[Cob] Problem oven

[Muddy Toenails]

Hi to everyone on the list-
i'm struggling quite a bit with this oven that i'm building...
Since we started laying the cob, two large sections of the oven have fallen.
we have fixed and rebuilt those two sections (luckily, we were still in the
building stage so the cob didn't dry too much between the old and the new
cob), but a new problem has arisen;
a long, deep fissure has appeared over the oven mouth. There is maybe a
foot's worth of material that looks like it is in danger of falling.
We have not yet pulled out the sand from inside the oven. i'm wondering what
i can do to prevent this section from falling away from the oven!
Thanks in advance for your advice,
h.