Penetrometer (floor)
The penetrometer measures the penetration resistance of the soil in the field up to a depth of about 1 m as a pressure in the range from 0 to 5 MPa (1 MPa = 0.1 kN / cm² 10.1972 kg / cm²).
construction
The penetrometer consists of
- a stainless, metal probing rod of 90 to 110 cm in length, which can be extended with similar rods if necessary.
- At the bottom end, the probe rod carries a probe cone with a base area that is larger than the cross section of the probe rod.
The cone can have different opening angles; usually the angle is 60 °. Measurements with different opening angles cannot be compared with one another. - The probe rod carries a dynamometer between the handles or below.
The dynamometer is based on a compression spring or a pressure gauge box with
- a pointer display, which is often equipped with a drag pointer to display the maximum value; or
- a writing device, then one speaks of an analog penetrograph; or
- a digital recording of measured values, known as a digital penetrograph or penetrologger, and partly equipped with a GPS receiver .
If the measured force is related to the base area of the probing cone, a pressure measured value results.
variants
In the case of smaller designs, the probing rod usually does not have a separate probing cone and goes directly into a pressure spring balance, the housing of which also represents the handle. Such devices are well suited for horizontal measurements such as in the wall of a floor profile .
Related to the penetrometer is the impact probe as a small variant of the ram sounding . With her, a probing rod with a mostly permanently mounted probing cone is not driven into the ground by hand pressure, but by hitting a falling weight. The drop weight encompasses the probing rod as a thick metal tube and is dropped over a defined distance along the probing rod onto an abutment (anvil) fixed to the probing rod. Here the force measurement is replaced by counting the impacts of the falling weight and this number is noted together with the achieved penetration depth.
commitment
Usually the penetrometer is pressed vertically into the ground. A spirit level on the handle is used to drive the penetrometer vertically into the ground. This ensures that the probing rod does not rub against the wall of the bore but can move freely in the space opened by the cone. The friction on the bore wall is one of the sources of error when measuring the penetration resistance with the penetrometer.
In the case of a hand penetrometer without a writing device, the (maximum) penetration resistance is manually noted at each measuring point for specified depth steps of 5 or 10 cm. The measuring points lie as a grid or in distinctive cutting lines on the test area; With regard to the distance between the measuring points, preliminary examinations are useful, which allow a statement to be made about the variability of the measured values in the area.
Influencing variables of the penetration resistance in the soil
The penetration resistance of the soil is a sum variable and depends on many soil properties:
-
Soil type
The proportions of sand, silt and clay have no clearly directed influence. They act on the resistance to penetration via the shear strength and the highly variable soil moisture and water conductivity over time. -
Storage density
A higher storage density leads to a higher penetration resistance; an example of this is the plow or cultivation bottom of agricultural areas. - Storage densification (consolidation)
Actually loosely abutting mineral particles in the entire ground body by a [iron] crusting (iron rind, Ortstein, Orterde) solidifies be. A high penetration resistance is then measured, although the storage density is low and the water conductivity is also high. - Moisture ( soil water tension )
A high level of soil moisture leads to lower penetration resistance with the same storage density.
Caution: This can also lead to the fact that a soil that has dried out on the surface but is still moist with increasing depth "hides" a significantly higher storage density at a greater depth when measured with the penetrometer. - Shear strength
The resistance to penetration increases with the shear strength. -
Humus content
A significantly higher humus content lowers the resistance to penetration. -
Coarse soil content (stones, gravel or gravel)
A higher coarse soil content increases the penetration resistance with the same or slightly lower storage density, because the displacement of the stones requires more force than the displacement of small mineral particles.
Every change in one of these factors leads to deviating penetration resistances, without the deviations being able to be identified on a factor-specific basis. Therefore, penetration resistance values are not normally distributed and they do not change continuously, but often abruptly over area and depth.
evaluation
With the penetration resistance, the penetrometer delivers measured values which, with the same soil moisture and comparable humus and coarse soil contents in soils without crust or putty structure, reveal the areal and depth-related distribution of lanes, tramlines, headlands, plowed or cultivated floors or noticeably dense subsoils ( soil compaction ). In this way, the physical effects of different soil cultivation methods can be identified or recultivation measures such as the loosening of compacted subsoils can be monitored. It is difficult to derive unambiguous statements from the measurements of the penetration resistance about root penetration as for agricultural crops or about growth conditions for trees in urban streets and parks, because these depend on many other factors. In addition, roots do not only grow forward into the soil in the direction of the root tip, but also laterally (“radially”), which cannot be recorded when measuring with the penetrometer.
When evaluating, one should rather compare measurements made under the same conditions without taking into account the specific physical unit than derive properties or possible uses from exceeding or falling below measured values classified as critical.
Due to its broad sensitivity, measurement with the penetrometer is the method of choice for spatial and time-related (soil water content) statements on the homogeneity of the penetration resistance of soils such as the selection of representative areas.
swell
- DIN 19662 (2011): Soil quality - Field studies - Determination of the penetration resistance of soils with the penetrometer. - Beuth, Berlin.
- Hartge, KH, H. Bohne, HP Schrey & H. Extra (1985): Penetrometer measurements for screening soil physical variability. Soil & Tillage Research, Volume 5, pp. 343-350.
- Hartge, KH & R. Horn (1989): The physical investigation of soils. Enke Verlag, Stuttgart - (Chapter 14: Determination of the penetration resistance with the impact probe)
- Schrey, HP (1987): A qualitative-quantitative representation of the depth-dependent distribution of penetration resistance. Announcements German Soil Science Society, Volume 55, pp. 239–244.