Micropile
One micropile by the allowable stress method of FHWA NHI-05-039, Micropile Design and Construction (2005): the grout-to-ground bond capacity of the bond zone, the structural capacity of the cased length and of the uncased bond zone in compression and tension, buckling where the pile crosses a void or very soft soil, and the bearing of the top plate on the cap by ACI 318-19 22.8. Service loads use the ASCE 7-22 2.4.1 combinations, the cap bearing the 2.3.1 combinations.
The FHWA manual is not on disk: its equations, the αbond ranges of its Table 5-3 and the strain-compatibility limit are written from knowledge and are marked "to confirm against the code text" in the report.
Inputs of the micropile #
- Loads: unfactored axial load per load type, compression positive, tension negative.
- Micropile: the casing (common API N80 sizes), its yield, the cased length, the casing area left at the threaded joint as a fraction (from the supplier's joint detail), a corrosion allowance on the outside face, the center bar and its yield, the grout strength.
- Bond zone: grouted diameter and length, the ground and the grouting method (Type A to D), the ultimate bond αbond (yours, from the geotechnical report and tests; the Table 5-3 range is shown) and the factor of safety (2.0).
- Buckling: an unsupported length (0 = laterally supported), K and a factor of safety.
- Cap connection: cap f'c, the square top plate and the cap concrete beyond it (for A2).
Auto-design varies the bond length (10 to 60 ft), the casing and the center bar; loads, ground, αbond, grout and factors of safety are kept.
Checks performed #
| Check | Basis |
|---|---|
| Bond capacity, compression and tension | αbond π Db Lb / FS (FHWA NHI-05-039 Ch. 5); αbond against Table 5-3 (advisory) |
| Cased length, compression | 0.40 f'cg Ag + 0.47 Fy (Abar + Acasing at the joint) |
| Uncased bond zone, compression | 0.40 f'cg Ag + 0.47 Fy Abar |
| Tension | 0.55 Fy Abar |
| Fy | Least of bar and casing yield, not more than 87 ksi (grout strain 0.003) |
| Buckling (where entered) | Euler over K Lu with the steel EI, divided by the factor of safety |
| Cap bearing | ACI 318-19 22.8.3.2: 0.65 × 0.85 f'c A1 √(A2/A1), √(A2/A1) ≤ 2, factored load |
The drawing is an elevation of the cap, the cased length and the bond zone with the bar.
Limitations #
- Group effects and spacing, settlement, lateral load and bending, the plunge length, the grout-to-bar bond and the cap design (punching, reinforcement) are not covered.
- Buckling ignores the lateral support of the soil (conservative over the length entered); a rational analysis (Davisson, p-y) may give more.
- FHWA NHI-05-039 not on disk: every FHWA equation and range is to confirm against the code text.
Verification examples #
Verification status: 8 values compared in 7 examples (0 published, 7 hand calculations from the cited equations): 8 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: written without the code document on hand, and confirmed only in part through published examples.
No published micropile example is on disk. Hand calculations (closed form) check the bond capacity, the cased and uncased compression, tension, the 87 ksi limit, the joint and corrosion area, Euler buckling and the cap bearing.