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Water Mains with Underbores

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Precision Estimating for Complex Crossings

When a proposed water main crosses a live road, the installation method depends on the type of roadway and traffic conditions.

  • For standard streets, open-cut trenching is generally permitted with appropriate traffic control measures.
  • For busy roads or highways, where road closures are not feasible even at night, an underbore is required to install the water main without disturbing surface traffic.

At WSE Sydney, we provide accurate cost estimates for water main underbores, ensuring every aspect of the job is factored in—including excavation, boring methods, casing pipe requirements, and backfilling.

1️⃣ Underbore Options – Choosing the Right Method

There are two primary methods for underboring a water main:

🔹 Horizontal Directional Drilling (HDD)

HDD is the preferred method when variable depth levels are required throughout the bore.

  • Ideal for complex designs where the bore must change depth along the alignment.
  • Can follow the long section profile provided by the designer.
  • Suitable for bores through varied ground conditions, including clay, shale, sand, and rock.
  • Allows precision control over entry and exit depths.

💡 Example: If the design shows a water main crossing with depths of 2.0m at Chainage 30, 1.5m at CH45, and 3.0m at CH70, HDD can be used to achieve these depth variations. A standard underbore cannot accommodate such level changes.

🔹 Auger Bore

A conventional underbore is suitable when the bore:

  • Maintains a steady grade (without depth variations).
  • Is through consistent ground conditions (e.g., uniform clay or sand).
  • Requires less complex depth control compared to HDD.

This method is simpler and more cost-effective but cannot handle situations requiring elevation changes.

2️⃣ Bore Pits – Understanding Setup & Cost Implications

Every underbore requires:

  • A Launch Pit – Where the boring machine is placed.
  • A Receiving Pit – Where the bore exits.

💡 Key Considerations for Bore Pits:

  • Excavation & Shoring: The water infrastructure contractor (not the boring company) is responsible for preparing both pits.
  • Pit Dimensions:
    • Launch Pit: 6m – 10m long and 3m – 4m wide.
    • Receiving Pit: Usually smaller than the launch pit but still requires shoring.
  • Shoring Requirements:
    • Bore pits require shoring boxes, which involve panels and arches to keep the pit stable.
    • Shoring setup is time-consuming and costly but is necessary for safety and compliance.
  • Backfilling & Compaction Testing: Once boring is complete, the pits must be backfilled in layers with compaction testing performed at each stage.

3️⃣ Ground Conditions & Rate Variations

The cost of an underbore depends heavily on the ground conditions encountered along the bore path.

💡 Different rates apply for:

  • Clay or Sand – Easier to bore, with lower costs.
  • Shale or Soft Rock (up to 30 MPa) – Moderate cost increase.
  • Hard Rock (30 – 50 MPa or higher) – Requires specialized boring equipment, significantly increasing costs.
  • Extremely Hard Rock (60 MPa and above) – May require pre-drilling or additional equipment, making the bore much more expensive.

💰 Each strength level of rock has a different boring rate, making accurate ground condition assessment critical for estimating project costs.

4️⃣ Free Bore vs. Casing Pipes – Understanding the Difference

Depending on the project specifications, underbores can be free bores or require casing pipes.

🔹 Free Bore (No Casing Pipe Required)

  • The bore is drilled, and the carrier pipe is pulled directly through the hole.
  • Only applicable when the bore is self-supporting (i.e., the surrounding ground conditions provide adequate stability).
  • Less common, as most bores require casing pipes for additional protection.

🔹 Casing Pipe (Steel Pipe Encasement)

  • In many cases, a steel casing pipe is required to protect the carrier pipe.
  • The casing pipe is larger than the carrier pipe and must be welded together in sections.
  • Boring contractors typically provide casing pipes for an additional cost.
💡 Key Cost Considerations:
  • Casing Pipe Welding: Since casing pipes are steel, they must be welded together before being installed.
  • Carrier Pipe Installation: Once the casing pipe is in place, the carrier pipe (water main) is inserted inside.
  • Spacer Installation: Because the casing pipe is larger than the carrier pipe, spacers are placed around the carrier pipe to:
    • Keep it centered inside the casing.
    • Ensure even spacing along the entire length.
  • Grout Filling: The gap between the carrier pipe and casing pipe is grout-filled using laser bore grout to:
    • Prevent movement.
    • Provide additional stability.
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5️⃣ Project Planning – Ensuring Accurate Estimates

To avoid costly surprises, it’s essential to plan for:

✅ Boring Method Selection – Whether HDD or standard underbore is required.
✅ Ground Condition Analysis – Ensuring the correct boring rates are applied.
✅ Bore Pit Costs – Factoring in excavation, shoring, and backfilling.
✅ Welding & Casing Pipe Requirements – Planning for additional material and labor costs.
✅ Traffic Control & Duration Planning – Accounting for road closures and work schedules.

Why Choose WSE Sydney for Underbore Estimating?

💡 We cover every detail in our estimates, ensuring all potential costs are accounted for:

✔️ Bore Method & Depth Analysis – We determine whether HDD or standard underbore is required.
✔️ Ground Condition & Material Costing – We apply the correct boring rates based on soil and rock conditions.
✔️ Pit Excavation & Shoring Estimation – Factoring in the full cost of launch and receiving pits.
✔️ Casing Pipe vs. Free Bore Planning – Identifying if additional casing pipes and welding are needed.
✔️ Grout Filling & Spacer Costs – Including all necessary materials for carrier pipe stability.

Get an Accurate Estimate for Your Underbore Project

Avoid unexpected costs by working with WSE Sydney, specialists in water main underbore estimation.

📞 +61 451 404 645
📧 tenders@watersewerestimatingsydney.com.au

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