Ground Improvement Parameters: micropiles
Design parameters for ground improvement techniques — stone columns, soil mixing, jet grouting, dynamic compaction, vibro-compaction, preloading with PVDs, deep soil mixing, micropiles, and ground freezing — with bearing capacity, settlement reduction, column dimensions, applicable soils, and QC methods
| technique | applicable soil types | QC method | area replacement ratio max (dimensionless) | area replacement ratio min (dimensionless) | bearing capacity increase factor max (dimensionless) | bearing capacity increase factor min (dimensionless) | column diameter m max (m) | column diameter m min (m) | environmental impact | improvement depth max m (m) | liquefaction mitigation | modulus improvement factor max (dimensionless) | modulus improvement factor min (dimensionless) | notes | production rate m per day (m/day) | relative cost | settlement reduction factor max (dimensionless) | settlement reduction factor min (dimensionless) | spacing m max (m) | spacing m min (m) | undrained shear strength gain pct max (percent) | undrained shear strength gain pct min (percent) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| micropiles | any_soil_or_rock | load_testing_and_integrity_testing | 0.2 | 0.05 | 6 | 2 | 0.3 | 0.1 | low | 30 | no | 50 | 10 | Small diameter drilled and grouted piles (FHWA definition: D<300mm); Types A-D per FHWA NHI-05-039; gravity/pressure grouted; steel casing or bar reinforcement; capacity 10-200 kips typical; applicable to restricted access sites and underpinning; CASE 1 reinforcement (structural load) vs CASE 2 (soil reinforcement/reticulated); production 10-15m/day; load tests required per AASHTO; undrained shear gain listed as 0 as micropiles do not treat surrounding soil | 10 | high | 0.6 | 0.2 | 2 | 0.6 | 0 | 0 |
| micropiles | karst_or_rock | load_test_and_core | 0.15 | 0.03 | 8 | 3 | 0.25 | 0.1 | low | 25 | no | 80 | 15 | Rock socketed micropiles; grout bond in rock controls capacity; socket length 3-6x diameter typically; high capacity possible (200-600 kips); useful for bridge underpinning over karst geology | 8 | very_high | 0.7 | 0.3 | 2.5 | 0.8 | 0 | 0 |
| micropiles | loose_sand_existing_structure | load_test_and_PDA | 0.15 | 0.05 | 7 | 3 | 0.2 | 0.1 | low | 20 | no | 100 | 20 | Micropiles for underpinning existing structures in loose sand; end-bearing and skin friction in densified zone; can be installed at angles through existing footings; pre-augering through obstructions; high capacity relative to small diameter; vibration limits during installation | 12 | high | 0.65 | 0.3 | 2 | 0.8 | 0 | 0 |
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