  In this first paper of a series, we report on the use of quasar spectra
obtained with the UVES high resolution spectrograph and available through the
ESO VLT archive to build the first sample of sub-DLA systems, absorbers with HI
column densities > 10^{19} cm^{-2} but lower than the classical definition of
damped Ly-alpha systems (DLAs) 2x10^{20} cm^{-2}. A systematic investigation of
the properties of these systems and a comparison with those of the DLAs (Paper
II of this series; Peroux et al. 2003b) is expected to provide new clues on the
association of high column density absorbers with galaxies and on the overall
evolution of the neutral hydrogen gas mass and metal content in the Universe.
In the spectra of 22 quasars which were found suitable for a sub-DLA search, we
identified 12 sub-DLAs and 1 borderline case between the DLA and sub-DLA
systems in the redshift interval z = 1.8-4.3. We measured the column densities
of HI and up to 16 ions of low-, intermediate- and high-ionization. We further
investigated the significance of the ionization corrections in the
determination of the chemical abundances from the low-ionization ions in the
sub-DLA HI column density range. Using the predictions of different ion ratios
as a function of the ionization parameter computed with the CLOUDY software
package, we have estimated that with the exception of one case, the ionization
corrections to the abundances of 9 systems for which we were able to constrain
the ionization parameter, are lower than 0.2 dex for all of the elements except
AlII and ZnII down to HI column densities of log N(HI) = 19.3 cm^{-2}. We
finally present the first sub-DLA chemical abundance database which contains
the abundance measurements of 11 different elements (O, C, Si, N, S, Mg, Al,
Fe, Ni, Zn, and Cr).
