After much searching on the net I have found virtually nothing on the effectiveness of artillery on troops in foxholes and trenches.
I wish I still had a copy of U.S. Army FM-640.
My recollection is that 6-40 was straight up gunnery and that all of the munitions effectiveness information was in a separate set of classified tables. Of course that was a long time ago . . . .
Unclassified information is few and far between but some shows up open source. Usually it has to do with either the US Army or an author, like David Isby, digging into Soviet artillery norms. Data on the light pieces will be hard to find but you should be able to do some interesting guess work for NATO calibers if you have 122/152mm information.
For instance, here's a test described in Major (Retired) George A. Durham's article Who Says Dumb Artillery Rounds Can’t Kill Armor?
The third test was against a simulated US mechanized
infantry team in defensive positions. The target area consisted
of a forward defense area with a tank ditch 250 meters long,
minefields and wire obstacles. The infantry was dismounted
and had prepared positions with overhead cover. The fighting
vehicles and tanks were in supporting positions, dug in with
both “hull down” and “turret down” positions.
For this test, a 24-gun 155-mm battalion was used to achieve
the Soviet criteria of 50 percent destruction. To accomplish
these effects, the fire plan for each of the three iterations of the
test required 2,600 HE rounds with a mix of PD and VT fuzes.
In each iteration, 50 percent of the infantry fighting positions
were destroyed and and about 50 percent of the personnel
were wounded or killed.