Designing foundations in Anchorage requires adherence to both the IBC and ASCE 7 standards, where assigning the correct seismic site class depends on accurate subsurface data. The CPT (Cone Penetration Test) provides a near-continuous stratigraphic log, making it especially relevant for the variable alluvial and glacio-lacustrine deposits beneath the city. Unlike the SPT, the CPT records tip resistance and sleeve friction every few centimeters, offering a high-resolution picture of layering. For projects near Ship Creek or the Turnagain Arm, we routinely pair this test with a [MASW survey](/masw-vs30/) to cross-validate shear-wave velocity profiles, ensuring the site class assignment meets the code requirements for seismic design.
The CPT delivers a continuous stratigraphic record at 2 cm intervals, capturing thin lenses of silt or sand that a standard boring would simply miss.
Methodology and scope
Local considerations
Anchorage sits at latitude 61°N on the Pacific Ring of Fire, a region that produced the 1964 magnitude 9.2 earthquake. That event unleashed widespread liquefaction and lateral spreading along the coastal plains. A standard SPT log may not capture the thin, loose sand interbeds that trigger liquefaction under cyclic loading. The CPT's continuous pore pressure trace, however, reveals those critical layers directly. By correlating the cone resistance with the Robertson & Wride (1998) liquefaction triggering curves, our engineers produce a layer-by-layer factor-of-safety profile that helps avoid catastrophic foundation failure during a seismic event.
Applicable standards
ASTM D5778-20 (Standard Test Method for Electronic Friction Cone and Piezocone Penetration Testing of Soils), ASCE 7-22 (Minimum Design Loads – Seismic Site Class), IBC 2021 Chapter 18 (Soils and Foundations, referencing CPT for site characterization)
Associated technical services
Standard Electronic Friction Cone (CPT)
Measures tip resistance and sleeve friction at 2 cm intervals. Ideal for stratigraphic profiling and preliminary liquefaction screening in the glacial outwash deposits common to the Anchorage Bowl.
Piezocone (CPTu) with Pore Pressure Dissipation
Adds u2 pore pressure measurement and dissipation tests to estimate in-situ permeability and consolidation coefficient. Essential for foundation design in the Bootlegger Cove Formation clays.
Seismic Cone (SCPT) for Shear-Wave Velocity
Integrates a geophone array into the cone to measure Vs directly. Provides the shear-wave velocity profile required for ASCE 7 site class assignment without a separate geophysical survey.
Typical parameters
Frequently asked questions
What is the cost range for a CPT sounding in Anchorage?
A standard CPT sounding to 20 m depth typically ranges between US$150 and US$250 per test point, depending on site accessibility, mobilization distance, and whether pore pressure dissipation tests are included.
How does the CPT differ from the SPT for Anchorage soils?
The CPT provides a continuous profile at 2 cm intervals, whereas the SPT yields a disturbed sample and N-value every 1.5 m. In stratified deposits such as those under Spenard, the CPT captures thin sand lenses that the SPT would skip entirely, making it superior for liquefaction assessment.
Can the CPT be performed in frozen ground or permafrost?
Standard electric cones cannot penetrate frozen soil. In Anchorage's discontinuous permafrost zones, we first thaw the upper 1–2 m with a heated probe or conduct the test during the summer thaw season. For deep frozen layers, alternative geophysical methods are recommended.
What soil parameters are derived from the CPT in addition to stratigraphy?
Beyond the basic tip resistance and friction ratio, the piezocone provides pore pressure (u2) for dissipation analysis, yielding in-situ horizontal conductivity and consolidation coefficient. With the seismic cone, we also obtain shear-wave velocity (Vs) for site class and modulus estimation.