## 9 Desiccation
[83] also touched upon the effect wetting and drying cycles on desiccation. It was found that after an initial wetting and drying cycle, cracks formed were smaller. After a subsequent cycle of wetting and drying numerous cracks then developed. This was attributed to the change in the bimodal pore structure, i.e., the formation of increased macro pores. These findings concur with hypothesis made by [PERSON], et al. [19] and therefore would suggest an important link between the microstructre of compacted soils and desiccation cracking present on embankments. Many detailed microstructural desiccation studies presented in literature are conducted in laboratory conditions on relatively small samples of soil. This is extremely limiting as desiccation cracks interact with vegetation and are not limited by size in the same way as laboratory samples. However, recent publication of work by [PERSON], et al. [84] describes field testing investigating the development and evolution of desiccation on the Achilles Bionics test embankment. The study was the first prolonged crack survey on transport embankment with meteorological and hydrological data. The data was primarily collected on the southern side of the embankment and produced several interesting findings:  
* the poorly compacted panel had the greatest number of cracks with 11 of the 19 cracks present on the slope,
* a linear cracking style was observed which contrasts with polygonal style generally seen in the laboratory (thought to be a result of root density),
* wind data showed a sheltering of the northern slope while there was increased solar radiation exposure on the southern face, which resulted in increased vegetation diversity (hypothesized to result in preferential crack formation on the southern slope),
* the upper section of the southern slope was subject to higher negative pore pressures during drying periods which was attributed to drainage under gravity, increased wind exposure, and surface runoff. Measurement of the developing cracks on the embankment showed four stages of crack propagation: initiation of a crack, expansion of the crack to its maximum width and depth, minor contraction, and closure during winter. Similar patterns have also been observed in other poorly compacted manmade geostructures in heritage assets, such as Baille Hill mound (Figure 14). These observations are also in agreement with laboratory data reported by [PERSON], et al. [18], where the water content at which desiccation cracking was initiated was directly impacted by compaction conditions. These novel findings demonstrate the holistic nature of engineered soils and the link between the soil pore structure and its behaviour, which has significant implications for its future performance. Compacted soils that have a larger macro pore structure are likely to be more vulnerable to the onset of desiccation cracking. The predicted changes in rainfall and surface temperatures associated with climate change means these preferential pathways will allow water to enter slopes at greater rates and make them vulnerable to pore water pressure changes. This is particularly poignant for historic geostructures due to the reduced compactive effort during their construction.