technicalhow-to
Deep Foundations: Selecting the Right System
Saida Bilqiis Bajeh, GMNSE
10/5/2023
10 min read
# Deep Foundations: Selecting the Right System
## Introduction
Selecting the appropriate deep foundation system is one of the most critical decisions in structural engineering. The right choice impacts project cost, schedule, and long-term performance.
## Foundation System Comparison
### Drilled Piers (Drilled Shafts)
**Best For:**
- Variable soil conditions
- Limited headroom or access constraints
- Proximity to existing structures
- Moderate to heavy loads (100-1,000+ tons per pier)
**Advantages:**
- Flexible diameter and depth
- Lower vibration during installation
- Can be verified by visual inspection
- Effective in most soil types
**Disadvantages:**
- Slower installation than driven piles
- Requires dewatering in high water table
- Weather-dependent concrete placement
- Equipment setup time
**Typical Cost:** $150-300 per linear foot (24-36" diameter)
### Driven Piles
**Best For:**
- Good load test data available
- Uniform soil conditions
- Open sites with space for equipment
- Time-sensitive projects
**Advantages:**
- Rapid installation
- High production rates (20-30 piles/day)
- Immediate load-carrying capacity
- Lower material costs
**Disadvantages:**
- Noise and vibration concerns
- Difficult in obstructed subsurface
- Less flexibility in final elevation
- Potential for pile damage during driving
**Typical Cost:** $35-65 per linear foot (12-14" square concrete piles)
### Caisson Foundations
**Best For:**
- Bridge piers and abutments
- Large concentrated loads
- Below water table construction
- Rocky or erratic soil profiles
**Advantages:**
- Excellent for major structures
- Can penetrate difficult materials
- High capacity (1,000+ tons each)
- Well-suited for underwater construction
**Disadvantages:**
- Specialized equipment required
- Higher cost per unit
- Complex construction process
- Weather and water dependent
**Typical Cost:** $400-800 per linear foot (8-12' diameter)
## Decision Matrix
When evaluating foundation options, consider these factors:
| Factor | Drilled Piers | Driven Piles | Caissons |
|--------|---------------|--------------|----------|
| Load Capacity | High | Medium-High | Very High |
| Installation Speed | Medium | Fast | Slow |
| Noise/Vibration | Low | High | Low-Medium |
| Cost | Medium | Low | High |
| Versatility | High | Medium | Medium |
| Verification | Easy | Difficult | Easy |
## Case Study: Riverside Commercial Complex
For the Riverside project, we faced several constraints:
- **Soil Profile**: 15' of loose sand over dense glacial till
- **Water Table**: 8 feet below grade
- **Adjacent Structures**: Historic building 25 feet away
- **Load Requirements**: 400-600 tons per column
### Our Solution
We selected drilled piers based on:
1. **Vibration Control**: Driven piles would risk damaging adjacent historic structure
2. **Variable Conditions**: Uncertain till depth required flexible system
3. **High Water Table**: Permanent casing for top 15' controlled groundwater
4. **Quality Assurance**: Visual inspection and cross-hole sonic logging
### Final Design
```
Pier Configuration: 42-inch diameter drilled piers
Depth: 42 feet (penetrating 12' into glacial till)
Capacity: 650 tons per pier (1.5 safety factor)
Installation: 16 piers over 3 weeks
Total Cost: $380,000 ($23,750 per pier)
```
## Geotechnical Investigation
No foundation selection is complete without thorough investigation:
### Minimum Requirements
- **Boring Frequency**: One per 5,000-10,000 SF of building area
- **Depth**: Minimum 1.5x expected foundation depth
- **Testing**: SPT values, moisture content, Atterberg limits
- **Advanced Testing**: CPT, shear wave velocity for seismic design
## Conclusion
Foundation selection requires balancing technical requirements, site constraints, and project economics. Early geotechnical investigation and collaboration between geotechnical and structural engineers leads to optimal solutions.
*Need foundation engineering expertise? [Contact our team](/contact) to discuss your project.*
## Introduction
Selecting the appropriate deep foundation system is one of the most critical decisions in structural engineering. The right choice impacts project cost, schedule, and long-term performance.
## Foundation System Comparison
### Drilled Piers (Drilled Shafts)
**Best For:**
- Variable soil conditions
- Limited headroom or access constraints
- Proximity to existing structures
- Moderate to heavy loads (100-1,000+ tons per pier)
**Advantages:**
- Flexible diameter and depth
- Lower vibration during installation
- Can be verified by visual inspection
- Effective in most soil types
**Disadvantages:**
- Slower installation than driven piles
- Requires dewatering in high water table
- Weather-dependent concrete placement
- Equipment setup time
**Typical Cost:** $150-300 per linear foot (24-36" diameter)
### Driven Piles
**Best For:**
- Good load test data available
- Uniform soil conditions
- Open sites with space for equipment
- Time-sensitive projects
**Advantages:**
- Rapid installation
- High production rates (20-30 piles/day)
- Immediate load-carrying capacity
- Lower material costs
**Disadvantages:**
- Noise and vibration concerns
- Difficult in obstructed subsurface
- Less flexibility in final elevation
- Potential for pile damage during driving
**Typical Cost:** $35-65 per linear foot (12-14" square concrete piles)
### Caisson Foundations
**Best For:**
- Bridge piers and abutments
- Large concentrated loads
- Below water table construction
- Rocky or erratic soil profiles
**Advantages:**
- Excellent for major structures
- Can penetrate difficult materials
- High capacity (1,000+ tons each)
- Well-suited for underwater construction
**Disadvantages:**
- Specialized equipment required
- Higher cost per unit
- Complex construction process
- Weather and water dependent
**Typical Cost:** $400-800 per linear foot (8-12' diameter)
## Decision Matrix
When evaluating foundation options, consider these factors:
| Factor | Drilled Piers | Driven Piles | Caissons |
|--------|---------------|--------------|----------|
| Load Capacity | High | Medium-High | Very High |
| Installation Speed | Medium | Fast | Slow |
| Noise/Vibration | Low | High | Low-Medium |
| Cost | Medium | Low | High |
| Versatility | High | Medium | Medium |
| Verification | Easy | Difficult | Easy |
## Case Study: Riverside Commercial Complex
For the Riverside project, we faced several constraints:
- **Soil Profile**: 15' of loose sand over dense glacial till
- **Water Table**: 8 feet below grade
- **Adjacent Structures**: Historic building 25 feet away
- **Load Requirements**: 400-600 tons per column
### Our Solution
We selected drilled piers based on:
1. **Vibration Control**: Driven piles would risk damaging adjacent historic structure
2. **Variable Conditions**: Uncertain till depth required flexible system
3. **High Water Table**: Permanent casing for top 15' controlled groundwater
4. **Quality Assurance**: Visual inspection and cross-hole sonic logging
### Final Design
```
Pier Configuration: 42-inch diameter drilled piers
Depth: 42 feet (penetrating 12' into glacial till)
Capacity: 650 tons per pier (1.5 safety factor)
Installation: 16 piers over 3 weeks
Total Cost: $380,000 ($23,750 per pier)
```
## Geotechnical Investigation
No foundation selection is complete without thorough investigation:
### Minimum Requirements
- **Boring Frequency**: One per 5,000-10,000 SF of building area
- **Depth**: Minimum 1.5x expected foundation depth
- **Testing**: SPT values, moisture content, Atterberg limits
- **Advanced Testing**: CPT, shear wave velocity for seismic design
## Conclusion
Foundation selection requires balancing technical requirements, site constraints, and project economics. Early geotechnical investigation and collaboration between geotechnical and structural engineers leads to optimal solutions.
*Need foundation engineering expertise? [Contact our team](/contact) to discuss your project.*