Extension rods are steel pipe sections 1-1.5 m long that screw together to lengthen an auger string, and a string of 25 m of common 33.7 mm pipe weighs about 60 kg, so a deep string needs a hoist or several people to pull. The pipe itself takes far more twist than two people can apply, so the weak points are the joints, and the biggest risk is dropping the string down the hole when you unscrew a rod.
Part of Deep Wells
Extension rods are the sections of pipe or bar between an auger and its handle. Each added rod extends the reach by its length and also adds weight, torsion and a joint that must hold.
The String Is Heavy, And Every Trip Pulls The Whole Of It
Hand augering is a loop: turn, lift the whole string out, empty the auger, lower it again. Each rod added makes every trip longer and heavier.
Weight. Take common steel pipe of 33.7 mm outside diameter and 3.2 mm wall (a standard 1-inch nominal size, recalled; measure and weigh yours). The inner diameter is 33.7 − 2 × 3.2 = 27.3 mm, the steel area is π ÷ 4 × (33.7² − 27.3²) = 307 mm², and with steel at 7,850 kg/m³ the mass is 307×10⁻⁶ × 7,850 = 2.4 kg per metre.
| Depth | Rods (1.5 m each) | String mass |
|---|---|---|
| 9 m | 6 | 6 × 1.5 × 2.4 = 22 kg |
| 18 m | 12 | 12 × 1.5 × 2.4 = 43 kg |
| 25 m | about 17 | 25 × 2.4 = 60 kg |
These masses are the rods only, with the auger and a full load of soil on top. Two people lift 22 kg without trouble; 60 kg pulled up from a hole is a back injury and a drop hazard. Use a tripod and pulley (Drill Rig Construction) from about the depth at which the string passes what one person lifts comfortably.
Twist. Two people on a T-handle with a 0.5 m arm, each pushing 200 N, apply a torque of 2 × 200 × 0.5 = 200 N·m (an assumed effort). For a hollow pipe the polar moment is J = π ÷ 32 × (33.7⁴ − 27.3⁴) mm⁴ = 72,100 mm⁴, and the shear stress at the outer surface is τ = T × r ÷ J = 200,000 N·mm × 16.85 mm ÷ 72,100 mm⁴ = 47 MPa. Mild structural steel yields in shear at about 135 MPa (a recalled figure for a grade with 235 MPa tensile yield), so the pipe has a margin of about three. Rods fail at their joints, which are thinner and cut by threads, rather than in the barrel.
What You Need
- Pipe sections of equal length — 1-1.5 m, steel, straight
- Joints — threaded couplings, or a square socket held by a cross pin
- A rod clamp or fork — a flat steel fork or a notched plank that rests across the hole collar and holds the string while you unscrew a rod
- A T-handle with a clamp, or a hoist hook for the top rod
- Grease or tallow for threads
Method: Add, Hold, Pull, Count
Step 1: Check each rod. Straighten any that are bent. A bent rod whips and snags.
Step 2: Add a rod when the handle nears the ground. Screw it on tight by hand, then snug with a wrench. Make sure the joint turns clockwise to tighten.
Step 3: When pulling, hold the string. As each rod comes up, rest the string on the clamp laid across the collar of the hole before you unscrew the top rod. Never unscrew a rod while holding the string only by hand.
Step 4: Lay rods in order. Stack them in the order they come out so that they go back the same way.
Step 5: Count the rods. Record the number in the string at the bottom so the depth is known without measuring.
Worked Example: How Many Trips
Assume each fill of a bucket auger advances the hole 0.5 m (a figure to replace with your own count). A 20 m hole then needs 20 ÷ 0.5 = 40 trips. The average depth over the job is 10 m, so the average string is 10 ÷ 1.5 = 6.7 rods, and 40 trips unscrew about 40 × 6.7 = 267 joints on the way up and screw the same number on the way down: about 530 joint operations. A hoist and a rod clamp turn that from the hardest part of the job into the quickest.
Common Mistakes
- Joints that unscrew in the hole. Turn clockwise only, and tighten each joint.
- Dropping the string. Use the clamp at every rod.
- Mixing rods of poor fit. Slack joints wobble and waste effort.
- Pulling by hand beyond a few rods. Use the hoist.
Safety
A dropped string is heavy and long
A string of rods dropped down a hole is lost and can injure feet and hands as it goes. Keep feet clear of the collar; use the clamp at every rod; never let rods stand loose where they can fall over onto someone. Pulling a heavy string by hand is a back injury waiting to happen: use a hoist. The hole is not entered, and it is covered whenever work stops.
What’s Next
- Learn next: Well Casing — once the hole reaches the water layer
- Combine with: Auger Types — the tools on the end of the string