16.4 Foundation Layout & Pipe Laying

Foundation Layout & Pipe Laying

Building Layout

A surveyor sets the control points and benchmarks for a building. The contractor then transfers those points to the excavation. Because excavation destroys corner stakes, crews build batter boards: horizontal boards nailed to stakes set far enough outside the excavation that the machine will not disturb them. String lines stretched between nails or saw kerfs on the batter boards mark the building lines. After digging, the strings are re-stretched, and a plumb bob drops each corner back to the bottom of the excavation.

Squaring Corners

• 3-4-5 method: measure 3 feet along one line and 4 feet along the other. The corner is square when the distance between the two marks is exactly 5 feet. For greater accuracy on larger layouts, use multiples such as 6-8-10, 9-12-15, or 12-16-20 feet.
• Equal diagonals: a rectangle is square only when both diagonals are equal. For a 40-foot by 60-foot building:

$$d = \sqrt{40^{2} + 60^{2}} = \sqrt{5{,}200} \approx 72.11 \text{ ft}$$

That is about 72 feet 1-5/16 inches. If one diagonal is longer, adjust the corners until both match.

Footing and Foundation Excavation

• Dig footings to the elevation shown on the plans. The bottom must be undisturbed native soil or compacted fill that meets the specification.
• Do not over-excavate below grade. If you dig too deep, never fill the over-dig with loose spoil. The engineer will usually require lean concrete or compacted structural fill.
• Excavate wide enough beyond the footing for formwork and workers, as the plans or foreman direct.
• Clean loose material from the footing bottom with a smooth-edged cleanup bucket, not a toothed digging bucket.
• If workers will enter an excavation 5 feet or deeper, Subpart P protection is required (Chapter 18).

Laying Gravity Pipe

Storm and sanitary sewers flow by gravity, so the pipe must be laid to an exact grade.

Direction of Laying

Pipe is normally laid starting at the downstream (lowest) end and working upgrade, with bell ends facing upstream. Each new spigot is pushed into the bell of the pipe already in place. This keeps the joints in line with the flow and lets water drain away from the work.

Grade Control

• Offset and cut stakes give line and grade at intervals along the trench (Section 16.2).
• A pipe laser set inside the pipe at the downstream manhole projects a beam at the design percent grade. A target placed in the end of each new joint shows when that joint is on line and grade.
• Grade rods or machine control check the trench bottom before bedding goes in.

Invert Calculations

The invert is the inside bottom of the pipe. Each invert elevation equals the downstream invert plus the grade times the distance:

$$\text{Upstream invert} = \text{Downstream invert} + (\text{grade} \times \text{distance})$$

For a pipe at 0.40 percent over 300 feet starting from an invert of 100.00 feet:

$$100.00 + (0.0040 \times 300) = 100.00 + 1.20 = 101.20 \text{ ft}$$

The trench subgrade must be lower than the invert by the pipe wall thickness plus the bedding. With a 0.25-foot wall and 0.50 feet of bedding, the subgrade at the upper end is $101.20 - 0.25 - 0.50 = 100.45$ ft.

Bedding, Haunching, and Backfill

Avoid driving heavy rollers over the pipe until the cover the specification requires is in place.

Handling Heavy Pipe

Pipe is heavy. The Caterpillar Performance Handbook (from American Concrete Pipe Association data) lists approximate weights for round reinforced concrete pipe:

An 8-foot joint of 60-inch pipe therefore weighs about $8 \times 1{,}295 = 10{,}360$ pounds.

Safe pipe-handling rules:

• Check the excavator's or backhoe's lift chart at the actual radius and height. Lift only from the manufacturer's designated lifting point, never from bucket teeth.
• Use rated rigging and tag lines, and follow Chapter 6 for excavator object handling.
• OSHA 1926.651(e) prohibits employees from being underneath loads handled by lifting or digging equipment. Workers in the trench or trench box stand clear until the pipe is lowered into place.
• Push joints home with a bar or come-along, or with the bucket against a timber block across the pipe end, as the manufacturer allows. Never strike the bell with steel.
• When a trench box is moved forward, make sure it does not drag the installed pipe or pull joints apart.