  The Planck satellite has detected cluster candidates via the Sunyaev
Zel'dovich (SZ) effect, but the optical follow-up required to confirm these
candidates is still incomplete, especially at high redshifts and for SZ
detections at low significance. In this work we present our analysis of optical
observations obtained for 32 Planck cluster candidates using ACAM on the 4.2-m
William Herschel Telescope. These cluster candidates were preselected using
SDSS, WISE, and Pan-STARRS images to likely represent distant clusters at
redshifts $z \gtrsim 0.7$. We obtain photometric redshift and richness
estimates for all of the cluster candidates from a red-sequence analysis of
$r$-, $i$-, and $z$-band imaging data. In addition, long-slit observations
allow us to measure the redshifts of a subset of the clusters
spectroscopically. The optical richness is often lower than expected from the
inferred SZ mass when compared to scaling relations previously calibrated at
low redshifts. This likely indicates the impact of Eddington bias and
projection effects or noise-induced detections, especially at low
SZ-significance. Thus, optical follow-up not only provides redshift
measurements, but also an important independent verification method. We find
that 18 (7) of the candidates at redshifts $z > 0.5$ ($z > 0.8$) are at least
half as rich as expected from scaling relations, thereby clearly confirming
these candidates as massive clusters. While the complex selection function of
our sample due to our preselection hampers its use for cosmological studies, we
do provide a validation of massive high-redshift clusters particularly suitable
for further astrophysical investigations.
