Geotechnical Testing in The 21st Century – Piezocone Penetration Testing (CPT)

Geotechnical Testing in the 21st Century – Piezocone Penetration Testing (CPT)

By Andrew Drevininkas

Introducing the second in our Guest Article Series on Advanced Geotechnical Testing

Inertia can sometimes prevent progress.  In Ontario, there is a massive historical database and a lot of momentum that supports the use of the Standard Penetration Test (SPT) as a benchmark for geotechnical investigations.  The SPT test has changed little since its invention in 1902 and remains at the core of standard industry practice, despite evidence to suggest that it is not suitable for all soil conditions.  Our three-part guest article series on Advanced Geotechnical Testing will provide you with insight into alternative testing methods and how they can deliver meaningful value on your projects.– Mark Tigchelaar, P.Eng.- President

Geotechnical Testing in the 21st Century – PiezoCone Penetration Testing (CPT)

The biggest risk for a development is the unknown, or poorly understood, subsurface conditions below the site. The type and number of geotechnical in-situ tests are the basis of minimizing risk (cost, schedule) for the proposed development. Yet this risk is typically not well understood by owners or other design consultants.

All geotechnical investigations include in-situ testing. In Ontario, this testing typically consists of the Standard Penetration Test (SPT) method, which involves hammering a tube into the ground and counting the number of blows required to penetrate the soil. It is a useful tool to determine the compactness/consistency and type of soil, obtain samples for basic soil testing and approximate basic strength/stiffness parameters for design. But, like all tools, it has its limitations that many consultants and owners are not aware of.

In sands, SPT can be a useful tool to assess soil strength and stiffness, however below the groundwater table sands and silts are commonly disturbed at the bottom of the drill string, in which case low SPT values are obtained indicating a very loose or loose compactness when in fact the soil may actually be compact to dense. While this can often be overcome with hollow stem augers with water balancing or mud rotary drilling, these drilling practices are not commonplace. Purchasers of geotechnical testing work may not be made aware of the drilling methods used during the investigation, and may not be aware of the implications of that work on their results.

In clays, SPT results are generally meaningless when assessing the soil stiffness and strength. This means a lot of sites are classified as having weak soils when they are actually stronger than reported. In weak and sensitive clays and cohesive silts, SPT hammer energy tends to remould soft soils and underreport the relative strength. In addition, typical sampling intervals with SPT are generally on the order of every one to five feet, meaning resolution is quite poor using SPT on these soils.

An advanced method to properly investigate a site to assess soil strength and stiffness is the PiezoCone Penetration Test (CPT). CPT is a quick, efficient, and cost-effective investigation tool. It consists of an instrumented probe of load cells and strain gauges that continuously measure the tip resistance, porewater pressures, and friction as the probe is pushed into the ground. Correlations for strength and stiffness are more accurate due to the repeatable and accurate CPT measurements. The probe measurements are not disturbed below the groundwater table in sands.  In clays which can often have variations in its layered deposits, CPT provides a continuous profile and is able to clearly identify and classify silt and sand layers, which can be very useful in some situations. Deep CPT probes can be carried out in less than half the time of a traditional deep SPT borehole.

As with all investigation tools there are limitations. The CPT can generally be pushed into very soft to very stiff clays and very loose to compact sand, so it is important to know the local geology of the site before using CPT in order for it to be cost-effective. In addition, although CPT can identify soil types with good accuracy, the CPT cannot provide samples for lab analysis. CPT combined with SPT can often provide excellent data on a site and improve accuracy of the CPT when normalized with SPT lab testing for moisture content and grain size.

The value of CPT is illustrated in two local sites as follows:

• For a proposed commercial development, the owner contracted a geotechnical engineer to undertake SPTs to more than 15 metres in sands and silts below the groundwater table. These tests indicated very low strength soil all the way down to depth where harder soils were encountered. Given this information, foundation options were limited to deep foundations to about 15 metres. One day of CPT soundings was carried out and it was determined that the sands and silts below the groundwater table were actually compact to very dense at a depth of about 5 metres, shortening the proposed pile lengths and allowing for a ground improvement design at the site to avoid costly foundations. SPT sampling disturbance was to blame for underreporting the soil strength.

• For a proposed residential development, an owner contracted a geotechnical engineer to undertake SPTs. These tests indicated weak silty clays to more than 30 metres in depth. CPT was subsequently used to provide continuous strength and stiffness profiles of the silty clays and allowed for a targeted approach to obtain a limited number of undisturbed samples to confirm the CPT results. The CPT results were provided within a few days after completion to allow the structural design of the proposed raft foundation to be expedited. The undisturbed laboratory test results were available a few weeks later and confirmed the CPT interpretation. SPT testing was inadequate on this project for providing raft reactions for an appropriate raft design.

CPT can be used to design cost-effective foundation options for developments by providing better and more accurate data on in-situ soil conditions. An understanding of the local geology and enhanced investigation techniques are needed to minimize costs and risks for foundation design.

Andrew Drevininkas is the president of Downunder Geotechnical Limited. His firm specializes in advanced geotechnical in-situ and laboratory testing of soils in Ontario. Using this experience Downunder Geotechnical helps clients with cost-effective geotechnical design for foundations and floor slabs. For more information please contact Andrew at office@downundergeotechnical.com.