In addition to CPT and drilling techniques, SOCOTEC also applies special (in-situ) measuring techniques.
Field Vane Test (FVT)
The Field Vane Test (FVT) is performed in accordance with NEN-EN-ISO 22476-9 class 1 FVTa. SOCOTEC has a so-called “protected version” of A.P. van den Berg’s FVT; see the figure below to the right. The advantage of this is that it is relatively easy to push through intermediate sand layers, no drilling is therefore necessary if intermediate sand layers are present in the underground.
It is a fully digital FVT, where the torque is applied and measured in the FVT itself, and not transmitted through rods. This results in the highest accuracy measurements.
An FVT is used to determine the undrained shear strength of the underground (silt, clay and peat) in situ. The undrained shear strength of the underground is important if loads have to be absorbed relatively quickly. This could include, for example, a water barrier that has to withstand a high wave, embankments for a road structure, but also a crane hardstand. Especially in the case of a crane hardstand with silt, clay or peat layers in the underground, the undrained shear strength is essential. If an undrained failure mechanism is the standard, the surface area of the required pad platform is directly proportional to the undrained shear strength. Halving the undrained shear strength therefore means doubling the surface area of the pads and of course vice versa! Problems with crane hardstands often arise because the undrained shear strength has been estimated too optimistically beforehand.
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Hydraulic Profiling Tool (HPT)
The Hydraulic Profiling Tool (HPT) is a measurement method for generating a continuous relative permeability profile of the underground.
While probing with the HPT, water is pumped into the ground through a circular filter at a constant flow rate. This flow rate is kept as constant as possible. The (varying) pressure required for this is measured (HPT pressure), as is electrical conductivity. Dissipation tests (pressure measurement over time at the same level with no water infiltration) are performed during the testing to determine the hydrostatic pressure at the test site. The HPT pressure recorded is corrected for this hydrostatic pressure. This corrected HPT pressure is an indication of the relative permeability of the underground.
On the basis of water permeability measurements on site at different depths, an estimate can be made of the absolute water permeability, and the relative permeability profile can be translated into an absolute water permeability profile. The water permeability measurements can consist of a combination of proven techniques such as slug tests, single well tests, pump tests, permeability tests, grading curves, information from the Dinoloket database of the underground in the Netherlands, etc.
The applications are legion, namely everywhere where insight is desired into the permeability of the underground. Think of dewatering, modelling piping for water barriers, the spread of contaminants, infiltration facilities, etc.
The advantage of an HPT probe is that continuous information is obtained about the full depth over which HPT probe is performed. Regular tests such as a slug test or grading curve only provide insight into permeability at a specific depth (so very local). In a full-size pump test, information is usually obtained about the average permeability of the total aquifer (kD value). An HPT probe provides insight into the distribution of that average permeability within that aquifer.
In geohydrological studies, it is always important to collect as much information as possible from diverse sources, and to compare and combine these with each other. An HPT probe is an important part of this and can make a significant contribution.
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The Standard Penetration Test (SPT) can be performed in accordance with NEN-EN-ISO 22476-3:2005. It is hardly or never used in the Netherlands, but it is used abroad.
With an SPT, a thick-walled hollow tube with a diameter of 50.8 mm is driven into the ground in a borehole over a length of 450 mm. The number of blows required to drive the pipe 150 mm into the ground is counted. The sum of the number of blows over the second and third 150-mm stretch is called the SPT or N value. When performing an SPT, sample material is also obtained, but this is far from undisturbed.
The disadvantages of an SPT are obvious. It is a very dated test, which only provides discontinuous and indirect information about the underground. Although there are many correlations with underground parameters, they are all of low quality.
The advantage of an SPT is that it is known worldwide and can be requested. However, if other survey techniques such as CPTs are possible, these are greatly preferred.
Coil probes are actually normal, electric CPTs where adhesion to the rods is minimised during probing by injecting bentonite. Large probing depths can be reached in this way (up to 80 to 100 m). Probing to these depths is becoming more important because of the ever-higher buildings and thus greater impact on the deeper soil layers.
While the rods are being pulled, the probe borehole can be immediately sealed with bentonite or injection grouting to prevent short-circuiting of flows towards an aquifer. This can be important, for example, in urban areas where there is often contamination in the upper layers or in a water extraction area with associated protection zone. A coil probe offers much greater certainty with regard to the complete sealing of the probe borehole than, for example, sealing with using pressure mapping. After all, with pressure mapping, there is a real risk that the rigid casing will not follow the course of the CPT borehole and will find its own way.
CPTs can be carried out with a MoStab to quickly take a virtually undisturbed soil sample at a predetermined depth, for example to check whether a water-inhibiting soil layer is present at a greater depth. This can eliminate the need for deep (more expensive) drilling.
With a probing unit, standpipes can also be pushed to depth using a casing with a lost tip. The advantage of this method is that it is cheap and fast. The disadvantage is that no soil characterisation is obtained. BRL SIKB 2100 can be met using special standpipes.