Which fault governs the Noun River bridge?
A 180 m bridge is planned across the Noun River near Foumban, on an alignment trending 080°. The site is underlain by weathered Pan-African granite and Miocene basalt, covered by 12 m of alluvium; groundwater is at 4 m and SPT N-values in the alluvium are 8–15. Mapping identifies two fault sets. F1 strikes 020° and dips 75° NW, carries left-lateral slickensides and 0.5–2 m of gouge, and intersects the river at proposed pier P2. F2 strikes 110° and dips 60° SW, carries right-lateral slickensides and 0.2–0.5 m of gouge, and lies 250 m downstream. A magnitude 5.5 earthquake occurred in 1970 about 25 km NE of the site, but no surface rupture was found. Regional seismicity is concentrated along the Cameroon Volcanic Line.
1. Interpret the kinematics of F1 and F2 and infer the local stress regime. Which fault should govern the bridge design, and why?
Read the worked answer
F1 is left-lateral and F2 right-lateral; both are steep strike-slip faults. Conjugate strike-slip faults of opposite sense indicate a strike-slip regime with horizontal maximum and minimum stresses and a vertical intermediate stress. The bisectors between the two fault strikes are about 065° and 155°; one is σ1 and the other σ3. The exact choice requires formal slip-vector inversion, but the regime is clearly strike-slip, not normal faulting.
F1 should govern design: it crosses proposed pier P2, has thicker gouge (0.5–2 m) and is directly on the alignment. F2 is 250 m downstream; it could displace an approach embankment but is less directly critical. The 1970 M5.5 event shows that the area is seismically active even without surface rupture, so both faults should be treated as capable. A paleoseismic trench and high-resolution geophysics across F1 are needed.
2. Outline the site investigation and design measures you would require before construction.
Read the worked answer
Site investigation should include: (1) seismic refraction and electrical resistivity to map the alluvium–bedrock surface and fault gouge; (2) cored boreholes at each pier to prove granite depth and gouge thickness; (3) a trench across F1 to check for Holocene displacement; (4) liquefaction assessment of the 12 m alluvium (SPT N=8–15, water table 4 m) using grain size and CPT; and (5) a seismic hazard study for the Cameroon Volcanic Line.
Design measures: found piers on deep piles or caissons socketed into fresh granite below the fault zone; avoid placing a pier in the F1 gouge; design for differential displacement and ground shaking; dewater during construction; and monitor slope stability. If the fault cannot be avoided, redesign the bridge to cross it in one span.