4. Estimate for Civil Engineering Works , Civil Engineering 5th Sem Notes CE 5002

 

4. Estimate for Civil Engineering Works

In civil engineering, estimating is crucial for determining the quantities and costs involved in executing various works like roads, embankments, canals, septic tanks, and community wells. Detailed estimates for these works help in proper budgeting, planning, and resource allocation for the project. The process involves careful measurements of dimensions, labor, materials, and other resources required.

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4.1 Earthwork

Earthwork involves activities such as excavation, backfilling, leveling, and grading, commonly done for projects like road construction, embankments, and canals. Accurate estimation of earthwork is crucial for cost planning.

4.1.1 Quantities for Roads, Embankment, and Canal

The calculation of earthwork quantities is performed using different methods depending on the nature of the cross-section and the shape of the land. Below are methods used to calculate earthwork for roads, embankments, and canals:

4.1.1.1 Mid Sectional Area Method / Mean Sectional Area Method

• Mid Sectional Area Method:

  • This method involves calculating the area of cross-sections at two points (at the beginning and end of the section) and then averaging these areas to estimate the volume of earthwork.
  • Formula: $$\text{Volume} = \frac{A_1 + A_2}{2} \times L$$ where:
    • $A_1$ = Area of cross-section at the starting point
    • $A_2$ = Area of cross-section at the ending point
    • $L$ = Length of the section

• Purpose: This method is useful for sections where the shape of the cross-section changes gradually. It provides an average estimate for the volume between the two sections.

4.1.1.2 Prismoidal and Trapezoidal Formula Method

• Prismoidal Formula Method:

  • The prismoidal formula is used when the cross-sectional areas are irregular or when the section changes significantly along the length.
  • Formula (Prismoidal Formula): $$\text{Volume} = \frac{L}{6} \left( A_1 + 4A_m + A_2 \right)$$ where:
    • $A_1$ = Area at the start of the section
    • $A_2$ = Area at the end of the section
    • $A_m$ = Area at the midpoint of the section
    • $L$ = Length of the section

• Trapezoidal Formula Method:

  • The trapezoidal formula is simpler and is used when the cross-sections of the embankment or excavation are more regular.
  • Formula (Trapezoidal Formula): $$\text{Volume} = \frac{L}{2} \left( A_1 + A_2 \right)$$ where:
    • $A_1$ = Area at the beginning of the section
    • $A_2$ = Area at the end of the section
    • $L$ = Length of the section

• Purpose: Both methods are used to estimate volumes of earth to be excavated or filled, especially when the sections change shape along the length of the embankment or canal.

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4.2 Detailed Estimate for Septic Tank and Community Well

4.2.1 Septic Tank

A septic tank is an underground chamber used for the treatment and disposal of domestic wastewater. The detailed estimate for constructing a septic tank includes excavation, concrete work, reinforcement, and labor costs.

• Steps for Estimating Septic Tank Construction:

  1. Excavation: Measure the volume of earth to be excavated for the tank. This can be done using the length, width, and depth of the excavation.
  2. Concrete Work: Calculate the quantity of concrete needed for the tank walls, floor, and cover.
  3. Reinforcement: Estimate the amount of steel reinforcement required for the concrete work.
  4. Labor: Include the labor charges for excavation, mixing, casting, and finishing the septic tank.

• Example: For a septic tank with dimensions 4m × 2m × 2m (length × width × depth):

  • Volume of excavation = 4 × 2 × 2 = 16 m³
  • Volume of concrete (for tank structure) = 4 × 2 × 2 (assuming 20 cm thickness of walls and base)

  Labor and material rates are then multiplied with the calculated quantities to get the detailed cost estimate.

4.2.2 Community Well

A community well is a water well built to provide water to multiple households or communities. The construction of a well involves excavation, brick or concrete work, lining the well, and providing a cover and safety measures.

• Steps for Estimating Community Well Construction:

  1. Excavation: Measure the depth and diameter of the well and calculate the volume of earth to be excavated.
  2. Wall Construction: Estimate the amount of material (bricks, stone, or concrete) required for lining the well.
  3. Cover and Safety Measures: Include the costs for a well cover, platform, hand pump, and safety measures.
  4. Labor: Estimate the labor required for excavation, lining, and other construction activities.

• Example: For a well with a diameter of 1.5m and depth of 10m, the volume of excavation can be calculated using the formula for the volume of a cylinder:

  $$V = \pi \times \left(\frac{D}{2}\right)^2 \times h$$

  where:

  • $D$ = Diameter of the well
  • $h$ = Depth of the well

  Then, estimate the quantities of bricks/concrete and other items required, and multiply with unit rates to arrive at the cost.

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4.3 Introduction to the Use of Computers/Software/Programs for Detailed Estimate Preparation of Civil Engineering Works

With the advancement of technology, the preparation of detailed estimates for civil engineering works has become more efficient and accurate through the use of computers and specialized software. These tools are widely used in modern civil engineering projects to automate the process of quantity takeoff, cost calculation, and reporting.

4.3.1 Benefits of Using Computers/Software for Estimating:

• Accuracy: Computers help reduce human errors in calculations by automating the measurement and costing processes.
• Speed: Estimating software allows for faster preparation of detailed estimates as it automatically updates quantities and costs based on design changes.
• Consistency: Software ensures consistent application of standards, unit rates, and cost factors across all parts of the estimate.
• Ease of Revisions: Any revisions or changes in design can be quickly updated in the software, and the estimate will be recalculated automatically.
• Cost Control: It allows for more accurate cost control as the software can generate reports that compare actual costs with estimated costs, providing insights into areas that need attention.

4.3.2 Common Software/Programs for Estimation:

1. AutoCAD: While primarily used for drafting, AutoCAD also supports 3D modeling and integrates with estimation software for generating quantities directly from the drawings.
2. Revit: A Building Information Modeling (BIM) tool that allows for detailed planning, design, and quantity takeoff from the 3D model.
3. Primavera P6: Used for project scheduling and cost management, often integrated with estimation tools.
4. MS Excel: Though not specialized software, MS Excel is widely used for creating estimation templates, and many estimators use advanced formulas and macros to manage quantities and costs.
5. CostX: A specialized software used for generating detailed estimates from 3D BIM models. It automates the process of quantity takeoff and cost estimation.
6. WinQS: Another estimation software used for generating bills of quantities and cost estimates. It offers features for material tracking and procurement planning.

4.3.3 Working Process Using Estimation Software:

• Importing Drawings: Drawings (AutoCAD, Revit, etc.) are imported into estimation software.
• Quantity Takeoff: The software extracts quantities directly from the drawing or 3D model, ensuring accurate measurements.
• Cost Calculation: Unit rates for materials, labor, and overheads are assigned to each item, and the software calculates the total cost.
• Report Generation: The software generates reports such as Bills of Quantities (BoQ), cost breakdowns, and summaries for review and analysis.