Vibracorer
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The Vibracorer (dt. Vibration core device (VKG), engl. Vibrocorer, American. Vibracorer) belongs to the standard equipment of the sea geology , as before, the heaviness or piston solders (gravity or piston corer) or box gripper (box corer).
Vibratory core drilling is a (geophysical-geotechnical) method to take samples of sediments from the seabed. The process comes from marine geology and is used in marine research. Today it is also used commercially and worldwide. a. in route and site exploration for oil and gas pipelines or offshore wind farms .
description
The vibracorer basically consists of a vibration drive unit and the core tube, which takes up the soil sample . The vibration drive unit is located in a guide frame that stands on the seabed .
functionality
In the case of the vibracorer, the core tube is initially only pressed into the ground by gravity (weight of the drive and the core tube itself). The vibration supports this process by overcoming the friction of the core tube in the sediment. At the inner and outer edge of the core tube, the vertical vibration creates a thin, turbulent-liquid layer that reduces the frictional resistance and thus accelerates the penetration into the sediment.
The German engineer and inventor Wolfgang Schmidt, Rostock, (born 1935) carried out extensive tests and developments for vibration core drilling as early as the 1980s. A construction was implemented to minimize the dead mass of the vibrating drive and at the same time maximize the total propulsive weight. The extensive separation / decoupling of the vibrating mass from the penetrating mass is unique worldwide. At the same time, with the Vibracorer VC (VKG), the breakout force required when pulling up the core tube is up to 4 times less than the penetration force applied.
A vibracorer is driven by an electric, hydraulic, pneumatic or electromechanical drive. Vibracorers work in a range between 11,000 to 100 movements per minute (approx. 200 Hz to 2 Hz). The amplitude of the vibration is only a few millimeters to centimeters.
The penetration speed and thus the effectiveness depend on the drive frequency (amplitude), the grain size distribution of the sediment and the water content. Both determine the natural frequency and thus the case of resonance. During core extraction, a core catcher prevents the sediment from slipping out of the core tube. The core catcher opens during the forward movement of the penetration and closes automatically during the backward movement due to the pressure of the sediment. At the same time, a valve at the upper end of the core tube closes when the sample wants to "sag" compared to the tube. The resulting negative pressure helps hold the core in position.
The quality of the drill core is also strongly influenced by the penetration speed. If this is too large, the sediment at the edges of the core is carried away by the skin friction, and any stratification that may be present is deformed. If the penetration speed is too slow, the sample is exposed to the vibration for an unnecessarily long time and the sediment is re-sorted.
The core is pulled out of the tube to examine the sample. To do this, the cutter and core catcher are dismantled. This process was very labor-intensive in the past and is now simplified by using a so-called "liner". For this purpose, a plastic tube, preferably transparent, is mounted in the core tube. This can be pulled out after use and divided into manageable lengths (usually 1 m).
The filling level of the core sample never reaches 100%. The core catcher does not close the lower end of the pipe perfectly. This can cause very soft or liquid sediments to run out
Gases contained in the sediment , which are dissolved or even visible as bubbles, escape in an uncontrolled manner and reduce the sample volume. Almost all systems have a smaller diameter at the cutting edge (core catcher) than in the liner (core tube)
The sampling and the description of the samples are based on various DIN standards :
- DIN 4020 geotechnical investigations for structural purposes
- DIN 4021 Digestion through prospecting and drilling, as well as taking samples
According to DIN 4021, the inner diameter ratio for drill cores of higher quality class Ci should be ≤ 3 percent in order to minimize the annular space between the liner and the soil sample and thus to keep water ingress and sample relaxation low. A Ci value = 0 would therefore be ideal, but this is difficult to achieve in terms of extraction technology. The Vibracorer VC (VKG) achieves a Ci value of approx. 1%.
Individual evidence
Gerd Siebenborn: Small bores according to DIN 4021 - a question of knowledge? Ed .: bbr 5/05, pp .: 37-41. 2005
Web links
Small bores according to DIN 4021 - a question of knowledge? Siebenborn, G .: Small bores according to DIN 4021 - a (ge) knowledge question ?, bbr 5/05, S .: 37–41 (2005)