United States Department of Agriculture
Natural Resources Conservation Service
Geological Services Unit
501 Felix, Bldg. 49SFort Worth, Texas 76115

Apr. 25, 2022
Lower Plum Creek Site 28, Caldwell County, Tx – Geologic Investigation

Subject:
To:

Brian Wenberg, State Conservation Engineer, Temple Texas
Dave Wolff, State Design Engineer, NRCS, Temple, Texas
Shawn Higgins, Design Engineer, NRCS, Temple, Texas

The geology of Lower Plum Creek Site 28 was investigated in two phases. In December 2014,
the right and left auxiliary spillways were investigated for sampling and Kh information. The
design investigation was done in March and April of 2019. The investigations were performed toobtain information for the upgrade of the structure due to reclassification from low hazard to highhazard. A site visit was conducted on April 21, 2022, with two additional hand-augered borings
taken. This is an update to the 2020 Geology report to reflect this field visit as well as to addresscomments from the 30% Design Review.
Watershed:
County:State:
Structure Class:Type of Equipment:

General Data
Lower Plum Creek
CaldwellTexas
Low reclassified to High
Mobile B-57, CME 45

Drainage Area:
Type of Structure:
Purpose:Direction of Valley:
Maximum Height of Fill:
Volume of Fill:
Length of Fill: 3
Storage Capacity:
Sediment pool:
Floodwater Storage:
Top of Dam (elev.):

Site Data
4691 acres (As-built 6/21/1962)
EarthenFloodwater Retarding
NE to SW
28 ft. (As-built 6/21/1962)
237,600 yrd3 (As-built 6/21/1962)
656 ft. (approximate)3283 Acre-ft. (As-built 6/21/1962)
199 Acre-ft. (Work Plan August 1960)
2892 Acre-ft. (Work Plan August 1960)
480.4 ft. (As-built 6/21/1962)

Auxiliary Spillway Crest (elev.):
Auxiliary Spillway Width (Left):
Auxiliary Spillway Width (Right):
Principal Spillway Crest (elev.):
Physiographic Description:
Topography:
Attitude of Beds:

474.5 ft. (As-built 6/21/1962)
100 ft. (As-built 6/21/1962)
400 ft. (As-built 6/21/1962)
460.2 ft. (As-built 6/21/1962)

Surface Geology and Physiography
Post-Oak Savannah
Rolling Hills
Strike: NE-SW
Dip: ~00 to 20 SE
GeneralThe majority of the site is underlain by the Eocene Wilcox Group (Ewi) mudstone andsandstone, with small portion of the watershed being underlain by the Eocene Carrizo Sand (Ec).
A slight amount of Quaternary Alluvium, (Qal), is deposited in a channel incised into the (Ewi).
Some regional faulting is in the immediate area as described on the Bureau of Economic
Geology, Geologic Atlas of Texas, Austin and Seguin Sheets, 1974. No faulting is directly under
the structure. Probabilistic ground motion values as measured in %g for this site indicate low
seismicity.
Wilcox Group
The Wilcox Group is mostly comprised of claystone, with varying amounts of sandstone, lignite,
and iron concretions. Claystone in the formation is massive to thin bedded, with some silt andvery fine sand laminations. It is pale brown to yellowish-brown in color and weathers toyellowish-brown. Sandstone are mostly medium to fine grained, moderately well sorted, crossbedded and occur in units 5 to 20 feet thick. Some lignite seams are present and the range inthickness from less than 1 to about 20 feet thick. Samples 250.6, 251.2, 252.2, 206.3, and 261.1
are representative of the weathered claystone material and samples 250.3, 250.4, 251.3 and 252.1
are representative of weathered silt and sandstone layers.
Carrizo Sand
The Carrizo Sand is comprised of medium to very coarse-grained sandstone. It tends to be thickbedded, light yellow to orange-brown in color and weathers to a yellowish brown. It can belocally banded with iron-oxide and ranges in thickness from 100 to 140 feet. No samples taken
are from this unit.
Alluvial Deposits
The alluvium is deposited in the channel incised into the Ewi. The material obtains a thicknessof up to 35 feet in hole 305. The material is comprised of moderately stiff, plastic clay, containsome fine-grained sand, with a grey to greenish-gray color. This was logged as CL, CH and SM

in the field in holes 305, 606, 607 and 608.
CulturalJust south of the site is FM 713, running east to west, and FM 86 is to the west of the site,running northeast to southwest. Lower Plum Creek Site 29 is located approximately 1 mile tothe southeast, and Lower Plum Creek Site 24 is located approximately 3 miles to the northwest.
Seismic
Probabilistic ground motion values as measured by %g indicates low seismicity for this site.
(Note: the term “low” is used as a descriptive term and not a classification term). A map of all
known seismic events with magnitudes (1811 to 2020) and faults within a 100km radius of thesite was constructed (TR 210-60, Earth Dams and Reservoirs, March 2019). Seismic event
locations are from published historic event, University of Texas Austin and the USGS
Earthquake Hazards Program. Fault locations are from the Texas Geologic Outcrop mas, Bureau
of Economic Geology. The Balcones fault zone is approximately 40km to the west of Lower
Plum Site 28 and is comprised of an area of mostly normal faulting. The fault zone has been
inactive for approximately 15 million years. No other significant, known faults are in thevicinity of Site 28.
Hazard curves attached are from the USGS Unified Hazard Tool program. Parameters used arefrom the Dynamic Conterminous U.S. 2014 (update)(v4.2.0) Edition, 760 m/s (B/C boundary)Site Class, Spectral Period is Peak Ground Acceleration with a time horizon return period of
2475 years. Peak Horizontal Ground Acceleration at this site is 0.025937407 g with an
Exceedance rate of 0.00040208253 yr-1.
The attached Total Deaggregation Plot (USGS) indicates the mean seismic contribution to thesite is from several earthquakes of magnitude 4.8-5.1, 30 to 100 km from the site. The events arealong the Balcones Fault to the northwest of the site.
The attached magnitude vs. distance vs. Papdopoulos and Lefkoppoulos (e-e’) curve is from thedeaggregation plot report mentioned above and the e-e’ data from the Magnitude-Distance
Relations for Liquefaction in Soil from Earthquakes, Bulletin of the Seismological Society ofAmerica, Vol. 83, June 1993. Magnitude/Distance points below and to the right of the e-e’ curve
are not considered susceptible to liquefaction. Points above and to the left of the e-e’ may besubject to liquefaction. The figure included in this report is intended for informational purposesand should not be considered a complete seismic evaluation for design.

Site
Rehabilitation site plans consist of “constructing an impact basin for the Need of the Project
existing principal spillway system; adding a new 30-inch hooded inlet principal spillway with an
impact basin; widening the right auxiliary spillway 36 feet; lowering both auxiliary spillway

crests 0.4 feet; regrading the inlet and outlet channels of both auxiliary spillways; raising the topof the dam 0.9 feet; flattening the upstream and downstream slopes to 3:1; reconstructing an
upstream wave berm and adding rock riprap for wave protection.” From: FINALSUPPLEMENTAL WATERSHED PLAN NO. VII and Environmental Evaluation for the
Rehabilitation of Floodwater Retarding Structure No. 28 of the Lower Plum Creek Watershed
Hays and Caldwell Counties, Texas. (May 2016)
Auxiliary Spillway
Eleven holes were drilled in the right Auxiliary Spillway to obtain samples for Kh determinationin 2014 to assist in planning. All holes were drilled using a Mobil B-57 rig using hollow stem
augers. All holes were drilled to at least existing Principal Spillway pipe invert elevation of 447
feet, to meet the “depth to valley floor” criteria in (TR 210-60, Earth Dams and Reservoirs,
March 2019). A geologic profile of the right auxiliary spillway is in attachment 7.
Eight holes were drilled in the left Auxiliary Spillway for the same reasons in 2014 and 2015.
These holes were also drilled using a Mobil B-57 rig with hollow stem augers and were also
drilled to principal spillway pipe invert elevation. A geologic profile of the left auxiliary spillway
is in attachment 7.
Auxiliary Spillway-Right
Sample No. Depth
250.5
20’-22’
251.2
10’-12.5’
252.1
5’-7’
253.1
5’-7.5’253.2
10’-12.5’
254.1
5’-7.5’254.2
10’-12.5’
255.1
5’-7.5’256.1
5’-7.5’
Auxiliary Spillway-Left
Sample No. Depth
260.1
0’-2.5’260.3
10’-12.5’
260.5
20’-22.5’
260.6
25’-27.5’
261.1
5’-7.5’261.2
10’-12.5’
262.1
5’-7.5’262.2
15’-17.5’
263.2
10’-12.5’
264.1
5’-7.5’

Location~12+31 CL AS-R 157ft L~13+77 CL AS-R 158ft L~15+25 CL AS-R 157ft L~16+76 CL AS-R 160ft L~18+26 CL AS-R 157ft L~19+74 CL AS-R 156ft L~21+28 CL AS-R 158ft L
Location~10+27 CL AS-L 17ft R~11+79 CL AS-L 20ft R~13+29 CL AS-L 18ft R~14+75 CL AS-L 21ft R~16+24 CL AS-L 18ft R

Type of sample
3” Push3” Push3” Push3” Push3” Push3” Push3” Push3” Push3” PushType of sample
3” Push3” Push3” Push3” Push3” Push3” Push3” Push3” Push3” Push3” PushFive additional holes were drilled in the outside cut of the right auxiliary spillway in 2019.
These holes were drilled to investigate material for borrow use as well as to investigate lithology
and slope stability in areas of proposed excavation for auxiliary spillway expansion.
Hole # Sample #270 270.1
270.2
270.3
270.4

Depth
1’-5’
7’-10’
10’-12’
13.5’-15’

Location~11+50CL AS-R 275’ RT

Type
small
small
3” push
SPT

271

270.1
270.2
270.3

2’-5’
5’-8’
8’-9.5’~10+80CL AS-R 295’ RT

small
small
SPT

270.4

13.5’-15’

272.1
272.2
272.3
273.1
273.2
273.3

2’-5’
7’-10’
13.5’-15.5’
2’-5’
10’-13.5’
13.5’-15’~9+70CL AS-R 290’ RT

274.1
274.2

0-5’
10’-11.5’~7+80CL AS-R 245’ RT

272
273

274

SPT

~8+80CL AS-R 265’ RT

small
small
3” push
small
small
SPT
small
SPT

Discussion
borrowborrowslope stability
6+9+8/ slope
stability
borrowborrow
11+22+29/
Slope stability
12+20+25/
Slope stability
borrowborrowslope stability
borrowborrow
14+27+33/
Slope stability
borrow
8+12+13/
Slope stability

Current Dam
Two holes were drilled on the dam at the principal spillway in 2019 to obtain samples for pipe
and inlet support, slope stability and foundation support characteristics. The follow table
outlines these holes, samples taken, and purpose.
Hole # Sample #304 304.1
304.2

305

Depth
5’-7.5’10’-12.5’

Location~20+95CL Dam

Type3” push3” push304.3

15’-17.5’3” push

304.4
304.5
304.6

30’-32.5’
35’-37.5’
42.5’-44’

3” push3” pushSPT

304.7

45’-46.5’

SPT

305.1

5’-7.5’~20+90CL Dam 95’DS

3” pushDiscussion
slope stability
slope stability
Pipe support
slope stability
Pipe support
slope stability
foundation
4+9+12
Foundation
6+19+27
Foundation
Classification

305.2
305.3
305.4

7.5’-10’10’-11.5’
12.5’-15’

3” pushSPT
3” push305.5

20’-21.5’

SPT

305.6

28’-29.5’

SPT

305.7

35’-36.5’

SPT

305.8

45’-46.5’

SPT

305.9

50’-51’

SPT

pipe support
pipe support
pipe,
Impact basin
10+14+19
classification
15+26+35
classification
12+17+25
classification
29+42+46
classification
29+50 for 6”
Classification

Downstream Toe
Eleven holes were drilled on the downstream toe of the dam in 2019. Samples were taken
between 5’ and 10’ to investigate toe drain properties and foundation support material. Further
samples were taken at lower depths primarily for classification purposes. The following table
outlines this information, along with blow counts for SPT intervals.
Hole # Depth
601 5’-7.5’
7.5’-10’20’-21.5’
25’-26.5’
30’-31.5’
35’-36.5’
40’-41.5’
602 5’-7.5’
7.5’-10’20’-21.5’
25’-26.5’
30’-31.5’
35’-36.5’
40’-41.5’
603 5’-7.5’
7.5’-10’20’-21.5
25’-26.5’
30’-31.5’
35’-36.5’
40’-41.5’
604 5’-7.5’
7.5’-10’15’-16.5’
25’-26.5’

Location~34+50CL Dam 93’DS~32+50CL Dam 95’DS~30+50CL Dam 95’DS~28+50CL Dam 95’DS

Type3” Push3” Push
SPT
SPT
SPT
SPT
SPT
3” Push3” Push
SPT
SPT
SPT
SPT
SPT
3” Push3” Push
SPT
SPT
SPT
SPT
SPT
3” Push3” PushSPT
SPTDiscussion
Toe drain/foundation support
Toe drain/foundation support
4+6+11, classification
4+13+20, classification
14+22+27, classification
11+22+30, classification
18+28+34, classificationToe drain/foundation support
Toe drain/foundation support
9+29+33, classification
5+7+12, classification
15+22+30, classification
17+22+25, classification
12+43+50 for 3”, classific.
Toe drain/foundation support
Toe drain/foundation support
7+9+12, classification
4+9+20, classification
15+22+36, classification
19+39+50 for 5.75”, classif.
20+21+50 for 4”, classif.
Toe drain/foundation support
Toe drain/foundation support
6+9+14, classification
22+36+43, classification

605

606

607

608

609
610

611

30’-31.5’
35’-36.5’
40’-41.5’
5’-7.5’7.5’-10’25’-26.5’
30-31.5’
35-36.5’5’-7.5’7.5’-10’25’-26.5’
30’-31.5’
35’-36.5’
40’-41.5’
5’-7.5’7.5’-10’25’-26.5’
30’-31.5’
35’-36.5’
40’-41.5’
5’-7.5’7.5’-10’35’-36.5’
40’-41.5’
5’-7.5’7.5’-10’
40’-40.25’
5’-7.5’10’-12.5’
35’-36.5’
40’-41.5’
7.5’-10’10’-12.5’
40’-41.5’~26+50CL Dam 95’DS~24+50CL Dam 95’DS~22+45CL Dam 91’DS~19+50CL Dam 83’DS~17+58CL Dam 89’DS~15+48CL Dam 90’DS~13+42CL Dam 95’DS

SPT
SPT
SPT
3” Push3” PushSPTSPT
SPT3” Push3” Push
SPT
SPT
SPT
SPT
3” Push3” Push
SPT
SPT
SPT
SPT
3” Push3” PushSPTSPT3” Push3” PushSPT
3” Push3” PushSPTSPT3” Push3” PushSPT

12+30+36, classification
50 for 6”, classification
21+32+41, classificationToe drain/foundation support
Toe drain/foundation support
12+24+50 for 6” , classif.
22+31+40, classification
15+25+50 for 5.5” , classif.
Toe drain/foundation support
Toe drain/foundation support
5+8+12, classification
17+26+27, classification
23+30+50 for 6” , classif.
50 for 6”, classificationToe drain/foundation support
Toe drain/foundation support
12+15+21, classification
30+43+39, classification
12+35+50 for 5” , classif.
29+50 for 6”, classificationToe drain/foundation support
Toe drain/foundation support
18+32+50 for 5”, classif.
50 for 4.5”, classificationToe drain/foundation support
Toe drain/foundation support
50 for 4”, lignite
Toe drain/foundation support
Toe drain/foundation support
17+32+47, classification
50 for 4”, classificationToe drain/foundation support
Toe drain/foundation support
13+21+32, classification

Upstream Toe
Six holes were drilled on the upstream toe in 2019. Samples were taken between 5’ and 10’ toinvestigate for foundation support characteristics. Deeper samples were taken mainly forclassification purposes. The following table outlines this information.
Hole # Depth
701 5’-7.5’
7.5’-10’15’-16.5’
20’-21.5’
25’-26.5’

Location~34+50CL Dam 82’ US

Type
3” Push3” Push
SPT
SPT
SPT

Discussion
foundation support
foundation support
6+10+16, classification
4+8+12, classification
6+9+12, classification

702
703
704
705

706

30’-31.5’
35’-36.5’
5’-7.5’7.5’-10’5’-7.5’7.5’-10’5’-7.5’7.5’-10’20’-21.5’
5’-7.5’7.5’-10’10’-11.5’
20’-21.5’
5’-7.5’7.5’-10’~32+50CL Dam 82’ US~30+50CL Dam 82’ US~28+50CL Dam 82’ US~26+50CL Dam 82’ US~9+45CL Dam 73’ US

SPTSPT
3” Push3” Push3” Push3” Push3” Push3” PushSPT
3” Push3” PushSPTSPT3” Push3” Push13+20+20, classification
15+23+29, classification
foundation support
foundation support
foundation support
foundation support
foundation support
foundation support
17+28+30, classification
foundation support
foundation support
6+9+13, classification
16+23+36, classification
foundation support
foundation support

Penetrometer Readings
Field penetrometer readings were obtained on most push tube samples. Readings were taken at
the base of the sample with units in TSF (tons per square foot). The following table outlines thisinformation.
Hole # Depth
304 7.5’
12.5’
17.5’
32.5’
37.5’
305 7.5’
10’15’
601 7.5’
10’602 7.5’
10’603 7.5
10’604 7.5’
10’605 7.5’
10’606 7.5’
10’607 7.5’
10’608 7.5’
10’

Location~20+95CL Dam~20+90CL Dam 95’DS~34+50CL Dam 93’DS~32+50CL Dam 95’DS~30+50CL Dam 95’DS~28+50CL Dam 95’DS~26+50CL Dam 95’DS~24+50CL Dam 95’DS~22+45CL Dam 91’DS~19+50CL Dam 83’DS

Material
CL
CL
CL
CL
CH
CH
SM
SM
CL
CL
CL
SM
SM
SM
SC
SM
CL
SC
CL
CL
CL
CL
SM
SM

TSF
4.5+
2.25
2.25
3.75
1.5
4.5+
1.5
4.5+
4.5+
4.5+
1.75
4.5+
4.5+
4.5+
2.5
4.0
4.0
4.5+
2.75
4.5
2.75
3.0
1.75
0.75

609
610
611
701
702
703
704
705
706

7.5’
10
7.5’
12.5’
10’
12.5’
7.5’
10’
7.5
10’
7.5’
10’
7.5’
10’
7.5’
10’
7.5’
10’

~17+58CL Dam 89’DS~15+48CL Dam 90’DS~13+42CL Dam 95’DS~34+50CL Dam 82’ US~32+50CL Dam 82’ US~30+50CL Dam 82’ US~28+50CL Dam 82’ US~26+50CL Dam 82’ US~9+45CL Dam 73’ US

SC
SC
CH
CL
CL
CL
CH
CH
CL
CL
CL
CL
SC
SC
SC
SC
CH
CH

1.5
1.0
4.0
4.0
2.5
1.0
2.5
2.25
3.75
2
3
2.75
3
4.5+
2.25
4
0.25
1.25

Water Levels
Water levels in most holes were recorded. The following table outlines water level and cave-in
information that was collected during drilling, immediately after drilling, and at least 24 hours
after drilling (if available). No water was introduced to during the drilling of any hole, and no
precipitation occurred while holes were open.
Hole #
270
271
272
273
274
304
305
601
602
603
604
605
606
607
608
609
610
611
701
702

WL(drilling)
n/a
n/a
n/a
n/a
n/a
36’
9’
n/a
15’
15’
10’
22’
20’
20’
35’
20’
10’
10’
20.5’
15’

WL(0hrs)
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
9.6’
n/a
n/a
4.4’
4.4’
2.9’
n/a
n/a

CI(0hrs)
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
25.2’
n/a
n/a
30.6’
33.4’
37.2’
n/a
n/a

WL(24hrs)
dry
dry
dry
dry
dry
n/a
7.6’
n/a
n/a
n/a
n/a
n/a
8.6’
7.2’
5.4’
4.2’
3.3’
2.7’
n/a
n/a

CI(24hrs)
9.3’
9.0’
9.2’
8.8’
7.0’
n/a
10.2’
n/a
n/a
n/a
n/a
n/a
21.1’
18.2’
27.3’
27.2’
16.2’
31.0’
n/a
n/a

703
704
705
706

15’
12’
n/a
n/a

n/a
n/a
10.6’
n/a

n/a
n/a
14.0’
n/a

n/a
n/a
n/a
n/a

n/a
n/a
n/a
n/a

Borrow
Seven borrow sampling holes were drilled in 2019 on the west side of the permanent pool, as
shown in attachment 2e. The five holes drilled on the outside cut of the right auxiliary spillway
were additionally sampled for borrow material. Samples of each horizon were taken, and then
combined into larger composite samples sent to the Soil Mechanics Laboratory for analysis. All
holes drilled in the borrow area and on the outside cut of the right auxiliary spillway were
backfilled with soil the day they were drilled. The following tables outline samples andestimated yardage of material. Volume is based on an estimate of 90% of an acre-ft being usable(approximately 1350 yds3/acre-ft usable). This conservative cubic yardage is used to account for
waste during construction.
Borrow Area East of Permanent Pool
Hole #
Material
Sample
176
CL
0’-7’

Volume9450 yds3

176

CL

7’-10’

4050 yds3

176

CL

10’-15’6750 yds3

177

CH

0’-3’

4050 yds3

177

CL

3’-8’

6750 yds3

177

CL

8’-15’

9450 yds3

178

CH

0’-2’

2700 yds3

178

CL

3’-5’

2700 yds3

178

CL

5’-11’

8100 yds3

178

SC

11’-15’5400 yds3

Description
clay, stiff, slight coarse gravel, very
Fine grained sand, gravel, slightly
Moist, plastic, reddish brown
Clay, stiff, organic brown- gray,
Moist, plastic
Clay, stiff, very fine grained sand,Light gray, moist to very moist,
Plastic to slightly plastic
Clay, silty, slightly moist, stiff,
Moderately plastic to plastic,
Dark brown
Clay, silty, slight gravel, moist,
Stiff, plastic, light brown
Clay, very stiff, sandy, moist to
Very moist, moderately plastic,
Light grayclay, black, top soil, moist, stiff,
Moderate to high plasticity
Clay, brown, slight CaCO3, very
Fine grained gravel, moist, stiff,
Plastic
Clay, stiff, light brown, moist,
Moderately stiff, plastic
Sand, clayey, very moist, plastic,
Medium density, light brown toLight gray

179

CH

0’-4’

5400 yds3

179

CL

4’-9’

6750 yds3

179

CL

9’-15’

8100 yds3

180

CL

0’-2’

2700 yds3

180

CL

2’-6’

5400 yds3

180CL

6’-10’

5400 yds3

180CH

10’-15’6750 yds3

181

SC

0’-5’

6750 yds3

181

CH

5’-11’

8100 yds3

181

CL

11’-15’5400 yds3

182

CL0’-1.5’2025 yds3

182

CL

1.5’-10’11475 yds3

182

CL

10’-15’6750 yds3

183

CH

0’-2’

2700 yds3

183

CL

2’-7’

6750 yds3

183

SC

7’-10’

4050 yds3

183CH

10’-13’4050 yds3

183

CL

13’-15’2700 yds3

Clay, silty, stiff, slightly calcareous,
Slightly moist to moist, dark brown
Clay, silty, slight gravel, moist, stiff,
Plastic, light brown
Clay, Very fine grained sand, silty,
Very moist, med. Density, light gray
Dark brown, roots, clay, slight sand,Silty, slight CaCO3, slightly moist to
Moist
As above, becomes lighter brown
With depth
Clay, silty, moist, slightly sand,Calcareous, light brown/gray, stiffClay with some very fine grained
Sand and silt, very moist, plastic,
Moderately stiff
alternating thinly bedded very fine to
Fine grained sand and sandy clay,
Slightly moist, loose sand, stiff clay,
Brown to gray-brown
Clay, moist, stiff, brown to light
Gray, plastic
Clay, moist, moderately stiff, light
Gray, very silty, some very fine
Grained sandclay, silty, slightly moist, roots,
Stiff, plastic, dark brown
Clay, silty, calcareous, fine grained
Gravel, moist, moderately stiff,
Light brown, increase in coarsenessOf gravel at 5’-8’
Clay with silty light gray lamination,
Moist to very moist, mod. Stiff,
Light brown to light gray with
Iron staining
Clay, silty, slightly moist to moist,
Roots, dark brown, stiff, plastic
Clay, silty, sandy, calcareous gravel,
Moist, stiff, light brownsand, clayey, moist, slightly muddy,
Plastic, moderately dense, dark grayTo brown
Clay, silty, slight gray silt, slight
Lignite, moist, plastic, light brown
With some light grayclay, very silty, slight very fineGrained sand, moist, mod. Stiff,
Light grayTotal potential borrow, area east of permanent pool:
Outside Cut of Right Auxiliary Spillway
Hole #
Material
Sample
270
CL
0’-1’

Volume1350 yds3

270

CL

1’-7’

8100 yds3

270CL

7’-10’

4050 yds3

271
271

SC
CL

0’-2’
2’-5’

2700 yds3
4050 yds3271

SC

5’-10’

6750 yds3

272

CL

0’-2’

2700 yds3

272

CL

2’-7’

6750 yds3272

SM

7’-10’

4050 yds3

273

SC

0’-2’

2700 yds3

273

CL

2’-9’

9450 yds3

274

SC

0’-7’

9450 yds3

Total potential borrow, outside cut right Aux. spillway:

160,650 yds 3
Description
Clay, fine, brown, slightly moist,
Plastic, stiff, brown to reddish brown
Clay, silty, moist, plastic, stiff,
Reddish brown
Clay, sandy, slightly moist to moist,
Stiff, light brownsand and clay, slightly moist, stiffclay, silty, slightly moist to moist,
Mod. Stiff, light gray/light brown
Sand, silty, slight clay, slightly
Moist, mod. Dense, slightly plastic
Clay and sand, slightly moist, stiff,
Roots, dark brown
Clay, silty, slight very fine grained
Calcareous gravel, slightly moist to
Moist, stiff, light brown
Very fine grained sand, silty, slightly
Moist, mod. Dense, light brown
Sand, slight clay, roots, slightly
Moist, dark brown
Clay, very silty, moist, plastic,
Mod. Dense, light brown
Very fine grained sand, silty,
Slightly moist, very slightly to
Slightly plastic clay, light brown,
Moderately dense
68850 yds 3

Resistivity
An electro-resistivity (ERI) survey was conducted during the investigation, due to a potential
concern for loose sand near the core trench. The ERI survey was conducted by NRCS geologist
Bryan Moffatt (PG) on Sept. 26, 2019. The equipment used as an Advanced Geosciences
Incorporated SuperSting R8 IP, 8 channel Earth Resistivity and IP Meter. One line was done
parallel across the dam, and the other was taken perpendicularly through it. Ten meter electrodespacings were used in each line. The perpendicular line was run through station 26+50 CL dam,
next to hole 605. The results of these lines can be seen in Attachment 4.

Interpretations and Conclusions
Borrow material sampled in the area east of the permanent pool totals an estimated 160,650 yds 3,
and in the outside cut of the right auxiliary spillway totals an estimated 68,850 yds 3. Total
potential borrow material in all sampled areas totals 229,500 yds 3. The Soil Mechanics Reportcontains information regarding the quality and suitability of sampled material. This report
indicated some dispersive soils found in Borrow Grid C, and recommended against using thatarea for borrow. The following borrow quantities are available based on sampled holes:
Total for Grids A and B (15’ sampled in each hole): 99,900 yd 3
Top 5’ of Grids A and B:
33,750 yd3
Top 10’ of Grids A and B:
67,500 yd3

Additionally, on Apr. 21, 2022, two more holes were hand augered to investigate for further
potential borrow areas. Hole 1101 was taken on the left of the pool area upstream of the dam.
This hole contained CH/CL material suitable for borrow. Field testing for dispersion indicatedno observable signs of dispersion in these soils. With this in mind, a minimum of another 6750
yd3(based on the hole being representative of approx. 1 acre, and 90% of 1 acre-ft.=~1350 yd 3)
of borrow material is available in this area, based on the top 5ft. of the sampled hole. Morematerial is expected to be available in this area as well, should it be required. Hole 1102 wastaken approximately 650 ft. directly downstream of the principal spillway. This borehole yielded
predominately sandy soils and was not deemed a good source of borrow material.
Kh was derived from where Kh=Ms= 0.78(UCS in Mpa) 1.09 for UCS< 10MPa, and Ms= UCS for
UCS > 10Mpa. Note: UCS in psf (4.78803E-05) = UCS in MPa (Part 628 Dams, NEH, Chapter
52, 2001, Table 52-3, Material strength number, Ms, for cohesive soil). Also, from ASDSO,
Guidance on the Selection of the Soil Erodibility Index, Kh for the NRCS/ARS Auxiliary
Spillway Erosion Model, Danny K. McCook, 2005, where in soil material: Kh=Ms x Kb x Kd x
Js, the factors Kb, Kd and Js are considered to be equal to 1, therefore Kh = Ms.
Ms values were calculated from laboratory shear test data. The following tables outline the holelocations, depth, type of sample and Kh, for samples from which Kh could be calculated. Where
field notes indicated blocky soils (in left auxiliary spillway), a range was calculated for Kh
values as Kb may be less than 1 in those materials.
Auxiliary Spillway-Right
Sample No. Depth
250.5
20’-22’251.1
0-2.5’
251.2
10’-12.5’
252.1
5’-7’
253.1
5’-7.5’253.2
10’-12.5’
254.1
5’-7.5’254.2
10’-12.5’

Location~12+31 CL AS-R 157ft L~13+77 CL AS-R 158ft L~15+25 CL AS-R 157ft L~16+76 CL AS-R 160ft L~18+26 CL AS-R 157ft L

Kh
0.45
0.19
0.62
0.26
0.22
0.14
0.33
0.26

255.1
256.1

5’-7.5’5’-7.5’

Auxiliary Spillway-Left
Sample No. Depth
260.1
0’-2.5’260.3
10’-12.5’
260.5
20’-22.5’
260.6
25’-27.5’
261.1
5’-7.5’261.2
10’-12.5’
262.1
5’-7.5’262.2
15’-17.5’
263.2
10’-12.5’
264.1
5’-7.5’~19+74 CL AS-R 156ft L~21+28 CL AS-R 158ft L

>1.66
>1.66

Location~10+27 CL AS-L 17ft RKh
0.04-0.07
0.34-0.67
0.26-0.52
0.15-0.29
0.12-0.23
0.38-0.73
0.11-0.22
0.45-0.90
0.39-0.78
0.24-0.48~11+79 CL AS-L 20ft R~13+29 CL AS-L 18ft R~14+75 CL AS-L 21ft R~16+24 CL AS-L 18ft R

In the 2016 soil mechanics report one auxiliary spillway sample tested as dispersive (sample
256.1, taken at a depth of 5-7.5 ft.) and one tested inconclusive (sample 260.2, also at a depth of
5-7.5 ft.). Sample 260.2 was recommended as being treated as dispersive as while the doublehydrometer test did not indicate dispersion, the crumb test did. All other samples tested as nondispersive.
A large rainfall event during the weekend of Oct. 16, 1998 resulted in auxiliary spillway flow forseveral hours. No information additional information regarding the flow was available. No
damage to the spillways was observed following this event. There may have been some small
amount of spillway flow during Aug. 2017, however this could not be confirmed. Since amajority of the soils are non-dispersive and considering that the site has been well maintained,with at least one flow event occurring with no noted damage, the presence of some areas ofdispersive or possibly dispersive soils 5 ft. below the surface does not appear to be negatively
affecting the auxiliary spillways.
The ERI indicates that there is a relatively high resistivity anomaly, approx. 60 ft. below the crestof the dam. This anomaly is being interpreted as a sand bar. The perpendicular transect was run
between holes 605 and 606, closer to 605. The anomaly is expected to be approximately 40 ft.
beneath the surface at these holes, which were drilled to 36.5 ft. and 40.5 ft. respectively.
Presence of water towards the bottom of these holes suggests that the presumed sand bar issaturated, however it is also possible there may be an additional confining layer between thebottom of the holes and the top of the anomaly. Both transects indicate this sand bar is not in
direct contact with the pool.

Michael Jugle, Geologist, NRCS, Temple, Tx

Attachments
1) Seismica. Hazard Curves and Input Data (2 pages).
b. Deaggregation Plot, Statistics and Contributors (3 pages).
c. Historical Earthquakes > Mw 2 and 100 km
d. Liquefaction Plot and Data (12 Pages)
2) Mapsa. Aerial Photo Location Map
b. Topographic Map
c. Geologic Outcrop Map
d. Soils Map and Properties (6 Pages)e. Hole Location Maps (2 Pages)
f. Resistivity Transect Map
3) Sample Logs
a. Auxiliary Spillway, Rightb. Auxiliary Spillway, Leftc. Principal Spillway
d. Outside Cut, Right Auxiliary Spillway
e. Upstream Toe
f. Downstream Toe
g. Borrow4) Resistivity Plotsa. Resistivity Line Parallel to CL Dam (2 Pages)
b. Resistivity Line Perpendicular to CL Dam
5) Sample Lists (7 Pages)6) Photos
7) Geologic Profiles

Attachment 2a
Aerial Location Map

Attachment 2b
Topographic Map

Attachment 2c
Geologic Outcrop Map

Attachment 2d, page 1
Soils Map and Properties

Attachment 2d, page 2
Soils Map and Properties

Attachment 2d, page 3
Soils Map and Properties

Attachment 2d, page 4
Soils Map and Properties

Attachment 2d, page 5
Soils Map and Properties

Attachment 2d, page 6
Soils Map and Properties

Attachment 2e
2014/2019 Hole Locations Map

Attachment 2e
1961 Hole Locations Map

Attachment 2f
Resistivity Transect Map

Attachment 2a
Aerial Location Map

Attachment 2b
Topographic Map

Attachment 2c
Geologic Outcrop Map

Attachment 2d, page 1
Soils Map and Properties

Attachment 2d, page 2
Soils Map and Properties

Attachment 2d, page 3
Soils Map and Properties

Attachment 2d, page 4
Soils Map and Properties

Attachment 2d, page 5
Soils Map and Properties

Attachment 2d, page 6
Soils Map and Properties

Attachment 2e
2014/2019 Hole Locations Map

Attachment 2e
1961 Hole Locations Map

Attachment 3a
Auxiliary Spillway Right, hole 250

Attachment 3a
Auxiliary Spillway Right, hole 251

Attachment 3a
Auxiliary Spillway Right, hole 252

Attachment 3a
Auxiliary Spillway Right, hole 253

Attachment 3a
Auxiliary Spillway Right, hole 254

Attachment 3a
Auxiliary Spillway Right, hole 255

Attachment 3a
Auxiliary Spillway Right, hole 256

Attachment 3a
Auxiliary Spillway Right, hole 257

Attachment 3a
Auxiliary Spillway Right, hole 258

Attachment 3a
Auxiliary Spillway Right, hole 259

Attachment 3a
Auxiliary Spillway Right, hole 259a

Attachment 3b
Auxiliary Spillway Left, hole 260

Attachment 3b
Auxiliary Spillway Left, hole 261

Attachment 3b
Auxiliary Spillway Left, hole 262

Attachment 3b
Auxiliary Spillway Left, hole 263

Attachment 3b
Auxiliary Spillway Left, hole 264

Attachment 3b
Auxiliary Spillway Left, hole 265

Attachment 3b
Auxiliary Spillway Left, hole 266

Attachment 3b
Auxiliary Spillway Left, hole 267

Attachment 3c
Principal Spillway, hole 304

Attachment 3c
Principal Spillway, hole 305

Attachment 3c
Principal Spillway, hole 305

Attachment 3d
Outside Cut, Auxiliary Spillway, hole 270

Attachment 3d
Outside Cut, Auxiliary Spillway, hole 271

Attachment 3d
Outside Cut, Auxiliary Spillway, hole 272

Attachment 3d
Outside Cut, Auxiliary Spillway, hole 273

Attachment 3d
Outside Cut, Auxiliary Spillway, hole 274

Attachment 3e
Upstream Toe, hole 701

Attachment 3e
Upstream Toe, hole 702

Attachment 3e
Upstream Toe, hole 703

Attachment 3e
Upstream Toe, hole 704

Attachment 3e
Upstream Toe, hole 705

Attachment 3e
Upstream Toe, hole 706

Attachment 3f
Downstream Toe, hole 601

Attachment 3f
Downstream Toe, hole 602

Attachment 3f
Downstream Toe, hole 603

Attachment 3f
Downstream Toe, hole 604

Attachment 3f
Downstream Toe, hole 605

Attachment 3f
Downstream Toe, hole 606

Attachment 3f
Downstream Toe, hole 607

Attachment 3f
Downstream Toe, hole 607

Attachment 3f
Downstream Toe, hole 608

Attachment 3f
Downstream Toe, hole 609

Attachment 3f
Downstream Toe, hole 610

Attachment 3f
Downstream Toe, hole 611

Attachment 3g
Borrow, hole 176

Attachment 3g
Borrow, hole 177

Attachment 3g
Borrow, hole 178

Attachment 3g
Borrow, hole 179

Attachment 3g
Borrow, hole 180

Attachment 3g
Borrow, hole 181

Attachment 3g
Borrow, hole 182

Attachment 3g
Borrow, hole 183

Attachment 3g
Borrow, hole 1101

Attachment 3g
Borrow, hole 1102

Attachment 4a
Resistivity Survey Parallel to CL Dam (Northwest Half)Attachment 4a
Resistivity Survey Parallel to CL Dam (Southeast Half)Attachment 4b
Resistivity Survey Perpendicular to CL Dam

Attachment 5
Sample List (Page 1)

Attachment 5
Sample List (Page 2)

Attachment 5
Sample List (Page 3)

Attachment 5
Sample List (Page 4)

Attachment 5
Sample List (Page 5)

Attachment 5
Sample List (Page 6)

Attachment 5
Sample List (Page 7)

Attachment 7
Geologic Profiles

Attachment 7
Geologic Profiles

Attachment 7
Geologic Profiles

Attachment 7
Geologic Profiles, Left Auxiliary Spillway

Attachment 7
Geologic Profiles, Right Auxiliary Spillway