Field Tool
Bore Planning Helper
Suggest a backreamer size, compute hole and pass volumes, estimate drilling fluid from the supplier's factor, and sketch the HDD entry path and setback against the first crossing.
Reamer 4.98 in. No issues found in the checks this tool runs (reamer ratio, level before the first crossing, entry geometry). Cover, separation from crossed utilities, pullback force and permit hole-size limits are not checked. The bundle OD is the tightest-packing lower bound, so the real ratio can be lower.
Good: No issues found in the checks this tool runs (reamer ratio, level before the first crossing, entry geometry). Cover, separation from crossed utilities, pullback force and permit hole-size limits are not checked. The bundle OD is the tightest-packing lower bound, so the real ratio can be lower.Suggested Backreamer
4.98 in
1.5 × 3.320 in bundle OD (2 × 1.660 in, tightest packing; rule of thumb, SME to verify)
Hole and Fluid
- Final hole, 4.98 in
- 35,060.7 in³ = 20.29 ft³ = 151.8 gal
- Drilling fluid estimate
- 749.1 gal (factor 3 × pass total)
| Pass | Diameter (in) | Volume (gal) |
|---|---|---|
| 1 | 4.00 | 97.9 |
| 2 | 4.98 | 151.8 |
| Total | 249.7 |
Entry Path
- Shortest setback, entry point to level
- 40.5 ft
- Path length to level
- 41.1 ft
- Pitch at entry
- 12.0° · 21.3%
- Governing radius
- 117.0 ft (rod)
- Arc drop · tangent drop
- 2.6 ft · 3.4 ft
- Level run before the first crossing
- 9.5 ft beyond the setback
This is the shortest setback, at the rod's minimum radius. A gentler curve needs a longer setback, so do not stake the entry point at exactly this distance. Setback is measured from where the bit enters the ground; add the rig's own offset (per the rig manufacturer).
This checks only that the path is level at depth D before the crossing. Separation from the crossed utility is per owner or utility spec (UG-05 step 1); confirm its depth by pothole (UG-04) after locates under the state one-call law.
Vertical scale × 3.5
Table View: Path Points
| # | x (ft) | Depth (ft) |
|---|---|---|
| 1 | 0.00 | 0.00 |
| 2 | 16.20 | 3.44 |
| 3 | 17.70 | 3.75 |
| 4 | 19.20 | 4.04 |
| 5 | 20.71 | 4.31 |
| 6 | 22.22 | 4.56 |
| 7 | 23.74 | 4.79 |
| 8 | 25.25 | 5.00 |
| 9 | 26.77 | 5.19 |
| 10 | 28.30 | 5.36 |
| 11 | 29.82 | 5.51 |
| 12 | 31.35 | 5.64 |
| 13 | 32.87 | 5.75 |
| 14 | 34.40 | 5.84 |
| 15 | 35.93 | 5.91 |
| 16 | 37.46 | 5.96 |
| 17 | 38.99 | 5.99 |
| 18 | 40.52 | 6.00 |
| 19 | 50.00 | 6.00 |
Points the profile is drawn from: the entry point, the tangent end, then the arc.
Planning sketch only. The engineer's bore plan, rod and product manufacturer data and owner specifications govern.
See It on the Map
The map scenes are schematic. The profile in this tool is drawn only from the values you enter.
Example: two 1¼ in SDR 11 HDPE conduits pulled together as a bundle under a two-lane road, in clay. After a 4 in pilot, one backream at 4.98 in (1.5 × the 3.32 in tightest-packing bundle OD) pulls the bundle back through a swivel and breakaway link to a pulling head or staggered pulling harness that holds both conduits. Breakaway link rating and pulling head per the conduit and pulling-head manufacturers. Some permits cap the reamer or hole size relative to the product, at 1.5 × or another figure; ask how yours applies it to a bundle. Sample site layout: 150 ft bore, first potholed utility 50 ft from the entry point. Your bore plan, rod and conduit data and permit govern.
Show the Math
Conduit Count and Bundle OD
D_b = k_n × d (with n = 1, D_b = d) k_1 = 1; k_2 = 2; k_3 = 1 + 2 ÷ √3 = 2.1547005 k_4 = 1 + √2 = 2.4142136 k_5 = 1 + √(2 × (1 + 1 ÷ √5)) = 2.7013016 k_6 = 3; k_7 = 3
n is the number of conduits (1 to 7), d the conduit OD (in), k_n the packing factor and D_b the bundle OD (in): the diameter of the smallest circle that holds n equal circles of diameter d. For n = 2 to 6, k_n = 1 + 1 ÷ sin(π ÷ n), a ring of n; n = 7 is a ring of 6 around 1. The count stops at 7 because those packings are proven and have closed forms. Below, the product OD od is D_b.
Tightest possible packing: the smallest circle these conduits can fit in. A real bundle is the same size or larger, so a reamer sized from it is the minimum. Use a measured bundle OD or the manufacturer's figure when you have one.
Reamer and Volumes
reamer = od × factor, with od = D_b d_ream = planned reamer if entered, else the suggested size ρ = d_ream ÷ od V_in³ = (π ÷ 4) × d² × (12 × L) V_ft³ = V_in³ ÷ 1,728 V_gal = V_in³ ÷ 231 fluid = k × (total of all passes, gal)
od is the product OD (in): the bundle OD D_b, which is the conduit OD with one conduit. Here d is a pass diameter (in), L the bore length (ft) and k the fluid factor (gallons of fluid per gallon of hole). Each pass is a cylinder of its diameter over the bore length, in the order listed. With no passes listed, the tool uses one pass at d_ream. The final hole is a pass at d_ream.
Entry Path
t = θ × π ÷ 180, with 0 < θ < 90 R = max(rod radius, product radius) of those entered h_arc = R × (1 − cos t); x_arc = R × sin t; s_arc = R × t h_tan = D − h_arc (the path works only when h_tan ≥ 0) x_tan = h_tan ÷ tan t; s_tan = h_tan ÷ sin t X = x_tan + x_arc; s = s_tan + s_arc; pitch = 100 × tan t % level run before the first crossing = C − X
θ is the entry angle, D the depth of the level path, R the governing bend radius, X the setback from the entry point on the surface (not from the rig) to where the path is level, s the path length to level and C the horizontal distance from the entry point to the first potholed crossing. The profile is drawn from the entry point, the tangent end and 16 equal angle steps along the arc; when C is beyond X, a level segment runs to the crossing.
Status
- Critical: the reamer is not larger than the product (d_ream, or the last listed pass, is at or below the bundle OD D_b, which is the conduit OD with one conduit); the path is not level before the first crossing (X more than C); or the curve alone needs more depth than D (h_arc more than D). The bundle OD is the smallest possible bundle, so the real margin is at most what the tool shows.
- Warning: the planned reamer is below the rule of thumb (ρ below the factor); the last listed pass is not d_ream; or the fluid factor, rod bend radius, product bend radius, conduit OD, bore length, entry angle or depth is blank.
- Good: No issues found in the checks this tool runs (reamer ratio, level before the first crossing, entry geometry). Cover, separation from crossed utilities, pullback force and permit hole-size limits are not checked. With two or more conduits: The bundle OD is the tightest-packing lower bound, so the real ratio can be lower.
- A reamer factor of 1 or less is not an input error: it makes the reamer no larger than the product, so the status is Critical.
Draft rule, pending the owner's review: the impossible-geometry Critical, the last-pass checks and the blank product radius, bore length, entry angle and depth Warnings go beyond the program plan. The comparisons use a tolerance of 1e-9.
Defaults and Where They Come From
- Reamer factor 1.5 × OD: a rule of thumb, SME to verify. The engineer or the reamer manufacturer sizes the reamer.
- Conduit OD presets are the HDPE conduit outside diameters in the Duct Fill tool (SDR, outside-diameter controlled), SME to verify (actual OD per manufacturer): 1 in 1.315 in, 1¼ in 1.660 in, 1½ in 1.900 in, 2 in 2.375 in and 4 in 4.500 in. Each is the OD of one conduit. For a bundle of 2 to 7 equal conduits, choose the number of conduits and the tool estimates the bundle OD as the tightest packing. For a bundle of mixed sizes, or a bundle OD you measured, set the number of conduits to 1 and enter that OD.
- Fluid factor: per fluid supplier, for the soil. FAHMY supplies no factor for a real job. The example shows 3 for illustration only, the low end of one drilling fluid supplier's published clay range.
- Rod and product bend radius: per manufacturer data. The example shows a rod radius of 117 ft (one rod maker's published minimum) and a product radius of 6.92 ft: one conduit's in-bore minimum, twice the DR 11 field-bend ratio of 25 × OD (50 × 1.660 in = 83 in, rounded up), per one PE pipe maker's HDD note. Each conduit follows the bore path, so it is not scaled to the bundle OD. Enter your own rod's and conduit maker's values.
- Target cover over the product: per permit.
- The opening example (proposed from industry research, owner review pending) is two 1¼ in SDR 11 HDPE conduits pulled together as a bundle under a two-lane road, in clay. The 150 ft bore and the first potholed utility 50 ft from the entry point are a sample site layout. The bundle OD is 3.32 in, the tightest packing of two conduits of 1.660 in; a real bundle is the same size or larger. After the 4 in pilot (one compact rig's minimum bore diameter), one backream at 4.98 in (1.5 × the bundle OD) pulls the bundle back through a swivel and breakaway link to a pulling head or staggered pulling harness that holds both conduits. Breakaway link rating and pulling head are per the conduit and pulling-head manufacturers. The planned reamer is left blank, so the suggested 4.98 in is used. It is the minimum for the smallest possible bundle, so a stock reamer smaller than this is below the rule of thumb; the engineer picks the reamer. The example lists two passes, one for each hole size. Some permits cap the reamer or hole size relative to the product, at 1.5 × or another figure; ask how yours applies it to a bundle. Some permits require grouting the space around the bundle; check yours. The 12° entry angle is one compact rig's published entry angle. The 6 ft depth to the path is below the entry ground; on the level part of the path, with level ground, the top of the 4.98 in hole is deeper than 5 ft. Required cover is per your permit, measured from the top of the conduits or casing up to the road surface, or to the lowest point of the crossed grade such as a ditch; some state DOTs require 5 ft, some more. While a field holds its example value, its tag reads "Example". No value in it is a default for a real job.
Assumptions
- The reamer factor is a rule of thumb (SME to verify); the engineer or reamer manufacturer sizes the reamer.
- Hole volume is a perfect cylinder of the pass diameter over the bore length; real holes wash out or close in.
- The fluid factor comes from the fluid supplier for the soil; FAHMY supplies no factor.
- The path is a straight tangent then one circular arc, in a vertical plane, from a level surface.
- Depth is to the path; cover over the product is less, and target cover is per permit.
- Setback is measured from where the bit enters the ground, so the rig's own offset is extra (per the rig manufacturer).
- The level run before the first crossing checks only that the path is level at depth D before the crossing. Separation from the crossed utility is per owner or utility spec (UG-05 step 1); confirm its depth by pothole (UG-04) after locates under the state one-call law. The tool does not check locate or tracking tolerance.
- The radius is the larger of the rod and product limits entered.
- The space around a bundle is not even. Centred, the gap is 0.83 in at the sides and the largest void is about 1.87 in across (for the example's two 1¼ in conduits in a 4.98 in hole). An empty bundle tends to ride at the top of the hole, leaving a void about 3.2 in across below it. Some permits require grouting voids; check yours.
- The bundle OD is the tightest packing of n equal conduits: the smallest circle they fit in. A real bundle is the same size or larger. Mixed sizes are entered as a measured bundle OD with n = 1. For a bundle, the product bend radius is one conduit's (each conduit follows the bore path, so it is not scaled to the bundle OD; SME to confirm), and pullback force and the breakaway link rating are per the conduit and pulling-head manufacturers.
- The setback shown is the shortest one, at that minimum radius. A gentler curve needs a longer setback, so do not stake the entry point at exactly this distance.
- Out of scope: pullback force and safe pull strength (per the conduit manufacturer), exit-side geometry, tracking and steering, soil and fluid mix design, and bundles of mixed conduit sizes (set the number of conduits to 1 and enter the measured bundle OD).
Sources
- UG-07 step 2: backreamer rule of thumb, status verify.
- Plastics Pipe Institute, Handbook of PE Pipe, Ch. 12 (2026): typical reamer 1.2 to 1.5 × the product diameter.
- HDD Consortium guidelines (2004 reprint): the lesser of OD + 12 in or 1.5 × OD; for products 8 in or less, OD + 4 in.
- One rig maker's guidance: 1.5 × up to 9 in, 1.3 × at 10 in and larger.
- Some permits cap the hole: one state DOT allows at most 1.5 × OD for pipe up to 12 in.
- One drilling fluid supplier's published guidance (trade press, 2022): 2 to 3 × the hole volume in sand, 3 to 5 × in clay. Use your supplier's factor.
- UG-05 step 1: bend limits per rod and product manufacturer data; target cover per permit; clearance at each potholed crossing per owner or utility spec.
- UG-04: potholing to confirm the depth of a crossed utility.
- Plastics Pipe Institute: the bore hole should be at least 1.5 × the OD of the conduit or conduit bundle (TN-63).
- For several conduits, PPI recommends a staggered pulling harness with a breakaway swivel. The breakaway rating for a bundle is per the conduit and pulling-head manufacturers.
- Tightest packing of equal circles in a circle (bundle OD factors k_1 to k_7, exact by geometry): Friedman, "Circles in Circles"; optimality for n = 5 to 7 proved by Graham (1968).
- Unit definitions (exact): 1 US gallon = 231 in³ ; 1 ft³ = 1,728 in³ ; 1 ft = 12 in.
Planning sketch only. The engineer's bore plan, rod and product manufacturer data and owner specifications govern.
FAHMY tools give planning estimates. The owner's specifications, the authority having jurisdiction and the manufacturer's data govern every job. Values marked "Planning assumption", "Planning default", "Typical maximum", "Rule of thumb" or "Example" are editable starting points, not requirements; "Per owner spec" fields take the owner's value.