Design And Installation Of — Asce 20-96 Standard Guidelines For The

For a project engineer designing a new 36-inch storm drain or a 12-inch gas main, here is the practical workflow dictated by ASCE 20-96:

Step 1: Geotechnical Investigation The standard requires a soil classification per the Unified Soil Classification System (USSC). A minimum of one soil test per 500 linear feet of trench is recommended.

Step 2: Determine Pipe Stiffness Calculate the pipe stiffness factor (PS) or D-Load for rigid pipes. Compare this to the required strength based on depth of cover. For a project engineer designing a new 36-inch

Step 3: Select Installation Type Choose Installation Type 1 through 4 based on available bedding materials and required long-term deflection. (Type 4 is rarely used today due to cost; Type 3 is the industry gold standard for critical utilities).

Step 4: Calculate Deflection (for Flexible Pipes) Use the Iowa Deflection Formula (modified by ASCE 20-96): Δy (%) = (DL * K * Wc * 100) / (144 * PS + 0.061 * E') Where E' is the soil modulus of reaction (depends on bedding type). The standard provides lookup tables for E' values. A key warning in the standard remains relevant:

Step 5: Specify Trench Geometry The guideline explicitly prohibits a trench width exceeding the pipe outside diameter plus 24 inches, unless structural reinforcement is added. Wider trenches impose higher loads on the pipe.

In 1996, trenchless technology was still emerging. ASCE 20-96 was prescient in dedicating a chapter to it. It covers: For a project engineer designing a new 36-inch

A key warning in the standard remains relevant: “Trenchless installation does not eliminate the need for proper bedding; it merely relocates the soil-disturbance zone.”

| Test Type | Purpose | ASTM Reference | |-----------|---------|----------------| | Static load test (compression) | Establish ultimate capacity | ASTM D1143 | | Static load test (tension) | Uplift resistance | ASTM D3689 | | Lateral load test | p-y curve calibration | ASTM D3966 | | PDA / Dynamic test | Driving stresses & capacity | ASTM D4945 |

| Mistake | ASCE 20-96 Prevention | | :--- | :--- | | Using native clay as bedding for a flexible pipe | Section 4.3 requires sand or gravel (ASTM C33) for Type 3 and 2 installations. | | Over-compacting the haunch (side of pipe) | Section 5.7 warns that excessive compaction of the haunch can displace the pipe centerline. | | Ignoring groundwater buoyancy | Section 3.4 provides uplift calculations for empty pipes in high water tables. | | Assuming HDD soil friction is zero | Section 6.2.1 mandates a safety factor of 2 for pullback force vs. pipe tensile strength. |