What are Cone Penetration Tests?

Cone Penetration Testing (CPT) is a method used to determine the geotechnical engineering properties of soils and delineating soil stratigraphy. It was first developed in the 1940s in Holland to investigate soft soils and has since developed to become one of the most used and accepted methods for soil investigation worldwide.

This in situ testing method consists of pushing an instrumented cone, with the tip facing down, into the ground at a controlled rate of approximately 2 cm/second where the data is recorded at regular intervals during penetration. The cone records cone resistance, q_c, sleeve friction, f_s and pore water pressure, u₂ at 10 mm depth intervals which are measured by transducers in the cone and are connected to the surface via an umbilical cable. The test provides a continuous output of real time data from the sensors which can be interpreted to provide a profile of soil type and strength. Each CPT typically takes between 30 minutes and three hours depending on the soil type and depth of test.

CPT Rigs and Site Locations

CPTs are carried out from ground surface level using a special rig, designed to house all of the equipment, including wheeled rigs, crawler rigs (ranging in size and weight from 1.5 to 22 tonnes), excavator mounted CPT rigs and detachable rams which enable projects to be completed in a variety of locations including sites with restricted access, offshore and overseas.

Wheeled CPT rigs are ideal for testing on hardstanding sites such as roads and car parks. However, a selection of them operate exclusively off-road and are multi-wheel drive with specialised tyres giving them increased versatility to access non-hardstanding sites when required or they can move between both with ease.

Tracked crawler rigs range in size and weight from 1.5 tonnes to 20 tonnes. These rigs have low ground bearing pressure and are best suited to soft, boggy sites where they can easily track between testing locations regardless of ground conditions.

Smaller rigs can be craned onto pontoons for nearshore work, into basements or tracked to other areas where access is restricted, including indoor sites.

Excavator mounted CPT rigs are very versatile and are ideal for rail investigations including testing on the track bed, embankments and cuttings. These rigs have also been used successfully on other sites with difficult access such as river banks. These CPT frames are attached to an excavator which provides the required reaction force for testing and can be manoeuvred quickly and easily between testing locations.

Other versatile rigs include detachable CPT rams which can either be attached to plant or bolted to a concrete floor for testing inside basements or areas where standard rigs simply do not fit. The hydraulic rams and power pack can be carried by hand and set up for each individual test inside buildings and have been utilised successfully in many specialised projects. An advantage of these mobile rams is that they can be stripped down and packed into a single crate for transportation overseas.

CPTs can also be carried out on floating pontoons, jack up barges, nearshore multi-cat survey vessels, along with much larger offshore DP vessels, operating in many different environments ranging from rivers, estuaries, lakes and the open sea.

CPT Cones

Advances in CPT equipment over the years has evolved so the technique can be used not only for geotechnical investigations, but also for geo-environmental investigations.

More information can be obtained from the soil in addition to the CPT cone data by choosing a specialised CPT cone. Such cones include;

• Gamma Cone which measures variations in naturally occurring gamma radiation.
• Seismic piezocone (SCPTU) which is a probe that combines standard CPT along with geophones to measure seismic velocity within the ground.
• Intrusive Magnetometer System which is used to survey for UXB (unexploded bombs) across a range of environments.
• Video Cone Penetrometer which records a high-resolution view of the soil in real time.
• MIP (membrane interface probe), a logging tool that measures volatile hydrocarbon and solvent contamination along with soil electrical conductance and permeability.
• HPT (hydraulic profiling tool), a logging tool that measures the pressure required to inject a flow of water into the soil as the probe is advanced into the ground.
• WASTAP, an intrusive water sampling system which allows you to take discrete water samples at depth.
• OIP (Optical Image Profiler), a direct push tool used for the delineation of non-aqueous phase liquid (NAPL) hydrocarbon fuels and oils.

There are many benefits to using CPTs for site investigation. Apart from being a proven soil investigation technique around the world, they are also cost effective, rapid and produce reliable data instantaneously for the client. The variety of CPT rigs ensures tests can be carried out pretty much anywhere. With no boring, cuttings or spoils, the method is, and perhaps most importantly, environmentally friendly.

This blog was provided by our colleagues at In Situ Site Investigation, who were acquired by CTS in August 2022, read the news story here.