Small strain elastic behaviour of unsaturated soil investigated by bender/extender element testing

The aim of this project was to investigate very small strain elastic behaviour of soils under unsaturated conditions, using bender/extender element (BEE) testing. The behaviour of soils at very small strains has been widely studied under saturated conditions, whereas much less work has been performed on very small strain behaviour under unsaturated conditions. A suction-controlled double wall triaxial apparatus for unsaturated soil testing was modified to incorporate three pairs of BEEs transmitting both shear and compression waves with vertical and horizontal directions of wave transmission and wave polarisation. Various different techniques for measuring wave travel time were investigated in both the time domain and the frequency domain and it was concluded that, at least for the current experimental testing programme, peak-to-first-peak in the time domain was the most reliable technique for determining wave travel time. An experimental test programme was performed on samples of compacted speswhite kaolin clay. Two different forms of compaction were employed (i.e. isotropic and anisotropic). Compacted kaolin soil samples were subjected to constant suction loading and unloading stages at three different values of suction, covering both unsaturated conditions (s= 50kPa and s= 300kPa) and saturated conditions (s=0). Loading and unloading stages were performed at three different values of stress ratio (η=0, η=1 and η=-1 ). In some tests a wetting-drying cycle was performed before or within the loading stage, with the wetting-drying cycles including both wetting-induced swelling and wetting-induced collapse compression. BEE tests were performed at regular intervals throughout all test stages, to measure shear wave velocity Vs and compression wave velocity Vp and hence to determine values of shear modulus G and constrained modulus M. The experimental test programme was designed to investigate how very small strain shear modulus G and constrained modulus M varied with unsaturated state variables, including how anisotropy of these parameters developed either with stress state (stress-induced anisotropy) or with previous straining (strain-induced anisotropy). A new expression has been proposed for the very small strain shear modulus G of an isotropic soil under saturated and unsaturated conditions. This expression relates the variation of G to only mean Bishop’s stress p* and specific volume v, and it converges to a well-established expression for saturated soils as degree of saturation approaches 1. The proposed expression for G is able to predict the variation of G under saturated and unsaturated conditions at least as well as existing expressions from the literature and it is considerably simpler (employing fewer state variables and fewer soil constants). In addition, unlike existing expressions from the literature, the values of soil constants in the proposed new expression can be determined from a saturated test. It appeared that, in the current project at least, any strain-induced anisotropy of very small strain elastic behaviour was relatively modest, with the possible exception of loading in triaxial extension. It was therefore difficult to draw any firm conclusion about evolution of strain-induced anisotropy and whether it depended upon the same aspects of soil fabric as evolution of anisotropy of large strain plastic behaviour. Stress-induced anisotropy of very small strain elastic behaviour was apparent in the experimental test programme. An attempt was made to extend the proposed expression for G to include the effect of stress-induced anisotropy. Interpretation of the experimental results indicated that the value of shear modulus was affected by the values of all three principal Bishop’s stresses (in the direction of wave transmission, the direction of wave polarisation and the third mutually perpendicular direction). However, prediction of stress-induced anisotropy was only partially successful, and it was concluded that the effect of Lode angle was also significant.

An investigation of the possible health-promoting modes of action of regular- and super- doses of phytase in the broiler chicken

The overall objective of this thesis was to study the effects of regular and high (super-) doses of phytase in the gut of broilers, with the aim of documenting the mechanism of their action leading to improvements in animal health. Phytase is often supplemented to commercial broiler diets to facilitate the hydrolysis of plant phytate and release of phosphorus for utilisation. Although not the original intention of its addition, phytase supplementation leads to improvements in growth performance parameters and enhanced nutrient utilisation. Further benefits have also been observed following the addition of super-doses of phytase which are not explained by an increase in phosphorus release, and thus have been termed ‘extra-phosphoric effects’. Using diets formulated to be adequate or marginally deficient in available phosphorus (aP; forming the negative control, NC), phytase was supplemented at 1,500 and 3,000 FTU/kg phytase in the first study (both super-doses) and the partitioning of nutrients within the body was investigated. It appeared that there were some metabolic changes between 1,500 and 3,000 FTU/kg, switching between protein and fat accretion, potentially as a consequence of nutrient availability, although these changes were not reflected by changes in growth performance parameters. However, the loss of the NC treatment without phytase on day 12 limits the comparison of the phytase within the NC treatment, but does allow for comparison of each dose at adequate or low dietary aP levels. As expected, a greater degree of phytate hydrolysis was achieved with 3,000 than with 1,500 FTU/kg phytase, but changes in carcass accretion characteristics were greater with 1,500 than 3,000 FTU/kg. Using these findings and the observation that there were no further changes in the parameters measured by increasing phytase from 1,500 to 3,000 FTU/kg (aside from phytate hydrolysis), 1,500 FTU/kg phytase was selected as the super-dose to be used in subsequent studies. The next study considered the influence of regular (500 FTU/kg) and super doses (1,500 FTU/kg) of phytase from within the gut. Overall, it was observed that changes were occurring to the gut environment, which ultimately would influence the absorptive capacity and conditions for further phytate hydrolysis. Dietary treatment influenced gut conditions such as pH, intestinal morphology and bacterial populations which can subsequently influence nutrient utilisation and potential for growth. The subsequent study was designed to investigate the effects within the gut in more detail. The release of nutrients from phytate hydrolysis and their bioavailability within the digesta can influence conditions within intestine, facilitating enhanced absorption. One of the parameters investigated was the expression of genes involved in the transport of nutrients in the intestine. Overall, there were few significant dietary treatment influences on gene expression in the intestine, however there was a dose-dependent response of phytase on the expression of the jejunual divalent mineral transporter. This indicates a change in divalent mineral bioavailability in the intestine, with correlations with inositol phosphate esters (IPs) being identified. This is likely explained by the IPs produced by phytase hydrolysis and accumulating in the digesta, differing between regular and high doses of phytase. It became apparent that interactions between the products of phytate hydrolysis (IP3, IP4) and minerals in the digesta had the potential to influence the gut environment and subsequent nutrient bioavailability and overall phytase action. The final study was designed to increase the content of the IPs, and investigate the influence of phytase under these conditions. As the complete hydrolysis of phytate to myo-inositol has been reported to be beneficial due to its proposed insulin mimetic effects, myo-inositol was also supplemented to one of the diets to see if any further benefits would be observed when supplemented alongside super-doses of phytase. Neither increased concentrations of the higher IP esters (IP6, IP5 and IP4) nor myo-inositol (myo-) had any effect on broiler growth performance, however there were still apparent beneficial influences of phytase supplementation. The results suggest considerable and important interactions between minerals and IP esters within the digesta, which ultimately have the potential to influence gut conditions and thus nutrient utilisation and growth performance. Reduced concentrations of blood glucose in the high IP ester diet with additional phytase supplementation suggest some insulin-like effects of myo- production. Additionally, the lack of effect of myo- supplementation on blood glucose and insulin concentrations suggests a difference between the structure of phytase-produced myo- and supplemented myo-. Although there were no improvements in growth performance by increasing phytase from 500 to 1,500 FTU/kg, there were changes occurring at the level of the gut and expression of genes in the intestine, influencing nutrient utilisation and the partitioning of nutrients within the body. There are many factors to be considered when supplementing phytase, with dietary nutrient content and nutrient release and IP production during phytate hydrolysis having an influence on phytase action, nutrient absorption and conditions within the gut. Super-doses of phytase may be beneficial for maintaining optimal gut conditions, clearing IP esters from the digesta, reducing their potential to form complexes with minerals and other nutrients, ultimately influencing the efficiency of production.