CAT E120B and 312B L Excavators with Caisson Attachments: Field Adaptations and Engineering Insights

[MikePhua]

Exploring the Versatility of Mid-Size CAT Excavators
The Caterpillar E120B and 312B L excavators represent two generations of mid-size hydraulic machines that have proven their worth in urban construction, foundation work, and utility trenching. While both models are no longer in production, they remain active in fleets across North America thanks to their robust design and adaptability. This article focuses on their use with caisson drilling attachments—a specialized configuration that transforms these excavators into foundation drilling rigs.
Machine Profiles and Core Specifications
- CAT E120B

• Operating weight: ~27,000 lbs
  
• Engine: Mitsubishi 4D31T, approx. 90 hp
  
• Boom: Standard mono boom
  
• Hydraulic flow: ~40 gpm
  
• Bucket breakout force: ~22,000 lbs
  
• Common use: Utility trenching, light demolition, foundation prep
  

- CAT 312B L

• Operating weight: ~28,000–30,000 lbs
  
• Engine: CAT 3064T, approx. 92 hp
  
• Boom: Long reach (L) configuration
  
• Hydraulic flow: ~44 gpm
  
• Bucket breakout force: ~24,000 lbs
  
• Common use: Site prep, caisson drilling, slope grading
  

Terminology Explained
- Caisson Attachment: A rotary drilling tool mounted to an excavator, used to bore deep cylindrical holes for concrete foundations
- Breakout Force: The maximum force the bucket can exert when digging
- Hydraulic Flow Rate: Volume of hydraulic fluid moved per minute; affects attachment performance
- Mono Boom: A single-piece boom design offering strength and simplicity
Field Story: Denver’s Urban Foundation Challenge
In Denver, Colorado, a contractor retrofitted a CAT 312B L with a caisson attachment to drill foundation shafts for a mixed-use building. The site had limited access and required precise hole placement between existing structures. The 312B L’s long reach and stable platform allowed the operator to drill 24-inch diameter shafts to depths of 20 feet without repositioning the machine. The project was completed ahead of schedule, saving over $15,000 in crane rental costs.
Attachment Integration and Hydraulic Considerations
Adding a caisson attachment to an excavator requires careful hydraulic matching. Key parameters include:

• Minimum flow rate: 40–45 gpm
  
• Operating pressure: 3,000–3,500 psi
  
• Auxiliary circuit: Must support bi-directional flow
  
• Mounting: Reinforced quick coupler or custom bracket
  

Recommendations:

• Install a flow control valve to prevent over-speeding the drill
  
• Use high-pressure hoses with abrasion-resistant sleeves
  
• Add a case drain line to protect the motor from pressure spikes
  
• Monitor hydraulic oil temperature during extended drilling
  

Structural Reinforcement and Wear Points
Drilling caissons places unique stress on the excavator’s boom and swing frame. Operators should inspect:

• Boom pivot pins for elongation or cracking
  
• Swing bearing for excessive play
  
• Undercarriage for uneven wear due to stationary drilling
  
• Hydraulic pump for cavitation signs
  

Preventive measures:

• Grease all pivot points daily during drilling
  
• Use boom-mounted vibration dampeners
  
• Reinforce boom with gussets if drilling in rocky soil
  
• Install a counterweight if using long-reach boom with heavy drill
  

Operator Visibility and Safety Enhancements
Open ROPS configurations like those on the E120B offer excellent visibility but expose the operator to dust and debris. For drilling applications:

• Install polycarbonate shields around the cab
  
• Use wireless cameras to monitor drill alignment
  
• Add LED floodlights for low-light conditions
  
• Equip operators with vibration-dampening seats and noise-canceling headsets
  

Real-World Comparisons and Legacy Value
While newer models like the CAT 315C or Komatsu PC138 offer improved electronics and emissions compliance, the E120B and 312B L remain popular due to:

• Simpler hydraulics
  
• Lower maintenance costs
  
• Compatibility with older attachments
  
• Proven reliability in harsh conditions
  

A contractor in Illinois shared that his E120B, despite being over two decades old, still outperformed newer machines in tight urban lots due to its compact swing radius and predictable controls.
Recommendations for Buyers and Retrofitters

• Verify hydraulic specs before purchasing a caisson attachment
  
• Inspect boom welds and pivot points for fatigue
  
• Consider upgrading to synthetic hydraulic fluid for better heat resistance
  
• Use GPS or laser alignment tools for precision drilling
  
• Maintain a log of drilling depth, soil type, and torque for each shaft
  

Conclusion: Engineering Adaptability in Action
The CAT E120B and 312B L excavators demonstrate how legacy machines can be repurposed for specialized tasks like caisson drilling. With proper hydraulic tuning, structural reinforcement, and operator training, these excavators continue to deliver value far beyond their original design. In the hands of a skilled crew, they become not just digging machines—but precision tools for shaping the foundations of tomorrow.