Macropore

Macropores are large diameter (more than 50 nm, see macroporous materials) conduits in the soil, created by such agents as plant roots, soil cracks, or soil fauna. Macropores increase the hydraulic conductivity of the soil, allowing water to infiltrate faster or for shallow groundwater to flow faster.

Macropores or cracks play a major role in infiltration rates in many soils as well as preferential flow patterns, hydraulic conductivity and evapotranspiration. Cracks are also very influential in gas exchange, influencing respiration within soils. Modeling cracks therefore helps understand how these processes work and what the effects of changes in soil cracking such as compaction, can have on these processes.

Modelling Methods

Basic crack modeling has been undertaken for many years by simple observations and measurements of crack size, distribution, continuity and depth. These observations have either been surface observation or done on profiles in pits. Hand tracing and measurement of crack patterns on paper was one method used prior to advances in modern technology. Another field method was with the use of string and a semicircle of wire (Ringrose-Voase and Sanidad, 1996). The semi circle was moved along alternating sides of a string line. The cracks within the semicircle were measured for width, length and depth using a ruler. The crack distribution was calculated using the principle of Buffon's needle.

Disc permeameter

This method relies on the fact that crack sizes have a range of different water potentials. At zero water potential at the soil surface an estimate of saturated hydraulic conductivity is produced, with all pores filled with water. As the potential is decreased progressively larger cracks drain. By measuring at the hydraulic conductivity at a range of negative potentials, the pore size distribution can be determined. While this is not a physical model of the cracks, it does give an indication to the sizes of pores within the soil.

Horgan and Young Model

Horgan and Young (2000) produced a computer model to create a two-dimensional prediction of surface crack formation. It used the fact that once cracks come within a certain distance of one another they tend to be attracted to each other. Cracks also tend to turn within a particular range of angles and at some stage a surface aggregate gets to a size that no more cracking will occur. These are often characteristic of a soil and can therefore be measured in the field and used in the model. However it was not able to predict the points at which cracking starts and although random in the formation of crack pattern, in many ways, cracking of soil is often not random, but follows lines of weaknesses.

Araldite-impregnation imaging

A large core sample is collected. This is then impregnated with araldite and a fluorescent resin. The core is then cut back using a grinding implement, very gradually (~1 mm per time), and at every interval the surface of the core sample is digitally imaged. The images are then loaded into a computer where they can be analysed. Depth, continuity, surface area and a number of other measurements can then be made on the cracks within the soil.

Electrical Resistivity Imaging

Using the infinite resistivity of air, the air spaces within a soil can be mapped. Samouëlian et al. (2003) from Orleans, France, produced a resistivity metre that improved the soil contact metre-soil contact and therefore the area of the reading. This technology can be used to produce images that can be analysed for a range of cracking properties.

Problems

There are difficulties with a range of crack modeling techniques. It is difficult to model the random formation of cracks and the influences and changes in cracking due to shrink-swell complications of some soils. Conventional methods are cumbersome, time consuming and often limited in the depth and accuracy of measurements. Conventional methods are often more qualitative than quantitative and are often subjective.

olutions

The use of a combination of methods and models to investigate coil cracking is currently the best approach; however this can increase the time required to produce measurements, and is still not perfect. With advances in 3-D imaging, tomography, and other technologies there is likely to be some technology in the future that will accurately and precisely measure cracking in the soil.

References

* Horgan, G.W. and Young. I.M. (2000) An empirical stochastic model for the geometry of two-dimensional crack growth in soil (with Discussion). Geoderma 96 263-276
* Ringrose-Voase, A.J. and Sanidad. W.B. (1996) A method for measuring the development of surface cracks in soils: application to crack development after lowland rice. Geoderma 71 245-261
* Samouëlian, A, Cousin. I, Richard. G, Tabbagh. A and Bruand. A. (2003) Electrical resistivity imaging for detecting soil cracking at the centimetric scale. Soil Science Society of America. 67 1319-1326