CPT vs. Boreholes: Which Ground Investigation Method Is Right for Your Project?

The decision between cone penetration testing and conventional borehole drilling is one of the most common questions at the start of a ground investigation. The short answer: both have a role, and the best investigations often combine them. Here’s how to think through the choice.

What CPT Gives You

CPT (ASTM D5778-20) advances an instrumented cone into the ground at a constant rate of 2 cm/second, recording tip resistance (qc), sleeve friction (fs), and pore-water pressure (u2) continuously with depth. The result is a high-resolution, undisturbed profile of the ground that can resolve soil layers as thin as 1 cm.

Because CPT is continuous, it captures every transition between soil types, such as thin sand lenses within a clay deposit or a soft zone within otherwise competent fill, that a borehole sampling programme might miss entirely between sample intervals.

CPT does not recover physical samples. This is the fundamental limitation: you get an accurate, quantitative log of the ground, but not a jar of material you can describe, smell, or send to a laboratory for particle size analysis.

What Boreholes Give You

Conventional drilling, whether rotary coring in rock or auger/shell-and-auger in soil, recovers physical samples. Those samples can be described visually by a geologist, tested in a laboratory for strength and classification properties, and archived as evidence of ground conditions at a specific location.

The limitation of borehole sampling is resolution and disturbance. Disturbed samples from split-spoon or bulk sampling give you material to classify, but lose the in-situ structure. SPT N-values, recorded at 1.5 m intervals, give you a point measurement of penetration resistance, missing everything between those intervals.

When to Use CPT

• Where continuous profiling is more valuable than sample recovery: soft clays, reclaimed fill, stratified deposits
• Liquefaction assessment: CPT-based methods (Robertson, 2009; Boulanger & Idriss, 2014) are the international standard
• Piezometric profiling via dissipation testing: CPT measures pore pressure in real time
• Foundation design parameter derivation where sample disturbance would compromise laboratory results
• Sites with good access and relatively uniform soils where drilling is unnecessarily slow and expensive

When Boreholes Add Value

• Rock: CPT refusal in hard material tells you rock is present; coring tells you its lithology, strength, fracture spacing, and weathering
• Unusual soils requiring visual identification: volcanic ash layers, organic deposits, coralline material
• Laboratory testing, such as compaction testing, Atterberg limits, or UCS, is a project requirement; all require physical samples
• CPT refusal has occurred and you need to understand what caused it