12.4 Setting Out Methods: Laser Levels, Optical Instruments & Boning Rods

Setting Out Methods: Laser Levels, Optical Instruments & Boning Rods

Setting out an underground drainage network requires transferring architectural design lines and engineering invert levels onto raw, uneven earth with uncompromising accuracy. In structural framing, a 5 mm variance might be absorbed by timber packers; in gravity drainage, a 5 mm vertical error over a 10-metre run at $1:60$ represents a catastrophic $30\%$ deviation in gradient, transforming an approved self-cleansing line into a sluggish silt trap or a high-velocity separator.

A registered Certifying Drainlayer must be fully competent in deploying both modern electronic instruments and traditional optical and mechanical leveling techniques. While self-leveling pipe lasers represent standard trade equipment on modern commercial and residential developments, certifying drainlayers must retain mastery of optical automatic levels (dumpy levels) and traditional sight rail and boning rod setups. When electronic lasers fail, battery supplies deplete, or deep trench excavations present line-of-sight obstructions, mechanical principles provide an infallible, physics-based method for verifying grade and alignment under NZBC Clause G13 and AS/NZS 3500.2:2021.

---

1. Rotating Pipe Laser Levels (The Modern Trade Standard)

Dedicated pipe lasers (such as Leica Piper, Topcon TP-L series, or Spectra Precision) are compact, ruggedized, waterproof instruments engineered specifically to operate inside manholes, pipes, and narrow trench excavations.

+-------------------------------------------------------------+
|               PIPE LASER OPERATION IN TRENCH                |
|                                                             |
|   Manhole Base or Trench Start                              |
|   +---------------+                                         |
|   | Pipe Laser    | === Red/Green Collimated Beam ===>      |
|   | Digital Grade |   (Set to -1.670% for 1:60)             |
|   +---------------+                                         |
|      ^ Centering Legs                                       |
|   ====================================================      |
|   Granular Bedding                                          |
|                                                             |
|                     Newly Laid Pipe Barrel                  |
|                  +--------------------------+               |
|                  |      Target Plate        |               |
|                  |  +--------------------+  |               |
|                  |  |         |          |  |               |
|   Beam Strikes-> |  |----+----*----+---- |  |               |
|   Centre Cross   |  |         |          |  |               |
|                  |  +--------------------+  |               |
|                  +--------------------------+               |
+-------------------------------------------------------------+

Operating Principles and Features

• Digital Grade Input: The instrument features internal electronic pendulum sensors driven by precision stepper motors. The drainlayer inputs the required gradient directly via the digital interface as a percentage (e.g. entering -1.670% for a $1:60$ fall, or +1.670% if shooting uphill).
• Centering Feet & Self-Centering Geometry: The laser housing is fitted with interchangeable screw-in legs sized specifically to match standard pipe bores (DN 100, DN 150, DN 200, DN 225, DN 300). When placed inside the bell end or barrel of a starter pipe, the feet automatically centre the laser diode on the exact central horizontal axis of the pipe.
• Grade Target Plate: A translucent plastic target marked with crosshairs and pipe diameter rings is slotted into the leading spigot or bell of each new pipe length as it is placed on the bedding. The pipe layer adjusts the pipe until the laser dot hits dead-centre on the crosshair target, ensuring both horizontal alignment (line) and vertical gradient (level) are perfect simultaneously.

Thermal Refraction Hazards ("Trench Shimmer")

A critical trade hazard when using pipe lasers over distances exceeding 20 metres is thermal refraction:

• When a trench or newly laid pipe is exposed to intense sunlight, or conversely when cold morning air enters a warm pipe, air density layers form inside the barrel.
• The laser beam bends (refracts) as it passes through air layers of varying temperature and density, causing the laser dot to drift upwards, downwards, or shimmer unpredictably.
• Mitigation: Certifying drainlayers must operate a small 12-volt air extraction blower or ventilation fan to circulate air through the pipe bore, establishing uniform temperature and ensuring a straight, true laser beam.

---

2. Optical Automatic Levels & Metric Staff Reading

The optical automatic level (commonly referred to in the trade as a "dumpy level" or "auto-level") is the foundational leveling instrument of civil engineering and drainage. It utilizes an internal optical compensator (a magnetically or pneumatically damped pendulum prism) that automatically levels the line of sight (line of collimation) within a small operational window.

Reading the Metric "E-Face" Leveling Staff

Metric leveling staffs are divided into alternating black and red blocks and graduations on a white background:

+-------------------------------------------------------------+
|                 METRIC E-FACE STAFF READING                 |
|                                                             |
|   Top of Meter Marker (e.g. Large '1')                      |
|   Decimeter Block (e.g. '04' = 1.400 m)                     |
|                                                             |
|   1.450 m --+  [E-Pattern Limb Top]                         |
|             |  [Solid Block = 10 mm]                        |
|   1.440 m --+                                               |
|             |  [Open Space = 10 mm]                         |
|   1.430 m --+                                               |
|             |  [Crosshair Position = 1.424 m]               |
|   1.420 m --+-- ---- -- ---- -- ---- -- ---- Crosshair Line |
|             |  [Estimate mm: 4 mm above 1.420 line]         |
|   1.410 m --+                                               |
|             |                                               |
|   1.400 m --+                                               |
+-------------------------------------------------------------+

• Metres: Indicated by large red or black whole-meter numerals (e.g. 1, 2).
• Decimetres: Indicated by two-digit numerals (e.g. 04 represents $0.4\text{ m}$, giving $1.400\text{ m}$ if below the 1 meter mark).
• Centimetres: Each alternating solid bar or open space of the "E" shape represents exactly $10\text{ mm}$ ($0.010\text{ m}$). The three horizontal bars of the letter "E" and the two spaces between them total $50\text{ mm}$ ($0.050\text{ m}$).
• Millimetres: The observer visually interpolates (estimates) within the $10\text{ mm}$ block from $1$ to $9\text{ mm}$ ($0.001$ to $0.009\text{ m}$). A high-calibre certifying drainlayer consistently reads a staff to within $\pm 1$ to $2\text{ mm}$.

Leveling Terminology

• Backsight (BS): The very first staff reading taken after setting up the instrument, always taken on a point of known elevation (such as a Benchmark or Temporary Benchmark, TBM).
• Foresight (FS): The final reading taken from an instrument setup, taken on a temporary change point (CP) before moving the instrument to a new tripod location.
• Intermediate Sight (IS): Any staff reading taken on points of interest (ground surface, trench floor, pipe invert) between the Backsight and the Foresight.
• Change Point (CP): A solid, immovable point (e.g. steel peg, prominent rock, or kerb) upon which the staff is held while the tripod is moved to a new position. The foresight is read on the CP from the old position, and the backsight is read on the identical CP from the new position.

The Collimation (Height of Instrument) Method

The Collimation Method is the fastest and most common method for calculating site levels:

$$\text{Height of Instrument (HI)} = \text{Known Benchmark RL} + \text{Backsight (BS)}$$ $$\text{RL of Target Point} = \text{Height of Instrument (HI)} - \text{Intermediate Sight (IS) or Foresight (FS)}$$

The Rise and Fall Method & Mathematical Checks

The Rise and Fall Method compares consecutive staff readings. Because a higher staff reading indicates a lower ground elevation:

$$\text{Difference} = \text{Previous Staff Reading} - \text{Current Staff Reading}$$

• If difference is positive, ground has risen: $\text{Rise} = +\text{Difference}$.
• If difference is negative, ground has fallen: $\text{Fall} = -\text{Difference}$.

$$\text{Current RL} = \text{Previous RL} + \text{Rise} \quad \text{or} \quad \text{Current RL} = \text{Previous RL} - \text{Fall}$$

The Three-Point Mathematical Closure Check

To prevent mathematical errors in field books, certifying drainlayers must perform the three-point check on every leveling sheet:

$$\text{Sum BS} - \text{Sum FS} = \text{Sum Rise} - \text{Sum Fall} = \text{Last RL} - \text{First RL}$$

All three calculations must yield the identical number. If they do not, an arithmetic mistake exists in the reductions.

---

3. Traditional Sight Rails and Boning Rods (The Traveler Method)

Before optical lasers, drainage across the globe was set out using timber sight rails and boning rods (also known as travelers). In deep, wet, or hazardous trench conditions where electronic gear cannot be safely stationed, sight rails remain an essential, robust trade technique.

+-------------------------------------------------------------+
|              SIGHT RAILS AND TRAVELER GEOMETRY              |
|                                                             |
|   Sight Rail 1                                              |
|   [RL = IL1 + T]                                            |
|       +======+                  Sight Rail 2                |
|       |      |                  [RL = IL2 + T]              |
|      ===    ===                     +======+                |
|       |      |                      |      |   Sight Rail 3 |
|      ===    ===                    ===    ===  [RL=IL3 + T] |
|       |      |                      |      |       +======+ |
|       |      |                      |      |       |      | |
|   -- - - - - - - Line of Sight - - - - - - - - - - - - - -  |
|       |      |                      |      |      ===    ===|
|       |      |                 +---------+ |       |      | |
|       |      |                 | Traveler| |       |      | |
|       |      |                 | Rod (T) | |       |      | |
|       |      |                 +----+----+ |       |      | |
|       |      |                      |      |       |      | |
|       |      |                      | T    |       |      | |
|       |      |                      |      |       |      | |
|       *======*                      *======*       *======* |
|   Invert 1                      Invert 2       Invert 3     |
|   ========================================================= |
|   Continuous Design Gradient Line (Fall / Run)              |
+-------------------------------------------------------------+

Components and Setup

1. Sight Rails: Horizontal timber boards (typically $150 \times 25\text{ mm}$ dressed pine) securely nailed across twin upright stakes driven into firm ground spanning or offset from the proposed trench line. Sight rails are set up at minimum three points along a pipeline (upstream head, intermediate station, and downstream outfall).
2. Boning Rod (Traveler): A lightweight T-shaped rod made of wood or aluminum. The crosshead is perfectly perpendicular to the vertical leg. The vertical length of the traveler ($T$) is fixed and uniform (commonly $2.000\text{ m}$, $2.500\text{ m}$, or $3.000\text{ m}$ depending on trench depth).

Calculating Sight Rail Elevations

The elevation of each horizontal sight rail is calculated directly from the design invert level:

$$\text{Sight Rail RL} = \text{Design Invert Level (IL)} + \text{Traveler Length } (T)$$

To nail the rail at the exact height using an optical level:

$$\text{Required Staff Reading on Top of Rail} = \text{Height of Instrument (HI)} - \text{Sight Rail RL}$$

The Three-Rail Alignment Check

A fundamental advantage of the sight rail method is instant, self-checking visual verification:

• When looking across the tops of the three sight rails, they must form a single, unbroken, inclined plane of sight.
• If any upright stake is knocked by an excavator, settles in soft clay, or is miscalculated, the three rails will not align in a straight plane. The defect is immediately obvious to the naked eye before a single pipe is laid.

Sighting with the Traveler in the Trench

• The drainlayer places the foot of the boning rod on the pipe invert.
• An observer sights across the tops of the upstream and downstream sight rails.
• When the horizontal crosshead of the boning rod aligns precisely with the line of sight across the two sight rails, the pipe is on exact design grade.
• If the traveler crosshead is above the line of sight, the trench must be dug deeper; if below, the trench has been over-excavated and requires compacted bedding.

---

4. Stringlines and Line Levels

For short branch drains (under 10 metres), subfloor suspended piping, or connecting gully traps to a nearby inspection junction, stringlines fitted with specialized line levels provide a quick setting-out method.

Rules for Stringline Accuracy

1. High-Tension Braided Nylon: Never use twisted cotton or polypropylene cord that stretches, sags, or absorbs moisture. Pull braided nylon stringline taut until it emits a high-pitched twang.
2. Midpoint Placement of Line Level: A line level weighs between $10$ and $20\text{ grams}$. If placed near an end post, its weight pulls the stringline down, creating an asymmetric catenary curve. The line level must always be suspended at the exact geometric midpoint between the two stakes, where catenary deflection is symmetrical.
3. Distance Limits: Stringlines must never be used for continuous runs exceeding 10 metres, nor for gradients flatter than $1:60$ ($1.67\%$).

---

5. Instrument Calibration: The Two-Peg Test

An optical automatic level must be calibrated before critical drainage work. Even a brand-new level can develop collimation error (where the internal line of sight is not truly horizontal when the circular bubble is centred) due to road transit vibrations, temperature fluctuations, or accidental knocks.

+-------------------------------------------------------------+
|                   THE TWO-PEG TEST GEOMETRY                 |
|                                                             |
|   SETUP 1: Instrument Midway between Peg A and Peg B        |
|                                                             |
|             Line of Sight (Inclined by error e)             |
|         - - - - - - - - - * - - - - - - -                   |
|        /                  |                  /              |
|     Staff A            Tripod              Staff B          |
|      |===|             Midway               |===|           |
|      |   |             (15 m)               |   |           |
|     Peg A                                  Peg B            |
|     *===* <--- True Height Diff = a1 - b1 ---> *===*        |
|                                                             |
|   SETUP 2: Instrument Moved Close to Peg A                  |
|                                                             |
|     Tripod                                                  |
|     at Peg A       Line of Sight (Inclined by error e)      |
|       |       - - - - - - - - - - - - - - - - - - - - - -   |
|     Staff A                                        Staff B  |
|      |===| (a2)                                     |===| (b2)
+-------------------------------------------------------------+

Two-Peg Test Field Procedure

1. Drive two firm wooden pegs (Peg A and Peg B) into level ground approximately $30\text{ metres}$ apart.
2. Setup 1 (Instrument at Midpoint): Set up the optical level exactly halfway between Peg A and Peg B ($15\text{ m}$ from each). Take staff reading $a_1$ on Peg A and staff reading $b_1$ on Peg B.
   • Because the distances to Peg A and Peg B are identical, any collimation error ($e$) tilts the line of sight by the exact same amount at both pegs. The errors cancel out!
   • True Height Difference: $$\Delta H = a_1 - b_1$$
3. Setup 2 (Instrument Close to Peg A): Move the instrument and set it up within $2$ to $3\text{ metres}$ of Peg A. Take staff reading $a_2$ on Peg A and staff reading $b_2$ on Peg B.
4. Calculate Collimation Error:
   • If the instrument has zero collimation error, reading $b_2$ must equal $a_2 - \Delta H$.
   • Expected Reading on Peg B: $$b_{2\text{ (expected)}} = a_2 - \Delta H = a_2 - (a_1 - b_1)$$
   • Collimation Error: $$\text{Error } (e) = b_2 - b_{2\text{ (expected)}}$$
5. Tolerance and Adjustment:
   • If the error is less than $3\text{ mm}$ over $30\text{ metres}$, the level is within acceptable trade tolerance.
   • If the error exceeds $3\text{ mm}$, the instrument requires adjustment. With the staff still on Peg B, remove the reticle cap and turn the reticle adjusting screw with an allen key until the horizontal crosshair aligns with $b_{2\text{ (expected)}}$.

---

6. Worked Leveling Run & Field Book Record

Below is a complete, real-world drainage leveling run recorded by a certifying drainlayer establishing inverts along a proposed foul drainage pipeline.

Field Book Reduction (Rise and Fall Method)

• Benchmark (TBM 1): Top of boundary survey peg. Known elevation = $\text{RL } 50.000\text{ m}$.
• Station 1: Ground level at proposed gully trap.
• Station 2: Invert of proposed gully trap outlet.
• Station 3: Invert at inspection junction (Station $15.0\text{ m}$).
• Station 4 (CP 1): Change point on top of kerb.
• Station 5: Invert of council sewer connection stub.

Arithmetic Closure Verification

1. Check 1 (BS vs FS): $$\text{Sum BS} - \text{Sum FS} = 3.265 - 4.075 = -0.810\text{ m}$$
2. Check 2 (Rise vs Fall): $$\text{Sum Rise} - \text{Sum Fall} = 1.030 - 1.840 = -0.810\text{ m}$$
3. Check 3 (RL Difference): $$\text{Last RL} - \text{First RL} = 49.190 - 50.000 = -0.810\text{ m}$$

All three checks balance exactly to $-0.810\text{ m}$, confirming that the field book reductions contain zero arithmetic errors.

---

7. Setting Out Trade Traps

+-------------------------------------------------------------------------+
|                        DRAINLAYER TRADE TRAPS                           |
|                                                                         |
| [!] THE TRIPOD SHIFT WITHOUT A FORESIGHT                                |
| Picking up the level tripod to move to a new vantage point without      |
| establishing a firm Change Point (CP) and taking a foresight reading.   |
| The entire leveling run is instantly ruined.                            |
|                                                                         |
| [!] THE TRIMMED BONING ROD TRAP                                         |
| An apprentice cuts 50 mm off the bottom of a wooden traveler rod to     |
| 'clean up a splintered foot.' Every pipe laid that afternoon ends up    |
| 50 mm too shallow, creating backfall into the foundation.               |
|                                                                         |
| [!] THE TWO-PEG BLIND SPOT                                              |
| Using an optical level that has been rolling around in the back of a    |
| trade ute for six months without performing a Two-Peg Test. A 5 mm      |
| collimation error compounds over multiple setups, destroying grades.    |
+-------------------------------------------------------------------------+