1. Overview
Foundation depth for compressor installations is governed by multiple factors that must be evaluated simultaneously. Unlike static structures where depth is often set by frost or bearing stratum alone, compressor foundations also require depth for mass ratio compliance, dynamic stability, and vibration isolation from adjacent foundations.
Factor 1
Frost Penetration
Base below local frost depth to prevent heaving
Factor 2
Bearing Stratum
Reach competent soil for bearing capacity
Factor 3
Mass Ratio
Depth often driven by mass requirements
Factor 4
Adjacent Foundations
Avoid overlap of stress bulbs and vibration zones
Governing factor: For compressor block foundations, the mass ratio requirement almost always governs depth, resulting in foundations significantly deeper than what frost depth or bearing stratum alone would require. Typical block depths range from 6 to 12 feet for reciprocating compressors in the 500 to 5,000 HP range.
2. Depth Determining Factors
The foundation depth is the maximum value from all governing criteria. Each must be checked independently.
Governing depth determination:
D_final = max(D_frost, D_bearing, D_mass, D_stability, D_adjacent, D_mfr)
Where:
D_frost = Local frost depth + 6" to 12" safety margin
D_bearing = Depth to reach competent bearing stratum
D_mass = Depth required for mass ratio (ACI 351.3R)
D_stability = Depth for overturning stability (FS ≥ 1.5)
D_adjacent = Depth to clear adjacent foundation stress zones
D_mfr = Manufacturer minimum depth requirement
Mass ratio depth (typically governs):
D_mass = (R_m x W_machine) / (L x W x gamma_c)
Where:
R_m = Required mass ratio (3-5 for reciprocating)
W_machine = Total machinery weight (lbs)
L, W = Foundation plan dimensions (ft)
gamma_c = Concrete unit weight (150 lb/ft^3)
Depth Criteria Summary
| Criterion | Typical Range | Code Reference | Commonly Governs? |
|---|---|---|---|
| Frost depth | 1-6 ft (varies by region) | Local building code | Rarely for compressors |
| Bearing stratum | 2-10 ft (site-dependent) | Geotech report | Sometimes (poor soil sites) |
| Mass ratio | 6-12 ft (recip); 4-8 ft (centrif) | ACI 351.3R | Usually (reciprocating) |
| Overturning stability | 3-6 ft | ACI 351.3R, ASCE 7 | Rarely (mass governs first) |
| Adjacent foundations | Clear zone = 2x depth | ACI 351.3R | Affects location, not depth |
| Manufacturer minimum | 3-6 ft | Vendor drawing | Sometimes (small machines) |
Minimum Depth Rules of Thumb
Quick depth estimates (before detailed analysis):
Reciprocating compressors:
D_min = max(L/5, W/3, 3 ft)
D_typical = 6 to 12 ft (for 500-5,000 HP)
Centrifugal compressors:
D_min = max(L/5, W/3, 2 ft)
D_typical = 4 to 8 ft (for 1,000-10,000 HP)
Height-to-depth rule:
D ≥ 0.5 x H_total (total height of machine above base)
This ensures CG of combined system stays in lower half of block.
3. Soil Investigation
A geotechnical investigation is mandatory for compressor foundation design. The investigation must characterize both static bearing capacity and dynamic soil properties needed for vibration analysis.
Required Geotechnical Data
| Parameter | Purpose | Test Method | Typical Values |
|---|---|---|---|
| Allowable bearing | Static design | Plate load test, SPT correlation | 1,500-6,000 psf |
| Shear modulus (G) | Dynamic spring constants | Crosshole, SASW, resonant column | 2,000-20,000 psi |
| Poisson's ratio (v) | Dynamic spring constants | Crosshole seismic | 0.25-0.45 |
| Soil density (rho) | Dynamic analysis | Shelby tube, sand cone | 100-130 pcf |
| SPT blow count (N) | Stratum identification | Standard penetration test | N > 15 for adequate bearing |
| Water table depth | Construction, buoyancy | Piezometer, boring observation | Variable |
| Soil classification | Settlement, swell potential | USCS classification | SC, CL, SP, GP, etc. |
Boring Requirements
Minimum boring program for compressor foundations:
Number of borings: 1 per foundation (minimum)
Small foundations (< 400 sq ft): 1 boring at center
Large foundations (> 400 sq ft): 2 borings at opposite corners
Boring depth:
D_boring = max(2 x B, D_foundation + 15 ft, 25 ft)
Where B = smallest foundation plan dimension
SPT sampling interval:
0-15 ft: every 2.5 ft
15-50 ft: every 5 ft
Below 50 ft: every 10 ft
Soil stratum identification:
Competent bearing stratum: N ≥ 15 (cohesionless) or
q_u ≥ 2,000 psf (cohesive)
Rock: Requires coring; RQD and unconfined compressive strength
Dynamic soil properties: Standard geotechnical reports often omit the dynamic shear modulus (G) and Poisson's ratio needed for vibration analysis. Specifically request these values from the geotechnical engineer. If field testing (crosshole, SASW) is not feasible, correlations with SPT blow count can provide preliminary estimates: G (psi) = 325 x N^0.68 (for sands) per Imai and Tonouchi (1982).
4. Frost Depth Considerations
Foundation bases must extend below the local frost penetration depth to prevent frost heave, which can cause differential settlement and equipment misalignment.
Regional Frost Depths (United States)
| Region | Frost Depth | Key States | Notes |
|---|---|---|---|
| Gulf Coast | 0-6" | TX, LA, MS, AL, FL | Frost rarely governs |
| Southern | 6-18" | OK, AR, TN, NC, SC | Shallow frost; mass governs |
| Mid-continent | 18-36" | KS, MO, KY, VA | Check local code |
| Northern | 36-60" | OH, PA, NY, MI, IA, NE | May affect top-of-foundation elev. |
| Far North | 60-84" | MN, WI, ND, MT | Still less than mass ratio depth |
| Permafrost | Continuous | AK (north) | Special pile foundations required |
Frost protection requirement:
D_base ≥ D_frost + 6" (minimum safety margin)
D_base ≥ D_frost + 12" (recommended for dynamic equipment)
Where:
D_base = Depth of foundation base below finished grade
D_frost = Local frost penetration depth per building code
Effect of waste heat:
Compressor foundations near operating equipment may experience
reduced frost depth due to waste heat. However, do NOT rely on
this reduction for design — use full frost depth.
Frost-susceptible soils (require extra caution):
Silts (ML, MH) — most frost-susceptible
Silty clays (CL) — moderate frost susceptibility
Clean sands (SP, SW) — low frost susceptibility
Gravels (GP, GW) — very low frost susceptibility
5. Overturning Stability
The foundation must resist overturning moments from wind, seismic, and dynamic unbalanced forces. The safety factor against overturning is evaluated about the base edges.
Overturning safety factor:
FS_overturn = M_resisting / M_overturning ≥ 1.5
Resisting moment:
M_resisting = W_total x (B/2)
Where:
W_total = W_machine + W_foundation + W_soil_on_block
B = Foundation width in the direction of overturning
Overturning moment (about base edge):
M_overturn = F_wind x H_cg + F_seismic x H_cg + F_dynamic x H_cg
Where:
F_wind = Wind force on exposed equipment (ASCE 7)
F_seismic = 0.2 x S_DS x W (simplified seismic, ASCE 7)
F_dynamic = Peak unbalanced force (from vendor)
H_cg = Height of force application above base
Sliding safety factor:
FS_slide = (mu x W_total + P_passive) / F_horizontal ≥ 1.5
Where:
mu = Coefficient of friction (concrete on soil)
= 0.30 (clay), 0.40 (sand), 0.55 (gravel)
P_passive = Passive earth pressure on embedded sides
Depth Effect on Stability
| Increased Depth Provides | Mechanism | Benefit |
|---|---|---|
| More mass | W_fdn = L x W x D x 150 | Higher resisting moment |
| Lower CG | Combined CG moves down | Reduced overturning arm |
| Passive resistance | Deeper soil engagement | Increased sliding resistance |
| Embedment effect | Confining pressure on sides | Increased dynamic stiffness |
Adjacent Foundation Clearance
Minimum separation between compressor foundations:
S_min = max(D1 + D2, 2 x D_max, 5 ft)
Where:
D1, D2 = Depths of adjacent foundations
D_max = Greater depth of the two foundations
Stress bulb overlap check:
The pressure bulb extends approximately 2B below the base
(B = foundation width). Adjacent foundations should not have
overlapping influence zones at the bearing stratum level.
Vibration isolation distance:
For reciprocating compressors, maintain minimum 10 ft clear
between foundation edges, or place isolation joints if closer.
6. Worked Examples
Example 1: Depth Determination for Multiple Criteria
Given:
1,500 HP reciprocating compressor in Oklahoma City, OK
Machine weight: 65,000 lbs
Foundation plan: 18 ft x 8 ft
Mass ratio required: 4:1
Local frost depth: 18 inches
Soil: Medium dense sand (N = 25), bearing = 3,500 psf
Geotech report: Competent sand starts at 3 ft depth
Check 1: Frost depth
D_frost = 18" + 12" safety = 30" = 2.5 ft
Check 2: Bearing stratum
D_bearing = 3.0 ft (top of competent sand)
Check 3: Mass ratio
D_mass = (4.0 x 65,000) / (18 x 8 x 150)
D_mass = 260,000 / 21,600 = 12.0 ft
Check 4: Minimum depth rule
D_min = 8 / 5 = 1.6 ft (smallest plan dimension / 5)
Check 5: Manufacturer minimum
D_mfr = 4.0 ft (per vendor foundation drawing)
Governing: D = max(2.5, 3.0, 12.0, 1.6, 4.0) = 12.0 ft
Use D = 12 ft (mass ratio governs, as typical for reciprocating)
Example 2: Overturning Stability Check
Given (from Example 1):
Foundation: 18 ft x 8 ft x 12 ft deep
W_machine = 65,000 lbs
W_foundation = 18 x 8 x 12 x 150 = 259,200 lbs
W_total = 324,200 lbs
Machine CG height above base: H_cg = 15 ft (12 ft depth + 3 ft above top)
Wind force on equipment: F_wind = 5,000 lbs (ASCE 7, 115 mph)
Seismic: S_DS = 0.15g, F_seismic = 0.2 x 0.15 x 324,200 = 9,726 lbs
Overturning about 8-ft edge (wind from long side):
M_resist = 324,200 x (8/2) = 1,296,800 ft-lbs
M_overturn = 5,000 x 15 + 9,726 x 15 = 220,890 ft-lbs
FS = 1,296,800 / 220,890 = 5.87 ≥ 1.5 OK
Overturning about 18-ft edge (wind from short side):
M_resist = 324,200 x (18/2) = 2,917,800 ft-lbs
M_overturn = 5,000 x 15 = 75,000 ft-lbs (wind only from this direction)
FS = 2,917,800 / 75,000 = 38.9 ≥ 1.5 OK
Both directions well above 1.5; mass ratio depth provides ample stability.
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