  We explore the possible nature of high-velocity features in Type~Ia
supernovae by presenting synthetic spectra generated from hydrodynamic models
of interaction between the supernova and a compact circumstellar shell. We use
TARDIS to compute the spectra and compare model spectra to data from SN~2011fe
at 2, 5, and 9 days after the explosion. We apply abundance models to the
shells that consist of either a hydrogen, helium, or carbon-oxygen substrate
with overall solar abundance of metals and depletion or enhancement of calcium
abundance. We vary the calcium abundances in the shell and the ejecta to study
the effect on the photospheric and high-velocity components of the calcium
near-infrared triplet. The substrate leaves no imprint upon the spectra, but
helium substrates are discouraged by demanding excessive calcium abundances. We
find that we can approximately reproduce the blended high-velocity and
photospheric velocity features at 2 and 5 days. The 9 day spectrum shows a
distinct high-velocity component of the line. We are unable to reproduce this
specific feature with the current models. We also explore helium-shell double
detonation models, finding they tend to give no high-velocity features or
excessively strong features. A very carefully chosen helium-shell mass might
give a reasonable reproduction of the observed calcium features. We discuss the
implications of our models for the nature of high-velocity features and their
significance to the evolution and explosion of SN~Ia.
