Seismic Design Fundamentals | Pipeline Earthquake Engineering Guide

1. Overview

Earthquakes affect buried pipelines through two fundamentally different mechanisms: transient ground deformation (TGD) from seismic wave propagation, and permanent ground deformation (PGD) from fault displacement, liquefaction-induced lateral spreading, or earthquake-triggered landslides. Pipeline seismic design must evaluate both mechanisms and ensure that the pipe can withstand the resulting strains without loss of pressure containment.

Unlike buildings and bridges where force-based design is standard, buried pipelines are best evaluated using strain-based criteria. This is because buried pipelines deform with the surrounding ground, and the critical question is whether the pipe can accommodate the imposed ground strains without failure.

Wave Propagation (TGD)

Transient Strain

Seismic waves pass through the ground, imposing temporary strains that rarely cause failure in modern welded steel pipe.

Fault Crossing (PGD)

Permanent Displacement

Surface fault rupture imposes large, permanent displacements that require strain-based design.

Liquefaction (PGD)

Ground Failure

Saturated loose soil loses strength, causing lateral spreading, buoyancy uplift, and bearing failure.

Landslide (PGD)

Mass Movement

Earthquake-triggered slope failures impose large transverse and axial displacements on pipelines.

Historical performance: Welded steel pipelines have performed remarkably well during earthquakes when subjected to wave propagation alone. The vast majority of seismic pipeline failures have been caused by permanent ground deformation: fault rupture, liquefaction, or landslide. Design focus should prioritize PGD hazards.

2. Seismic Hazard Types

2.1 Seismic Ground Motion Parameters

Seismic hazard is characterized by ground motion parameters obtained from ASCE 7 or USGS seismic hazard maps. The two most important parameters for pipeline design are Peak Ground Acceleration (PGA) and Peak Ground Velocity (PGV).

Parameter	Symbol	Use in Pipeline Design
Peak Ground Acceleration	PGA (g)	Ground curvature, bending strain, liquefaction triggering
Peak Ground Velocity	PGV (cm/s)	Ground strain from wave propagation (primary parameter)
Peak Ground Displacement	PGD (cm)	Fault crossing, lateral spreading displacement
Spectral Acceleration	Sa (g)	Above-ground facilities, not buried pipe

2.2 ASCE 7 Site Classification

Site class determines the amplification of ground motion based on soil conditions at the pipeline location. Softer soils amplify ground motion and increase seismic demand on the pipeline.

Site Class	Description	V_s30 (ft/s)	Wave Velocity C_s (ft/s)
A	Hard rock	> 5,000	8,000
B	Rock	2,500–5,000	5,000
C	Dense soil / soft rock	1,200–2,500	2,500
D	Stiff soil	600–1,200	1,500
E	Soft soil	< 600	600

3. Wave Propagation Analysis

Seismic body waves (P-waves and S-waves) and surface waves (Rayleigh and Love waves) propagate through the ground, creating transient ground deformation. A buried pipeline, being much longer than the seismic wavelength, deforms with the ground and experiences axial and bending strains.

3.1 Axial Ground Strain

Ground Strain from Wave Propagation: ε_ground = PGV / C_s Where: ε_ground = Maximum ground strain (in/in) PGV = Peak Ground Velocity (in/s) C_s = Apparent wave propagation velocity (in/s) The apparent wave propagation velocity depends on site class and wave type. For design purposes, use the values from ALA Guidelines Table 5.1. Example (PGV = 40 cm/s, Site Class D): PGV = 40 × 0.3937 = 15.75 in/s C_s = 1,500 ft/s = 18,000 in/s ε_ground = 15.75 / 18,000 = 0.000875 = 0.0875%

3.2 Bending Ground Curvature

Ground Curvature from Surface Waves: κ = PGA × g / C_s² Where: κ = Ground curvature (1/in) PGA = Peak Ground Acceleration (g) g = 386.4 in/s² Bending strain in pipe: ε_bend = (D/2) × κ Example (PGA = 0.40g, C_s = 18,000 in/s): κ = 0.40 × 386.4 / (18,000)² κ = 4.77 × 10²&sup7; /in ε_bend = (20/2) × 4.77e-7 = 4.77 × 10²&sup6; Bending strain is typically much smaller than axial strain and can often be neglected for buried pipeline design.

Soft soil amplification: For Site Class E (soft soil), C_s = 600 ft/s versus 5,000 ft/s for rock. This means the ground strain in soft soil is over 8 times higher than in rock for the same PGV. Site class is the most important factor in wave propagation strain analysis.

4. Fault Crossing Design

When a pipeline crosses an active fault, surface rupture can impose large permanent displacements on the pipe. Unlike wave propagation strains (typically less than 0.1%), fault crossing strains can reach 1-5% or more, requiring strain-based design methodology.

4.1 Fault Displacement Components

Fault Displacement Resolution: For a pipe crossing a fault at angle β: Axial component: δ_axial = δ_fault × cos(β) Transverse component: δ_trans = δ_fault × sin(β) Where: δ_fault = Total fault displacement (inches) β = Crossing angle (degrees) β = 90° → purely transverse (perpendicular crossing) β = 0° → purely axial (parallel to fault)

4.2 Axial Strain from Fault Crossing

The axial component of fault displacement is absorbed over an anchorage length determined by pipe-soil interaction. The soil friction gradually transfers the fault displacement from the ground into axial strain in the pipe.

Anchorage Length and Axial Strain: L_a = √(2 × A × E × δ_axial / f_s) ε_axial = δ_axial / L_a Where: L_a = Anchorage length (inches) A = Pipe metal cross-section area (in²) E = Modulus of elasticity (29 × 10&sup6; psi) f_s = Axial soil friction per unit length (lb/in) δ_axial = Axial fault displacement (inches)

4.3 Optimal Crossing Angle

The crossing angle significantly affects the pipeline strain demand. For strike-slip faults, the pipe should cross such that fault movement puts the pipe in tension (not compression), because steel pipe has much greater tensile capacity than compressive (buckling) capacity.

Fault Type	Optimal Crossing Strategy	Worst Case
Strike-slip (right-lateral)	Cross to produce tension	Compression side causes local buckling
Strike-slip (left-lateral)	Mirror of right-lateral strategy	Same buckling concern
Normal (extensional)	Nearly perpendicular crossing	Parallel crossing causes maximum axial strain
Reverse (compressional)	Avoid if possible; use offsets	Any crossing produces compression

5. Liquefaction Assessment

Liquefaction occurs when saturated, loose granular soil loses shear strength during seismic shaking, effectively behaving as a liquid. This creates several hazards for buried pipelines: buoyancy uplift (the pipe floats in liquefied soil), lateral spreading on sloping ground, and loss of bearing support causing settlement.

5.1 Susceptibility Screening

The following conditions must all be present for liquefaction to occur: saturated soil (water table above pipe), loose granular soil (sands, silty sands), and sufficient seismic shaking (PGA above triggering threshold).

PGA (g)	Water Table < 10 ft	Water Table 10–30 ft	Water Table > 30 ft
< 0.10	Low	Low	Low
0.10 – 0.20	Moderate	Low	Low
0.20 – 0.30	High	Moderate	Low
> 0.30	Very High	High	Moderate

5.2 Pipeline Hazards from Liquefaction

Buoyancy Uplift Force (liquefied soil): F_buoy = γ_soil × A_trench − W_pipe Where: γ_soil = Liquefied soil unit weight (~100–110 pcf) A_trench = Trench cross-section area (ft²) W_pipe = Pipe weight per foot (including contents) If F_buoy > 0, the pipe will float upward. Lateral Spreading Displacement: Empirical estimate per Youd et al. (2002): log(D_H) = f(M, R, ground slope, liquefiable thickness) Typical values: 1–10 ft of lateral movement

Liquefaction is a PGD hazard: Like fault crossing, liquefaction-induced ground deformation is permanent and can impose very large strains on buried pipelines. A pipeline designed to withstand wave propagation can still fail due to liquefaction-induced lateral spreading or buoyancy uplift.

6. ALA Strain Criteria

The American Lifelines Alliance (ALA) Guidelines establish strain-based acceptance criteria for pipeline seismic design. These criteria recognize that buried pipelines are displacement-controlled structures where strain capacity governs design.

Allowable Strain Limits

ALA Tensile Strain Limit: ε_t_allow = 2% for standard girth welds (ECA-qualified) ε_t_allow = 4% for enhanced inspection girth welds ALA Compressive Strain Limit (local buckling): ε_c_allow = 0.175 × t/D Where: t = Wall thickness (inches) D = Outside diameter (inches) Example (20" OD, 0.500" wall): ε_c_allow = 0.175 × 0.500/20.000 = 0.00438 = 0.438% Compressive strain limit is typically much smaller than tensile limit, making compression the governing case for most pipe sizes.

Comparison of Strain Limits by D/t Ratio

Pipe Size	D/t Ratio	Compressive Limit (%)	Tensile Limit (%)
12" x 0.375"	34.0	0.515	2.0
16" x 0.375"	42.7	0.410	2.0
20" x 0.500"	40.0	0.438	2.0
24" x 0.500"	48.0	0.365	2.0
30" x 0.625"	48.0	0.365	2.0
36" x 0.500"	72.0	0.243	2.0

D/t ratio is critical: As pipe diameter increases relative to wall thickness, the compressive strain limit decreases. Large-diameter thin-wall pipe is more susceptible to local buckling under seismic loading. For fault crossing applications, thicker wall pipe (lower D/t ratio) provides significantly more compressive strain capacity.

7. Mitigation Strategies

Design Approaches for Fault Crossings

Strategy	Application	Benefit
Increased wall thickness	All fault crossings	Increases compressive strain capacity (0.175 t/D)
Optimized crossing angle	Strike-slip faults	Puts pipe in tension rather than compression
Loose backfill	Fault zone ±200 ft	Reduces soil restraint, spreads strain over longer length
Oversized trench	Fault zone	Allows pipe movement before engaging soil
Isolation valves	Both sides of fault	Limits product release if pipe fails
Fault avoidance	Route selection	Eliminates hazard entirely (preferred when feasible)

Liquefaction Mitigation

Method	Mechanism	Typical Cost
Ground improvement (densification)	Increases soil density above liquefaction threshold	High
Deep burial	Places pipe below liquefiable layer	Moderate
Concrete ballast	Prevents buoyancy uplift	Low–moderate
Sheet pile containment	Prevents lateral spreading at pipe crossing	High
HDD undercrossing	Places pipe in non-liquefiable stratum	Moderate–high

Pipeline Materials for Seismic Zones

Material selection significantly affects seismic performance. Ductile materials that can accommodate large strains without fracture are preferred for seismically active areas.

Material	Strain Capacity	Joint Weakness	Seismic Performance
Welded steel (butt weld)	2–4% tensile	Girth weld defects	Excellent
HDPE (butt fusion)	High (>5%)	Fusion quality	Excellent
Ductile iron (restrained)	Moderate	Joint pull-out	Good
Cast iron	Very low (<0.1%)	Brittle fracture	Poor
Asbestos cement	Very low	Brittle joints	Very poor

Weld quality is paramount: For steel pipelines in seismic zones, girth weld quality directly determines the pipeline's tensile strain capacity. Standard radiographic inspection allows 2% tensile strain, while enhanced inspection with Engineering Critical Assessment (ECA) allows up to 4%. All girth welds in fault crossing zones should receive 100% radiographic or ultrasonic inspection.

Pipeline Seismic Design

ε = PGV / C_s

0.175 × t/D

2 – 4%

Contents

Use this guide when:

1. Overview

Transient Strain

Permanent Displacement

Ground Failure

Mass Movement

2. Seismic Hazard Types

2.1 Seismic Ground Motion Parameters

2.2 ASCE 7 Site Classification

3. Wave Propagation Analysis

3.1 Axial Ground Strain

3.2 Bending Ground Curvature

4. Fault Crossing Design

4.1 Fault Displacement Components

4.2 Axial Strain from Fault Crossing

4.3 Optimal Crossing Angle

5. Liquefaction Assessment

5.1 Susceptibility Screening

5.2 Pipeline Hazards from Liquefaction

6. ALA Strain Criteria

Allowable Strain Limits

Comparison of Strain Limits by D/t Ratio

7. Mitigation Strategies

Design Approaches for Fault Crossings

Liquefaction Mitigation

Pipeline Materials for Seismic Zones

Pipeline Seismic Design

ε = PGV / Cs

0.175 × t/D

2 – 4%

Contents

Use this guide when:

1. Overview

Transient Strain

Permanent Displacement

Ground Failure

Mass Movement

2. Seismic Hazard Types

2.1 Seismic Ground Motion Parameters

2.2 ASCE 7 Site Classification

3. Wave Propagation Analysis

3.1 Axial Ground Strain

3.2 Bending Ground Curvature

4. Fault Crossing Design

4.1 Fault Displacement Components

4.2 Axial Strain from Fault Crossing

4.3 Optimal Crossing Angle

5. Liquefaction Assessment

5.1 Susceptibility Screening

5.2 Pipeline Hazards from Liquefaction

6. ALA Strain Criteria

Allowable Strain Limits

Comparison of Strain Limits by D/t Ratio

7. Mitigation Strategies

Design Approaches for Fault Crossings

Liquefaction Mitigation

Pipeline Materials for Seismic Zones

ε = PGV / C_s