When Should a Project Use an Underbore Instead of Open Trench Excavation?
In the world of civil infrastructure, especially for water and sewer installations, one major question that comes up is:
“Should we use open trenching or underboring?”
Let’s break it down.
🛠️ Who Decides?
👉 The designer, not the contractor or estimator, usually decides whether a section of pipe should be open trenched or underbored.
As estimators, our job is to price what’s specified — but we also provide insights if both methods are being considered.
🔍 What is Open Trenching?
Open trenching is the traditional method where a trench is dug to lay the pipe.
It’s straightforward, cost-effective, and ideal for:
Shallow to moderate depths
Open or undeveloped land
Areas with low traffic
However, open trenching isn’t always possible.
🚧 When is Underboring Required?
Underboring (also called trenchless excavation) becomes the only or preferred option when:
🏠 1. Crossing Existing Structures
If a new pipe needs to run under an existing house, building, or structure, you obviously can’t dig through it.
➡️ Solution: Underbore it.
🛣️ 2. Crossing Highways or Main Roads
Busy roads like motorways, highways, or arterial roads cannot be shut down for days.
➡️ Solution: Trenchless underboring avoids road closures.
🧱 3. Deep or Rock-heavy Sections
When sewer mains run very deep (e.g. 5–7m) or pass through solid rock, open trenching becomes dangerously expensive and time-consuming.
➡️ In these cases, underboring is often safer and more economical.
🔁 4. When Designers Want Both Options Priced
Sometimes, the designer will request pricing for:
Option A: Open trench
Option B: Underbore
This allows them to compare the cost-benefit of each and decide which one is more viable based on the specific site constraints.
⚙️ Types of Underboring Methods
There are several trenchless techniques, including:
| Method | Use Case |
|---|---|
| Auger Bore | Steel casing pushed with rotating auger inside |
| Laser Bore | High-precision for shallow/deep bores |
| Free Bore | Pipe laid directly without casing |
| Cased Bore | Pipe inside steel casing with spacers & grout |
Each has its own cost, accuracy, and setup time — all of which we consider when estimating.
🧠 Real-World Example
Let’s say a designer proposes a deep sewer line crossing under a busy Sydney motorway.
Open trenching would require road closures, detours, and extreme depth excavation through rock.
Underboring, though initially costlier, saves time, avoids traffic disruption, and reduces risk.
✅ Underbore wins.
💡 Summary
| Factor | Use Open Trench | Use Underbore |
|---|---|---|
| Accessible land | ✅ Yes | ❌ Not needed |
| Under a building | ❌ No | ✅ Required |
| Busy roads | ❌ No | ✅ Required |
| Deep or rocky ground | ❌ Difficult | ✅ Preferred |
| Time-sensitive | ❌ Slower | ✅ Faster |
| High precision | ⚠️ Not ideal | ✅ Better with laser guidance |
📋 At WSE Sydney, We Know What to Look For
When estimating, we:
Understand both methods in detail
Recognize high-cost triggers like rock, depth, or traffic conditions
Provide clear breakdowns of trenchless vs open trench costs
📄 All we need is your tender documents.
We’ll do the rest — accurately and efficiently.
📧 Need a quote or unsure which method is right for your job?
Reach out: tenders@watersewerestimatingsydney.com.au
📞 +61 451 404 645
