Railroad Bridge Approach Embankment Settlement Mitigation-Paper

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The paper describes settlement mitigation for a 1,520-ft-long railroad bridge approach embankment in the San Diego River floodplain. The site contained soft, compressible silts and clays, liquefiable soils, and shallow groundwater. Up to 12 ft of new fill was planned in a narrow right-of-way beside an operating railway, buildings, roads, and numerous utilities, making settlement a major risk.<br /><br />Design analyses found that conventional fill could cause nearly 5 in. of settlement at the railway and 1.3 in. near the property line. Lightweight cellular concrete fill (LCCF) reduced the embankment’s load, but alone did not adequately protect adjacent structures. Stone columns were also evaluated; rigid inclusions (RIs) were selected because they better controlled settlement beneath both the embankment and nearby buildings.<br /><br />The final design combined an LCCF mechanically stabilized earth wall with RIs installed in a triangular grid at roughly 7-ft spacing. More than 800 inclusions, at least 18 in. in diameter and up to 65 ft deep, transferred loads through compressible soils to stronger underlying layers. A crushed-gravel load-transfer platform distributed stresses and protected the LCCF. Limited compaction grouting was used where access restrictions prevented RI installation.<br /><br />Construction took place alongside live train traffic and existing structures. Real-time data acquisition tracked drilling and grouting parameters, while grout samples were tested to verify strength. Settlement was monitored at more than three dozen locations. Analyses predicted maximum settlement of about 0.5 in. at the railway and under 0.125 in. at the property line; measured settlement was 0.125 in. or less. The project demonstrates that combining load reduction with RI ground improvement can effectively limit total and differential settlement in constrained railroad environments.

Keywords

railroad bridge

settlement mitigation

embankment

San Diego River floodplain

lightweight cellular concrete fill

rigid inclusions

ground improvement

compressible soils

liquefiable soils

load transfer platform