ARTIFICIAL GROUND FREEZING FOR EAST SIDE ACCESS NORTHERN

BOULEVARD CROSSING FROZEN SOIL ARCH QUEENS, NEW YORK

Adam Curry, P.E., Moretrench American Corporation, Rockaway, NJ, USA

Gregory Ziegler, V.P., P.E., Moretrench American Corporation, Rockaway, NJ, USA

Abstract: The CQ039 Northern Boulevard Crossing Contract was one part of the larger MTA Capital

Constructions East Side Access Project connecting the Long Island Railroad with NYCs Grand

Central Station. This contract required the construction of a SEM tunnel between two 85-ft deep

access shafts. Difficulties included working as deep as 55 feet below the groundwater table while

mining through variable subsurface conditions with preclusions to dewatering and settlement. In

addition, the tunneling would occur directly beneath an active subway line, an elevated rail line (both

supported on piles), and Northern Boulevard, a main artery through Queens, New York. In order to

create both a groundwater cutoff and a temporary excavation support during the tunneling

operations, the tunneling contractor selected an artificially frozen soil arch, installed horizontally from

the two access shafts.

In addition to a discussion of the overall approach, this paper includes a detailed description of the

drilling and surveying procedures used for the installation of the horizontal freeze pipes. It also

summarizes the results of the various tests performed at the site to verify the performance of the

freeze system and the data collected while monitoring the freeze formation. The paper closes with a

synopsis of the results and an update on the project.

I INTRODUCTION

The New York City Metropolitan Transit Authority awarded the CQ-39 contract to Schiavone Kiewit

JV to construct a new tunnel structure below Northern Boulevard connecting the Queens open-cut

area to the existing bellmouth north of Northern Boulevard. The CQ-39 contract was part of the

larger East Side Access Project. The goal of the project is to connect the Long Island Railroad

(LIRR) lines in Queens to a new LIRR terminal beneath Grand Central Terminal in Manhattan. The

main intent is to increase the LIRRs capacity and shorten travel time for Long Island commuters

traveling to the east side of Manhattan.

The proposed tunnel utilized Sequential Excavation Method (SEM) tunneling methods with a frozen

arch to act as temporary structural support and groundwater cutoff. The SEM method was developed

in the 1950s for work in consolidated formations and has been advanced and adapted to become

suitable for an array of soft ground conditions. As its name implies, SEM construction utilizes

excavations in short incremental lengths with benches sequenced to maintain weight against the

face and reduce the span of the crown. Initial support is provided with a spray-on shotcrete lining,

typically reinforced with steel arches applied immediately upon excavation. Generally completed by

hand, there is no mobilization and setup of a Tunnel Boring Machine (TBM).

Figure 1: SEM Excavation East Side Access Northern Boulevard Crossing

Artificial ground freezing is the conversion of in situ pore water into ice through the circulation of cold

liquid through a system of pipes installed in the ground to impart compressive strength and

impermeability to a soil. The frozen in situ pore water acts as a bonding agent creating a frozen soil

mass with noticeably improved compressive strength and decreased permeability compared to the

surrounding unfrozen soils (Powers, Corwin, Schmall, Kaeck, 2007).

A typical ground freezing system for a shaft consists of a series of freeze pipes installed along the

perimeter of the proposed excavation, extending into the subsurface strata. Within each of the freeze

pipes, a smaller diameter feed pipe is installed permitting the downward circulation of the cooling

liquid which then flows back to the surface through the annulus of the larger pipe. Calcium chloride

(brine) is chilled by a series of electrically powered refrigeration plants. As the circulation of the brine

progresses, cylindrical columns of frozen soil grow around each pipe. The diameter of the columns

increases with time, forming a virtually water-tight impermeable barrier.

II DESIGN OF GROUND FREEZING SYSTEM

The design process was a joint effort between the Owners Engineers and the Construction Team.

The design process included several independent yet integrated components summarized below:

Thermal analysis to determine the required freeze pipe spacing and temperature

profile.

Structural analysis to ensure the frozen mass will have sufficient strength to act as

temporary structural support during excavation prior to shotcrete.

The geology in the general area in sequence was Fill (Stratum 1), Organic Deposits (Stratum 8),

Mixed Glacial Deposits (Stratum 2, 3, and 4), Glacial Till (Stratum 5), Decomposed Rock (Stratum 6)

and Bedrock (Stratum 7). Figure 2 provides a geologic profile.

Figure 2: Geologic Profile

The predominant soil unit in the frozen arch was Stratum 4. Other partial components were Stratum

3 at higher elevations, in parts of the crown; and Stratum 5 closer to the base of the arch. Stratum 6

consisted of decomposed or weathered rock. The base of the arch was keyed into rock (Stratum 7).

From generally established knowledge of the relevant strengths of frozen soils, it was evident that

Stratum 3 and Stratum 5 (both sands/silts, with gravel) would have greater frozen strength properties

to those in Stratum 4. Frozen soil strength testing was therefore limited to Stratum 4.

In order to describe the stress and deformation behavior, the following tests were performed by CDM

Consultants in Bochum Germany.

Testing of unfrozen and thawed samples

o Uniaxial compression tests

o Triaxial undrained, consolidated compression tests

Testing of frozen samples

o Uniaxial compression tests (T = -10 C, T = -15 C)

o Triaxial compression tests (T = -15 C)

o Uniaxial creep tests (T = -15 C)

The following table summarizes the results of the lab tests on Stratum 4:

TemperatureUniaxial Compressive

Strength

Shear

Parameters

2 week 2 month 3 months 6 months 2 week 2 month 3 months 6 months 2 week 2 month 3 months 6 months

Stratum 4 -10 4.95 10.0 / 2.14 330 325 320 320 1.18 1.13 1.08 1.05 0.49 0.47 0.45 0.44

Stratum 4 -15 6.85 10.0 / 2.2 390 385 380 375 1.63 1.55 1.5 1.45 0.68 0.65 0.63 0.61

Layer

o/MN/m2MN/m2

Time dependent values

Time Dependent Frozen Soil Parameter

Youngs Modulus of Elasticity Compressive Strength (allowable) cohesion intercept (allowable)

MN/m2 MN/m2

Short term values

(Lab Conditions)

oC MN/m2

Table 1: Results of Frozen Soil Tests

Based on the results of the soil investigation the following criteria needed to be met to achieve an

acceptable frozen arch:

Arch minimum thickness of 6 feet

Arch temperature of -32 degrees Celsius at the freeze pipes and -1 degrees Celsius at the

intrados and extrados

The thermal analysis on this project was conducted using the finite element (FEM) heat transfer

program Temp/W, developed and distributed by GeoSlope of Calgary, Alberta. Several models were

evaluated as part of the thermal analyses for this project. The results of the analyses are

summarized below:

4.0 ft. nominal spacing yields closure in 16 days and a 6 ft. thickness in 35 days.

5.0 ft. nominal spacing yields closure in 25 days and a 6 ft. thickness in 46 days.

6.0 ft. nominal spacing yields closure in 34 days and a 6 ft. thickness in 55 days.

7.0 ft. nominal spacing yields closure in 46 days and a 6 ft. thickness in 64 days.

Based on the results it was determined that 43 freeze pipes on approximately 3.5 foot spacing would

be the basis for the design.

III CONSTRUCTION

The initial ground freezing approach required the installation of (81) horizontally drilled holes

approximately 120 feet in length from the newly constructed Queens open-cut area to the existing

excavation located to the north of Northern Blvd. The arrangement consisted of 43 freeze pipes, 3

temperature monitoring pipes, 22 compensation grout holes/heat pipes, 3 draining wells, and 10 void

grout holes. In addition, 4 vertical freeze pipes were also required. The purpose of the void grout,

compensation grout, and heat pipes were to mitigate heave or settlement during the construction

process.

Combinations of track mounted geotechnical dual rotary drill rigs and horizontal skid mounted coring

rigs were utilized to drill the holes. The contractor anticipated encountering obstructions that included

boulders, jet grout, concrete filled steel pipe piles, slurry wall reinforcement, and rock. In the event

that an obstruction was encountered and was not able to be penetrated, the contractor had the

ability to telescope drill casing giving a higher chance of success. As a contingency, the contractor

supplied a horizontally mounted sonic drill rig to penetrate the obstructions in the event that

traditional rotary or coring methods were unsuccessful.

Horizontal boreholes penetrated the slurry wall in varying depths from 20 to 55 feet below the water

table. Given the nature of the silty sands to flow under these conditions it was critical for the drilling

to be performed utilizing a groundwater control device (GWCD). The GWCD consisted of a steel

trumpet cored and mounted in the slurry wall. Attached to each trumpet was a combination of

chambers, valves, and seals that allowed drilling to occur with minimal to no soil loss.

Figure 3: Groundwater Control Device

It was important to accurately survey the borehole upon completion to ensure that adequate pipe to

pipe spacing was achieved. The contractor chose to utilize a Reflex Gyroscope to perform the

borehole surveys. Several factors needed to be considered when choosing the correct instrument. In

addition to accuracy, the gyroscope needed to be able to survey on a flat horizontal plane as well as

inside a steel casing. These criteria eliminated standard inclinometers and magnetic survey

instruments. The reflex gyro utilized a digital micro gyro which consisted of a silicon microchip and

an integrated circuit assembled inside a nonmagnetic package. This combination allowed the

instrument to provide both azimuth and dip on any plane and in any environment. The probe

transmitted the data to a field PC utilizing Bluetooth technology. Once processed the output could be

plotted in AutoCad for easy interpretation.

Figure 4: Surveying Using Gyroscope

Prior to any freeze pipe installation it was required by specification that the contractor demonstrate

the chosen means and methods for settlement control. A trial grout program was established which

consisted of a combination of void grouting and compensation grouting. Void grouting was

conducted utilizing a non-cementitious grout injected through the end of the drill casing in stages as

the casing was retracted. Compensation grout consisted of repeat injections of both cementitious

and non-cementitious grout through Tube-A-Manchette (TAM) pipes. The trial grout program was

considered successful once the contractor demonstrated the ability to lift the subway structure as

measured by total stations with prisms inside the subway structure.

In addition to the trial grout program, an additional eight (8) void grout holes were completed prior to

the start of freeze pipe installation. It was believed based on previous work in the area that voids

existed underneath the subway structure that would result in uneven heave or settlement during the

construction work. The void grouting would reduce the number and size of voids allowing for more

predictable settlement and heave. In total 19,100 gallons of non-cementitious grout was injected

prior to the start of freeze pipe installation.

The first freeze pipe was installed utilizing high speed rotation coring methods. Subsequent survey of

the pipe indicated a much higher deviation downward than would be accepted based on the design

and that the soils near the top of the arch were less dense than anticipated. It was determined to do

an additional 10 void grout holes to consolidate the soils prior to continuing with the freeze pipe

installation. An additional 40,000 gallons of non-cementitious grout was injected beneath the subway

box.

Figure 5: Freeze Pipe Installation High Speed Coring Rig

Freeze pipe installation was completed after approximately twelve months including the time

required by the general contractor to excavate and brace a portion of the bellmouth that was

previously unexcavated. In total 62 freeze pipes were installed utilizing several different types of drill

rigs to overcome access and different strata.

Figure 6: Installing Freeze Pipes with Geotechnical Drill Rig

In addition to freeze pipes, it was necessary to install several temperature monitoring pipes.

Generally, these pipes are drilled and constructed similar to freeze pipes but instead of circulating

brine they are equipped with RTD sensors, thermocouple wire, or fiber optic wire. The temperature

probes were used to measure the change in the ground temperature as the freezing progressed.

Typically, the locations of the temperature pipes were selected based on the as-built surveys of the

pipes and installed in areas which had the greatest pipe deviation so that the worst case scenario

could be monitored. Given the excavation sequence, the monitoring pipes had to be chosen before

all of the freeze pipes were drilled. In total five temperature monitoring pipes were installed.

The drill casing used to advance the borehole was left in place and a 4 inch schedule 40 steel pipe

with welded joints was installed inside each casing. The pipe was pressure tested to safeguard

against leaks and the annulus between the casing and the pipe was grouted with a non-shrink grout.

The grout provided proper heat transfer between the circulating brine and the surrounding soils.

All of the freeze pipes were connected to a common supply and return header pipe which could

circulate the chilled brine back to the refrigeration plant at a rate of approximately 20 gpm through

each freeze pipe. A single refrigeration plant with a rating of 312 tons of refrigeration was utilized.

The freeze plant used ammonia as a primary refrigerant to chill the 28% calcium chloride solution to

temperatures ranging between -25 to -30 degrees Celsius. Refrigeration was accomplished with two,

400 horesepower electrically-powered compressors contained within the custom-made freeze plant

manufactured specifically for ground freezing operations.

The actual freezing took approximately 20 weeks to achieve the closure. During that time, constant

data acquisition was maintained. The key components to the freezing were temperatures and

groundwater levels. For this project, the pressure inside the frozen arch was monitored with the use

of pressure transducers hooked up to wells installed within the arch footprint. A gradual increase in

groundwater pressure inside the arch would indicate that the frozen mass had achieved closure

resulting from the frozen soil mass growing inwards increasing the pore pressure on the unfrozen

soils. Groundwater levels were also measured in a series of vertical piezometers across the entire

site. Daily records of the ground temperatures recorded in the temperature pipes provided data that

could be used to verify assumptions made in the initial thermal model. The use of actual ground

temperatures in revised calibrated models aided in the determination of the in-situ wall thickness and

temperature.

IV CONCLUSION

The General Contractor completed the final lining for the SEM tunnel on February 25, 2013. The

freezing system operated as designed for approximately 455 days.

Figure 7: Completed Tunnel

This project represents the ability for artificial ground freezing to act as both structural support and

groundwater cutoff in highly variable urban soils.

REFERENCES

Powers, J., Corwin, A., Schmall P., and Kaeck W., 2007. Construction Dewatering and Groundwater

Control New Methods and Applications. John Wiley & Sons Inc, United States, pp 491-500

Andersland, O.B., and Ladanyi, B. 2004. Frozen Ground Engineering, Second Edition. John Wiley &

Sons, Inc., Hoboken, NJ.