## 9 Desiccation  
The predicted increase in surface temperature is expected to result in an increase in the formation of desiccation cracks during summer months. Desiccation cracks act as preferential pathways for precipitation to percolate into the ground. This results from an increase in near-surface permeability and pore water pressures, which is important for historic geostatructures, e.g., reactivation of relict failures [7]. [PERSON], et al. [18] suggests that factors affecting the formation of desiccation cracks can be categorised as follows:  
* Internal factors--mineral composition, fines content, water retention characteristics, and tensile strength
* External factors--temperature, relative humidity, and wetting and drying cycles  
[PERSON], et al. [81] highlighted that desiccation cracks in clay forms due to stress build-up at flaws within a material. This is because cracks form when the tensile strength of the soil is exceeded by negative pore pressure-induced tensile stresses [7]. These flaws include micro cracks, delaminations, foreign inclusions such as air bubbles, or high-stiffness particles (sand and gravel). Other influences of desiccation formation include flaw distribution, soil fracture energy, desiccation rate, and particle size distribution. [PERSON], et al. [18] showed that the formation of desiccation cracks is intrinsically linked to compacted soils bimodal pore structure. Testing using image analysis investigated the effect of desiccation on Xiaush compacted soil during air-drying while maintaining measurement of water content. Results showed that for soil samples compacted dry of optimum, random cracks were initiated evenly on the soil surface. In comparison, slurry samples showed branching sub-cracks producing an orthogonal pattern. Furthermore, Mercury Intrusion Porosimetry (MIP) demonstrated that changes in soil microstructure and increase in macro pores (interaggregate pores), when compacted dry side of optimum, were the main influence of the observed desiccation cracking pattern. It is postulated that as the soil volume shrinks, the macro pores also shrink, and there is a rearrangement of aggregates. These macro pores have a larger pore size, resulting in higher strain energy prior to cracking, and therefore can be seen as planes of weakness allowing for crack initiation. It was also shown that soil specimens compacted wet of optimum presented a similar cracking pattern to material prepared as a slurry. This again yields from the pore structure, which is unimodal for both materials. Consequently, it was shown that with an increasing compaction water content, both the water content at the initiation of cracking and crack density decreased, while crack width increased. Therefore, it is likely that soils compacted at lower densities have an increased probability for the development of greater crack density, which is initiated at higher water contents. This is relevant to historic compaction techniques used in the construction of geostructures in the UK, as this research suggests they may be more vulnerable to initiation of desiccation cracking at higher water contents. Both [PERSON], et al. [82] and [PERSON], et al. [83] introduced the use of X-ray Computed Tomography (X-ray CT) to measure volumetric changes during shrinkage and internal crack formation. Prior to this, volumetric measurements of desiccation cracks were primarily undertaken via the Mercury Intrusion Porosimetry (MIP) method. However, [PERSON], et al. [83] showed that the use of X-ray CT and image analysis produces results which are similar to that of MIP. This is advantageous as it is a non-destructive technique and therefore allows for the measurement of the development of volume changes during the drying process. Use of X-ray CT and image analysis by [PERSON], et al. [82] was able to show the increased accuracy when calculating volumetric shrinkage strain and volume compared to traditional geometric measurements, which underestimated shrinkage up to 90%. [PERSON], et al.