Cone Penetration Tests (CPTs)
In principle, CPTs are a fast, cost-effective and reliable method for gaining insight into the structure and bearing capacity of the underground.
The advantage of this technique is that it is standardised across the entirety of Europe and the majority of the world. Not only the equipment to be used, but also the method of implementation. The result is unambiguous, accurate, can be reproduced and is not subject to human interpretation (which is the case for drilling).
To carry out a standard probe, a cone-shaped measuring body is pressed into the ground with rods by using a lorry, for example, as counter pressure. The cone in fact measures the mechanical resistance of the soil whilst penetrating the various soil strata. The soil structure can be derived from the result of the measurement, a CPT graph, and layers with high resistance (sand, gravel) and low resistance (clay, peat, silt) are visible.
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During probing, not only the cone resistance is measured but also the local (jacket) adhesion, the pore pressure, conductivity, temperature, etc. The parameters that we measure depend on the type of survey required.
For example, the relation between the cone resistance and the local adhesion (friction coefficient) is a clear indication of the type of soil. If the pore pressure is also measured in non-cohesive soils, insight is also gained in the prevailing water pressure at a certain depth. Moreover, pore-pressure probing also shows the presence of thin water-inhibiting, cohesive interfering layers (of clay or peat) in an aquifer.
We can, for example, detect explosive remnants of war (ERW) with a magnetometer probe, but also determine the length of a sheet pile wall or foundation pile.
With a HPT probe (Hydraulic Profiling Tool), a continuous permeability profile can be generated of the underground.
SOCOTEC carries out various different types of probes and test, such as probes to test the pore pressure and/or adhesion, dynamic penetration tests, seismic probes, probes to test the conductivity, but also the aforementioned magnetometer probes and HPT probes, etc. Each type has its own, specific research objective.
Probes to test the pore pressure and/or adhesion
CPTs are carried out in accordance with NEN-EN-ISO 22476-1. Said standard distinguishes 4 CPT classes: 1 through 4. Class 1 has the greatest accuracy, class 4 the lowest. In all 4 classes, both the tip resistance and local adhesion are measured (indicated by TE1). From class 3 and higher, the slope is also measured. With class 1 CPTs, the pore pressure (indicated by TE2) is always measured. Class 2 and class 3 CPTs are able to optimally measure pore pressure. Class 4 is never used in the Netherlands.
The advantage of pore pressure CPTs
The advantage of carrying out CPTs with pore pressure is that in the case of non-cohesive soil types, immediate insight is obtained into the prevailing water pressure at a certain depth. More insight is also gained into thin water-inhibiting cohesive interfering layers (clay or peat) within an aquifer. These layers cannot be detected or can be detected to a lesser extent with a normal CPT with measurement only of the tip resistance and local adhesion. This can be important in the design of dewatering for a construction pit.
A dissipation test as an additional option
A dissipation test can also be carried out during execution of pore pressure probing. With a dissipation test, the development of the pore pressure over time is measured at the same depth. A dissipation test provides more insight into water permeability at a certain depth. In more cohesive layers, the consolidation coefficient (CV value) can be derived on the basis of a dissipation test.
Pore pressure can be measured at several positions of the cone, namely halfway to the tip (u1), directly behind the tip (u2) or behind the adhesive jacket (u3). Depending on the application, a specific position is chosen. Normally this is the u2 position.
A class 1, 2 or 3 CPT
SOCOTEC uses class 3 CPTs as its standard. These are extremely well suited to the design of building foundations and for general soil exploration. Class 2 or even class 1 CPTs are carried out for more specific applications, such as the testing of water barriers or analyses of settling. On the basis of a class 1 CPT, a relationship can be established (Nkt factor) between the undrained shear strength (su) as determined on the basis of laboratory tests and the CPT resistance qnet (cone resistance corrected for pore pressure).
Dike cone CPTs
A “dike cone” is a very accurate cone with a measurement inaccuracy of 7.5 kPa (more than 4 times more accurate than a class 1 cone) with an a-factor of 0.99. The latter means that the cone resistance does not need to be compensated for with respect to pore pressure and in fact qnet is measured directly.
A normal CPT cone is constricted directly behind the cone tip in order to be able to install the adhesive jacket (see figure below).
This means that the pore (over) pressures that can arise during probing, and which can reach high values in clay and peat layers, can also act in the gap between the cone tip and the cone housing, and consequently the total cone resistance (qc) decreases.
To obtain the actual cone resistance (qnet), this will have to be corrected for the pore pressure at the u2 position (just behind the cone tip) depending on the a-value. The a-value is the ratio between Ac and An. The real cone resistance qnet is essential in determining the Nkt factor, the relationship between the cone resistance and the laboratory-determined undrained shear strength su.
When using the dike cone, no correction is necessary for the measured pore pressure, since the a-factor is virtually 1. The qnet is therefore measured directly and with a considerably higher accuracy than with a class 1 cone. No inaccuracies are introduced by the pore pressure correction, allowing a better correspondence with the undrained shear strength su determined in the laboratory.
A “disadvantage” of the dike cone is that, due to its high accuracy, it has a limited measuring range, which means that it is only possible to probe in Holocene deposits and not in sand with higher cone resistances. This need not be a problem because probing in sand with the dike cone has no added value, since sand reacts as if it is drained and an su cannot be determined from sand.
Mechanical CPTs are performed in accordance with NEN-EN-ISO 22476-12:2009 test type TM1. This type of CPT is carried out if very solid sand/gravel layers have to be passed through or if there is a considerable risk of rod breakage.
- The method of performing a mechanical CPT
Mechanical probing is actually the predecessor of electric probing and was invented in the middle of the last century by Dr. ir. Begemann. The disadvantage of mechanical probing is that only cone resistance and local friction (adhesion) can be measured. The cone is also pushed to depth with steel rods from a heavy CPT truck. The pressure required to push the rods to depth is measured in the wagon with a pressure transducer. By using inner rods, which are virtually frictionless in the outer tube, the pressure on the cone tip can be measured directly, without needing to be corrected for the total adhesion on the outer rods. With an electric cone, this is done with strain gauges in the cone, which makes it possible to measure even more directly. Nowadays, with mechanical probing, the pressure on the inner rods is measured electrically and registered every 2 cm. The slope cannot be measured during probing; thus the depth reached cannot be corrected for the slope.
- The advantage of mechanical probing
The major advantage of mechanical probing is that it is often possible to probe at a considerably greater depth. After all, it is possible to transfer almost the full weight of the vehicle (approx. 20 tonnes) to the inner rods. The maximum tip load with a standard electric cone is only 10 tonnes.
In addition, the cost of a mechanical cone is considerably lower than an electric cone (by more than a factor of 10). More risk can therefore be taken when penetrating very solid sand or gravel layers, because the costs in the event of breakage are considerably lower.
It is good to bear in mind that the cone resistance measured with a mechanical cone is often lower than with an electric cone (due to the cone’s shape). When building on top of existing buildings, for which the original foundations were often designed on the basis of mechanically performed CPTs, some margin in the bearing capacity of the existing foundation piles can often be found if additional electric CPTs are made.
Impact probes are carried out in accordance with NEN-EN-ISO 22476-2:2005 and are mainly used if even firmer sand or gravel layers have to be passed through or in places that are difficult to reach and thus inaccessible to heavy equipment, while the underground needs to be explored at even greater depth.
Combination CPTs can also be carried out, whereby a normal electric probe is initially used until it gets stuck, for example, in a very solid sand/gravel layer. Subsequently, the solid sand/gravel layer is investigated with the aid of impact probes. After this layer in passed, the CPT continues with a normal electric cone. In South Limburg, for example, CPTs have been carried out to a depth of more than 50 m through solid layers of gravel.
Seismic CPTs are carried out if more insight is desired into the dynamic behaviour of the underground. Consider, for example, the design of foundations in earthquake-prone areas, but also for dynamically loaded foundations such as machine foundations or wind turbines. Seismic CPTs can also be useful when determining the softening sensitivity of the underground (useful in earthquake zones or when determining the probability of settlement flow on underwater slopes).
SOCOTEC has developed a method whereby the strike of the hammer required can be given automatically and in a controlled manner. The eliminates the irregularities caused by an uncontrolled strike of the hammer. It is also far less demanding on workers.
Netherlands Code of Practice: NPR 9888
Above all in areas subject to earthquakes, such as the north of the Netherlands, it is important to construct in such a way that dynamic behaviour of the ground during vibrations is made clear in the design phase. According to Netherlands Code of Practice NPR 9888, it must be determined whether there are softening sensitive layers present that can eventually result in a loss of bearing capacity or compaction of soil layers and thus settlement.
The risk of this can be determined amongst other things by carrying out seismic CPTs. The design of the foundation can then be geared to this (see also Earthquakes in Groningen).
This type of CPT is often used to detect explosive remnants of war (ERW) or to demonstrate that they are not present.
However, magnetometer CPTs can also be used to determine the length of a steel sheet wall or foundation piles.
A magnetometer measurement in fact measures a change in the magnetic field in the soil by the presence of ferrous objects (such as ERW or a sheet wall). The degree to which the magnetic field changes is proportional to the size of the ferrous objects.
Whilst carrying out a magnetometer measurement, other probing parameters such as cone resistance, local friction, slope and pore pressure can also be measured.
Ball CPTs and T-bar penetrometers
Ball CPTs and T-bar penetrometers are so-called full-flow penetrometer measurements, in contrast to probes that are carried out with a cone.
Full flow penetrometer tests with ball CPTs and T-bar penetrometers differ from the standard CPTs carried out with a cone in two ways:
- The larger surface area (100 cm2) of the ball CPT and T-bar penetrometer means that the resistance of the underground can be determined in a more reliable and uniform way than with a standard tapered cone, whose surface area is 10 cm2 or 15 cm2. The larger surface area provides a higher resolution for weaker soil layers (clay or peat)
- The residual undrained shear strength (residual strength after collapsing) of weak soil layers (clay or peat) can be determined by carrying out cyclic tests. This is done by moving the ball CPT or the T-bar over a certain depth zone; the resistance is measured during both the upward and downward cycle of the test
As a result, measurements carried out with ball CPTs or T-bar penetrometers are excellent for using the resistant measured to calculate the undrained peak shear strength and residual undrained shear strength of weak soil layers (clay and peat layers).
Using the Nkt factor, the resistance measured with the ball CPT or T-bar penetrometer must then be tested against the results of Field Vane Testing (FVT) or the results of the Triaxial tests or Direct Simple Shear tests on undisturbed soil samples carried out in the lab.
To carry out penetrometer tests with a ball CPT or T-bar penetrometer, the same measuring body and data acquisition system are used as for CPTs that are carried out with a tapered cone. Only the tapered cone is then replaced by a ball CPT or a T-bar penetrometer.
The disadvantage of ball CPT and T-bar penetrometer is that these measurements cannot determine the local friction/adhesion so that the friction coefficient cannot be determined, making it (very) difficult to classify the various types of soil.
The ball CPT can be carried out with or without pore pressure measurement.
To compensate for not being able to classify the types of soil during Ball CPTs and T-bar penetrometers, they can be carried out in combination with standard class 1 or class 2 CPTs with pore pressure and/or friction measurements.
CPTs With Conductivity Measurement
The electrical conductivity of the underground is measured with an EC probe. On the basis of the electrical conductivity, more insight can be obtained into the possible presence of contaminants and/or the chloride content (location of freshwater/salt water boundary).
Since the conductivity of the underground depends on the type of soil, during the execution of a conductivity measurement, the standard probing parameters such as cone resistance, local friction and slope can be measured, supplemented by pore pressure and temperature.