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Sewer Estimates for Underbores
In cases where a sewer main cannot be installed using open trench excavation—either due to traffic conditions, site constraints, or existing underground services—designers specify an underbore method. Underbores are complex and require specialized machinery, excavation of bore pits, shoring, and precise alignment.
Proper estimating is critical, as underestimating bore length, ground conditions, or shoring needs can lead to significant financial losses.
1. Why Choose Underbore Over Open Trenching?
Sewer runs are typically designed as underbores when:
✅ Roads cannot be closed for open trenching (e.g., major highways, arterial roads).
✅ Existing services make excavation risky (gas lines, electrical conduits, deep stormwater).
✅ Sewer needs to cross a railway line or waterway.
✅ Tight site conditions prevent safe open excavation.
When an underbore is specified, there are two main boring methods:
2. Types of Sewer Underbores
A. Auger Bore
✔ Works on a steady grade but has some tolerance in accuracy.
✔ Cheaper than laser bore.
✔ Suitable for cases where minor grade variations are acceptable.
✔ Uses mechanical augers to cut through the ground while advancing the bore pipe.
B. Laser Bore
✔ Provides precise alignment with zero tolerance for grade variations.
✔ Used when the bore must connect directly into a sewer manhole.
✔ More expensive but ensures high accuracy.
✔ Often required for sewer grades that must maintain a perfect fall.
💡 Choosing the Right Boring Method
- If the sewer requires extreme accuracy, laser bore is the best option.
- If minor tolerance is acceptable, auger bore can be used to reduce costs.
- If the bore is terminating into a sewer manhole, a laser bore is recommended as it can core directly into the manhole for a precise connection (at an additional cost).
3. Understanding Casing Pipes & Free Bore
When performing an underbore, the sewer pipe is either installed inside a casing pipe or directly bored through the ground without casing (free bore).
A. Casing Pipe (Steel)
✔ Used when the ground is unstable and cannot support the bore hole.
✔ Provides extra protection for the sewer pipe.
✔ Requires spacers and grout filling to hold the sewer pipe in place.
✔ Welding of casing pipe sections is required before installation.
💰 Cost Considerations for Casing Pipe:
🔹 Material: Steel casing pipes have different costs.
🔹 Pipe Welding: Additional cost for welding casing pipe sections together.
🔹 Spacers: Used to prevent movement of the sewer pipe inside the casing.
🔹 Grout Filling: The void between the casing pipe and the sewer pipe must be grouted to ensure stability.
B. Free Bore (No Casing Pipe)
✔ If the ground is self-supporting, a free bore can be used without casing.
✔ Requires approval by the designer—not a decision that can be made on-site.
✔ Common in rock, shale, and stable clay formations.
✔ Cheaper than casing bore, but requires careful ground assessment.
💰 Cost Considerations for Free Bore:
🔹 Bore length & ground conditions impact cost.
🔹 Boring rate differs for different soil types (e.g., rock, clay, shale, sand).
4. Bore Pits – Critical Cost Components
Every underbore requires two pits:
A. Launch Pit (Entry Pit)
✔ Larger & deeper than receiving pits.
✔ Requires strong shoring (panel & arch shoring systems).
✔ May need a ramp for machinery access if deep.
✔ Excavation, shoring, and backfilling costs must be accurately estimated.
B. Receiving Pit (Exit Pit)
✔ Smaller than the launch pit but still requires proper excavation & shoring.
✔ Must be aligned precisely with the bore to ensure correct pipe placement.
💰 Cost Considerations for Bore Pits:
🔹 Excavation & shoring → Costs increase significantly for deep pits.
🔹 Backfilling & compaction → Must meet Sydney Water standards (compaction testing required).
🔹 Traffic control duration → If bore pits are in live roads, additional traffic control costs must be included.
5. Ground Conditions & Boring Rates
The cost of an underbore depends heavily on the type of ground being bored through.
| Ground Type | Boring Rate & Cost Impact |
|---|---|
| Soft Sand | Fast boring, low cost |
| Clay | Moderate boring speed, medium cost |
| Shale | Slower boring, higher cost |
| Hard Rock | Very slow boring, highest cost |
💰 Incorrect ground condition assumptions can result in massive cost variations.
👉 Example: If a bore is assumed to be in clay, but the site has hard rock, the boring rate will be much slower, requiring more machine hours and increasing costs significantly.
6. Additional Cost Factors for Sewer Underbores
A. Spacers & Grout Filling
✔ Spacers hold the sewer pipe in place within the casing pipe.
✔ Grout filling is required to stabilize the pipe and prevent movement.
✔ Laser bore grout is used in some cases for precise stabilization.
B. Geotechnical Testing & Compaction
✔ After backfilling bore pits, compaction testing is required.
✔ Sydney Water specifications require compaction testing every 150mm layer.
✔ Depths over 3m require Level 1 Geotechnical Supervision.
C. Traffic Control
✔ Sewer underbore jobs usually take longer than open trenching, meaning more days of traffic control.
✔ Traffic control is a major cost item → If underestimated, it can result in huge financial losses.
7. Key Takeaways for Accurate Sewer Underbore Estimating
🔎 Choosing the Right Bore Type:
- Laser bore → High precision, higher cost.
- Auger bore → Slight tolerance, cheaper option.
🔎 Casing Pipe vs. Free Bore:
- Casing pipe → More expensive, required in unstable ground.
- Free bore → Cheaper, but must be designer-approved.
🔎 Critical Cost Items to Include:
✔ Bore pits (launch & receiving) → Excavation, shoring, backfilling.
✔ Ground condition rates (sand, clay, shale, rock).
✔ Pipe welding, spacers, and grout filling (if casing pipe is used).
✔ Traffic control duration.
✔ Geotechnical testing & compaction compliance.
💰 Underestimating any of these factors can result in massive cost blowouts.
At WSE Sydney, we specialize in accurate underbore estimating, ensuring that all ground conditions, bore types, and structural requirements are properly costed—minimizing financial risk for contractors.
📧 tenders@watersewerestimatingsydney.com.au
📞 +61 451 404 645