How to Accurately Estimate the Amount of Material Needed

One of the most common questions we receive is how to estimate the quantity of erosion control blankets (ECBs), turf reinforcement mats (TRMs), and high-performance turf reinforcement mats (HPTRMs) needed on a slope or in a channel. Inaccurate material quantities are also a common mistake on construction plans or in bidding documents. Typically, the quantities listed can be significantly less than what is actually needed to complete the project. This can ultimately lead to project overruns, change orders, and project delays.

Common mistakes that can lead to material shortage include using the 2D plan view to determine coverage areas and not including additional materials for trenching, overlaps, curves, and waste.

Using a slope stabilization example, we are going to compare two methods of estimating material. For this example, the given includes a 500-ft section of a roadway relocation that requires a large cut in a hill adjacent to the site. The slope is 60-ft high and on a 2H:1V grade.

Note - Erosion control materials are typically measured in square yards (SY), in lieu of square feet (SF). As a reminder, there are nine SF in one SY.

Method 1

This method uses measurements from the 2D construction plan sheet or 2D CAD to calculate the quantity of materials needed. It is commonly used to estimate the quantity listed in bid documents or construction plans.

Slope Area 2D Plan View Calculation

Slope width = 500-ft, Slope height = 60-ft and Slope Grade = 2H:1V.

The 2H:1V grade states for every change in one vertical (V) foot there is 2-ft of horizontal (H) slope distance.

Based on a Slope Height = 60-ft the slope horizontal length would be 2X the vertical height.

Slope Horizontal Length = 2 X 60-ft = 120-ft.

See Figure 1 for slope cross-section and Figure 2 for slope layout in 2D plan view.

From looking at the 2D plan view, it would be logical to assume that the roadway slope area to be protected would be:

Slope Area = Slope Width X Slope Horizontal Length = 500-ft X 120-ft = 60,000 SF =

60,000 SF ÷ 9 SF/SY = 6,666.7 SY

Rounding up to 6,667 SY of material quantities.

As mentioned, this is typically the quantity listed in the bidding documents or within the construction plans. However, this quantity does not take into account the true slope length based on the 2H:1V grade, trenching, overlaps and waste. This can lead to a shortage of material quantities needed.

Let’s recalculate the materials needed for this slope using the true slope length and taking into account the materials needed for trenching, overlaps and waste.

Method 2

This method uses measurements from the slope area 3D view calculation and areas for trenching, material overlaps, and waste.

Slope Area 3D View Calculation

Using basic trigonometry we can calculate the true slope length and slope area as depicted in Figure 3.

Slope Length = L = = 134.2-ft

Slope Area = Slope Width X Slope Length = 500-ft X 134.2-ft = 67,100.0 SF =

67,100.0 SF ÷ 9 SF/SY = 7,455.6 SY

Round to 7,456 SY of material quantity needed to cover the slope face.

In comparing Method 1 vs. Method 2 there is a 789 SY or an 11.8% increase in quantity. However, this is not the final quantity because we need to calculate the materials needed for trenching, overlaps, and waste.

Trench Area

Propex suggests that all of the material edges along the outer limits of coverage be placed within a trench. This is to prevent the materials from being undermined during flow/runoff events and/or from being displaced by wind uplift. The outer limits include trenches constructed along the top of the slope, the bottom of the slope, and lateral edges. The size of the trench can vary based on site conditions and project design requirements. A typical trench is 1-ft wide X 1-ft deep. This would require 2-ft of material per linear foot of trench. See figures 4 and 5 below for examples of trench layout.

Propex may suggest that the top and/or toe of the slope trench be offset from the slope hinge point as depicted in the example shown above. If this applies, the offset horizontal distance should be added to the material per linear foot of trench. For this example, the trench is placed at the edges of the upper and lower slope limits.

Trench quantities are calculated as follows:

• Top of Slope = 500-ft X 2-ft / per foot of Trench = 1,000 SF = 1,000 SF ÷ 9 SF/SY = 111.1 SY
• Toe of Slope = 500-fT X 2-ft / per foot of Trench = 1,000 SF = 1,000 SF ÷ 9 SF/SY = 111.1 SY
• Lateral Edge Left Side = 134.2-ft X 2-ft / per foot of Trench = 268.4 SF =
• 268.4 SF ÷ 9 SF/SY = 29.8 SY
• Lateral Edge Right Side = 134.2-ft X 2-ft/ per foot of Trench = 268.4 SF =
• 268.4 SF ÷ 9 SF/SY = 29.8 SY
• Total trench area equals 111.1 SY + 111.1 SY + 29.8 SY + 29.8 SY = 281.8 SY
• Round to 282 SY of material quantity needed for all of the outer limit trenches

Material Overlap Areas

ECBs, TRMs, and HPTRMs require the material to be overlapped along the edges. This overlap can vary based on the project and manufacture requirements. A typical edge overlap is 3-inches as depicted in Figure 5 and in the photo of the overlap example. A typical overlap for an end of the roll is 6-inches as depicted in Figure 6 and the photo of the end of the roll example. While you can set up a mathematical equation to account for the additional material needed for the overlaps, it can be quickly estimated by using a percentage of the combined slope and trench areas. Historically, using 3 percent of this combined slope and trench area will provide an accurate calculation of material overlap area.

Using the combined 3D slope and trench areas, the quantities for the overlaps can be calculated as followed:

Material Overlap Area = (3D Slope Area + Trench Area) X 3% = (7,456 SY + 282 SY) X 0.03 = 232.1 SY

Round to 232 SY of quantity needed for material overlap.

Material Waste Area

When estimating materials needed, it important to account for material waste due to end of rolls, installation damage, excessive curves, extra cuts, etc. This can be a harder material area to estimate. Based on Propex’s experience, using 3%-5% of the combined 3D slope, trench, and overlap areas will provide a close estimate for material waste area. For projects with a lot of curvatures in the channel or slope, use the higher amount of 5%.

Using the areas previously calculated, and assuming no curvatures along the slope, the material needed to account for waste can be calculated as followed:

• Material Waste Area = (Slope Area + Trench Area + Overlap Area) X 3% =
• (7,456 SY + 282 SY + 232 SY) X 0.03 = 239.1 SY

Round to 239 SY of quantity needed for material waste

Total Quantity Summary

The total quantity of materials needed to cover this slope can be calculated by summing together all of the areas calculated.

Total Area = Slope Area 3D + Trench Area + Overlap Area + Waste Area

Total Area = 7,456 SY + 282 SY + 232 SY + 239 SY = 8,209 SY

Summary

In comparing the initial 2D area of 6,667 SY in Method 1 to the total area of 8,209 SY calculated in Method 2, there is a 1,542 SY difference or 23.1% increase. This is a significant increase of material required to cover the slope from the quantities calculated in Method 1. Not calculating the quantities correctly can lead to project overruns, change orders, and potential project delays.

While the example provided was for a slope, this same technique can be used to calculate material quantities for channels as well.

Pro Tip – When listing material quantities in bid documents and construction plans, use footnotes stating that the quantities provided include materials needed for trenching, overlaps and waste.

Table 1 can be used as guidance in determining the total square yards of materials needed for slope and/or channels.