I did some searches of the Official Records today and came up with a number of hits referring to Flat Creek. It is difficult to know to what the authors were referring, a village called Flat Creek, the creek itself, or the Flat Creek bridge on the E. Tenn. & Virgina RR.

My best guess is that the site is on the Rutledge/Knoxville Road, approximately 5 miles below the present day Blain on US 11W, perhaps on Flat Creek itself.

Below is what I have found.

Hope this helps.

George Martin

KNOXVILLE,
November 17, 1864.

The garrison at Strawberry Plains has not retreated, but the enemy is between us and them, reported in strong force. Skirmished with them this evening near Flat Creek, four miles this side of the Plains, our force finally retiring. Many rumors of large force coming from Virginia--nothing reliable.
J. AMMEN,
Brigadier-General.
Lieut. Col. G. M. BASCOM,
Assistant Adjutant-General, Louisville.
(Same to General Steedman.)
OR139P1, p. 887

Most likely referring to the Flat Creek bridge located approximately 5 miles east of Strawberry Plains on the East Tennessee & Virginia RR

STRAWBERRY PLAINS,
November 17, 1864.

Enemy, somewhat re-enforced, occupies very strong works on opposite side of the river. To-day we burned railroad bridge over Flat Creek between this point and Knoxville. Have been skirmishing with him since yesterday, but the number and composition of my troops will not justify an assault upon the works. Enemy is too near supports and we too far to run much hazard. Am a little uneasy about other end of the department. Hope General Echols has returned from Richmond. We still gather in prisoners from the late engagement, and have more flags, wagons, mules, &c., than I at first reported.
J. C. BRECKINRIDGE,
Major-General, Commanding.
General R. E. LEE.
OR1V39P1, p. 892

Reference is to the Flat Creek bridge located approximately 5 miles east of Strawberry Plains on the East Tennessee & Virginia RR

HEADQUARTERS TWENTY-THIRD ARMY CORPS,
13 Miles from Knoxville and 4 Miles from Blain's Cross-Roads,
December 8, 1863.

COLONEL: I did not overtake the command last night until it had arrived, with the Ninth Army Corps, at Flat Creek, near this place.
OR1V31P1, p. 374

This place would be 5 miles below the present day Blain on US 11 West, 3 miles due North of the Flat Creek bridge.

FLAT CREEK, KNOX COUNTY, TENN,
December 26, 1863.

Brig. Gen. S. P. CARTER:

SIR: Unable to reach home, I have been staying for the last ten days at the house of Maj. Gaines McMillan, OR1V31P3, p. 507

General Spears, who is on the Knoxville and Rutledge road at Flat Creek
OR1V32P2, p. 140

I would thus conclude, the best evidence is that the locale, Flat Creek, would be located on US 11W, 5 miles below Blain

Again, I would point out that Flat Creek drains into the Clinch River ~12 miles SSW from Tazewell and that Confederate troops were camped somewhere South of Tazewell on the Clinch. One would believe that your gggrandfather may have been returned to camp and buried there.