry trench technology project the size of the WTC is inherently complicated, the level of

“Mega Projects Of New York City: Geotechnical Aspects – Specialty Seminar” Presented by ASCE Metropolitan Section Geotechnical Group May 13, 2010, New York City

DESIGN AND CONSTRUCTION ISSUES ENCOUNTERED DURING RECONSTRUCTION OF THE WTC SUBGRADE John S. Lizzo, Principal Geotechnical Engineer, PANYNJ, New York City, NY Raymond E. Sandiford, Chief Geotechnical Engineer, PANYNJ, New York City, NY

Abstract: The WTC site is a 16 acre office complex located on the west side of lower Manhattan in New York City, approximately 0.5 miles north of Wall Street. The buildings comprising the WTC site originally contained 10 million square feet of office space, of which 2 million square feet was contained in each of the iconic towers WTC-1 and WTC-2. Due to the destruction which occurred on September 11, 2001, the entire site required reconstruction. At the start of the project it was decided that in lieu of reconstructing in kind, that the complex would be upgraded based on current program requirements which did not exist when construction was originally completed in the early 1970’s . In order to provide the program space required for the upgrades, the portion of the original basement space which extended from street level down approximately 75-ft to bedrock only on the west side of the site, had to be expanded to extend to bedrock also on the east side of the site. This paper summarizes the design procedure and construction issues which were encountered when constructing the basement space from street level down approximately 75-ft to bedrock for the east side of the WTC site.

I INTRODUCTION

The office complex on the west side of lower Manhattan in NYC officially known as the “World Trade Center” (WTC) was originally conceived in the early 1960’s to rejuvenate the business district of lower Manhattan. The WTC site is located approximately 0.5 miles north of Wall St. along the Hudson River, and occupies 16 acres bounded by Vesey St. to the north, Church St. to the east, Liberty St. to the south, and West St to the west.

The WTC was designed, constructed, owned, and operated by the Port Authority Of New York and New Jersey (PANYNJ). The original construction of the WTC was the first major project in the United States to use the slurry trench method for below grade reinforced concrete wall construction. While any slurry trench technology project the size of the WTC is inherently complicated, the level of complication was magnified based on the subgrade conditions at the site, which consisted of old landfill dating back to the 1600’s that was used to extend the existing shoreline of Manhattan island at that time, and also by the location in the dense urban environment of lower Manhattan in NYC. The original construction was completed in the early 1970’s and by the 1980’s with the improving economy the WTC became the economic engine encouraging other commercial development such as the World Financial Center office complex (WFC), and residential development such as Battery Park City (BPC).

Due to the destruction which occurred on September 11, 2001, the entire WTC required reconstruction. Early in the project it was decided that in lieu of reconstructing in kind, that the site would be reconstructed based on current program requirements which did not exist during the original construction. For the original WTC, only the basement on the west side of the site was


constructed down to bedrock by the slurry trench method. The program requirements for the reconstruction made it necessary to also construct the basement on the eastside of the site down to bedrock . The east basement was constructed using the slurry trench method of construction and encountered complications which were different but just a difficult as those for the original construction. This paper summarizes the design procedure and construction issues which were encountered when constructing the basement space from street level down approximately 75-ft to bedrock for the east side of the WTC site.

II BACKGROUND

To better understand issues regarding the reconstruction of the WTC, it is important to understand some of the features of the original construction and the program requirements for the reconstruction.

A. The Original Construction

The Port Authority Of New York and New Jersey (PANYNJ) is a bi-state agency chartered to construct, own, operate and maintain interstate transportation infrastructure between the states of New York and New Jersey in order to facilitate the economy of the region. As such the PANYNJ owns and operates port facilities in New York harbor; Teterboro, La Guardia, Kennedy, and Newark Airports; and the bridges and tunnel crossings between New York and New Jersey. Based on the PANYNJ charter to facilitate the economic growth of the region, the PANYNJ was chosen as the agency to construct, own, and operate the WTC.

Prior to the WTC construction the 16 acre site bound by Vesey St., Church St., Liberty St., and West St., was mostly occupied by small electronic parts stores, and the area was commonly referred to as “Radio Row”. The area was also the location for the 20 story building which was the headquarters for a privately owned small commuter railroad between NYC and NJ known as the “Hudson And Manhattan Railroad” (H&MRR). Through NYC using the rite of eminent domain, and the PANYNJ agreeing to assume ownership and operation of the H&MRR, the PANYNJ was able to obtain the land for the WTC.

After the PANYNJ assumed ownership of the H&MRR, it was renamed the “Port Authority Trans-Hudson Railroad” (PATH). Once the PANYNJ acquired the 16 acre site, the NYC streets through the site were de-mapped in order to develop the area as a whole as an oasis of open space in the congestion of lower Manhattan (See Fig. 1). Prior to being de-mapped, Greenwich St. bisected the site into the east and west sides with the No. 1 subway line for NYC running underneath Greenwich St. within the WTC site as a buried structure. Therefore while at grade Greenwich St. was “removed”, the No. 1 subway line remained in service below grade continuing to bisect the site into the east and west sides. The 20 story headquarters for the H&MRR was constructed in the early 1900’s. The basement of the H&MRR building was constructed by a series of interlocking hand-dug caissons which were approximately 8-ft. thick and 30-ft wide and were filled with concrete after excavating down to bedrock. While the building above grade was demolished as part of the site redevelopment, the basement was incorporated into the subgrade of the facility.

THE ORIGINALWTC SITE

WEST ST.

CHURCH ST.

World Financial Center (WFC)

VESEYST.

LIBERTY ST.

N

Path Tracks

Orig. Towers

H&MGreenwich St./ No. 1 Subway

Fig. No. 1“Mega Projects Of New York City: Geotechnical Aspects – Specialty Seminar”Presented by ASCE Metropolitan Section Geotechnical Group May 13, 2010 , New York City


The slurry trench method of construction was used to construct the basement on the west side of the site down to bedrock approximately 75-ft below grade, and became the location for the iconic twin towers WTC-1 and WTC-2 . The tracks of the H&MRR originally crossed from the west side to the east side of the site, with the station within the basement of the H&MRR building. When the WTC site was constructed, as PATH the tracks and station were rerouted and located within the west basement. The east basement was smaller in area than the west basement and did not go down to bedrock, extending approximately 35-ft below grade with the basement walls constructed by more conventional means of a braced excavation. The basement of the original H&MRR was incorporated into the east basement and was the location for the loading docks for the facility. The No. 1 subway line remained as a buried structure in the sliver of soil left between the east and west basements. There were only three (3) points where you could cross from the east basement to the west basement :

• Through the two H&MRR tunnel sections which existed below the No. 1 subway line.

• Through the passage way underneath the No. 1 subway line the PANYNJ constructed for PATH passengers to move between the PATH station and Church St.

B. West Basement Reconstruction

Early in the design phase for the reconstruction of the WTC, it was acknowledged that the site could not simply be rebuilt in kind due to new program requirements which were necessary to commemorate the events and the lives lost on September 11, 2001, as well as to address how the neighborhood around the WTC had changed since it was originally constructed. The following is a summary of the major design requirements incorporated into the reconstruction of the west basement (see Fig. 2) .

1. The WTC Memorial

It was decided to use the areas in the west basement which were originally occupied by towers WTC-1 and WTC-2 as the site for a memorial (the Memorial) to commemorate the events and lives lost on September 11, 2001 . At grade the Memorial would be a tree lined park like area highlighted by two reflecting pools with water cascading down below grade. In memory of the original twin towers, the pools are located at the same locations and cover the same square footage as WTC-1 and WTC-2 . Below grade the Memorial is a museum with artifacts and displays associated with September 11, 2001 .

2. World Trade Center Tower 1

To replace the original iconic towers WTC-1 and WTC-2, a new iconic tower WTC Tower 1 (initially known as the Freedom Tower) was located at the corner of West St. and Vesey St. in the west basement. WTC Tower 1 was designed to a height of 1800-ft (+) to eventually become the tallest building in the United States, and the new iconic symbol for the WTC site. While WTC Tower 1 will contain mostly office space, it will also contain observation decks and restaurants.

WEST BASEMENT PEDESTRIAN

CONCOURSE

TOWER 1

TEMP EGRESSHELIX RAMPP.A.C. BUILDING

MEMORIAL ZONE

Path Tracks

WTC 1

WTC 2

N

Fig. No. 2“Mega Projects Of New York City: Geotechnical Aspects – specialty Seminar”Presentation by ASCE Metropolitan Section Geotechnical GroupMay 13 2010 New York City


3. The Pedestrian Concourse Under West Street

When the original WTC was constructed the only development west of the site were West St., which was a highly trafficked thorough fare with 3 north and 3 south bound lanes, and abandoned piers along the Hudson River. Due to the lack of development west of the WTC, the original design did not facilitate the easy flow of pedestrians across West St. to the Hudson River. However since the original WTC construction the west side of West St. has become highly developed to include the World Financial Center office complex (WFC), Battery Park City residential buildings (BPC), and a vibrant river front promenade frequented by those working in the area, tourists, and residents. In order to accommodate the new requirement for the easy pedestrian flow between the WTC, the WFC, BPC, and the river front, a pedestrian concourse was designed under West St. to connect the WTC to the buildings of the WFC.

4. The Path Station

Due to its vital function of transporting commuters between NJ and Manhattan, it was imperative that the PATH station be restored. While several plans were investigated to realign and/or relocate the PATH tracks, the conclusion was that it was best to restore the tracks in their previous location in the west basement.

5. The Temporary PATH Entrance

The current temporary entrance/exit to the PATH station in the west basement is located at the corner of Vesey St. and Greenwich St. in the west basement. This will also be the location for a helical below grade ramp for emergency entrance and egress to the site, as well as a future Performing Arts Center (PAC) above grade.

6. The Vehicle Security Center

Based on the need for additional security for vehicles entering the below grade space of the reconstructed site, new state of the art screening equipment will be required. However due to the program demands for subgrade space in the west basement and the newly constructed east basement, the required space was not available. Therefore a new subgrade space south of the west basement along Liberty St. is necessary to screen vehicles entering the subgrade area, namely the Vehicle Security Center (VSC). The basement area for the VSC will be constructed by the slurry trench construction method, and will eventually be tied into the west basement and newly constructed east basement. In the future it is planned to construct an office tower (WTC Tower 5) above the VSC as part of the replacement of the 10 million square feet of office space from the original construction.

C. East Basement Reconstruction

1 The New Transportation Hub

The new east basement would be highlighted with an arched transportation hub with winged spires to create a wide open grand space inside similar to other great transportation hubs such as Grand Central Station. The WTC Hub would function as the main “mixing bowl” for


commuters, tourists, and area residents to transfer between the different public transportation lines that converge in the area (see Fig. 3) .

2 New WTC Tower 2, Tower 3, and Tower 4

Despite the size of the 1800-ft tall WTC Tower 1 in the west basement, three more towers (WTC Tower 2, WTC Tower 3, and WTC Tower 4) are required in addition to the tower (WTC Tower 5) planned for future construction over the VSC to replace the 10 million square feet of office space from the original WTC. The three additional towers will range in height from 900-ft (+) to 1100-ft. (+) above street level, making each roughly the height of the Empire State Building. The foundations for the buildings will be founded on bedrock, and most of the associated space required for the daily support and operation of the buildings will be located below grade.

3 Bus Parking

Since the original WTC construction, the amount of bus traffic around the WTC site had increased significantly due to the increased work force commuting to the rejuvenated business district of lower Manhattan, and the number of tourists visiting the WTC. Due to the lack of parking facilities, after the morning rush hour many of the buses would idle along West St. waiting for the return trips to start with the evening rush hour adding to the congestion of the already crowded streets. When the site is reconstructed it is expected the volume of traffic will increase even more due to the increased number of tourists wanting to visit the WTC site and the Memorial. Without adequate parking facilities these buses would further add to the congestion. Accordingly it was decided to add parking for 47 buses in the subgrade space of the east basement.

4 Retail Space

Since the original construction of the WTC, lower Manhattan not only became a rejuvenated business district, it also became a residential area not only with the construction of BPC, but also with the conversion of other commercial properties to condominiums. The increased number of residents in the area created a need for additional retail shopping. Accordingly a 50% increase in retail space from 430,000 sq-ft. to 660,000 sq-ft was included in the WTC reconstruction.

5 Underpinning Of The NYCTA No. 1 Subway Line

The original design for the construction of the east basement included leaving the No. 1 subway line within the WTC site as a below grade structure on grade on the sliver of soil left between the east and west basements. Similar to the original WTC construction there would be underpasses from the PATH station in the west basement to the WTC Hub in the east basement, as well as several underpasses for vehicles traveling between the east and west basements. However even with the extra subgrade volume added to the WTC site with the construction of the new east basement, the combination of the subgrade space required for the program elements described earlier in this section, and the subgrade space required to run an office complex of this size (i.e., loading docks, MEP equipment, heating/cooling plant, etc) was even greater. Therefore it became necessary to excavated underneath the No. 1 subway line to bedrock to create the required additional subgrade volume. When

EAST BASEMENT

NYCTA DEY ST.CONCOURSE

TOWER 4 TOWER 3

TOWER 2

WTC HUB THE OCULUS

TEMPRETAINING WALLS

MIXED USE UNDER SUBWAY•VEHICULAR RAMP•MEP SPACE•RETAIL SPACE

GREENWICH STREET &UTLTS. ABOVE SBWY.

Jet Grout Closure

Jet Grout Closure

N

Fig. No. 3“Mega Projects Of New York City: Geotechnical Aspects – Specialty SeminarPresentation by ASCE Metropolitan Section Geotechnical GroupMay 13, 2010 New York City


construction is finished the portion of the No. 1 subway line within the WTC will have changed from a below grade structure on grade, to being structurally supported within the basement of the reconstructed WTC site.

6 The New Basement Limits

The original WTC construction included two basement areas:

• The west basement constructed by the slurry trench method and excavated down to bedrock approximately 75-ft below grade.

• The east basement with less area than the west basement, and constructed with braced excavations to depths of approximately 35-ft below grade.

• The No. 1 subway line was left as a below grade structure on grade between the two basements.

• The two basements were connected at three points, the underpass for the PATH station, and two vehicle ramps which were actually left over railroad tunnel sections from the original alignment for the H&MRR.

The final construction will connect the east basement to the west basement, as well as add the basement space for the VSC, to create one mega-basement excavated down to bedrock (See Fig. No. 4).

• The walls of the east basement will run along Liberty St., Church St., and Vesey St. .

• Only the West St. and Vesey St. walls of the west basement will remain. The Greenwich St. wall will be removed when the area underneath the No. 1 subway line is excavated. The Liberty St. wall will be removed when the VSC is constructed.

• While the basement space will be separated by partition walls for the many subgrade program spaces, the basement walls supporting the soil and water outside the basement from grade down to bedrock will run continuous around the perimeter of the site.

III DESIGN OF THE EAST BASEMENT

While there are many papers which can be written about the recovery efforts after September 11, 2001 and the reconstruction of the WTC site, the focus of this paper is on the design and construction of the east basement.

A. The Design Procedure For The East Basement Wall

As described in the previous sections, to create the required subgrade space for the reconstruction, it was required to enlarge the east basement by increasing the square footage and excavating down to bedrock.

THE RECONSTRUCTEDWTC SITE

WEST ST.

CHURCH ST.

World Financial Center (WFC)

VESEYST.

LIBERTY ST.WESTBSMNT

EASTBSMNT

PEDESTRIANCONCOURSE

V.S.C.

N

Path Tracks

Orig.Towers

H&MGreenwich St./ No.1 Subway

Fig. No. 4“Mega Projects Of New York City: Geotechnical Aspects – Specialty Seminar”Presented by ASCE Metropolitan Section Geotechnical GroupMay 13, 2010, New York City


1. The Stratigraphy

In the project datum El. 300 is 2.653-ft above NGVD 1929. Existing grade at the site varies from El. 315 (+) at Greenwich St. to El. 325 (+) along Church St., with bedrock at El. 240 (+), 75-ft to 85-ft below grade. The approximate soil stratigraphy from grade to bedrock is 10-ft to 20-ft of miscellaneous granular fill, 30-ft to 50-ft of red-brown sandy silt, 5-ft to 15-ft of till, all underlain by mica-schist bedrock, with the design water level at El. 300 .

2. Wall Thickness

As the basement was excavated it was laterally supported by prestressed tiebacks grouted into bedrock. The bottom of the wall would be socketed a minimum of 2-ft into solid rock classified as 2-65 per NYC Building Code in order to provide a water cutoff at the bottom of the wall, and to provide the bearing to support the vertical component of the prestressed tiebacks.

As with all projects, schedule is a priority and the WTC reconstruction is no different: the shorter the schedule the better. One way to shorten the construction schedule of the east basement was to limit the number of levels of the prestressed temporary tiebacks by designing the wall to span as far as possible between points of support. For the original WTC construction, the slurry trench method of construction was limited to 3-ft wall thicknesses. However equipment today can construct 4-ft thick walls. Since the incremental cost and time for excavating a 3-ft thick wall versus a 4-ft thick wall was not significant when compared to the cost and time for adding an additional level of tiebacks, it was decided to construct the east basement with a 4-ft thick wall. In addition to also help maximize the span between tiebacks, the wall was reinforced with a much rebar as possible, with the inside face of the basement wall which would be subjected to mostly tension loads designed with two layers of rebars, and the outside face of the wall which would be subjected to mostly compressive loads designed with one layer of rebars. For the inside and outside faces of the walls, the rebars were spaced as close as 9-in. o.c. in both the vertical and horizontal directions. The result was that the basement could be excavated to a depth of 20-ft before the first level of tiebacks had to be installed. Subsequent levels of tiebacks were spaced vertically from 15-ft at the mid-height of the wall, to 10-ft at the bottom. The east basement would require four (4) to five (5) levels of tiebacks for temporary lateral support, while the original west basement construction required five (5) to six (6) levels for equivalent depths.

The heavily reinforced 4-ft thick wall section also allowed the walls to be designed for permanent lateral support from floor slabs at 20-ft o.c., as opposed to floor slabs at 10-ft o.c. as for the original construction. The increased floor height helped accommodate program requirements for the reconstruction.

3. Temporary Tiebacks For Lateral Support

As the basement was excavated the walls were laterally supported with prestressed temporary tiebacks, except for the last level which was installed as a prestressed permanent tieback. Even though except for the last level the tiebacks are temporary, the time they would be required to provide lateral support would be of the order of several years and not months. Therefore all of the tiebacks were installed with double corrosion protection because the incremental cost versus single protection was insignificant when


compared to costs which could be incurred if tieback corrosion became an issue. Since the last level of tiebacks would go almost from the wall into bedrock, and the heads would be encased in concrete in the wall, they would not be susceptible to corrosion and could be considered permanent. By installing the lowest level of tiebacks as permanent support eliminated the need for a buttress slab to support the toe of the wall as required for the original west basement design. The basement walls and floor slabs for the finished basement will replace the support of the temporary tiebacks.

The lock off loads for the tiebacks were designed based on two criteria.

• Not to overstress the wall by using an excessive lock off load and pulling the wall into the soil behind the wall.

• To prevent excessive deflections of the wall during excavation to avoid overstressing the wall, causing settlements behind the wall, and overstressing the tiebacks as a result of the elongations after lock-off.

At the start of the design the criteria was set to limit the deflection of the wall to a maximum of 1-in. either into the soil from tieback prestressing, or towards the basement due to excavation.

4. Design Calculations

Due to the balancing which was required for the design of the tiebacks, it was decided to use a computer program which could model the deflections of the wall both backward into the soil as tieback prestress loads were applied, and forward into the basement as the area was excavated. In addition, to also model the effects of the deflections from each stage of excavation on the wall stresses, the program would have to be able to cumulatively add the deflections and stressed from one stage to the deflections and stresses from the previous stage. Accordingly the finite element program PLAXIS was used for the design calculations.

After using the PLAXIS program for a short time, it was realized that a small change in some input parameters could have a significant effect on the calculated deflections and stresses of the wall. Therefore two methods were used to verify the PLAXIS results.

• Records still remained from instrumented panels from the original WTC construction summarizing measured deflections and stresses. The PLAXIS program was first run for one of these panels, starting with typical input values for the soil stratigraphy behind the panel. The input parameters were then adjusted until the PLAXIS calculated deflections and stresses closely matched the measured values from the instrumented west basement panel. These “calibrated” input soil parameters were then used for the design of the east basement panels.

• A finite element beam-column program was also used to analyze the final stage of construction. While the program could not be used to estimate wall deflections at each stage of construction, the program could be used to calculate the wall stresses and tieback loads for the final stage. The beam-column program was used because it was one for which there was more experience checking the results of the analysis with hand calculations for simple designs, thereby providing confidence in the


program results. Simplified hand calculations were also performed for the final stage to verify the PLAXIS results.

Once the input soil parameters were “calibrated”, and the analysis procedure verified by alternate methods, the analysis procedure was then used to complete the design for the basement panels around the perimeter of the east basement.

B. Panel Widths And Depths

For practical purposes wall panel widths for slurry trench construction are limited to a minimum width of 15-ft (+) and a maximum width of 22-ft (+). While the alignment of the east basement wall was defined by the project limits, the widths of the individual panels were chosen by maintaining a uniform panel width between 18-ft to 22-ft for as many panels as possible between working points, and then filling in the remaining space at the ends with smaller or larger panels. The goal was to make construction more efficient by maintaining as much uniformity as possible. Each panel contains four (4) to five (5) levels of tiebacks in two vertical columns for a total of eight (8) to ten (10) tiebacks per panel. Each tieback was locked off based on the per-linear foot of wall tieback load calculated from the PLAXIS program and the contributing area for each tieback (see Fig. 5).

The design required a minimum embedment for each panel of 2-ft into class 2-65 bedrock as defined in the NYC Building Code. However it was also recognized that rock socket excavation is time consuming and expensive during slurry trench construction, and that the elevation of bedrock is not uniform, even over the width of a single basement wall panel. Based on the initial panel widths, borings and rock cores were drilled at each panel location. Based on the embedment requirement, and the boring/coring information, the following criteria were used to determine the bottom elevation of each panel.

• A general criteria was set to limit the depth of a panel rock socket to a maximum of 4-ft. Starting at the lowest bedrock elevation within the panel width, the bottom elevation of the panel was set to 2-ft lower. This elevation was then held until the depth reached 4-ft below bedrock. At this point the bottom of the panel was stepped up back to a minimum of 2-ft below bedrock. Sometimes two (2) and up to three (3) steps were added to the bottom of a panel when required to balance the requirements for a minimum embedment of 2-ft into bedrock with the expense of bedrock excavation during slurry trench construction.

• For every set of rules, there are exceptions. The future excavation for the east basement will be El. 238 to allow for the installation of a subdrain system and slab on grade at a finish elevation of El. 240 (+). Therefore to excavate to El. 238 up to the face of a basement panel, the panel would need a minimum bottom elevation of El. 236 . After the initial panel depths were chosen, they were compared to the minimum requirement of El. 236 . If they were much higher (i.e. El. 248), due to the time that would be required to excavate an additional 12-ft of rock socket along the width of a panel it was decided that is would be more economical to embed the panel to El. 246 and when excavating in front of this panel to step out 5-ft to 8-ft before excavating to El. 238 . The bedrock within this step would be left at its insitu elevation, and the vertical rock face would be reinforced with permanent rock

Typical Stratigraphy Used For Design

Φ = 32° C = 0 γ = 120 pcf

20-ft to 30-ft Granular/Misc. Fill

Φ = 32º C = 0 γ = 120 pcf

30-ft to 50-ft Rd-Br Sandy Silt

Φ = 34º C = 0 γ = 120 pcf

5-ft to 15-ft Till φ = 40º C = 0

γ = 130 pcf 2-ft

Approx.

75-ft.

Slurry Wall

Rock Anchor Tieback (Typ)

Mica-Schist BedrockFig. No. 5

“Mega P

rojects Of N

ew Y

ork City: G

eotechnical Aspects -

Specialty S

eminar”

Presentation by A

SC

E M

etropolitan Section G

eotechnical Group

May 13, 2010 N

ew Y

ork City


anchors to support the vertical rock face and the basement panel above. If the initial panel depths were only slightly above El. 236 (i.e. El. 238), it was decided to excavate a deeper rock socket for that panel to reach El. 236 instead of leaving a small rock step in front of the panel .

• If the elevation of class 2-65 bedrock resulted in the initial panel depths being only slightly lower than El. 236 (i.e. El. 228 to El. 236), then the panels were excavated to reach solid bedrock and provide a water cut off. However if solid class 2-65 bedrock was significantly deeper than El. 236 (i.e. El. 200 to El. 210), a decision had to be made as to how deep to excavate the panel. In such cases it was decided to excavate the panel not deeper than El. 220, and provide grout tubes within the panel in order that after the panel concrete had been tremie placed, but before the final basement excavation depth was reached, the fractured rock between bottom of the panel and class 2-65 bedrock could be pressure grouted to create a water cut off.

For the majority of the panels, the elevation of bedrock was such that the initial panel depths were at El. 236 or lower, providing water cut off and allowing excavation to El. 238 up to the face of the panel. Only along the west end of Liberty St. next to the No. 1 subway (El. 250 +) and along the mid-point of Vesey St. (El. 245) was bedrock higher requiring a reinforced rock step to be excavated. Only along Church St. approximately half way between the H&MRR wall and Liberty St. was there a location when solid bedrock was as low as El. 200 and the bottom of the panel was set to El. 228 with grout tubes incorporated into the panel. There was also a section along the north half of the H&MRR wall where the elevation of bedrock (El. 245 +) required a reinforced rock step for excavation since the original hand-dug shafts used for the H&MRR wall construction only went down to solid bedrock.

The procedure described above was used to choose the panel widths and depths shown on the contract drawings in order that the Contractors all started with uniform information when bidding. However the contract drawings also allowed the Contractor to modify the panel widths and depths to best fit his operations and equipment as long as the same wall alignment was used, and each panel had a minimum embedment of 2-ft into bedrock. Any Contractor modifications would have to be submitted to the Engineer for review and approval.

C. Incorporation Of The H&MRR Building Basement Wall

The complete basement for the H&MRR building was incorporated into the original east basement for the WTC. However the only portion of the H&MRR basement which would fit into the reconstruction was the section along Church St. . The NYCTA R/W subway line was located directly behind the H&MRR basement along Church St., therefore there was no room to construct a new wall behind the H&MRR basement wall, and then remove the H&MRR wall during excavation. Also it would be counterproductive to construct a new basement wall down to bedrock to replace an existing basement wall down to bedrock. As a result it was decided to incorporate the Church St. wall for the H&MRR basement into the reconstructed east basement, and remove the other three walls of the H&MRR basement during excavation.


Historical documents detailing the construction of the H&MRR basement were reviewed to incorporate the Church St. wall of the H&MRR basement into to the new WTC east basement construction. The H&MRR basement was constructed with hand-dug excavated concrete filled caissons down to bedrock with approximate dimensions of 8-ft thick and 30-ft wide. While there is some reinforcement in the H&MRR basement panels, due to the wall thickness the reinforcement was minimal. To determine the condition of the H&MRR wall concrete, and to confirm that the H&MRR panels were founded on sound bedrock, three (3) cores were drilled through three (3) different panels along Church St. (1 per panel). Each core showed the concrete to be solid and in excellent shape, and each panel to be founded on solid bedrock equivalent to class 2-65 or better per NYC Building code. The delineation from concrete to bedrock was very clear with no broken of fractured rock in between.

Based on the historical and concrete/rock core information, the H&MRR basement wall was modeled in the PLAXIS program to design the tieback loads required for excavation following the same design procedure as for the new east basement wall panels. While the H&MRR wall has five (5) levels of tiebacks similar to the new basement wall panels, due to the panel widths the H&MRR panels have three vertical columns of tiebacks spaced at approximately 10-ft. o.c. . The new east basement panels were designed to come as close as possible to the north and south end of the H&MRR basement Church St. wall. The small gap that would remain would be closed off by several overlapping jet grout columns prior to excavation. The jet grout would serve as a temporary closure during construction with a permanent cast in place concrete closure doweled into the new basement panel and the H&MRR panels as excavation progressed.

D. Extension Of East Basement Wall Underneath The No. 1 Subway

As noted earlier, it was required to excavate down to bedrock underneath the No. 1 subway line to create the required basement space. This would require that the new east basement wall extend underneath the No. 1 subway line and connect with the remaining west basement Vesey St. wall in the north, and the west basement Liberty St./VSC wall in the south. The No. 1 subway line was vital to the public transportation of lower Manhattan, and therefore would have to remain in service during east basement construction. Therefore the slurry trench method of construction could not be used to extend the east basement wall under the No. 1 subway line.

In lieu of the slurry trench method of construction, top down construction was chosen for basement panel wall construction underneath the No. 1 subway line (see Fig 6). This was accomplished by drilling to install a jet grout mass beneath the No. 1 subway structure. The jet grout mass consisted of three (3) overlapping rows of 4-ft (+) diameter columns spaced at 3-ft. (+) o.c. to form a mass approximately 9-ft thick from flush with the invert of the subway down to bedrock. Where gaps existed along the edges where jet grout columns could not be drilled through the walls of the subway structure, specialty columns ranging up to 6-ft in diameter were required to seal the gaps. The jet grout mass was designed with 10.75-in diameter concrete filled minipiles in the front and 2.25-in diameter bars in the back as reinforcement for additional shear capacity in order that the jet grout mass could span excavated heights of 10-ft. . Once 10-ft. of jet grout is exposed, a section of permanent wall will be constructed and laterally supported with tiebacks prior to the next level of excavation. This sequence of excavation will continue until the basement excavation reaches bedrock

Closure beneath NYCTA No. 1 TunnelClosure beneath NYCTA No. 1 Tunnel

Cast In Place Wall

Jet Grout Closure

No. 1 Subway

Fig. No. 6“Mega Projects Of New York City: Geotechnical Aspects - Specialty SeminarPresentation by ASCE Metropolitan Section Geotechnical GroupMay 13, 2010 New York City


and the last section of permanent wall can be constructed. With each lift of permanent wall, the ends of each lift are doweled and cast flush to the basement panels on each side to create a permanent closure. Prior to casting the first section of permanent wall, a series of minipiles were drilled and socketed into bedrock within the alignment of the basement panel. The minipiles were required to support the vertical component of the tiebacks until the panel construction could be completed and seated into bedrock.

Similar to the basement panel widths and bottom elevations shown on the contract drawings, a layout of jet grout columns were shown on the contract drawings to present uniform information for bidding. However the contract drawings required the Contractor to survey the actual field conditions and develop a column layout that best suited his equipment and would provide a complete cutoff to the limits shown on the contract drawings. It was the Contractor’s responsibility to provide complete closure, and repair any leaks at no additional cost.

E. Underpinning Of The No. 1 Subway Line

Prior to excavating under the No. 1 subway structure, the structure first had to be temporarily underpinned with 10.75-in. O.D. concrete filled minipiles socketed into bedrock. The design required approximately 500 minipiles in bents of three (3) to five (5) piles at 10-ft. o.c. drilled through and at the sides of the subway structure (see Fig 7).

The design and construction of the temporary minipile support of the No. 1 subway line was a separate contract from that of the east basement wall panels and basement excavation, with its own design and construction issues. Therefore while the design and construction of the underpinning of the No. 1 subway and the design and construction of the east basement had to be closely coordinated, a discussion of the No. 1 underpinning will have to wait for another paper.

IV. CONSTRUCTION OF THE EAST BASEMENT

A. Slurry Panel Construction

A total of 85 four foot thick panels approximately 75-ft to 85-ft deep from grade to bedrock were constructed for the new WTC east basement. As noted in section III.B, while the contract drawings included basement panel widths and depths to provide uniform information for bidding, the contract drawings provided the Contractor with the option and criteria for modifying panel widths and depths to best suit his equipment and construction sequence in order to meet the required contract schedule. The Contractor was also provided with the same boring/coring information used to determine the panel depths shown on the contract drawings. The Contractor exercised this option prior to starting construction, and submitted alternate panel widths and depths following the same alignment presented on the contract drawings to the Engineer for review and approval. By providing clear criteria on the contract drawings and available boring/coring information, it was possible for the Contractor and the PANYNJ to agree upon revised panel widths and depths in a relatively short period of time. Agreeing to alternate panel widths and depths was mutually beneficial to the Contractor and the PANYNJ because the alternate plan satisfied contract

NYCTA No. 1NYCTA No. 1--Line Tunnel UnderpinningLine Tunnel Underpinning

Temporary

Permanent

Fig. No. 7“Mega Projects Of New York City: Geotechnical Aspects – Specialty Seminar”Presentation by ASCE Metropolitan Section Geotechnical GroupMay 13, 2010 New York City


requirements and provided the Contractor with a sequence of construction that would allow him to meet the contract schedule.

The Contractor started basement panel construction with a slurry trench method which did not vary significantly from the method used for the original WTC construction. A trench approximately 8-ft. wide was excavated 15-ft to 20-ft deep to remove any obstructions which may have been present in the miscellaneous fill. The trench was then backfilled with a material referred to as “flow fill”, which was a weak sand cement mix with a strength of approximately 300 psi. The flow fill was self supporting when excavated, but also provided easy excavation for construction of the cast in place concrete guide wall. The guide wall was used to maintain the alignment for the wall panels, as well as the vertical control for checking the panel depths and setting rebar cages. Each panel was excavated with a clam shell bucket similar to the equipment used for the original WTC construction (see Fig. 8) . Steel I-beams were used for the end stops.

The Contractor used a steel star shaped chisel to remove obstructions which were not removed by pre-trenching (see Fig. 9). The chisel would simple be dropped up and down until it broke the obstruction into small enough pieces that it could be excavated. This is a brute force method which has not changed significantly since the early days of slurry trench construction, but over the years has proven to be the most cost effective way for removing obstructions. While the pre-trenching did remove most obstructions, there were still several panels where unknown steel and concrete obstructions were present which were removed using the steel chisel.

One construction method which was different than the original WTC basement construction was the use of a hydromill for excavation of the panel rock sockets. Hydromills were not invented during the original construction, so the rock sockets were excavated by dropping the steel chisel to break up the bedrock. This was very time consuming and required switching between the chisel and the clam shell bucket to excavate the rock sockets. The hydromill consisted of concentrically rotating 4-ft diameter wheels with high strength cutting teeth for grinding and excavating the panel rock socket (see Fig. 10). At the same time the hydromill would circulate the bentonite slurry in the trench to remove cutting material and provide fresh clean slurry. The hydromill was approximately 8-ft. wide and was used to cut the rock sockets in three (3) bites. The hydromill also included sensors to tell the operator how deep the hydromill was below the slurry elevation, as well as pressure gauges for each cutting wheel which could be used as an indication of the strength of rock being excavated. The hydromill was a very effective and accurate method for excavating the rock sockets for each panel.

Fortunately, panel construction went smoothly, and there were few incidents.

• Despite pre-trenching along the alignment of the wall, there were panels where obstructions were encountered. However as noted previously the steel chisel was effective in breaking up the obstructions within the width of the basement panel in order that it could be excavated and panel construction continue.

• There were only several panels where slurry was lost when an unknown abandoned utility was penetrated. The Contractor was able to seal the breach by dropping cement/sand bags into the trench which flowed to the breach and held long enough to maintain the slurry level in the trench and allow for the placement of concrete.

The ClamshellThe Clamshell

Fig. No. 8“Mega Projects Of New York City: Geotechnical Aspects - Specialty Seminar”Presentation by ASCE Metropolitan Section Geotechnical GroupMay 13 2010 New York City

The ChiselThe Chisel

Fig. No. 9“Mega Projects Of New York City: Geotechnical Aspects - Specialty SeminarPresentation by ASCE Metropolitan Section Geotechnical GroupMay 13 2010 New York City

TheThe HydromillHydromill

Fig. No. 10“Mega Projects Of New York City: Geotechnical Aspects - Specialty Seminar”Presentation by ASCE Metropolitan Section Geotechnical GroupMay 13, 2010 New York City


• Despite the revised panel widths and depths mutually agreed to by the Contractor and PANYNJ, there were panels where the Contractor requested to revise the bottom elevation of the panel based on actual bedrock elevation encountered during panel construction. By the Contractor submitting his proposed revision to the Engineer to show how it would satisfy contract requirements based on existing conditions, having the boring/coring information at each panel, and having an open dialogue with the Contractor for discussing all options, requests to revise to panel depths during construction were able to be resolved to the mutual satisfaction of both sides within 24-hrs., and therefore did not slow the panel construction schedule.

• While the H&MRR wall panels extended down to bedrock, the basement of the H&MRR building extended down to El. 264 which was approximately 20-ft above bedrock. When this lower portion of the H&MRR wall was exposed during excavation it was discovered that since it was below grade in a stiff till material, that it was not formed as well as the portion above El. 264. The excavation was able to be completed to bedrock, however for the long term stability a liner wall had to be poured in front of the lower portion of the H&MRR wall. This was considered a typical design modification which is required when incorporating a 100 year old structure with minimum drawings indicating construction details into new construction.

• To date the east basement has been excavated to bedrock in front of all new east basement wall panels and existing H&MRR wall panels and water leaks at the bottom of the panels have not been encountered. There were several minor leaks between the panel joints which were easily sealed by drilling into the leak and injecting grout.

B. Tieback Installation

For new basement wall panels, steel trumpet pipe sleeves were attached to the rebar cages and cast into the wall panels. The trumpets extended from the front to the back of the panel to eliminate the need for drilling through the newly constructed panels. For the existing 8-ft. thick H&MRR wall panels, a trumpet pipe sleeve was drilled and grouted approximately 2-ft into the wall, and the remainder of the wall was drilled through for the tieback installation. For the tieback installation, whether drilling through a new basement wall panel or an existing H&MRR basement wall panel, a steel casing was drilled down to bedrock and seated 2-ft into bedrock to create a seal. A rock socket was then drilled past the bottom of the steel casing to create the bond zone for the tieback.

1. Ground Settlements

In order to minimize settlements resulting from the tieback installation, it was important to control the return flow of water and soil up the casing during the drilling operation. Accordingly a tapered rubber doughnut gasket was placed around the steel casing to seal the gap between the steel casing and the steel trumpet to prevent the loss of water and soil between the annular space while drilling. The gaskets also had fittings through which chemical or cement grout could be pumped to seal the flow of water and soil. Even when the flow of water and soil was well controlled, settlements due to ground loss and ground disturbance seem inevitable. Therefore any time a slurry trench construction method is used, good construction quality controls should be used to minimize settlements, and


contingency plans developed to mitigate settlements if they become excessive. For the WTC east basement settlements occurred along Vesey St. and Liberty St., but were within sidewalk areas which will be reconstructed, and will be regraded at that time. Settlements up to 2.5-in. also occurred along Church St. under the R/W and the A/C/E subway lines. While the settlements did not result in the disruption of subway service, they did require that the soil beneath the station be treated with compaction grouting to restore the soil strength and stabilize the settlements.

2. Obstructions During Drilling

A total of 800 tiebacks have been installed for the east basement and approximately 5% have encountered obstructions when drilling the steel casing down to rock. The obstructions varied from unknown steel or timber construction behind the H&MRR wall left over from the hand dug caisson construction, to the steel casing getting stuck in a fractured rock/gravel layer above solid bedrock. Sometimes the Contractor was able to drill through the obstruction, and sometimes the casing could not be advanced or sheared off at a joint, resulting in the casing being abandoned. Experience showed that in such instances in lieu of pulling the casing, it was best to proceed as follows:

• When it is determined that the casing cannot be advanced further, flood the casing with water and cap it as soon as the drill rods are removed. The casing can remain this way while as drilled information is obtained, such as, centerline elevation, vertical angle, horizontal angle, etc. .

• If it is determined that it will not be possible to install the tieback through the existing casing, fill the casing with grout and abandon it in place. Review the as drilled information for the casing to determine where the obstruction was encountered, and determine a new location and alignment for the tieback as close as possible to the original location which will allow the tieback to avoid the obstruction during drilling. Drill a hole and grout in a trumpet pipe sleeve at the new location. At the original tieback location fill the wall pocket at the head of the tieback with grout, and restore rebar across the pocket if required, prior to stressing the relocated tieback.

The above procedure is preferable to pulling the steel casing and attempting to reuse the trumpet pipe sleeve. When the casing is pulled the tendency is for a significant volume of soil to flow through the trumpet before it can be sealed, resulting in a large void forming behind the wall which could result in ground settlements. For the WTC east basement, there was only one instance where the casing was pulled resulting is a large inflow of soil. After the trumpet was sealed, drill rods were used to pump grout behind the wall at the tieback location to fill any voids which may have formed.

Despite best efforts, it should be expected that obstructions will be encountered resulting in some original tieback locations being abandoned in place, and a relocated tieback being installed. Therefore contingency plans should be available for grouting abandoned drill casing in place and drilling at a new location.

3. Tieback Testing

The Contract drawings contained tabulated information for the design load, test load, and lock off load for each tieback in order that each tieback could be checked by either a performance test or


proof test followed by a lift off test. All testing was performed following standard Post Tensioning Institute recommendations. The Contract drawings specified the required double corrosion protecting, and the minimum rock socket diameter and length. However the installation and testing of the tiebacks were based on a performance specification, and it was the responsibility of the Contractor to determine the rock socket diameter, length, and grout strength required for successful installation and testing of the tieback. If based on test results it was determined that the tieback was not acceptable, the Contractor was required to replace the tieback at no additional cost. To date out of the 784 tiebacks installed of the 810 that are required, only 11 were determined to be unacceptable and require replacement.

Strand anchors were used for the temporary tieback installation. In the beginning for the sake of uniformity it was thought to be better to limit the number of tieback sizes (i.e. number of strands in each tieback), and just specify different test and lock off loads for each tieback as required, thereby avoiding eventually installing the wrong size tieback. However the steel wedges used to lock off the strands require the tieback to be locked off at a minimum of 50% of the design load for the tieback in order for the wedges to be pulled down hard enough to hold the strands. Initially with a one size fits all tieback design, for some tiebacks the specified lock off loads were less than 50% of the tieback capacity. This resulted in requiring only some strands of the tieback being used for prestressing in order to maintain adequate loads per strand in order for the associated wedges to properly lock off the tieback. Unfortunately this occurred on the top two levels of tiebacks being installed along the south end of the H&MRR wall. To expedite the schedule these tiebacks were installed with low head room equipment within the remaining portions of the H&MRR basement prior to the basement being demolished during excavation. The installation of the strand anchors which varied from 80-ft to 100-ft long with low head room equipment resulted in extra rotation of the tiebacks as they were lowered down the steel casing. The combination of the extra rotation of the tiebacks during installation, along with only some of the strands being stressed, resulted in extra binding along the length of the tieback which resulted in the elongation of the tiebacks during proof testing being less than the minimum requirement of an elastic elongation equivalent to 80% of the free length. These tiebacks required additional testing and analysis before it was determined they would be acceptable. One simple detail which aided in making this decision was the fact that the drill casing was cut 2-in to 3-in short of the tieback anchor plate prior to testing. Therefore even if the tieback was binding in the casing, since the casing did not have enough skin friction to support the test or prestress load, and was not bearing on the anchor plate, the load would still have to be transferred to the rock socket. In addition the elongation was long enough to verify that the load test and prestress loads were transferred far enough down the steel casing that the loads had transferred past the assumed failure plan behind the wall.

When the tiebacks were designed to only have the required number of strands so all strands were stressed during testing, and they were installed with no overhead restrictions and thereby avoid extra twisting, the tiedowns regularly tested within allowable limits.


C. Construction Of Jet Grout Mass Beneath The No. 1 Subway Line

Prior to installing the jet grout columns beneath the No. 1 subway line, the Contractor was required to perform the following tasks.

• While the Contract drawings included a pattern of jet grout columns to provide a mass approximately 9-ft. thick from the invert of the subway down to bedrock, the Contractor was required to perform a survey of the area of installation to determine the pattern to provide the jet grout mass required based on actual details of the subway structure. This included determining when columns greater than 4-ft in diameter would be required to fill in gaps underneath the subway walls.

• The contractor was required to perform a test section in an open area of the east basement prior to excavation to show that columns of the required size, strength and overlap could be constructed as required for the jet grout pattern developed by the Contractor. Based on this test program the Contractor determined the installation parameters and sequence of installation for the jet grout columns.

The proposed jet grout pattern by the Contractor, and the jet grout test section results had to be reviewed and approved by the Engineer prior to starting construction. The contractor used a double jet process with varying injection pressures and rotation and lift rates to adjust for jetting in till and silt material to obtain the required results.

While the service for the No. 1 subway could not be interrupted, weekend shutdowns (general outages, G.O.’s) were available for construction of the jet grout mass. While subway service was temporarily suspended from 10 PM Friday night to 5 AM Monday morning, the Contractor could work around the clock to install the jet grout columns beneath the subway structure. For the majority of columns, the Contractor was able to install threaded pipe sleeves flush with the invert of the tunnel during a series of weekend G.O.’s . On subsequent G.O.’s the Contractor was then able to connect additional drill casing to the threaded shoe in the invert and extend the drill casing through an opening in the subway roof. The Contractor was then able to perform the jet grouting in a continuous procedure from on top of the subway using full headroom equipment, and contain and control the return of spoils on top of the subway structure and eliminate the risk of spoil spreading out along the subway tracks, which would require extensive clean up prior to restoring service. When there were columns which could not be constructed from the top of the subway box because of clearance problems, the Contractor was forced to construct the columns from within the subway tunnel. While working from within the tunnel complicated the installation due to working with low headroom equipment and for controlling and disposing of the return spoils, the Contractor was able to successfully install the columns.

An additional complication was trying to close gaps in the jet grout mass beneath the wall of the subway structure where a column could not be installed directly over the area. In such cases the Contractor covered the area with large diameter columns from inside and outside the tunnel which would reach underneath the tunnel wall and overlap with adjacent columns on the other side. When required the Contractor also installed a series of angles columns in these locations to ensure closure of the jet grout mass.

The Contractor used a double jet process and varied the injection pressure and rotation and lift rates when jetting in the till and silt layers to obtain the required results. Jetting began 2-ft into


grout column, a length of pipe filled with grout was left in place above the subway invert. As the grout level settled, the pipe was refilled in order to maintain a positive head above the invert in order to maintain the grout flush with the tunnel invert.

When jet grouting underneath the No. 1 subway, settlements were a concern due to liquefying too much soil, as well as heave due to building up too much pressure beneath the structure. Accordingly an extensive monitoring program including laser targets was used to monitor settlements and heave during jet grouting. By maintaining a minimum spacing of six feet (two columns) between freshly grouted columns, and reducing injection pressures and rotation and lift rates as a column neared the subway invert, the Contractor was able to successfully install jet grout columns for both Vesey St. and Liberty St. without causing any settlements or heave to the subway structure.

To date only the first lift of the permanent wall panels at Vesey St. and Liberty St. have been constructed. For both Liberty St. and Vesey St. a good seal was obtained against the subway invert, and the adjacent wall panels. Due to construction scheduling issues, the first lift of exposed jet grout at both Liberty St. and Vesey St. was left uncovered for approximately four (4) months prior to construction of the first lift of the permanent basement panel. No leaks were encountered at the Liberty St. wall. However approximately a week prior to construction of the first lift of the Vesey St. permanent wall panel, a significant leak developed through the jet grout. An extensive repair program requiring shotcrete and cast in place concrete to reinforce the face of the jet grout, prior to performing grouting within the jet grout mass to seal the leak was performed. For construction of future lifts of the permanent basement panels, the construction schedule will be better coordinated to limit the time the jet grout face is exposed in order to limit the risk of leaks forming.

The results for how well the remainder of the excavation in front of the jet grout masses and the construction of the permanent panels works will have to be reported in another paper.

D. Excavation Of Bedrock

The original design for the east basement required only excavating down to bedrock. However after construction began, further changes to the program required excavation of bedrock beneath the WTC Transportation Hub and the No. subway structure to create the required below grade space. The rock excavation is being performed by controlled blasting and entails detailed reviews of the Contractor’s blast plans, as well as extensive monitoring of blast induced vibrations at sensitive structures around the site. As of the date of this paper, the rock excavation work has just begun. The results of the controlled blasting program will have to be reported in another paper.

V CONCLUSIONS

This paper provides a broad overview of some of the many issue regarding the reconstruction of the WTC site, focusing on the construction of the new east basement. By summarizing features of the original construction and comparing them to program requirements for the new construction, the paper highlights some of the design choices which were made. Any subgrade construction the magnitude of the WTC east basement using the slurry trench method of construction in inherently


complicated. The Construction of the east basement of the WTC east basement was further complicated due to its location in lower Manhattan, and the coordination required with all of the other project and stake holders on site.

The paper also provides a broad overview of some of the design and construction issues which were encountered during construction to highlight the complexity of the project. While such high tech tools such as computer aided 4-D modeling and high resolution 3-D AutoCAD models and animations help to define and visualize issues, they are not enough to solve the issues. In the end as cliché as it may sound, the best way to resolve construction issues for an extremely complex project in a timely and efficient manner is to start with a well designed set of contract drawings, and for both the Contractor and owner to maintain an open and professional dialogue during construction. An adequate budget should be provided to address field issues as they arise. Both the Contractor and owner should be prepared to expect the unexpected and work together to develop mutually acceptable solutions.

Documents

DESIGN AND CONSTRUCTION ISSUES … › images › files › geotech › 2010...While any slurry trench technology project the size of the WTC is inherently complicated, the level of