Bridge Foundations in Liquefiable Alluvium: Triggering Analysis and Foundation Choice
A 6-span, 240 m long road bridge is to cross a river on alluvial deposits. Pier loads are 8 MN, and spread footings at 3 m depth are proposed. The upper 12 m consists of loose to medium-dense silty sand with SPT N = 6–9, fines content 8–12%, and groundwater at 2.0 m. CPT tip resistance is 3–5 MPa in the same interval. Below 12 m, N values exceed 30 in dense gravel. The design earthquake has PGA 0.25g and moment magnitude 6.5. The river is 60 m wide, and the bridge must remain open for emergency vehicles after the design event. The client has asked whether spread footings are acceptable and what ground improvement or foundation alternative would satisfy the performance requirement.
1. Assess liquefaction triggering at 6 m depth. Show the calculation and state whether the spread footing concept is viable.
Read the worked answer
Use the simplified Seed–Idriss approach at 6 m depth.
Total vertical stress: 6 m × 18 kN/m³ ≈ 108 kPa. Pore pressure: (6 − 2) × 9.81 ≈ 39 kPa. Effective stress: 108 − 39 ≈ 69 kPa. Depth reduction factor rd ≈ 0.97.
CSR = 0.65 × (0.25) × (108/69) × 0.97 ≈ 0.25.
SPT N = 8 at 6 m. The overburden correction Cn is about 1.2, giving (N1)60 ≈ 9.6. With 8–12% fines, the CRR curve gives approximately 0.13–0.15. The factor of safety is therefore about 0.13/0.25 to 0.15/0.25, or 0.5–0.6.
That is well below 1.0, so the sand is expected to liquefy at 6 m under the design event. The CPT values of 3–5 MPa are consistent with a loose, contractive deposit. Post-liquefaction settlement could be of the order of 100–300 mm, with lateral spreading toward the river adding more. A spread footing at 3 m depth bears within the liquefiable layer and would lose bearing capacity and suffer differential settlement. The concept is not viable for a bridge that must remain open after the design event.
2. Compare foundation and ground-improvement options and recommend one, with reasons.
Read the worked answer
Options:
Spread footings with ground improvement: vibro-replacement stone columns or deep soil mixing to 12 m, plus a gravel raft. Stone columns densify and drain, but they do not eliminate liquefaction beneath a heavily loaded pier and are difficult to verify under a 60 m river crossing. Deep soil mixing creates a composite block but is expensive and its design at 8 MN pier load is sensitive to column quality.
Driven or bored piles to the dense gravel below 12 m. This bypasses the liquefiable layer and transfers load to competent ground. For a bridge that must remain serviceable, this is the most robust option. Piles must be designed for downdrag and for lateral spreading loads from the liquefied crust, not just vertical load.
Base grouting or jet grouting under footings: possible but difficult to verify and prone to defects.
Recommendation: driven precast piles or bored piles socketed at least 2–3 m into the dense gravel, with pile caps designed for a 100–150 mm settlement allowance and approach slabs with joints that accommodate movement. The piles should be checked for lateral spreading: the river is only 60 m wide, so the free-face effect is significant. Ground improvement may still be used behind the abutments to reduce approach fill settlement, but it should not be the primary support for the piers.
3. What verification and monitoring would you specify, and what performance criteria would you set?
Read the worked answer
Verification must cover both the ground model and the constructed foundation.
Pre-construction: additional SPT and CPT at each pier, with shear-wave velocity measurements to confirm the depth to dense gravel. Re-run the liquefaction analysis with the final (N1)60 and fines values; do not rely on the preliminary numbers. If any pier has a thicker liquefiable layer than assumed, revise the pile lengths.
During construction: pile integrity testing on 100% of piles (low-strain) and static load testing on at least two piles, one at an abutment and one at a central pier. For driven piles, record set and hammer energy; for bored piles, use a tremie log and concrete volume check. Verify pile toe level against the gravel surface.
Post-construction monitoring: settlement plates on approach fills and abutments, surveyed quarterly for two years; piezometers in the liquefiable layer at two piers; and crack gauges on approach slabs. After any earthquake above 0.1g, inspect for settlement and lateral movement.
Performance criteria: vertical settlement of any pier less than 25 mm, differential settlement between adjacent piers less than 10 mm, no structural damage that prevents emergency vehicle access, and approach slab steps below 15 mm. If these are not met, the bridge should be closed pending reassessment.