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Including Practical Working Space & Offset Considerations
Hello builders and billers! 👷♂️👷♀️
You’re on-site. The surveyor is shouting measurements, the foreman wants to know how much soil to remove, and your BOQ says “as per IS 1200.” But what does that really mean when it comes to excavation for footings?
Today, let’s unravel the real meaning of IS 1200 – Part 1 (1992) and how it applies to excavation quantity for footings, including those oh-so-important offsets and working space that contractors often forget—or argue over in bills.
Let’s go clause by clause, and pit theory vs reality. 💪
📘 What IS 1200 (Part 1): 1992 Actually Covers
This standard lays out how to measure earthwork, including excavation, filling, trenching, lead, lift, working space, and more. It applies to all building and civil engineering works (excluding river valley projects).
The key clause for us today is:
🔹 Clause 4.1: “The measurement of earthwork shall be done in cubic metres… the authorized dimensions from which soil has been taken out shall be measured.”
📌 Translation? You measure only the volume that is approved in the drawing/spec—not what’s practically dug on-site—unless the contract allows it.
🧱 Case Study: Single Square Footing
Let’s assume you’re constructing a 2m x 2m footing, with a 1.5m depth. Converting to millimeters (mm):
- Length (L) = 2000 mm
- Breadth (B) = 2000 mm
- Depth (D) = 1500 mm
📐 CASE 1: Without Working Space (Strictly as per IS 1200)
✅ Code Reference:
Clause 4.2: “Measurement shall be done in cubic metres without allowance for increase in bulk.”
This means no buffer, no breathing space. You calculate just the footing footprint.
🔸 Calculation:
2000 × 2000 × 1500 = 6,000,000,000 mm³ = 6.00 m³
🧾 You use this value in:
- IS-compliant BOQs
- Government tenders (PWD, CPWD)
- Contracts that explicitly exclude working space
📊 Excel-Based Excavation Calculator
To make your life even easier, I’ve created a ready-to-use Excel sheet that automates excavation volume calculations for:
- Footings with and without working space
- PCC thickness inclusion
- Various offset sizes (300 mm / 600 mm)
- Batch calculation for multiple footings
It instantly gives you the total excavation volume — fully aligned with IS 1200 clauses and real site practice. This is perfect for QS teams, site engineers, and billing engineers who want speed + accuracy.
🛠️ CASE 2: With Working Space / Offset (Realistic Approach)
Site engineers, you know this: digging just enough for a footing is a nightmare. You can’t place formwork, waterproofing, or even stand inside the pit! So we add a buffer—called offset or working space.
✅ Code Reference:
Clause 4.2.5.1:
“Where working space is not specified, allow 600 mm measured from the face of the substructure at the lowest level.”
That’s 600 mm on all four sides—a generous yet reasonable space for practical work.
🔸 Updated Excavation Dimensions:
- Length = 2000 + 600 + 600 = 3200 mm
- Breadth = 2000 + 600 + 600 = 3200 mm
- Depth = 1500 mm
Volume = 3200 × 3200 × 1500 = 15.36 m³
⚠️ But wait!
Clause 4.2.5 also says:
“The authorized working space or actually excavated—whichever is less—shall be measured.”
So unless your drawing explicitly shows this 3200 x 3200 pit, you might not get paid for it in full.
A more commonly accepted practical offset is 300 mm on each side, which gives:
- Excavation = 2600 × 2600 × 1500 = 10.14 m³
Some people wrongly calculate from bottom of footing to natural ground only.
But this drawing makes it clear:
- Excavation starts from E.G.L. (Existing Ground Level) down to bottom of PCC
- So, total excavation depth = D + 100 mm
“As seen in the drawing, placing bottom steel bars, ties, and shuttering requires at least 300 mm working clearance on each side. Hence, additional excavation is not just justified—it’s essential.”
“If PCC thickness is provided (e.g., 100 mm), excavation depth = Depth of footing (D) + PCC thickness.”
🔍 Summary Table: With vs Without Offset
| Item | Length (mm) | Breadth (mm) | Depth (mm) | Volume (m³) | Notes |
|---|---|---|---|---|---|
| Without Offset | 2000 | 2000 | 1500 | 6.00 | Strict IS 1200 measurement |
| With 300 mm Offset | 2600 | 2600 | 1500 | 10.14 | Practical offset, often used in site billing |
| With 600 mm Offset | 3200 | 3200 | 1500 | 15.36 | Only if drawings/specs explicitly allow it |
🧠 What About Sloped Sides? (Battering or Benching)
Sloped excavation is often unavoidable in deep foundations, loose or collapsing soil, or when using manual excavation where vertical sides aren’t feasible.
✅ Code Reference:
Clause 4.2.6:
“Battering and benching shall be specified and measured along with the main item of excavation.”
📌 Let’s break it down:
| Term | Meaning |
| Battering | Forming continuous sloped sides in the excavation (like a trapezoid), to prevent collapse. |
| Benching | Cutting the soil in horizontal steps at regular intervals, used in very deep excavations. |
🔹 When Are They Used?
- When depth > 1.5 m
- In loose, silty, or clayey soil
- Where safe access and soil stability are required
- As per HSE guidelines or NBC India recommendations
🔸 Example: Trapezoidal Pit with Battered Sides
- Footing size: 2000 × 2000 mm
- Depth: 3000 mm
- Batter slope: 1:1
🔹 Top dimensions:
Length = 2000 + 2×3000 = 8000 mm
Breadth = 2000 + 2×3000 = 8000 mm
🔹 Volume (using average area method):
Avg Length = (2000 + 8000) / 2 = 5000 mm
Avg Breadth = (2000 + 8000) / 2 = 5000 mm
Volume = 5000 × 5000 × 3000 = 75.00 m³
Compare this to vertical excavation of same footing: 12.00 m³ ➝ That’s 63 m³ of additional soil!
🖼️ Real-Life Example of Benching on Site
Here’s an actual site photograph showing excavation carried out using benching technique:
🔹 Excavation Width: ~5.0 m
🔹 Excavation Depth: ~3.8 m
🔹 Visible stepped formation = benching for safety and stability
This clearly illustrates how horizontal benches are used to stabilize deep cuts in soil, allowing safe working conditions.
📝 Important: Such excavation methods must be referenced in soil reports or site-specific method statements and approved by the engineer to be billable.
⚠️ Key Notes:
- Extra excavation due to battering/benching is only payable if shown in drawings or approved.
- Always justify with soil reports or safety norms.
- Get it signed off by the Engineer/Consultant.
🧠 Typical Slope Recommendations:
| Soil Type | Slope (H:V) |
| Firm cohesive soil | 1:1 |
| Loose/fill/soft clay | 1.5:1 |
| Dry sand | 1:1 |
| Wet sand/silty soils | 2:1 |
| Rock (stable) | Vertical |
🧾 Field vs Tender vs Billing: When to Use What?
| Scenario | Volume to Use | Comments |
| IS 1200 standard, no working space | 6.00 m³ | Default unless contract allows extra space |
| Site reality (300 mm offset used) | 10.14 m³ | Acceptable in many private and EPC contracts |
| Drawings show pit with 600 mm offset | 15.36 m³ | Fully billable if shown or approved |
| Consultant allows practical working space | 10.14–15.36 m³ | Confirm % allowance in pre-bid or billing stage |
📌 Golden Tips for Engineers & QS Teams
✅ Always check the drawing notes — many engineers miss that “all footings to be excavated with 300 mm clearance” line!
✅ Get written approval for extra excavation if site conditions demand it.
✅ Separate line items for “Extra Excavation for Working Space” can help during billing.
✅ Use Clause 4.2.5.1 during disputes—it’s your backup when someone says “offset not allowed.”
✅ For sloped sides, Clause 4.2.6 is your shield—but only if slopes are shown or justified.
🎯 Final Thoughts
IS 1200 (Part 1) gives us the baseline rules for excavation measurement, but actual site work often goes beyond it. That’s why the standard wisely includes clauses for working space, battering, and realistic construction needs.
Your job as a QS or Engineer is to:
- Apply IS 1200 where required
- Adapt to practical conditions where permitted
- Negotiate and document every variation clearly
