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Hakskeen Pan is what is often referred to in different parts of the world as a playa, dried lake bed, alkali or clay pan, and more. In South Africa these systems are known generally as “pans” or even “salt pans”. The reason for a more general description in SA is that pan systems occur from the most arid areas in the west of the country all the way through to the very humid areas in the east. Pan systems in SA therefore have a very wide range of ecological characteristics and water regime. In the west these pans are dry for most years and in the east they are often wet permanently. Due to this very large climate and rainfall gradient from east to west the soil and biological characteristics vary significantly. For example, Hakskeen Pan (Figure 1) is dry in comparison to a pan (such as Chrissiesmeer) on the Mpumalanga Highveld (Figure 2). (Note: Search for both of these pans by name on the web and see how different they are!) Even though they are very different these systems have one aspect in common, namely: they are both systems into which water flows without any outflow. The only loss of water is through evaporation (from the water body itself) or evapotranspiration (loss of water through transpiration processes of plants).
Depressions with inflows of water but without outflows are called “endorheic” systems. In South African legislation these are defined as wetlands and are therefore protected against unauthorised activities. These activities are listed in different sets of legislation, regulations and guidelines and can be provided upon request – it is a long and detailed list though!
Various activities associated with the Bloodhound SSC project require specific authorisation and more information will be provided in a separate document.
Figure 1 Hakskeenpan as viewed from dunes on the eastern edge
Figure 2 One of the pan wetland systems near Chrissiesmeer in the dry season
The answer is a definite “No!”. Although the pan lies in an area with a very low rainfall (Figure 3) there is still a significant amount of rainfall at times – even though it may be very erratic in distribution and quantity. Very specialised organisms, such as the Triops in Figure 4, thrive in the short periods of inundation after rainfall events.
Figure 3 Mean Annual Rainfall for South Africa with Hakskeen Pan indicated in the North West
Figure 4 Horseshoe shrimp (Triops spp) found on the pan during periods of inundation
Apart from accessibility aspects Hakskeenpan is ideal for a land speed record attempt due to its flatness and hardness of the pan surface (under dry conditions!). To illustrate, the pan is approximately 20 km long and 5 km wide. Over this expanse the surface of the pan varies with less than 20 cm (8 inches) from one side to the other in its altitude above sea level (approximately 800 m high). Over distances of one kilometre the variation in surface level is less than 5 cm (2 inches). For a vehicle travelling at 1000 mph (1609 kph) a flat surface is critical!
The soils of the pan surface are interesting in the sense that they have a thin (2 cm) crust that is relatively soft and brittle with a very dense and hard subsoil (Figure 5). The vehicle can therefore “ride” on the harder subsoil by breaking up the thin crust with its weight and wheels.
Figure 5 Cracked and brittle crust overlying a dense and hard subsoil
Figure 6 Brittle crust sample from the pan
In the next few sections we will have a look at the science of assessing the pan floor to describe all the aspects/characteristics relevant to the Bloodhound SSC project.
This question can be answered in a number of ways but the easiest is through the use of an instrument called a “penetrometer” (Figure 7). This instrument is available in various forms but the one used for the assessment of the pan surface has a metal cone at the end of a metal rod. At the top is a load cell that measures the resistance force experienced when the rod and cone is pushed into the ground through a mechanism that resembles bicycle pedals. The force is measured in kPa and values of 4000 kPa and above indicate conditions similar to average roads. On the pan the penetration resistance was measured in duplicate at more than 40 separate points (Figure 8).
The results of the investigation indicate that the pan soils are brittle to a depth of 2 to 3 cm after which the density and strength increase drastically, with penetration resistance exceeding 4000 kPa (Figure 9). This is a good sign as it proves that the pan is suitable as a track for Bloodhound. In Figure 10 one point of investigation is shown that had a much lower penetration resistance. Fortunately this point falls outside of the designated Bloodhound track!
Figure 7 Operation of the penetrometer on Hakskeen Pan
Figure 8 Sample positions (in duplicate) on Hakskeen Pan
Figure 9 Mean penetration resistance values for the points investigated
To answer this question we have to delve a bit deeper into the physical, chemical and mineralogical characteristics of the pan soils. These characteristics are the result of a number of processes that have taken millennia to yield the specific soil composition.
The pan soils have been formed through factors that include 1) the weathering of predominantly tillite rock (originally formed under glacial deposition!) that makes up the pan floor, 2) the “washing” in (leaching processes) of different size fraction soil materials and salts from the surrounding landscape, 3) the blowing in (Aeolian processes) of predominantly fine grained sand material from the Kalahari and 4) in-situ formation of new minerals in the presence of the inherited minerals.
We can determine the size fractions (sand, silt and clay) of these minerals through a process known as “particle size analysis”. From the results we calculate the relative proportions of the sand, silt and clay and indicate whether the soils are sandy, loamy or clayey (or combinations thereof).
For the soils of Hakskeen Pan a small challenge was experienced in that a large proportion is made up of soluble salts such as NaCl (table salt). These salts dissolve during the process of particle size analysis and we have to compensate for the losses in the determination of the insoluble physical fractions. In the case of these soils a weak hydrochloric acid leach was performed first. After this the resultant soil material was sieved into different fractions down to 50 micron. The fraction from 2 mm to 50 micron (0.050 mm) is defined as sand. The fraction smaller than 50 micron was quantified through the use of a hydrometer to differentiate between silt (0.05 – 0.002 mm) and clay (smaller than 0.002 mm).
The results are provided in Figure 10 with the position of the samples indicated in Figure 11. From the data the soils of Hakskeen Pan are classified as being predominantly clay with areas being silty clay.
Figure 10 Particle size analysis results (percentage per fraction) of Hakskeen Pan soils
Figure 11 Sample points for soil texture analysis
The chemistry of the pan soils is a function of millennia of repeating wetting and drying cycles with the addition of soluble salts to the pan through leaching from the surrounding landscape and concentration through evaporation of water. Salts are composed of cations (positively charged ions) and anions (negatively charged ions) that precipitate (typically crystallise) from solutions (water with dissolved elements). The most mobile or soluble cation in these landscapes is sodium (Na+) with ammonium (NH4+), potassium (K+), magnesium (Mg+2) and calcium (Ca+2) becoming less soluble in that order. The anions are a bit more complicated in that they consist of chloride (Cl-) and fluoride (F-) as single element anions and nitrate (NO3-), carbonate (CO3-2), sulphate (SO4-2) and phosphate (PO4-3) as compound anions. The main determinants of the specific salts present in pan soils are 1) the concentrations of the specific elements/compounds in the parent materials and 2) the solubility of these elements/compounds.
The clay particles determined in the previous section have electrostatic charges due to a number of reasons. (Refer to soils science texts regarding isomorphous substitution in clay minerals and pH dependent charge characteristics of exposed hydroxyl groups on clay mineral edges.) The clay particles have a specific capacity to adsorb (electrostatic attraction of ions to a charged clay mineral surface) a certain quantity of ions – and these are called “exchangeable ions”. The specific ions depend on the polarity of the surface charge – isomorphous substitution typically yields a negative charge and at neutral to high pH surface charges are negative as well. In other words, the clay particles can adsorb a set quantity of ions with the rest remaining in solution in wet conditions – the latter being referred to as “soluble ions”. However, when the pan dries out the remaining free cations and anions react electrostatically and precipitate out of solution as specific salt mineral crystals (Figure 12). The mineralogy will be discussed in more detail a bit later.
Figure 12 Salt precipitation on the surface (top) and close-up of salt crystals (bottom)
The determination of the concentration of soluble cations and anions is done through adding a known volume of deionised water to a specific mass of soil. This suspension or paste is stirred vigorously after a set amount of time the sample is filtered to yield a solution with soluble ions. These ions are determined through various means with the most common being spectrometric techniques. The determined concentrations are recalculated to reflect a specific concentration per unit mass of soil. Together with these other parameters such as pH and electrical conductivity (EC) of the samples are measured.
The positions of the samples collected for analysis are provided in Figure 13 and the results are presented in Table 1. It is apparent that the EC, Na and Cl values for the pan soils are much higher than those of typical agricultural soils. Both these parameters are an indication of aridity and drainage conditions in the landscape. Sodium has a massive influence on the physical behaviour of the pan soils (explained later).
Table 1 Summary of chemical analysis results (cations)
Table 2 Summary of chemical analysis results (anions)
Figure 13 Position of samples collected for chemical analysis
The soil minerals found on the pan are, as discussed previously, a function of the interaction of weathering products from the underlying rock as well as introduced materials (through wind and leaching) from the surrounding landscape. Mineralogy of a soil sample is determined through 1) concentration of the clay fraction, 2) precipitation of this fraction on a glass plate and 3) determination of mineral crystals present through X-ray diffraction (XRD). Selected samples from the pan (Figure 14) were analysed and the results are provided in Table 3.
Figure 14 Soil mineralogy samples collected on the pan
Table 3 Mineralogy analysis results for samples from Hakskeenpan
The origin of the minerals found in the pan soils is as follows:
Several web resources exist for more information on the minerals discussed above. Type in the name in your web search engine and do some more research!
All soil clay particles have some form of electrical charge on their surface. If we assume our specific clay particle to have 20 negative charges distributed over its surface (10 on each side) it would look something like the particle in Figure 15. Sodium has a specific ionic radius (size) and one positive charge (valence). Calcium has a slightly different ionic radius (but not too different to Na) but has 2 positive charges (valence). In the event that Na ions adhere to the clay particle due to electrostatic forces (positive and negative attracts) we will have one Na ion for each negative charge. In the case of Ca this becomes one ion for every two negative charges. Therefore, Na dominated clays have a much larger surface concentration of ions than Ca dominated clays. This dense occurrence of Na causes, in solution, a repulsion of other Na dominated clay particles and each one of the particles tends to remain in suspension in water. In the case of Ca the clay particles can move much closer to each other where, at a critical distance, van der Waals forces take over and the clay particles stick together. The effect of Na causing a moving apart of clay particles is called “dispersion” or “deflocculation” and the effect of Ca is called “flocculation”.
Figure 15 Schematic representation of Na and Ca influences on dispersion and flocculation of clays
From the chemical analysis data (Table 1) of the pan soils we see that Na is by far the most dominant cation. Under such conditions the pan soils will be highly dispersive when wet. This is confirmed when one attempts to walk on the soils while wet as the soil material squishes through ones toes when walking barefoot. The result of the dispersion is that the soil particles settle at the same level as “water level”. Once the water evaporates the soil particles remain behind at the same flat level. The only change here is that the “swelling” properties that the clays had when wet now leads to a decrease in volume upon drying. This drying process then leads to the formation of roughly hexagonal shaped crust units. The crust therefore undergoes swelling and shrinking with every rainfall event followed by a dry spell. Any small surface disturbance of the pan crust is “reset” after an adequate wetting period.
The subsoils below the crust are a bit different. These soils, also having cracks between units, experience an infilling of dispersed materials into the cracks. The result is therefore a very dense subsoil where repeated wetting and drying cycles lead to a clogging up of any porous areas.
An additional aspect that is not always easy to grasp is the fact that rainfall water falling on the pan is similar to distilled water. From irrigation practices it is known that very pure water does not infiltrate into a high salt content soil as readily as water that also has a high salt concentration. The rainfall water therefore only wets the upper part of the soil profile and does not readily infiltrate into the subsoils. This leads to a very dynamic crust and a relatively static subsoil – all elements suited to a land speed record attempt. In later sections the “mechanics” (behaviour under loading – such as wheels of a heavy vehicle) of these soils will be discussed in more detail.
The fact that the soils are dispersive in the presence of high concentrations of Na is ideal for the “creation” of a pan system suitable for land speed records. The pan soil surface level is very similar to what a water level would have been. There are nuances on the pan surface in that certain areas exhibit more salt crystals and puffy crust than other areas (Figure 16). The flat surface is evident when one looks at fence lines on the pan (Figures 17 and 18). In these photographs, with a bit of imagination, the curvature of the earth can be seen in the dipping away of the fence in the distance!
Figure 16 Puffy salt crusts in certain pan areas (top) and hard crust surfaces in others (bottom)
Figure 17 Zoom on fence line across Hakskeenpan
Figure 18 Zoom on fence line (different section than Figure 18) across Hakskeenpan