A 6-story apartment building on Hamilton Street showed 8 cm of differential settlement within two years of completion. The culprit was a metastable soil structure that collapsed upon wetting — a classic collapsible soil failure. In Allentown, these loose, low-density soils often come from the Allentown Formation or older alluvial fans along the Lehigh River. Our lab team has processed dozens of samples from these deposits, running collapse potential tests under controlled moisture conditions. Before foundation design begins, we recommend coupling our collapsible soil evaluation with a density test using a sand cone to confirm in-place dry unit weights across the site.
Collapse strains between 2% and 8% are common in Allentown's colluvial silty sands — enough to crack foundations.
Method and coverage
Allentown grew rapidly after the Lehigh Canal opened in 1829, and many early neighborhoods were built directly on colluvial soils without any geotechnical investigation. These soils, often silty sands with low clay content, exhibit collapse strains between 2% and 8% when loaded and wetted. The key parameters we measure include initial dry density, water content, and the collapse index (I_c) per ASTM D5333. For a typical site in Allentown, we collect intact block samples or thin-walled Shelby tubes to preserve the natural metastable fabric. Our laboratory then runs incremental oedometer tests at the field water content and again after flooding, recording the vertical strain at each load step. Results are tabulated against the collapse potential chart by Lutenegger and Saber (1988).
Technical reference image — Allentown
Regional considerations
In Allentown, the risk of collapsible soil varies sharply between neighborhoods. Sites on the west side near Cedar Crest Boulevard, underlain by the Allentown Formation dolomite, rarely show collapse behavior. But east of the Lehigh River, in areas like the former Bethlehem Steel slag fills and alluvial terraces, collapse potential can reach 10% under moderate loads. A single heavy rain event or a leaking water main can trigger sudden settlement in these zones. Our collapsible soil evaluation identifies these high-risk pockets before construction, allowing designers to specify deep foundations, soil improvement, or controlled wetting to stabilize the ground.
We load intact samples in a fixed-ring consolidometer at field water content, then flood the specimen at a seating pressure of 5 kPa and continue loading to 200 kPa or greater. The collapse strain is measured as the vertical compression after wetting minus the compression under dry conditions at the same stress. Results are reported as collapse index, percent collapse, and a classification per ASTM D5333.
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Block Sampling & Thin-Walled Tube Retrieval
Because collapsible soils lose their metastable structure when disturbed, we prioritize high-quality sampling. For shallow colluvial deposits in Allentown, we excavate hand-carved block samples (30 cm cubes) and drive 76-mm thin-walled Shelby tubes with a hydraulic rig. Each sample is sealed, transported upright, and tested within 72 hours to preserve in-situ density and fabric.
Standards that apply
ASTM D5333-20 (Standard Test Method for Measurement of Collapse Potential of Soils), ASTM D2435-20 (Standard Test Methods for One-Dimensional Consolidation Properties of Soils), ASTM D1587-15 (Standard Practice for Thin-Walled Tube Sampling of Soils for Geotechnical Tests)
Quick answers
What is the difference between collapsible soil and expansive soil?
Collapsible soil undergoes a large volume decrease when wetted under load, because its loose granular structure collapses. Expansive soil, on the other hand, swells when wetted due to clay minerals like smectite. The two mechanisms are opposite, and both can occur in Allentown, though collapsible soils are more common in silty sand deposits along the Lehigh River.
How much does a collapsible soil evaluation cost in Allentown?
A standard collapsible soil evaluation, including field sampling of three locations and oedometer collapse testing per ASTM D5333, typically ranges between US$850 and US$2,420 depending on site access, sample depth, and number of tests. Additional boreholes or block samples will increase the total.
Can I build on collapsible soil without deep foundations?
Yes, if the collapse potential is low (I_c < 2%) and the wetting depth can be controlled. Techniques include pre-wetting the soil to collapse it before construction, dynamic compaction, or replacing the upper 1-2 meters with engineered fill. However, for moderate to severe collapse potential (I_c > 5%), deep foundations to bedrock or a mat slab designed for post-wetting deformation are typically required.
