Syntactic blending: bunker down

[Clai Rice]

At what point does a blend cease to be a blend and become its own phrase?

Google returns about 2,360 hits for "bunker down"; LexisNexis provides 350
hits for "bunker down" AND NOT "golf" (to eliminate uses like the following:

The Houston Chronicle, June 08, 2003, Sunday, 2 STAR EDITION, SPORTS 2;, Pg.
1, 3680 words, U.S. OPEN PREVIEW; Olympia Fields grumbling is par for an
Open course, STEVE CAMPBELL
... narrow hole with a bunker down the left side. The tee ...)

Out of the 350 we still have a few adjectival PP, such as "... in a concrete
bunker down the road from the Price Slasher ..." and ... investigate a
strange bunker down deep within the bowels of ...". Discounting a
conservative 2/10, we are still left with about 280 genuine examples, dating
back to:

The Times (London), December 8 1986, Monday, Issue 62635., 332 words, Arts
(Television): From bed to worse, MARTIN CROPPER
  With their arch nicknames and economically sketched characters, the boys
in light blue down at Blackwall Fire Station seemed to have been seconded
from a sit-com of unknown provenance. They had only to tuck in to beef curry
and tinned apricots for the alarm bell to ring; they had only to bunker down
round a blue video for their new female colleague to amble in.

--Clai Rice