I was a bit quick scanning Daijirin's "施設・建造物など...". 施設 is rather physical. Better drop that.

@@ -25,1 +25,1 @@
-<gloss>foundation (building, institution, etc.)</gloss>
+<gloss>foundation (building, etc.)</gloss>

"foundation (institution)" suggests that it can mean "an institution established with an endowment" 
(like 財団). But I don't think that's true, is it? What is meant by "institution" here, actually?

Daijr, GG5, 新和英中辞典, etc.

The refs are all over the place with sense splits. GG5 and 中辞典 combine ground/crust/etc. with foundation/base, but Daijirin and Kodansha split. Daijr has "bed" in with the earthy one, which makes sense. GG5 splits footing/foothold and support/constituency, but 中辞典 combines them.
I think the above works OK without getting too messy.
GG5 has ちばん as a reading; Daijr just mentions it.

@@ -14,0 +14,3 @@
+<r_ele>
+<reb>ちばん</reb>
+</r_ele>
@@ -16,1 +19,3 @@
-<gloss>(the) ground</gloss>
+<gloss>ground</gloss>
+<gloss>crust (earth)</gloss>
+<gloss>bed (gravel, river, etc.)</gloss>
@@ -20,5 +25,1 @@
-<gloss>bed (river)</gloss>
-</sense>
-<sense>
-<pos>&n;</pos>
-<gloss>foundation</gloss>
+<gloss>foundation (building, institution, etc.)</gloss>
@@ -32,0 +33,2 @@
+<gloss>footing</gloss>
+<gloss>foothold</gloss>

Kodansha:
地盤 [じばん] 
(1)大地の表面。地殻(ちかく)。ground <用例> ～沈下。
(2)建造物などの基礎となる土地。土台。foundation
(3)物事の根拠地。勢力の範囲。base <用例> ～をかためる。
.
河床 [かしょう] ★ 川の地盤。かわどこ。riverbed

I don't think "stronghold" is quite the right word. It's defined as
1. A place that has been fortified so as to protect it against attack
2. A place where a particular cause or belief is strongly defended or upheld
I think it's more like "constituency; support (electoral, etc.)" 
It corresponds to Kodansha's 勢力の範囲.
Anyway, I don't think that sense should come at the head of the definition.
Example:
当地では近年共和党の地盤沈下が続いている
The Republicans have been losing support here recently.

@@ -16,2 +16,0 @@
-<gloss>stronghold</gloss>
-<gloss>base</gloss>
@@ -20,0 +18,15 @@
+<sense>
+<pos>&n;</pos>
+<gloss>bed (river)</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>
+<gloss>foundation</gloss>
+<gloss>base</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>
+<gloss>constituency</gloss>
+<gloss>power base</gloss>
+<gloss>support (electoral)</gloss>
+</sense>