  We present wide-field mapping at 850 $\mu$m and 450 $\mu$m of the $z$ = 2.85
protocluster in the HS1549$+$19 field using the Submillimetre Common User
Bolometer Array 2 (SCUBA-2). Spectroscopic follow-up of 18 bright sources
selected at 850 $\mu$m, using the Nothern Extended Millimeter Array (NOEMA) and
Atacama Large Millimeter Array (ALMA), confirms the majority lies near $z$
$\sim$ 2.85 and are likely members of the structure. Interpreting the
spectroscopic redshifts as distance measurements, we find that the SMGs span 90
Mpc$^2$ in the plane of the sky and demarcate a 4100 Mpc$^3$ "pancake"-shaped
structure in three dimensions. We find that the high star-formation rates
(SFRs) of these SMGs result in a total SFR of 20,000 M$_\odot$ yr$^{-1}$ only
from the brightest galaxies in the protocluster. These rapidly star-forming
SMGs can be interpreted as massive galaxies growing rapidly at large
cluster-centric distances before collapsing into a virialized structure. We
find that the SMGs trace the Lyman-$\alpha$ surface density profile. Comparison
with simulations suggests that HS1549$+$19 could be building a structure
comparable to the most massive clusters in the present-day Universe.
