Quantifying Soil Strength

Soil strength was determined using the dynamic cone penetrometer (DCP) and pocket penetrometer. The DCP experiments showed that the average starting depth was highest at the AOF (123mm +/-47) indicating a soil surface with a medium level of hardness (299kPa) and lowest at the HTS (78mm +/-11) indicating a surface with a hard level of hardness (424kPa).

The topsoil thickness was deepest at the AOF (97mm +/14) indicating that the site had the most loose and unconsolidated topsoil (342kPa), which may be due to ploughing and shallowest at the CFT (25mm +/-12) indicating that it had the firmest and most consolidated topsoil (1965kPa).

Resistance at 10cm depth was chosen as the best indicator for showing a difference in strength at compacted and un-compacted sites (Singleton, Boyes and Addison, 2000). The mean resistance (Figure 4.7) was highest at the SFT (367kPa) and CFG (340kPa) which were similar resistances to four or more tractor passes at a clay site (316kPa) in Buenos Aires, Argentina (Botta, Jorajuria and Draghi, 2002) and under low grazing at a sandy loam site (365kPa) in Kinsella, Canada (Naeth et al., 1990). The mean resistance was lowest at the SOF (281kPa) and HOF (294kPa) which were similar resistance to no grazing at a sandy site (290kPa) (Sousa Neto et al., 2014) and under low grazing at a loamy sand site (215kPa) in Bukit Mahang, Malaysia (Majid, Awang and Jusoff, 1989).

Figure 4.7: Resistance at all fourteen sites.

The resistance measured in all tests in this study had a relatively small range, between 165 and 430kPa. In seventeen other studies the range in the level of resistance observed was much higher, between 150kPa and 2510kPa, at a clay loam site in Solohead, Ireland (Tuohy et al., 2015). Out of these studies, the mean resistance under no grazing was 563kPa, under low levels of grazing it was 907kPa and in twelve studies under high levels of grazing it was 1095kPa. These findings suggest that the compaction levels caused by machinery and animals in this study are relatively low.

Figure 4.8: Resistance at sites from the literature and in this study.

The first graph (Figure 4.8) shows that there is a 25% chance that, a random sample collected under zero tractor passes, will have a resistance of less than, or equal to 200kPa and a 100% chance that it will be under 296kPa.

However, the third graph shows that there is only a 25% chance that, a random sample collected at the AOF site, will have a resistance that is less than, or equal to 300kPa and a 100% chance that it will be less than or equal to 344kPa. This suggests that the AOF is slightly more compacted than has been observed in other studies under no compaction in the literature.

The mean maximum penetration resistance (MPR) was highest at the CFT (1965kPa) at a depth of 25mm (Figure 4.9) and the HTS (1100kPa) at a depth of 37mm. The lowest MPR was observed at the AOF (350kPa) at a depth of 73mm and the SFT (409kPa) at a depth of 80mm. The mean depth of MPR across all sites in this study was 50mm. There were slight differences by type of site the mean depth of MPR was 65mm+/-39 for Arable, 42mm+/-22 for Cattle, 48mm+/-23 for Horses and 60mm+/-25 for Sheep.

Figure 4.9: Depth of maximum penetration resistance at all sites.

The results suggest that the human induced pressures compact the soil in different ways depending on their weight and hoof surface area. Other studies have identified that sheep cause more surface compaction and cattle cause more compaction at greater depths however, this study suggests that

cattle and horses cause compaction at shallower depths and sheep at greater depths.

The pocket penetrometer results (Figure 4.10) showed that the mean unconfined compressive strength was highest at the STS (3.12kg/cm²) which was similar to a horse grazed site (3.60kg/cm²) in Nevada, USA (Davies, Collins and Boyd, 2014), a heavily cattle grazed silt loam site (2.85kg/cm²) in Eureka, USA (Gifford et al., 1977) and a cattle grazed loamy site (2.30kg/cm²) in Ketereh, Malaysia (Majid et al., 1989).

Figure 4.10: Unconfined compressive strength at all fourteen sites.

The unconfined compressive strength was lowest at the HOF (0.76kg/cm²) which was similar to under light cattle grazing at a sandy loam site (0.90kg/cm²) in Wycanna, Australia (Braunack and Walker, 1985) but lower than at a cattle grazed sandy loam site (1.90kg/cm²) in Willow Creek, USA and a loam site (2.11kg/cm²) in Valley Creek, USA (Clary, 1995). This suggests that the level of surface compaction at the HOF is very low and the site is less compacted than in most other studies. No unconfined compressive strength values were obtained at the CFT because there was ponded water at the site throughout the year.

Figure 4.11: UCS at sites from the literature and in this study.

The first graph (Figure 4.11) shows that there is a 50% chance that, a random sample collected under zero tractor passes, will have a UCS of less than, or equal to 0.51kg/cm² and a 100% chance that it will be under 1.02kg/cm². However, the fifth graph shows that there is only a 28% chance that, a random sample collected at the HOF site, will have a UCS that is less than, or equal to 0.50kg/cm² and a 100% chance that it will be less than or equal to 1.15kg/cm2. This suggests that the HOF is slightly more compacted than has been observed in other studies under no compaction in the literature.

The unconfined compressive strength measured in all tests, at all sites ranged between 0 and 5kg/cm² showing the total range of values possible with the pocket penetrometer. The normal range of strength values for a granular, cohesion-less soil like silt are 0.5-1.5kg/cm² and therefore the soil surface at the ATL, CFG, CTS, HTS, SFT and STS are likely to have been affected by the processes of capping and crusting. In sixteen other studies the mean strength under no grazing was 1.13kg/cm², low levels of grazing it was 1.64kg/cm² and high levels of grazing it was 2.38kg/cm². The very high UCS values at the Cattle and Sheep sites are indicative of very high soil strength and compaction.

There was no correlation (r²<0.3) between the surface, maximum or 10cm depth resistance and the unconfined compressive strength on the soil surface in this study. This may be due to high heterogeneity in compaction levels at site and because the dynamic cone and pocket penetrometer measurements were taken on different parts of the sampling area.