Keeping a tunnel boring on line and grade is a complex task that places heavy responsibility on the surveyor. Establishing survey control for an interconnected network of tunnels adds complexity to the survey requirements. Where multiple headings are driven at the same time by different construction contractors, the engineering survey must assure that the interconnecting tunnels will be tied together within the allowable construction tolerance for both line and grade. Under these conditions, it is important for the engineering-manager to develop a survey program which considers all steps in the sequence of survey operations, from the precontract stage to the setting of the last work point for construction. The survey program must assure construction of the tunnels within the specified construction tolerance and must be consistent with the survey methods used by the contractors.
The methods described in this chapter are representative of the present state-of-the-art and have proven successful with maximum daily tunneling rates of 100 ft (30 m) per day and average daily tunneling rates of 40 ft (12 m) per day.
A preliminary horizontal and vertical control survey is required to obtain general site data for route selection and for structure design. The preliminary survey can rely on existing survey records and monuments, with additional temporary monuments and bench marks placed where required. In addition, photogrammetric mapping, recording of seismic activity, and geophysical profiling are performed. If construction of a subaqueous tube is planned, hydrographic mapping and current velocity surveys should be included in the preliminary work. The width of the corridor to be surveyed may vary from 200 ft (60 m) to 0.5 mile (800 m), depending on the terrain and right-of-way conditions. A large-scale topographic map of the surveyed corridor is prepared to locate the horizontal and vertical projection of the tunnel centerline.
PRIMARY CONTROL FOR FINAL DESIGN AND CONSTRUCTION
After completion of route selection, a horizontal and vertical control survey of high-order of accuracy is conducted. Permanent monuments and bench marks, consisting of brass discs secured in concrete, are established at tunnel portals and over the tunnel alignment to serve as primary control during the final design stage and during construction (Fig. 11-1). Reference marks are set for each monument so that the monuments can be readily verified and, if necessary, reestablished in case they are disturbed or destroyed during construction.
Fig. 11-1. Monuments, bench marks, and centerline track and tunnel.
SURVEY WORK DURING CONSTRUCTION
During construction, the following survey work is performed:
Tunnel centerline location, tunnel stationing, and tunnel grade are transferred from the primary control monuments and bench marks located on the surface to the tunnel and are carried forward as the tunnel is constructed.
A construction control system is established that will assure tunnel driving or subaqueous tube placement within the allowable tolerance.
Observation wells are installed to monitor groundwater levels adjacent to tunnels and underground structures.
Surface movements are carefully monitored over the tunnels.
Special recording devices are installed to record vertical and lateral soil movement or stresses adjacent to tunnels or underground structures.
When construction of the tunnels is completed, permanent centerline monuments are placed in the completed tunnel at intervals of about 1,000 ft (300 m) and at all tangent-to-spiral and spiral-to-circular curve points. From these monuments measurements are taken laterally to critical clearance points to ensure that the clearance envelope is in accordance with design requirements for guideway installation.
DIVISION OF RESPONSIBILITY BETWEEN ENGINEER AND CONTRACTOR
Survey costs are small in comparison to the expenditures involved in tunnel driving. Nevertheless, if tunnel driving is held up because of faulty survey work or because of interference of the survey crew with driving operations, resulting monetary losses may be large. The following analysis of heading costs illustrates this point: Unit bid price of tunnel, $1,200 per foot (30 cm); average advance, 40 ft (12 m) per day; and average income, $2,000 per hour. Even if only one-third of the average income is related directly to heading time, preventing any delay in the tunneling operation is important to the contractor.
For this reason, specifications relating to tunnel-driving accuracy should be written as a performance specification, and the contractor should be assigned full responsibility for transferring line and grade from the primary surface control into the tunnel and for development of tunnel-construction control procedures. Tunnel construction crews should be equipped with state-of-the-art laser alignment instruments.
Thus, the survey-engineer performs all survey work before the start of construction, such as preliminary surveys and primary control surveys on the surface. During construction the survey-engineer’s responsibility should be confined to checking surveys involving coordination of survey work at contract interfaces. The engineer thus assures that the underground survey control of two adjacent contractors agrees at the interface.
Information concerning groundwater levels, as obtained from observation well readings, is of vital importance to the contractor’s tunneling operation. It is thus reasonable to include the installation of observation wells, maintenance of the wells, and periodic reading of water levels in the construction contractor’s contractual obligation. Water level records should be made available to the engineer-manager at the time of recording.
Monitoring of surface settlement, which serves as an indication of potential construction problems at the tunnel heading, is not of immediate practical concern to the contractor. Indeed, the probability of obtaining accurate and timely readings during times when heading problems are being encountered are lower than during times of normal operation. The contractor’s best people will be preoccupied with the construction problems at the heading during times of trouble and, therefore, will be able to spend a minimum of time on required surface-level readings. Therefore, levels over the tunnels should be run and evaluated by the survey engineer, and the results should be made available to the contractor. This is also recommended with respect to the installation and monitoring of special recording devices, such as subsurface settlement points, inclinometers, and strain gages.
PRIMARY CONTROL SURVEY NETWORK
To establish survey control for the construction of a tunnel or subaqueous tube, the control must be tied into the basic survey control network of the area. Accordingly, any major project in the United States should be tied to the national geodetic control network as maintained by the National Geodetic Survey (NGS). The advantages in tying the primary control for the project in several places to the national network include: The closures provide an independent check on the new survey; the resulting survey data are consistent and can be readily correlated with data from other surveys in the area; and two or more connections provide adequate orientation for the horizontal control survey. It should be noted that bench marks located in, or near, the construction area may settle, and the elevations should be checked periodically during construction of the project from reference bench marks located on firm ground outside the construction area.
Traverses for the primary control survey network should be run before the contract drawings are completed. Each traverse may be several miles in length. The primary traverses should approximate the future tunnel alignment and should be run between existing higher order triangu-lation stations located near the tunnel alignment (Fig. 11-1). Triangulation stations should be tied into the existing national geodetic control network by angular and distance measurement.
AH monuments should be described by narrative and sketch showing state plane coordinators, elevation, and bearings and distances to adjacent monuments. Horizontal and vertical control survey diagrams and monument descriptions should be included in the contract drawings. Permanent bench marks should be placed as required to carry elevation into the tunnel. If settlement over the tunnel is a concern as, for example, in urban areas, additional bench marks should be placed along the tunnel alignment at distances not exceeding 600 ft (180 m). The bench marks should be placed about 200 ft (60 m) from the centerline of the tunnel to avoid settlement caused by tunnel excavation. During construction of the tunnel, surface and subsurface settlement monitoring points should be set over the centerline of the tunnel and on adjacent buildings. Level loops for settlement monitoring surveys should be run through the settlement points and closed on permanent bench marks.
RELATIONSHIP OF CENTERLINE OF TRACK AND CENTERLINE OF TUNNEL
On a rapid transit system, centerline of track and centerline of tunnel are normally not identical because of clearance requirements. The centerline of track should be the basis for the control during overall layout of the work. During construction of the tunnel, however, it may be desirable from a practical standpoint that the centerline of tunnel rather than centerline of track be used as the basis of construction control.
The vertical and horizontal offset from centerline of track to centerline of tunnel will vary with the superelevation of track. The resulting tunnel centerline is a curve of complex mathematical definition and cannot be produced in the field using standard survey procedures.
Therefore, a tunnel centerline should be developed which is composed of tangent, circular, and transition spiral sections and approximates the complex theoretical tunnel centerline within a specified tolerance as, for example, 1/4 in. (6 mm). This centerline should be incorporated into the contract drawings, and all tunnel control should be based on this line. A computer printout listing the coordinates of points, tangent bearings, elevations of points, and slope at 5-ft (1.5-m) intervals on the tunnel centerline should also be incorporated into the contract documents.
Since the stationing along the centerline of tunnel and centerline of track will not agree because of different curve radii, station equations between centerline of tunnel and centerline of track should be incorporated at the beginning and the end of each construction contract, at tangent-to-spiral, at spiral-to-curve points, and at such points as vent shafts or cross passages. Stationing along the centerline of tunnel should start at Station 0+00 for each tunnel contract. Stationing of track proceeds through the entire system, which may comprise several tunnel contracts. If the rapid-transit system has a major junction from which several lines branch out in different directions, the Station 0+00 should be assigned to this junction point. Stationing then proceeds to the outlying areas, and future extensions of the system can be added without upsetting the sequence in stationing.
The working line is the survey line used by the contractor’s field personnel to establish shield or tunneling machine guidance in the tunnel. The working line may coincide with the centerline of tunnel or may run through the laser position points for the laser setup. Selection of the working line must be left to the contractor to suit his tunnel equipment and methods.
TUNNEL CONSTRUCTION CONTROL
Construction Control for Drill and Blast Method
Where the tunnel is excavated by drill and blast methods, the centerline is extended to the face before drilling for the next round is begun. The centerline location is marked on the face, and the drill pattern is centered on that mark. Surveyors also give centerline location for positioning steel sets.
Construction Control for Shields or Tunneling Machines
A shield or tunneling machine progresses in a sequence of “shoves.” After each shove, the shield or machine is stopped, and its location and attitude are determined. If the shield or machine is found to be off-line, adjustments of the steering mechanism are made to guide it back to its desired location. Where tunnel lining is erected in the tail of the shield, its location and attitude are determined and recorded. The decision of whether to install standard or tapered lining sections after the next shove is based on this record.
The most practical method of shield or machine control is by laser beam and double target. A laser tube, as shown in Fig. 11-2, is set up at a distance behind the shield or tunneling machine to emit a laser beam from a predetermined point of origin along a predetermined line to the targets mounted on the shield or tunneling machine. In the horizontal plane, the laser line is a chord line or a tangent to the tunnel centerline, as shown in Fig. 11-3. In the vertical plane, the laser line approximates the slope of the tunnel centerline as shown in Fig. 11-4.
After the tunnel is driven to the end of one laser beam line, the laser is moved to the next laser position point. The laser tube is then set to emit the beam along the next predetermined laser beam line.
Two targets, called the front target and the rear target, are mounted on the shield or tunneling machine, centered on a line parallel to its longitudinal axis and 4 ft (1.2 m) to 10 ft (3.0 m) apart. The rear target is transparent and the leading target opaque. The targets are intersected by the laser beam, which produces a bright red spot on the target. Theoretical points of intersection between laser beam line and targets are calculated in advance for each shield location. These theoretical points are plotted on the targets and connected by a curved line, as shown on Fig. 11-5. The shield or tunneling machine is guided by the tunneling crew maintaining coincidence of the actual laser line intersection points with the predetermined intersection points on the target.
Fig. 11-2. Control of tunneling machine shields by laser and double target.
Fig. 11-3. Plan of tunnel centerline and laser line.
Fig. 11-4. Profile of tunnel centerline and laser line.
Fig. 11-5. Laser target.
Calculation of Offsets to Laser Line from Tunnel Centerline
The laser line and centerline of tunnel are plotted in plan and elevation. Several trials may be necessary to find the best location of the laser line. The following guidelines are observed in locating the laser lines:
The longest unobstructed line of sight is sought, since it will reduce the required number of laser position changes.
A laser position is selected that is out of the way of passing tunneling equipment.
On tangent and flat curves, the laser line length is limited to about 1,000 ft (300 m) because of diffusion of the laser beam.
The target offsets for the projected laser line should not exceed the size of the laser targets, which is often restricted due to space requirements for other equipment.
At the end of each last line, an overrun is provided. This gives the heading engineer the opportunity to make the change of laser positions and targets at any convenient time while the shield or tunneling machine operates in the tunnel section covered by the overrun. Once the coordinates and the elevation of the laser position, as well as the lateral bearing and slope of the laser line, have been determined, horizontal and vertical target offsets are calculated as shown in Table 11-1. Then the target offsets are plotted on the target as shown in Fig. 11-5. The calculations are performed under the assumption that coordinates and elevation of points on the centerline of tunnel are given at 5-ft (1.5-m) intervals. Also given are the tangent bearing and slope of the tunnel centerline at each point. If these values are not given, they must be determined by standard survey methods. The calculations for target offsets are of a repetitive nature, and the use of a computer is helpful to reduce the time required for the calculations.
Positioning of Laser
The laser must be correctly positioned in its X, Y, and Z-coordinate positions at the laser position point, and the laser must be set to beam along the predetermined laser line. Several ways of mounting the laser tube are in use. One method is to mount the tube on the carriage of a transit, as shown in Fig. 11-2. The transit carriage is mounted on a laser-mounting bracket, which allows vertical and horizontal adjustments. Another way of mounting the laser tube is to secure it inside a pipe with adjustable wingbolts, as shown in Fig. 11-6. The pipe, in turn, is mounted by adjustable brackets to the tunnel lining, as shown in Fig. 11-7.
Regardless of the method of mounting the laser, checks have to be provided to alert the surveyors if the laser drifts off alignment or is hit by construction equipment. A good method of checking utilizes two control targets made of a piece of metal with a hole just large enough for the laser beam to pass through (Fig. 11-8). The control targets are set on laser beam line between the laser and the shield. The laser beam passes through the holes of both control targets. Should the laser move, the disturbance is quickly noticed.
Theodolites and appurtenant equipment are used to set the laser on the laser position point and to set the control targets. Typical equipment requirements for tunnel surveys include: Two theodolites complete with container, tripod, lateral adjusting trivet, illuminating accessories; one optical plumbing device to be used with the theodolite; one set of traverse equipment consisting of three targets with tribrachs interchangeable with the theodolite, battery boxes for illumination of targets; scales and survey platforms as required to use as work points in the tunnel; two levels complete with tripod and trivet; one laser for shield control system; one constant voltage transformer to ensure constant voltage to laser from fluctuating power supply due to varying demands of tunneling operations; four level rods, Philadelphia type, 7 ft (2.1 m) to 13 ft (3.9 m), with micrometer target; two rod levels; one steel tape, 300-ft (90-m) graduated in hundredths of a foot standardized by the National Bureau of Standards; four steel tapes, 100-ft (30-m) standardized, graduated in hundredths of a foot; six steel tapes, 50 ft (15-m) graduated in hundredths of a foot; four clamp handles for steel tapes; four tension handles for steel tapes; two tape thermometers and cases; folding rules graduated in hundredths of a foot; plumb bobs.
Table 11-1. Calculations for Horizontal and Vertical Offsets from Centerline Tunnel to Laser Line*
1. Point P at Station
2. Tangent Bearing
N3° 10 44W
N2° 57 7.9W
N2° 54 3.8W
N2° 50 41W
N2° 7 48.7W
3. Coordinates of Point P
N 468 451.359
N 468 593.646
N 468 603.633
N 468 613.620
N 468 701.035
4. Coordinates of Intersection Laser Line and Curve
Target observations and measurements should be recorded by the heading engineer after each shove. What follows are notes on a typical progress report, as shown in Fig. 11-9, illustrating the required records and the use of the shove jacks to maintain the shield on alignment.
Line 4. Rotation or Roll [1-1/2 in. (3.8 cm) clockwise] is defined as the rotation of the shield about its longitudinal axis. If roll is indicated, the laser targets are adjusted for roll by moving them along the slotted holes until the error in target location caused by roll is eliminated, in this case, a correction of 1-1/2-in. (3.8 cm) clockwise.
Line 5. Front target Deviation [vertical 1-1/4 in. (3.2 cm) high, horizontal 1/4 in. (6 mm) to the left]. Vertical or horizontal target deviation is the vertical or horizontal displacement of the shield at the location of the target from its desired location.
Rear Target Deviation [vertical 1-3/8 in. (3.5 cm) high, horizontal 3/8 in. (1.0 cm) to the left].
Line 6. Ring position at End of Push. Ring No. 309, no record of measured diameter and deviation on this report.
Ring Pitch [1 in. (2.5 cm)]. Ring pitch is determined by hanging a plumb bob from the lead flange at the crown of the ring. The distance from the lead flange in the invert of the ring to the plumb line is measured as ring pitch.
Ring Lead [Ring No. 310—1/8 in. (3 mm)]. Lead boards are maintained on the tunnel lining at springline within 50 ft (15 m) of the heading. On each board, a mark is so placed that the line joining the two marks is at right angles to the tunnel centerline. The ring lead is determined by measuring the distance from the marks on the lead boards along both springlines of the tunnel to the lead flange of the ring. The difference of both measurements is entered into the report as lead ring.
Horizontal Ring Deviation [Ring No. 310—9/16 in. (1.4 cm)]. A plumb line is set on laser line. Measurements are made to the left and right of the plumb line to the ring springline. The offset from laser line to tunnel centerline [0.434 in. (1.1 cm) for Ring 310 at Station 415+54 on Table 1] is subtracted from the measurement to the right springline and added to the measurement to the left springline. The resulting distances represent the distance from left to right springline to tunnel centerline [left 8 ft, 3-3/4 in. (2.53 m) right 8 ft 2-5/8 in. (2.51 m)]. The two distances are added to arrive at the actual ring diameter [16 ft, 6-3/8 in (5.04 m)]. The difference of the two distances is divided in half and represents the ring deviation [9/16 in. (1.4 cm)].
Fig. 11-9. Ring 311, Progress Report.
Fig. 11-10. Laser target mounted on tunneling machine.
Vertical Ring Deviation [Ring No. 310—3/4 in. (1.9 cm)]. A measurement is taken from the laser line to the top flange of the ring. The theoretical distance for this measurement is calculated in the office [2.25 ft (69 cm)]. The theoretical distance is subtracted from the actual measurement [2.319 ft (71 cm)] to arrive at the vertical ring deviation [0.06 ft (1.9 cm) = 3.4 in. (1.9 cm)]. Measurements to the invert of the ring are not included in the ring check because the presence of muck and mucking equipment in the invert make such measurements impractical. Invert flange elevation should be determined after the tunnel is driven.
Line 7. (Ring No. 309, tie rod installed; Ring No. 310, tie rod not installed). Indicates whether the measurements were made after the tie rod, pulling both springlines together, was installed or before.
Line 8 (not shown in Fig. 11-9). Ring Stationing (Ring 309, Stationing 415+51.5; Ring 310, Stationing 415+54).
The following illustrative data are provided for a contract: The shield is of 18-ft (5.49-m) outside diameter. Steel tunnel rings installed behind the shield are of 17.5-ft (5.34-m)-diameter and are made up of a key segment and six segments of 2.5-ft (76-cm) width.
From the data recorded on the Progress Report, the location and attitude of the shield or tunneling machine are determined. In this specific case (after shove for Ring 311), the shield is high [1-1/4-in. (3.2-cm) front target, 1-3/4-in. (4.4-cm) rear target]. It is to the left of tunnel centerline [1/4-in. (6-mm) front target, 3/8-in. (1.0 cm) rear target]. In the vertical and horizontal planes, the shield axis is not parallel to the tunnel axis (see Fig. 11-11 and 11-12).
The deflection of the shield, or machine axis, from the theoretical grade of tunnel centerline is called “inclination” or “pitch.” The algebraic difference of vertical front and rear target deviations serves as a measurement of shield inclination [(l-l/4 in.) - (1-3/8 in.) = -1/8 in. (3 mm)].
The deflection of the shield or machine axis from the horizontal plane is called “yaw.” It is measured as the algebraic difference of the horizontal front and rear target deviations [(l/4-in.) - (3/8 in.) = -1/8 in. (3 mm)] (Fig. 11-11).
After the location and attitude of the shield or tunneling machine are determined, the heading engineer has to decide how to adjust the steering mechanism of the shield or tunneling machine to guide it to the desired location. In the specific case described in the foregoing example, the heading engineer has to decide which jacks to engage to bring the shield back on line. No firm relationship between deviation and jack selection can be developed because every shield responds differently to jack pressure. A basic rule is to make corrections gradually over several shoves and not abruptly. In the example, the heading engineer engaged all jacks except the bottom three (Jacks 1, 9, 10) to make the shield dive (see Fig. 11-13 for jack location). Then he engaged Jacks 1, 9, and 10 again and disengaged Jacks 4, 5, and 6 to level the shield and give it a slight yaw to the right. During the last third of the shove, only Jack 5 was disengaged, keeping the shield on even keel.
The result was a reduction of vertical deviation of front and rear targets from 1-1/4 in. (3.2 cm) to 1 in. (2.5 cm) and 1-3/8 in. (3.5 cm) to 1 in (2.5 cm) respectively. Horizontal deviation of front and rear targets was reduced from 1/4 in. (6 mm) to 1/8 in. (3 mm) and from 3/8 in. (1.0 cm) to 1/8 in. (3 mm), respectively.
The following examples include a breakdown of manpower used for surveying and shield or tunneling machine control on typical projects:
Length and diameter of tunnel—Two 17.5-ft (5.34-m)-diameter tunnels of 5,200-ft (1590-m) length each.
Tunnel equipment—Two tunneling machines driving both tunnels simultaneously.
Average footage per 24-hour day—40 ft (12 m) per machine.
Maximum footage per 24-hour day—107 ft (32.6 m) per machine.
Manpower for transfer of line and grade to heading, maintenance of line in tunnel, and surface settlement readings: One party chief, one instrument man, and two rod men; one day shift every weekday and many weekends.
Tunnel machine control—One surveyor (heading engineer) on each machine each shift (four 6-hour shifts per day).
Fig. 11-11. Side view of shield.
Fig. 11-12. Plan view of shield.
Fig. 11-13. Location of push jacks and laser target.
Length and diameter of tunnel—Two 17.5-ft (5.34-m)-diameter tunnels of 1,640-ft (500-m)-length each.
Tunneling equipment—Two tunneling shields driving both tunnels simultaneously.
Average footage per 24 hour day—20 ft (6.1 m) per shield.
Maximum footage per 24 hour day—32.5 ft (9.91 m) per shield.
Manpower for transfer of line and grade to heading, maintenance of line in tunnel, and surface-settlement readings: One party chief, one instrument man, and two rod men. One day shift every weekday and many weekends.
Tunnel machine control—One surveyor (heading engineer) for both shields (1/2 surveyor per shield) for each shift (three 8-hour shifts per day).
Davis, R. E., Foote, F. S., Anderson, J. M., and Mikhail, E. M., Surveying, Theory and Practice, McGraw-Hill Book Co., Inc., New York, N.Y., 1982.
Encyclopedia of Science and Technology, McGraw-Hill Book Co., Inc., New York, N.Y., 1982.
Gossett, F. C., “Manual of Geodetic Triangulation,” Special Publication No. 247, U.S. Coast and Geodetic Survey, Washington, D.C., 1959.
Rappleye, H. S., “Manual of Geodetic Leveling,” Special Publication No. 239, U.S. Coast and Geodetic Survey, Washington, D.C., 1948.
Talbot, A. N., The Railway Transition Spiral, McGraw-Hill Book Co., Inc., New York, N.Y., 1927.