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Earthwork

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Earthwork

Cross Section and Borrow Pit Methods

- This lecture covers:
- Readings: 26-1 to 26-6, 26-8 to 26-10.
- Figures: 26-1 to 26-4, 26-6, and 26-7
- Plate B-5 page 893, and B-2 page 890
- Examples:26-1 and 26-3

Volumes

- Usage:
- Quantities of earthwork and concrete
- Capacities of some structures: tanks,..
- Quantities of water discharged by streams per unit time

- Units:
- 1 yd3 = 27ft3
- 1 m3 = 35.315ft3
- Acre-foot: volume of an acre of 1 foot depth

- More accurate than a single profile along the centerline.
- Done by measuring cross sections (profiles) at a right angles to the centerline, usually at intervals of 50, or 100 ft.
- Readings at each cross section are taken at the centerline and at critical points perpendicular to the centerline.
- Cross sections are drawn and design templates are superimposed, the difference in area is the area of cut or fill at that section (end area).

- End areas can be cut, fill, or transition (both).
- Use the areas to compute volumes, knowing the distance between the sections.
- The whole work can be done with photogrammetry and a computer software.

- Plate B-5
- Left page looks like Profile leveling, no intermediate points
- right page: in front of each station, a group of fractions that describe the point location, reading, and elevation, in the form:

99.2

7.4

52

Elevation

rod reading

distance from CL

- Simple cases: formulae in fig 26-2, and fig26-4
- End areas by coordinates: we will learn it through (traversing)

- Simple cases: formulae in fig 27-2, and fig 27-4

compute individual areas and add them up. After computing the

elevation at critical points, form a table:(mistakes!)

station H L C D E R G

24+00 0 C12.5 C15.8 C18.0 C10.1 C12.2 0

15 1533.8 20 0 33.3 15

Compute the areas and add them up.

- Done after computing the end areas, identify which is cut and which is fill. Two main methods:
- Average End Area: Multiply the average area of the two sections by the distance between them. See next slide
- Ve = A1+ A2 * L yd3
2 27

- Ve = A1+ A2 * L yd3

- Prismoidal Formula
- What is a prismoid? A solid with parallel ends joined by a plane or continuously wrapped surfaces
- Fits most earthwork problems
- VP = L(A1+4AM+A2)yd3
- 6*27
- Where AM is the area of computed section midway between stations.
- Prismodial Formula is more accurate, The difference is called CP: Prismoidal correction

- Compute end areas at stations, fill the first three columns in table 26-3.
- Compute the cut and fill volumes, one of the formulae.
- Multiply the fill volumes by an expansion factor.
- Compute the amount of soil to be borrowed or transferred out of the site, which is the difference between the cut and the fill.

- Not suitable for linear features, very useful for construction sites.
- The site is divided into equal squares of sides 20,50, or a 100 ft. Elevations are then measured at the corners of the grid, which are given titles that correspond to the coordinates of the corner in the grid, ex: 3-D, 4-A,..
- V = (hijn) A yd3
4*27

- The idea is to multiply each height by the number of complete squares it is common to.

(

)

The volume of any square, or part of a square is equal to the average

height(elevation difference) at the corners, times the area.

To compute the volume:

1- draw a line between the cut and the fill areas

2- compute the total volume

of all the complete cut squares,

do the same for the fill, use

the previous formula

3- Compute the

incomplete squares

separately and add them to the

squares.

4- Compute the difference

between the cut and the fill.,

pay attention to the expansion

factor.

Site 1

Site 3

Site 2

36

Assumed 36 for piles

200 ft

20

C

D

B

A