Tunneling techniques

APPENDIXTunneling TechniquesDrill-and-BlastDrill-and-blast is still used where the use of tunnel boring machines is not economicallyfeasible. Tunnel Nos. 1 and 2, as well as Tunnel stage 1, were excavated by drilling and ives are loaded in holes that are drilled in a specific pattern chosen to produce the mosteconomical and satisfactory breakage of rock. Drills may be individually mounted on bars orcolumns, with an adjustable clamp permitting movement. This method was used in the past(including for tunnel Nos. 1 and 2) and is still effective for small tunnels. However, for largetunnels, drills are now mounted on drill carriages known as “jumbos.” A drill jumbo is aportable carriage with one or more working platforms equipped with bars, columns, or booms tosupport several drills. The supports allow the drills to accommodate any drilling pattern. Thejumbo moves along the tunnel as excavation the holes are drilled, they are loaded with explosives. Detonation, usinginstantaneous and delay exploders, follows a specified sequence. Dynamite is used extensivelyfor blasting; a charge may be fired by a blasting cap or fuse. 15

Ammonium nitrate explosivesmay also be used. More than 7,000 blasts were needed to excavate stage 1 and more than 2million cubic yards of rock spoil were removed—enough to fill Madison Square Garden fivetimes. The drill-and-blast method poses severe safety hazards to tunnel workers. Un-detonatedexplosives pose a risk, as do the dust, gases, and noise associated with blasting. In addition,blasting disturbs and may fracture the rock around the tunnel, thus increasing the risk of fallingrock and most common shapes for tunnel cross-sections are circular, elliptical, horseshoe, andvertical wall with arch-roof. All three City water tunnels were designed with a circular ing blasting, the tunnel is ventilated. Gases, fumes, and dust created during thedrill-and-blast operation are removed. Mucking (loading broken rock or earth for removal fromthe tunnel) then proceeds. Mechanical muckers, such as power shovels, are typically is transported out of the tunnel by either narrow-gauge muck cars pulled by locomotives,diesel trucks, conveyor belts, or through a pipeline, as a ConstructionShaft construction is accomplished by three components: excavation, lining, and theinstallation of plumbing and equipment. Shaft excavation must first proceed throughunconsolidated rock or overburden typically situated on the bedrock. In areas where bedrock isclose to the surface, sheet piling can be used to keep the excavation from caving in before it can

A “round” consists of drilling holes into the tunnel face, loading the holes with explosives, and detonatingthe explosives.1575

be lined with concrete. In areas where there is a significant depth of overburden, other methodsmust be stage 1, most of the shafts were in areas where solid rock was close to the surface. Inthe stage 2 Brooklyn and Queens section of the tunnel, most of the shafts were in areas that hadsignificant depths of water-laden overburden. In these areas, contractors chose to use ground-freezing as the primary method of support for the overburden excavation. The freeze methodentails drilling small vertical holes in a circle approximately 10 feet around the proposed finisheddiameter of the shaft. Pipes containing cooling elements are inserted into the holes to graduallyfreeze the water-laden soil surrounding the pipes. Eventually, a frozen cylinder is formed aroundthe proposed shaft location. Excavation then proceeds, using clamshell buckets to remove theunfrozen soil in the middle. When solid rock is reached, the excavated shaft is lined withreinforced concrete. The cooling pipes are then removed, and shaft construction into solid rockbeneath is construction in solid rock can continue in one of two ways: either by using thetraditional drill-and-blast method, or by a technique known as raise boring. In cases where shaftsare constructed before the tunnel underneath is constructed, drill-and-blast techniques must beused. When the tunnel is already excavated beneath the shaft being constructed, contractors havethe option of using the raise-bore method, which involves drilling a relatively small vertical holedown into the existing tunnel. A raise-bore drill head is then installed at the bottom of the shaftand worked upward by diesel-powered machinery on the surface, to yield a fully excavated bit resembles a Christmas tree in shape, with a rod attached to the top. Muck and debrisfrom the drilling operation falls to the tunnel floor, where it can be hauled away to another shaftand disposed of. A major benefit of this method is that it is less disruptive to people living orworking near the shaft site: there is no need to blast, and muck is hauled away below groundinstead of through the streets. In addition, it is easier and less costly than the drill-and-blastmethod. Most of the stage 1 shafts were constructed using the raise-bore Tunnel shaft will be outfitted with one to four 48-inch diameter riser pipes linedwith stainless steel cladding, and will include a chamber (constructed in rock) that houses riservalves and other mechanical equipment. In addition, each shaft has a distribution chamber justbelow the street. The function of the system at each shaft is to supply local needs by providinginterconnections with the existing distribution system (water mains) through the Boring MachineStage 1 of the Tunnel was excavated by drilling and blasting through rock. Stage 2 isbeing excavated by a tunnel-boring machine—a technology that was developed in the 1960'ed to drilling and blasting, a tunnel-boring machine:• can excavate rock at a greater rate• results in a smoother tunnel wall76

• does not disturb the rock around and adjacent to the excavation. This allows for lesssupporting steel to safely anchor the surrounding excavation.• provides for less overbreak of rock, resulting in the use of much less concrete in thelining process. DEP engineers estimate that a tunnel excavated by a tunnel-boringmachine requires three cubic yards per foot, compared to nine cubic yards per footusing drill-and-blast.• requires much fewer personnel than drill-and-blast gh other projects (such as the January 1972 North River North Branch InterceptingSewer Project) were successfully completed using tunnel-boring machines in hard rock areas ofNew York City, DEP did not utilize tunnel-boring machines for construction of the Tunnel untilthe stage 2 Brooklyn section was started. DEP stated that an earlier attempt at using a tunnel-boring machine to construct the Richmond Tunnel discouraged it from doing so until much -boring machine use was discontinued in the Richmond Tunnel in the late 1960’s, due topersistent alignment problems after only 400 tunnel-boring machine used to construct the stage 2 Brooklyn section of the Tunnelexcavated an 18-foot, 11-inch-diameter hole, which was lined to form a 16-foot finisheddiameter tunnel. The machine had eight 261 kilowatt (350HP), 575 volt motors and a 2,088kilowatt (2800 HP) cutting head that contained 40 17-inch diameter cutters. A larger tunnel-boring machine (23-foot diameter cutting head) is being used to excavate the Queens portion ofstage 2. It is 85 feet long and contains approximately 50 19-inch diameter SupportThere are various methods of anchoring bolts in rock. In the Brooklyn section of stage 2,the contractor elected to use "swellex" bolts. These bolts resemble collapsed tubes that are slidinto drilled holes, then expanded by applying pressurized air in one end. The main advantage ofthis type of bolt is that they grip along their entire te LiningIn pressure tunnels, the parent rock is used as the primary system of containment forfuture internal pressures, thus minimizing the tunnel-lining structure. In most areas where rockis strong and tight, an un-reinforced concrete lining can be installed. In areas where rock qualityis deficient, steel pipe liners must be installed. For tunnels driven through solid rock, it isdesirable to delay the start of concrete lining until excavation is completed. In this way, muckingand lining operations will not interfere with each other, and greater operating efficiency can stages of the Tunnel will be lined with watertight concrete. A concrete lining, 17inches thick, was incorporated within the entire length of stage 1 and in the Brooklyn section ofstage 2. Three on-site concrete batching facilities were used to supply stage 1 e-mixed concrete was dropped as deep as 800 feet and hauled as far as five miles in thetunnel in specially designed agitator cars. The overall compressive strength for test specimens77

made in the field averaged 6,800 psi. Approximately 750,000 cubic yards of concrete were usedto line stage used for lining tunnels are generally known as "traveling" forms, and areconstructed of steel or a combination of steel and wood. (See photographs on page 79.) Theyare constructed of steel members that are lined with steel plate or wood to provide a surface thatconforms to the shape of the inside surface of the tunnel. Each form is mounted on a traveler ora jumbo on wheels on which it moves along rails. A traveler is equipped with adjustable jacks orscrew ratchets that permit the form to be expanded into position for a concrete pour, thencollapsed slightly to pull it away from the concrete and move it to a new location. Concrete ispumped into the top of the form through a pipe. As the concrete fills the space behind and abovethe form, the pipe is withdrawn until the entire space is ent for installing the full-circle, cast-in-place, concrete lining in the stage 2Brooklyn tunnel consisted of a specially designed traveling formwork. The formwork consistedof 540 feet of telescopic steel forms segmented into 30-foot sections. After the last 30-footsection of previously poured concrete lining had cured, the section was collapsed and movedforward to the beginning of the 540-foot formwork train, where the form was prepared foranother pour. The form was long enough to permit the continuous pouring of concrete whileallowing for proper concrete curing time. This procedure lined approximately 300 feet per day,overall. Approximately 90,000 cubic yards of 6,000 psi concrete was placed during constructionof this section of ngGrout consists of a cement and water mix. It is pumped under pressure into the areabetween the tunnel lining and the excavated wall to fill voids between the concrete and ng stops or reduces water infiltration through the concrete lining and repairs anyweaknesses that would compromise the load-carrying capacity of the ng usually occurs concurrently with the placement of concrete lining. Tunnelexcavation on a downhill heading with high groundwater inflows can damage excavationequipment. Effective pre-grouting is required in these areas. Contract documents for stage 2excavation required an advance drill hole to be a minimum of 30 feet ahead of the excavationface. When water inflows greater than 20 gallons per minute (gpm) are encountered through theadvance hole, grout holes are drilled into the face, and face-grouting operations are pre-grouted areas can then be excavated with minimal water inflow.78

Typical Concrete Form Liners79