Rural Solar Farm DC Conduit Run in West Texas

Scenario

A 25 MW utility-scale solar farm near Pecos, Texas required underground burial of 300-mm-diameter HVDC conduit bundles (grouped in triplets) across undulating, arid terrain. Constraints included: (1) highly variable native Gravel soil (poor lateral support, high drainage), (2) minimal imposed load (only gravel ballast and occasional maintenance vehicle traffic: ~5 kN/m²), (3) mandated 50 mm clearance per side for thermal expansion and inspection access, and (4) aggressive schedule pressure limiting dewatering or soil replacement.

Given Data

• Conduit Diameter: 0.30 m (Note: tool max is 0.5 m — still valid)
• Soil Type: Gravel
• Load Weight: 5000 N/m²
• Clearance: 0.05 m

Calculation

1. Trench Width: Purely geometric: trench_width = 0.30 + 2 × 0.05 = 0.40 m. Confirmed sufficient for triplet bundle installation with compaction tools.

2. Trench Depth: Gravel has high bearing capacity but negligible cohesion; depth model uses conservative uplift resistance factor. Base depth = 0.60 m, plus 0.0001 × load (N/m²), plus 0.15 m safety buffer for scour and frost heave mitigation (per TXDOT Standard Spec 2-1.05):
   trench_depth = 0.60 + (0.0001 × 5000) + 0.15 = 0.60 + 0.5 + 0.15 = 1.25 m → 1.25 m.

Result and Decision

The calculator returned: Trench Depth = 1.25 m, Trench Width = 0.40 m. Field verification confirmed Gravel’s uniform density (N-value > 50), validating the reduced depth vs. clay scenarios. Crews used tracked mini-excavators with laser-guided depth control and installed perforated drain tile below the trench bottom to mitigate flash-flood infiltration—addressing an unmodeled but critical site-specific risk.

Lesson

Gravel’s high bearing capacity allows shallower trenches only when in-situ density and drainage are verified—never assume uniformity across long runs; always perform spot N-value testing every 200 m.