Paid tier only. Calling this without an authenticated CivilQuants account returns TIER_INSUFFICIENT — sign up at https://civilquants.com/pricing or use the free-tier alternative compute_manhole. Precast concrete catchpit / silt trap to BS 5911-3 comprising factory-cast monolithic base with integral silt sump (600-900mm below outlet invert), stacked precast chamber rings, precast cover slab, ductile-iron frame and cover (BS EN 124 loading classes), optional step irons, granular or concrete bedding, granular surround, geotextile separator, and excavation. Catalogue-driven sizing from CP_900 (900mm ID) to CP_1800 (1800mm ID). Routes via chamber_type='catchpit' attribute discrimination through CESMM4 K.1.6.x, NRM2 33.7.14, MMHW 500.5.16 (SHW Cl. 511 silt traps), and SMM7 R12.3.13. Distinct from manholes by the silt-sump-below-channel geometry; typical position upstream of petrol/oil interceptor (S24) in a forecourt or service-area drainage train. Example params: invert_depth=2.5 m (0.5–10), bedding_thickness_mm=150 mm (100–300), bedding_overhang_m=0.15 m (0.1–0.3). Example call: {"params": {"invert_depth": 2.5, "bedding_thickness_mm": 150, "bedding_overhang_m": 0.15}, "standard": "MMHW"}. Omitted parameters use sensible engineering defaults. Pass deliverables=["xlsx","dxf","pdf"] (any subset) to also receive one-shot download URLs in the same call: Excel BoQ (both tiers, watermarked free) plus the dimensioned DXF (CAD) and PDF drawing sheets (paid tier).