Cone Penetration Tests (CPT) by SGS

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Cone Penetration Tests (CPT)

Cone Penetration Test CPT Test SoniCPT Piezometer

KEY HIGHLIGHTS



		Mechanical Cone Penetrometer
		Electrical Cone Penetrometer
		SoniCPT
		Foundations calculations

ELECTRICAL AND MECHANICAL CONE PENETRATION TESTS (CPT)

The Cone Penetration Test (CPT) is an in situ testing method used to determine the geotechnical engineering properties of soils and delineating soil lithology.

The test method consists in a hydraulic rig, pushing into the ground an instrumented cone tip, mechanical or electric, through several rods and at a constant rate of 2 cm/sec. The resistance needed to penetrate the ground is measured continuously.

The total force acting on the cone is called the cone resistance (qc) and qualifies the strength of the soils. The force acting on the sounding rods provides the total friction (Qst). Measurements with an electric cone, equipped with a friction sleeve, provide the local sleeve friction (fs).

When the exact groundwater level is required or when the groundwater has to be monitored, it can be necessary to install a piezometer which can be placed quickly by a CPT-truck if hard layers are absent. The thrust capacity of the sounding equipment is supplied by the truck’s ballast and/or by screw anchors (to develop extra reaction).

Based on this information, the following geotechnical parameters are calculated:

Effective friction angle (φ’)
Coefficient of consolidation (C)
Bearing capacity
Settlement behavior of a foundation

SGS provides for every investigation a clear and comprehensive report gathering the measurements and calculated parameters in tables and graphs.

Furthermore, in every report, SGS gives a description and interpretation of the stratigraphy, an indication of the groundwater table (measured in unprotected sounding hole), an adapted foundation advice and a clear site plan with the tests localization.

CPT Test Techniques:

Mechanical Cone Penetrometer

The mechanical cone penetrometer is the most common technique. The mechanical cone (top angle 60 °, section area 10 cm²) and the sounding rods, consisting of an inner rod and an outer tube, are pushed continuously or discontinuously into the ground.

Measurements of the cone resistance and the total friction are realized every 20 cm and registered electronically above the ground. These measurements are sufficient for providing foundation calculations.

Types of mechanical cones

Dutch cone with a conical mantle (M1)
Begemann cone (M2 – local sleeve friction)

Electric Cone Penetrometer

More complete information is received by the use of the electric cone. The cone resistance (qc) and the local friction (fs) is measured continuously every 2 cm. With this information, the friction ratio can be calculated enabling a better classification of soil layers (for example: a thin peat layer in clayey soils).

The measurements are made through the electric cone and the signals are transmitted to the surface through a cable inside the hollow penetrometer rods. This signal is then registered by computer. The test is carried out with continuous controlled rate of 2 cm/sec.

In addition to normal electrical cones (CPT-E) with section area of 10 cm² and 15 cm², SGS also proposes a piezocone (CPT-U) to gather additional pore water pressure data.

SoniCPT

This original technique is based on the “Sonic Scamp Drill”, used for drilling. SGS has developed a unique system to “vibrate’ through hard layer (such as sandstone) or in lose sediments.

Two excentric rotating engines produce vibrations, so that the soil becomes more fluid and the friction is reduced in a considerable way.

The soniCPT can be used with a mechanical cone only. When the CPT is blocked by a hard layer, the sonic can be interfere and without pulling out the sounding rods, pass through the hard layer and continue the measurements of the CPT. Obviously, the soniCPT saves time when results under hard layer(s) are required.