Crushing Expectations: Understanding Tile Strength Ratings in Drainage and Septic Systems

[MikePhua]

Introduction
In the world of underground infrastructure, the strength of buried pipe—often referred to as “tile” in agricultural and septic contexts—is a critical factor in system longevity and safety. Whether laying PVC for a septic outlet or dual-wall plastic tile for field drainage, understanding crush ratings helps ensure that the pipe can withstand soil pressure, surface loads, and environmental stress. This article explores the differences in crush ratings between Schedule 35 and Schedule 40 PVC and dual-wall corrugated plastic tile, with terminology, field anecdotes, and historical context to guide informed decisions.
Defining Crush Rating and Its Importance
Crush rating refers to the amount of force a pipe can withstand before structural failure. It is typically measured in pounds per square inch (psi) or pounds per linear inch (lbs/in) depending on the testing method.
Key factors influencing crush rating include:

• Pipe wall thickness
  
• Material composition
  
• Pipe diameter
  
• Soil cover depth
  
• Surface load (e.g., vehicles, equipment)
  

Terminology Explained

• Schedule 35 PVC: A lightweight pipe with thinner walls, commonly used in gravity-fed sewer systems.
  
• Schedule 40 PVC: A thicker-walled pipe with higher pressure and crush resistance, suitable for pressurized systems and shallow burial.
  
• Dual-Wall Corrugated Tile: A plastic pipe with a corrugated exterior for flexibility and a smooth interior for flow efficiency.
  
• Cover Depth: The amount of soil above the pipe, which affects load distribution and protection.
  

Comparing Pipe Types and Ratings

• Schedule 35 PVC
  • Wall Thickness: Thinner than Schedule 40
    
  • Crush Rating: Lower; not ideal for shallow burial under heavy loads
    
  • Use Case: Gravity sewer lines with adequate cover
    
• Schedule 40 PVC
  • Wall Thickness: Thicker and more rigid
    
  • Crush Rating: Higher; suitable for shallow burial and moderate surface loads
    
  • Use Case: Septic outlets, pressurized systems, and areas with minimal cover
    
• Dual-Wall Corrugated Tile
  

• Structure: Corrugated exterior with smooth interior
  
• Crush Rating: Varies by manufacturer; often rated for agricultural loads
  
• Use Case: Field drainage, stormwater management, and septic leach fields
  

Field Anecdote: The Rolling Hills Dilemma
In northwest Ohio, a contractor faced a challenge installing a septic outlet across a rolling field with only one foot of soil cover in some areas. Concerned about the crush rating, they debated between Schedule 35 PVC and dual-wall tile. Ultimately, they chose Schedule 40 PVC for its superior strength, ensuring the pipe could withstand farm equipment traffic and soil movement. The decision prevented future collapse and costly repairs.
Historical Context: Evolution of Drainage Tile Materials
Early drainage systems used clay tiles, which were brittle and prone to breakage under load. The 1960s saw the rise of concrete and asbestos-cement pipes, followed by the widespread adoption of PVC and HDPE plastic in the 1980s. These modern materials offered improved flexibility, chemical resistance, and crush strength, revolutionizing underground infrastructure.
Related Case: Agricultural Tile Failure in Iowa
A farmer in Iowa installed dual-wall tile with minimal cover to drain a low-lying field. After a wet season, heavy machinery compacted the soil, and several tile sections collapsed. The manufacturer later clarified that the tile required at least 18 inches of cover to meet its rated crush strength. The incident underscored the importance of matching pipe specs to field conditions.
Best Practices for Pipe Selection and Installation

• Verify crush ratings from manufacturer specifications
  
• Consider surface load and soil type when choosing pipe
  
• Maintain adequate cover depth—typically 18–24 inches for plastic tile
  
• Use Schedule 40 PVC in shallow or high-traffic areas
  
• Avoid sharp bends and improper bedding that can stress the pipe
  

Conclusion
Crush ratings are more than technical specs—they’re the backbone of reliable underground systems. Whether navigating rolling terrain or supporting heavy equipment, choosing the right pipe based on crush strength ensures durability and peace of mind. With a blend of engineering insight and field experience, contractors and landowners can build systems that stand the test of time—and the weight of the world above.