Estimating Small Buildings - Earthwork

General Overview

  • Course: Estimating Small Buildings

  • Institution: George Brown College

  • Relevant Division: Division 31 - Earthwork

Course Objectives

  • Key Outcomes: After completing this class, students should be able to:

    • Understand the construction process of excavating trenches.

    • Explain workspace and safety requirements for different types of soils.

    • Obtain existing grade and finished/proposed grade.

    • Decide the factors affecting the size of the trenches.

    • Calculate the volume of the excavation and backfill for trenches.

Earthwork Measurements (Div. 31)

Site Clearing & Grubbing (Div. 311000)

  • Measurement Units:

    • Site clearing shall be measured in square meters/square feet.

    • Includes removal of shrubs, trees (not exceeding 300mm girth), and other vegetable matter.

    • Trees to be removed must be enumerated and classified according to size:

    • 300mm to 900mm

    • 900mm to 1800mm

    • 1800mm to 2700mm

  • Topsoil Handling:

    • Stripping and disposal of topsoil shall be measured (bank measure) in cubic meters/cubic yards.

Understanding Bank Measure

  • Definition: Bank Measure is the net volume of soil in-place without any adjustments.

  • Swell and Compaction Factors:

    • Conversion Example: 1.0 cubic yard in natural condition (in-place yards) equates to:

    • After digging: 1.0 = 1.25 cubic yards (loose yards)

    • After compaction: 0.90 cubic yard (compacted yards)

    • Figure 2-2 illustrates typical soil volume change during earthmoving.

Excavation and Fill (Div. 31 23 00)

  • Measurement Units: Earthwork shall be quantified in cubic meters/cubic yards unless stated otherwise.

  • Types of Excavation and Backfill:

    • Bulk work

    • Basement excavations

    • Trenches

    • Column bases and pits

    • Trenches for mechanical and electrical trades

Definitions of Terms

  • Excavation: A hole left in the ground due to material removal.

  • Trench: A specialized type of excavation where the depth exceeds the width (illustrated in Figure 1).

Working Space Requirements

  • Required working space at footings/trench/face of mass excavation:

    • 150mm from the face of footing or 600mm from face of wall above, whichever is greater.

Trench Guidelines for Pipe Installation

  • Trench Excavation Allowances:

    • For pipes not exceeding 300mm in diameter: 300mm on each side of the pipe.

    • For pipes exceeding 300mm in diameter: 600mm on each side of the pipe.

Safety Considerations

Cave-Ins Prevention

  • Safety Regulations:

    • A straight cut trench should have a depth of up to 1.2 m depending on regulatory requirements, soil type, and moisture conditions.

    • Methods for Safety:

    • Sloping (cut backs)

    • Trench boxes

    • Shoring systems

Soil Types and Their Impact on Safety

  • Soil types dictate the strength and stability of trench walls. Awareness of soil types and changing conditions is crucial for excavation safety.

    • Soil Types Identified by the Construction Regulation:

    • Type 1: Hard soil, difficult to penetrate.

      • Properties: Smooth walls, no water release, stable until exposure to rain or prolonged sunlight.

      • Examples: Hardpan, consolidated clay, glacial tills.

    • Type 2: Moderately hard, can be excavated with some difficulty.

      • Properties: Vertical walls initially, cracks may form upon exposure to air.

      • Examples: Silty clay and less dense tills.

    • Type 3: Previously disturbed materials, easier to excavate.

      • Properties: Flows easily, might cave under vertical pressure.

      • Examples: Sand, granular materials, and wet clays.

    • Type 4: Very soft soil requires significant support.

      • Properties: High moisture content, sensitive to vibration, and easily disturbed.

      • Examples: Muskeg, quicksand, and silty clay with high moisture.

Trench Boxes and Shoring Systems

  • Trench Boxes: Designed to protect workers in case of a cave-in, not to support trench walls.

  • Shoring Systems: These provide structural support to trench walls, preventing soil movement and ensuring safety of underground utilities, roadways, etc.

Excavation and Backfill Measurements

  • Excavation for Trenches: Measured (bank measure) in cubic meters/cubic yards.

  • Backfill for Trenches: Measured similarly as excavation.

Disposal of Surplus Earth

  • Calculating Disposal Volume: It is determined by subtracting the volume of backfill from the volume of excavation.

Grade and Elevation Management

Understanding Site Plans

  • Importance of reading and interpreting existing grades and proposed site plans, which include critical elevation markings for infrastructure projects.

Example Calculation for Average Grade

  • A calculation example:

    • Average existing grade from raw data given.

    • Recording all relevant elevation data is crucial for determination of project requirements.

Depth of Backfill Considerations

  • When determining heights of backfill under various surface materials (e.g., interlock paving).

Quantity Takeoffs for Earthwork

Example Calculation of Dimensions and Cost

  • Includes detailed breakdown of quantities for:

    • Grubbing and clearing

    • Excavation and backfill calculations

    • Disposal costs and materials procurement

    • Potential cost implications for different project sectors, ensuring accurate estimations for budgets.

Trench Excavation Assumptions

  • Establishing assumptions about:

    • Site conditions, elevation, and soil types before any excavation permits are issued.

Recap on Project Elements

  • Summarization of key earthwork elements, including:

    1. Grubbing and Clearing measurements.

    2. Topsoil removal specifications.

    3. Detailed trench excavation calculations.

    4. Backfill and disposal estimates.