Wes Marshall, P.E. University of Connecticut March 2007 CE 276 Site Design Chapter 7 – Earthwork Wes Marshall, P.E. University of Connecticut March 2007

Chapter 7 Earthwork

Definitions

Definitions Subgrade The top of the material on which the surface material is placed Top of a fill situation & bottom of a cut Compacted subgrade must attain a specified density Undisturbed subgrade has not been changed or excavated

Definitions Base / Sub-base Imported material that is typically placed under pavements Typically a course or fine aggregate

Definitions Finished Grade Final grade after all landscaping has been completed (i.e. top surface of lawns, pavements, etc.) Typically designated with contours and spot elevations on a grading plan

Definitions Finished Floor Elevation (FFE) Typically the elevation of the first floor of a structure However, it can be used for any floor The relationship of FFE to the exterior grades depends upon the type of construction

Cut & Fill plan section

Other Definitions Cut & Fill Cut The process of removing soil plan section Cut The process of removing soil Proposed contours extend across existing contours in the uphill direction Cut

Other Definitions Cut & Fill Fill The process of adding soil plan section Fill The process of adding soil Proposed contours extend across existing contours in the downhill direction Fill Cut

Other Definitions Compaction Topsoil Borrow The densification of soil under controlled conditions, sometime with a specified moisture content Topsoil Typically the top layer of a soil profile that can range from ~ one inch up to ~ one foot Highly organic & subject to decomposition Therefore not suitable for structures Borrow When fill material needs to be imported to a site, it is sometimes referred to as borrow

Cut & Fill Factors that influence your cut & fill grading decisions… Cost Nature of the site including size & shape Aesthetics Intricacy of the grading plan Soil types & suitability for use Surrounding uses & elevations

Cut & Fill In situations where you cannot balance cut & fill… It is generally better to use more cut than fill (unless there are very specific soil requirements such as with an athletic playing field)

Cut & Fill Why? Importing soil tends to be more expensive than removing soil A fill condition is usually structurally less stable and more susceptible to erosion & settlement Buying soil, hauling, compacting

Calculating Cut & Fill 3 methods of estimating cut & fill volumes Average End Area Method Contour Area Method Grid Method Volumes ares

Calculating Cut & Fill Earthwork volumes are typically measured in cubic yards… Keep in mind that cubic feet (ft3) must be divided by 27 ft3/yd3 to convert to cubic yards (yd3)

Average End Area Method Take cross sections at regular intervals indicating both existing & proposed contours Calculate the amount of cut & fill at each cross-section based upon existing & proposed grades Multiply average of two adjacent cross-sections by the length between them

Average End Area Method V = [(A1 + A2) / 2] x L V = Volume A1, A2 = Cut/Fill area of cross sections L = Distance between A1 & A2

Average End Area Method Keep in mind that the vertical scale is usually exaggerated by 5 or 10 times This is needed because the vertical change is usually a lot smaller than the horizontal scale It helps you see what is really going on

Average End Area Method The Average End Area Method is best used in linear situations such as: Roads Paths Utility work

Contour Area Method Using the grading plan with existing & proposed contours: Establish the “no cut - no fill” limit line Separate the area of cut from the area of fill Keeping cut & fill separate… Measure the horizontal area of change for each contour line within limit line

V = A1h/3 + (A1+A2)h/2 + … + (An-1+An)h/2 + Anh/3 Contour Area Method V = A1h/3 + (A1+A2)h/2 + … + (An-1+An)h/2 + Anh/3 V = Volume A1, A2 , An = Area of horizontal change for each contour h = Vertical distance between areas

Contour Area Method V = i(A1 + A2 + … + An) If h equals i (the contour interval) Then the equation can be simplified as follows: V = i(A1 + A2 + … + An) V = Volume A1, A2 , An = Area of horizontal change for each contour i = Contour interval This form of the equation often overestimates volumes…

Contour Area Method

Step 1 No Cut - No Fill Line

Step 2 Line Between Cut & Fill

Step 3 Measure Area of Cut & Fill for each contour line

Repeat for Each Contour Line For the 72’ Contour Line Approximate area of Cut = 284 ft3 Approximate area of Fill = 1,056 ft3 Repeat for Each Contour Line

Total Cut & Fill Approximate Total Cut = 1,280 ft3 Approximate Total Fill = 3,216 ft3 Divide by 27 ft3/yd3 to find square yards Cut = 47.4 yd3 Fill = 119.1 yd3

Contour Area Method The Contour Area Method is best used for large, relatively uncomplicated grading plans as well as for calculating the volume of water in lakes or ponds

Grid Method Also known as the Borrow Pit Method Create a grid over the area to be graded Smaller cells → More accurate For each grid cell Find the average change in elevation by determining the elevation difference for all four corners of the grid cell The volume is calculated by Adding the averaged cut & fill volumes separately Then multiplying by the area of one grid cell

Step 1 Create Grid

Step 2 Find Avg. Change in Elevation (existing spot) proposed spot

Step 2 Find Avg. Change in Elevation Vavg = (h1 + h2 + h3 + h4) / 4 Vavg = 3.5+4.4+2.6+3.3 4 Vavg = 3.45 feet of cut Repeat for each grid cell…

Step 3 Add Cuts/Fill Separately & Multiply by Grid Cell Area Grid Cell #1 = 3.45 feet cut Grid Cell #4 = 2.30 feet cut Grid Cell #2 = 3.48 feet cut Grid Cell #5 = 2.35 feet cut Grid Cell #3 = 2.50 feet cut Grid Cell #6 = 1.78 feet cut Add cuts & fills separately In this case, the site is all cut Total of Grid Cells = 15.85 feet cut Multiply by the Area of one Grid Cell 15.85’(100’)(100’) = 158,500 ft3 → 5,870 yd3

Grid Method The Grid Method is best used for complex grading projects and urban conditions

Adjusting Cut & Fill Volumes For estimating purposes Cut & fill volumes are determined for the volume in between existing & proposed subgrades (not the finished grades) Since grading plans depict finished grade, we typically make adjustments…

Adjustment Principles The depth of the proposed surfacing material (i.e. pavement, topsoil, etc.) Increases the amount of Cut needed Decreases the amount of Fill needed

Adjustment Principles The removal of existing surfacing (i.e. pavement, topsoil, etc.) Decreases the amount of Cut needed Increases the amount of Fill needed

Adjusting Cut & Fill Volumes If we remove topsoil… Multiply the area by the depth For example… 6” of topsoil over 60,000 ft2 of area = 30,000 ft3 less cut or more fill If we are installing a pavement… 6” concrete slab w/ 4” gravel base = 49,800 ft3 more cut or less fill

Adjusting Cut & Fill Volumes If the depth of the Removed surface & proposed surface are the same Then no adjustment is needed... They cancel each other out

Adjusting Cut & Fill Volumes Another factor affecting cut & fill volumes is compaction Cut volumes (existing in-place soil) will typically yield less than their full volume when used as fill by 10 to 20% This means that 100 yd3 of material cut will only result in 80 to 90 yd3 of fill

Adjusting Cut & Fill Volumes Therefore, for estimating purposes, fill volumes should be increased by 10 to 20% (depending on the type of soil)

It saves money and is the most energy efficient option Balancing Cut & Fill Balancing cut & fill on-site is often a goal of the grading process… It saves money and is the most energy efficient option

On a balanced site, no earth needs to be hauled to or from a site Balancing Cut & Fill On a balanced site, no earth needs to be hauled to or from a site

Balancing Cut & Fill Taking into account compaction, we would then require a cut to fill ratio between 1.1 & 1.2 Achieving this is typically a process of trial & error

Use the Average End Area Method to find the volumes of cut & fill Stations 0+00 & 2+35 have no cut or fill

Use the Contour Area Method to find the volumes of cut & fill