The Cone Penetration Test (CPT) is one of the widely used and accepted test methods for determining geotechnical soil properties, as well as to assess whether soil layers are likely to liquefy under different levels of earthquake shaking. CPTs are used in variety of places and in different terrains. There are a variety of equipment available and depending on the types of soil conditions the choice of equipment may differ. CPT is used in a variety of industries such as Mining, Infrastructure projects, Environmental site investigations, Commercial and Residential constructions, Earthquake engineering etc.
CPT involves penetration of a hardened steel cone down into the test ground on a continuous defined rate of progress to assess the hardness level of the soil at a specific depth, using hydraulic rams mounted on either a heavily ballasted vehicle or using screwed-in anchors as a counter-force During the process, the variation in the pressure required to maintain the constant rate of progress gives us an indication if the variation in soil types (soils, sands, clays, and rocks) encountered during the downward movement. The steel cone contains an electronic measuring system that records tip resistance and sleeve friction (piezocones). The soil resistance is recorded using force sensors in the tip. Some cones also have a pore water transducer, which records water pressure in the soil. These readings can be used to determine ground water responses as the cone is pushed through the soils.
A cone penetration test typically takes between 30 minutes and three hours. As the cone goes into the ground, measurements are constantly sent back to the rig and recorded on computer. The on-board CPT computer software and equipment collects and displays real-time data, enabling accurate onsite display of results and data interpretation. This data gives a profile of the subsoil layers, often called a ‘trace’. This information can help in the design of foundations and ground improvements. By doing a test before and after ground improvement works, cone penetration test results can also be used to determine how much strength a soil has gained following ground improvement works.
In addition, the friction encountered by the sleeve around the cone gives an idea of the make-up of the soil. The cone tip pressure, friction of the sleeve, pore pressure and seismic readings are obtained simultaneously and the readings are displayed and recorded on computers.
CPT Rig Types:
The cone is pushed down using rigs, which come in 10-ton, 15-ton and 20-ton pushing capacities. This is an important variable which determines the maximum pressure required to perform a specific test, to a specific depth in a given geological area. The variation of the equipment occurs depending on whether the rigs are mounted on track, truck, trailer, anchored on ground or off-shore conditions. The rigs should be anchored properly to withstand the repulsive force, while applying pressure on the cone tips (sounding). The type of anchoring depends upon the soil surface conditions at that location, which in turn determines the need for light or heavy ballasted rigs to enable anchoring system to work effectively.
Cone penetration test rigs vary in size – from small portable rigs to large truck-mounted rigs. Each rig has benefits and limitations but they all conduct the same test. Depending on the applications, there are a number of test rigs in use world-wide. These rigs vary based on a variety of factors including effective downforce required, consistent feed rate or push rate control as per ASTM standards, type of push platforms, need for multiple rig anchoring options, extent of operating control required, rig maintenance options etc.
Another factor that decides the type of rigs required is the condition of the terrain that have to be traversed such as muddy, swampy, or sandy ground, or ground that is sensitive to weight-bearing loads. CPT rig have to be small to be more manoeuvrable in a woody environment, or in a location which is surrounded by structures such as bridge, abutments etc.
Truck Mounted CPT Rigs:
These are available in medium or heavy designs, which come with an in-built lab-type environment to suit all terrain and weather conditions. They can accommodate faster set up and also facilitate deeper pushes. Truck-mounted CPT rigs are therefore recommended for larger job sites, where setting up anchors may not be an economical option.
Additionally, Limited Slip Differential (LSD) is installed on the trucks to access steeper terrain and softer areas. Sometimes, a combined crawler/ truck mount combination provides a CPT sounding, either on or off the truck, giving accessibility to hard to reach terrains. Sounding from the truck allows for swift and economic road movement as well as for quick location change on site if the crawler rig is not needed.
Tracked CPT Systems:
Having a highly manoeuvrable platform with low ground pressure can make a big difference in specific applications. Tracked rig CPT platforms are designed to not only traverse and work in difficult terrains but also to be highly manoeuvrable around obstacles such as trees, rocks and galleys. Track rigs are usually designed in order to distribute the weight of the rig over more square inches of contact area. This helps to minimize damage to sensitive areas as well as help the rig not get stuck in less than optimum ground conditions.
The CPT has evolved over the years from a mere mechanical and electronic cones, to a variety of other CPT-deployed tools, to provide additional subsurface information. These include a geophone set to gather seismic shear wave and compression wave velocities. This data helps determine the shear modulus and Poisson’s ratio at intervals through the soil column for soil liquefaction analysis, low-strain soil strength analysis and to determine the soil’s behaviour under low-strain and vibratory loads.
Additional tools such as laser-induced fluorescence, X-ray fluorescence, soil conductivity/resistivity, pH, temperature and membrane interface probe and cameras for capturing video imagery are also increasingly advanced in conjunction with the CPT probe. An additional CPT deployed tool is a piezocone combined with a tri-axial magnetometer. This is used to attempt to ensure that tests, boreholes, and piles, do not encounter unexploded ordnance (UXO) or duds. The magnetometer in the cone detects ferrous materials of 50 kg or larger within a radius of up to about 2 m distance from the probe depending on the material, orientation and soil conditions.
Off-shore Marine applications:
For the last five decades, the CPT has played an essential role in offshore soil investigations though operating seafloor CPT has posed many a challenge, especially in deeper offshore areas. Marine CPT units are based on a wheel drive system and the straight rod as used in conventional land based CPT testing. The straight rods are clamped between two pairs of driving wheels which rotate, pushing the rods into the ground. The units typically come in a range of sizes from 2 tonnes up to 20 tonnes. The units can be lowered to the seabed from a vessel by using an auxiliary winch and wire; the CPT system is linked to base by a separate umbilical cable or by acoustic modems in deep water environments. The system is typically capable of operating in water depths of up to 2000 m. It can be configured to achieve penetration depths of up to 40 meters below the seabed level. Typical applications are:
- Pipelines (shallow and deep-water)
- Seabed foundation solution
- Ports and harbours – dredging and construction projects
- Cable route surveys
- Offshore wind farms
- Environmental surveys
As we have seen above, there are multiple applications for the CPT test rig and depending on the purpose, its design gets adapted suitably. Some of the factors that determine which rig has to be applied can be enlisted as follows:
- The maximum amount of pressure that will be required to perform a specific test, to a specific depth in a given geological area.
- The type of anchoring needed to reliably resist the upward force generated by the test (or ‘sounding’).
- The type of surface conditions, which decide the use of light or heavy Rig, as well as the possibility of anchoring the Rig down.
- The other primary factor that decides the design of CPT Rigs is the terrain that the Rig will have to traverse. Muddy, swampy or sandy ground, or ground that is sensitive to weight bearing loads, may require a tracked CPT Rig to work effectively.
- Ease of manoeuvrability decides whether to use a smaller rig for better access. In some cases, very small CPT Rigs, portable hand-operated CPT Rigs or even CPT Rigs that are mounted on the back of a drilling vehicle might be a good option.
- Special applications where seismic data also needs to be collected decides the appropriate equipment needed for the test.