ASCE 7-22 and the 2021 International Building Code (IBC) classify Manchester, New Hampshire with a site seismic hazard that demands a serious look at soil liquefaction potential, particularly given the city's position along the Merrimack River. The river deposited extensive layers of loose alluvial sands and silty sands across the valley floor, and when you combine that with a relatively shallow water table—often encountered within 10 to 15 feet—you have the basic ingredients for cyclic mobility during a significant earthquake. A standard geotechnical report that skips this analysis is leaving a gap that can translate into differential settlement, bearing capacity loss, or even flow failure. Our approach ties together in-situ SPT drilling data with grain size distribution from the grain size analysis lab to feed the simplified empirical procedures, giving you a defendable assessment for the local building official and the structural engineer.
Liquefaction is a function of the soil's state, not just its type—the same sand that stands firm at 98% relative density can flow at 40%, and that's what the testing captures.
Local geotechnical context
The Merrimack River's historic floodplain in Manchester is underlain by 20 to 40 feet of loose Holocene alluvium, with a groundwater table that fluctuates seasonally but typically sits less than 12 feet below grade in the Millyard and downtown districts. That's a textbook liquefiable profile. During the 1755 Cape Ann earthquake—estimated at magnitude 6 to 6.3—sand blows and lateral spreading were documented in coastal New Hampshire, and while Manchester is inland, the same seismic source zones (the Central New Hampshire Belt and the offshore Thatcher's Corner fault) can generate a design-level event. The IBC requires a site-specific liquefaction study when the mapped Ss exceeds 0.25g for sites with saturated granular soils, and Manchester's Ss is in the 0.30 to 0.35g range. Ignoring this hazard risks foundation tilting, utility trench collapse, and lateral spread toward the river channel. For critical structures, we pair the liquefaction analysis with stone columns as a densification mitigation, installing vibrated aggregate to increase relative density and provide drainage paths that dissipate excess pore pressure before it triggers instability.
Reference standards
ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, 2021 International Building Code (IBC) Section 1804.5: Liquefaction Potential and Soil Strength Loss, ASTM D1586 / D1586M-18 Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils, ASTM D2487-17 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), ASTM D4318-17e1 Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils
Frequently asked questions
At what depth does liquefaction typically occur in Manchester, NH?
Most liquefaction in Manchester occurs within the upper 30 to 40 feet, where the loose alluvial sands and silty sands deposited by the Merrimack River are found. The groundwater table is usually between 8 and 15 feet deep, so the saturated zone starts relatively shallow. We typically extend borings to 50 or 60 feet and sample continuously through the critical interval to capture any interbedded liquefiable lenses.
What's the cost range for a soil liquefaction analysis in Manchester?
A complete liquefaction analysis for a typical Manchester project—including two to three SPT borings with lab testing, CPT profiling, and the engineering report—runs between US$2,740 and US$3,690. The final cost depends on access constraints, depth requirements, and whether you need a MASW survey for shear wave velocity profiling.
Is CPT better than SPT for liquefaction assessment?
Neither is universally better; they complement each other. CPT provides continuous data and detects thin liquefiable seams that a 2.5-foot SPT interval might miss, and the Robertson method for CPT-based CRR is well-calibrated. SPT gives you a physical sample for fines content and plasticity testing, which directly adjusts the CRR curve. For Manchester's variable alluvial deposits, we often use both—SPT for index properties and CPT for a high-resolution stratigraphic profile.
Does the IBC require a liquefaction study for every project in Manchester?
Not for every project, but IBC Section 1804.5 requires a liquefaction potential study when the mapped spectral acceleration at short periods (Ss) exceeds 0.25g and the site contains saturated granular soils. Manchester's Ss is in the 0.30 to 0.35g range, so most commercial and industrial projects on the Merrimack River floodplain will trigger this requirement. Residential projects on shallow foundations may also need it if the building official determines the site class falls under Site Class E or F.
What mitigation methods do you recommend if liquefaction potential is high?
The mitigation strategy depends on the site constraints and the structure's tolerance for settlement. For most Manchester sites, we evaluate stone columns or vibrocompaction to densify the loose sands and create drainage paths that prevent pore pressure buildup. For smaller structures, a rigid mat foundation can bridge differential settlements, while deep pile foundations that extend through the liquefiable zone to competent bearing soil (glacial till or bedrock) eliminate the risk entirely. The report includes a comparative feasibility matrix for the options applicable to your site.
