During the peaceful years of 20's and 30's, the bunkers were often
built as cheap as possible. For instance, only ONE machine gun bunker
had the mg-pedestal installed (the factory prototype) and no weapons
had been installed. So after the mobilization began, the first thing
to do was to quickly design a new pedestal from wood and build them
by hand. The bunkers were not camouflaged and only occasionally guarded,
so the Soviet intelligence was able to find out the exact positions
and layouts for most of the fortifications before the war. This was
partly possible because no restrictions were made for people to visit
the bunker sites. (It was common to find detailed sketches and maps
about Finnish defenses from fallen Soviet officers.)
One has to remember that the Mannerheim Line did not exactly follow
the "Enckell Line". So the bunkers of "Muolaa"(Mu),
"Sikniemi"(Mu) and "Salmenkaita "(Mä) were
actually not a direct part of Mannerheim Line. Instead as the line
along the many isthmi between Lakes Muolaanjärvi and Vuoksi,
that portion of the Mannerheim Line had only field fortifications.
So at the end of October 1939, when the war started, the actual Mannerheim
Line had in the area between the Gulf of Finland and Vuoksi, some
80 km wide, 41 concrete fortifications (10 of them built in the 1920's
and not repaired/modernized during the 1930's) and in the Vuoksi-Suvanto-Taipale
area, some 55 km wide, 25 concrete fortifications (all of them built
during the 1920's) see the map below. If the
concrete fortifications had been dispersed evenly along the line,
the distance between two of them would've been some 2 km, i.e. an
average density of about 0.5 concrete bunkers / km.
And if counting concrete bunkers which had machine guns or cannons
(thus excluding passive concrete shelters), a total of 48 bunkers,
there was one such bunker for each 3 km of front, i.e. roughly 0,35
armed bunkers / km.
(Compare the above figure with the average number of
concrete fortifications / km in the following lines;
- some 10 in the Maginot Line, in the weaker part at the Belgian border
- Westwall had at least 15)
As the primary AT-obstacle, from a wide variety of possibilities,
the "rock obstacle"(also "rock rows") was chosen
to be the primary type. There were several sound reasons for that.
First, the soil of Finland has large deposits of granite (less expensive
than concrete, which was on short supply). Second, the shortage of
equipment capable of mixing the cement at the construction sites.
Picture source: "Talvisodan Historia
And third, the relatively low number of trucks in Finland (a nationwide
total of 13 000), which forced to the use of carts drawn by horses,
thereby limiting the practical transportation distance from quarry
to construction site.
Some AT-ditches were made and some "slope cuts" ("rinneleikkaus"
in Finnish) where in the base of the slope, soil was dug out to produce
a vertical "wall".
The main problem to arise was that the obstacles were tested by Finnish
Renault FT 17 and Vickers 6-ton tanks thus giving a wrong idea of
their effectiveness (which was usually overestimated). There handicap
became even greater when the ground was covered with snow.
The AT-obstacles of the Mannerheim Line were built in the most threatened
areas, such as "Inkilä", Summa and the nearby areas,
the isthmi between Lakes Muolaanjärvi and Vuoksi, and Taipale.
Also, the flooding of some areas was planned, but they produced only
small results, partly because of the exceptionally cold winter.
The sheer volume of work and too small resources resulted in a line
that practically lacked depth. Only in the Summa sector on the "Gateway
of Karelia" was the back line (2 km from the front-line) of the
main defense line with adequate field fortifications. In most places,
tactical depth was achieved by locating some front-line unit shelters
200 -300 meters back from the front.
The Mannerheim Line was a line, which used natural barriers. The
cold winter made this however in many cases a threat as the ice on
the lakes and Ladoga and Gulf of Finland could carry the weight of
Soviet light tanks (BT-series and T-26 types). This wouldn't have
been a serious problem if
a) the Finnish artillery had had enough ammunition
b) there would've been more AT-guns, and
c) the testing of "ice mines" would've been conducted before
1939 (the bitter experience led to the development of the effective
"Arsa-mines" after Winter War).
The result of the ice cover on the lakes and sea was, that Soviet
tanks could easily launch attacks against Finnish isles and make flanking
maneuvers, while the Finns tried to prevent this by both blowing and
sawing gaps in the ice, which after a few hours froze up again.
Also, the frozen lakes provided the Red Army good supply routes and
deleted the need of bridging equipment in many places (Taipale, for
instance). In many places the Finns were forced to build temporary
defensive positions on the ice of Lakes. These were built mainly of
snow and logs, but soon pulp cubes were noted to be quite useful after
first being watered and then allowed to freeze solid before covering
them with snow. However on most cases the breastwork offered cover
from small arms only, therefore the positions were manned during the
dark hours only, the Soviet air superiority and the abundance of enemy
artillery forced the Finns to withdraw from them before dawn.
The terrain was generally low characterized
by small rises with height varying 15 - 40 meters.
The soil on the line consisted of loose soil
types, covering the bedrock completely.
The soil in front of the line, if divided into
marshy ground and firm ground, was
app. 30 % marsh
app. 70 % firm ground.
The land in front of the line, if divided into
wood, cultivated and other, was
app. 75 % wood
app. 20 % cultivated fields
app. 5 % unused, open areas etc.
The trees in the woods, along the line, was
app. 36 % of the trees were between 1 - 40 years of age
app. 53 % were 40 - 80 years
app. 10 % was over 80 years
If the woods, along the line, are divided by
dominating tree type, then
app. 51 % was dominated by pine trees
app. 26 % by spruce trees
app. 20 % by deciduous trees
In average, one hectare of forest had app. 63
cubic meters of timber (range 55 - 80 m3).
(Source: P.Hovilainen "Mannerheim-linja..."