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Current Practice in Buried Structures & Pipelines April 2012. Presentation by: Howard Lum, P.E., S.E . Agenda. Design Criteria AASHTO Design Standards Concrete Box Culvert Design Steel Pipe Design Cut-and-cover Construction Tunneling Methods. Design Standards. Loads:
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Current Practice in Buried Structures & PipelinesApril 2012 Presentation by: Howard Lum, P.E., S.E.
Agenda • Design Criteria • AASHTO Design Standards • Concrete Box Culvert Design • Steel Pipe Design • Cut-and-cover Construction • Tunneling Methods
Design Standards Loads: • California Building Code • Caltrans/AASHTO LRFD Standards Concrete Box Culverts: • ACI 318 & AASHTO LRFD Steel Pipes: • AWWA M11 – Steel Pipe Design Guide Concrete Pipes: • AWWA M9 – Concrete Pressure Pipe
Vehicular Load • HS-20 (20-ton) semi-truck • Axle Load (LL) = 32 kips • Wheel Load = 32k/2 = 16 kips • Max Impact = 30% of LL
Projection of Wheel Loads • Live Load diminishes uniformly below grade • AASHTO equation: LL1 = 4 kips / Z12 (in ksf) LL2 = 16 kips / Z12 (in ksf)
Concrete Box Culvert Design • ACI 318 – LRFD • U = 1.2 (D + F + T) + 1.6 (L + H) where D: Dead; F: Fluid/Water; T: Temperature; L: Live; H: Soil/Ground Water F F
AASHTO LRFD Procedures • Determine Mu (moment), Vu (shear) and Pu (axial) based on factored loads • Assume concrete section, As (reinforcement), f’c (concrete compressive strength) & Fy (60 ksi) • Check Flexure: Mn = 0.9 (As*Fy)*(d - a/2) > Mu • Check Shear: Vn = 0.9 (Vc + Vs) > Vu • Check Axial Compression: Pn = 0.7 Pn > Pu
Vehicular Loads (HS-20) • Tire contact area (rectangle): • length in the direction of traffic = 10 inches • width of tire = 20 inches 10” 20” Direction of Traffic
AWWA M-11 Design • Internal Pressure (Hoop Stress) • Min. pipe thickness t = p*D/(2S) • p: internal pressure (psi) • D: outside diameter • S: allowable stress (20,000 psi)
AWWA M-11 Design • External Loads • Wc (dead load) = DH • Check axial stressand deflection • Deflection governs as mortar lining is rigid with low tolerance • Allow. Deflection = 2% of pipe D
Cut-and-cover Pipeline • Open excavation and temporary support prior to pipeline installation • Advantages: • Conventional excavation equipment • Lower cost of construction • Exposed ground conditions • Disadvantages: • Access and Right-of-way issues • Disturbance to neighborhood • Traffic control • Environmental issues
Sloped Excavation 68 feet 25 feet 18 feet
Tunneling Methods • Drill and Blast Tunneling • Micro-tunneling • Tunnel Boring Machine (TBM) • New Austrian Tunneling Method (NATM) or Sequential Excavation Method (SEM)
First Drilling Jumbo in USA Hoover Dam 1931
Micro-tunneling Micro-Tunneling Micro Tunneling TBM
Microtunneling • Diameter ranges from 18” to 108” (9 ft) • Length - less than 1,500 feet • Straight drives between shafts • Limitations: • Boulders and rocks • Very soft ground • Mixed face conditions • Construction debris or fill • Shaft construction
Tunnel Boring Machine (TBM) • First TBM invented in 1846 for Fréjus Rail Tunnel between France and Italy through the Alps • Adaptable to various ground conditions • Hard Rock vs. Soft Ground TBM • Soft Ground TBM: • Earth Pressure Balance (EBP) – up to 7 bars pressure • Slurry Shield (SS) – high pressure
