We perform total energy calculations based on the tight-binding Hamiltonian scheme (i) to study the structural properties and energetics of the extended {311} defects depending upon their dimensions and interstitial concentrations and (ii) to find possible mechanisms of interstitial capture by and release from the {311} defects. The generalized orbital-based linear-scaling method implemented on Cray-T3D is used for supercell calculations of large scale systems containing more than 1000 Si atoms.