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

April 28, 2014
Subject:Geology Report, Plum Creek Site 6, Hays County, Texas

To:John Mueller, State Conservation Engineer, Temple, Texas

During the month of August 2010, William York, Geologist, Fort Worth, Texas and DanielWilkinson, Physical Science Technician, Fort Worth, Texas performed a Geological
Investigation of Plum Creek Site 6, Hays County, Texas. Additional work was performed byBryan S. Moffatt, Geologist and Daniel Wilkinson August and September 2010. The purpose ofthe investigation was to acquire data for rehabilitation of the site. The following report is areconstituted summary of the investigation due to the departure of William York from theAgency.
General Data
Watershed:
Sub-watershed:
County:State:
Location:
Structure Class:Type of Equipment:
Investigated by:

Guadalupe River
Plum Creek (Porter Creek)
Hays
Texas
3 miles E and 1 mi. N of Kyle, Texas
N 30.002337 W -97.822603
High Hazard
Simco Power Auger
William York, Bryan Moffatt, Daniel Wilkinson
Site Data

Drainage Area:
Type of Structure:
Purpose:Direction of Valley:
Sediment Storage Acre/Ft:
Floodwater Storage Acre/Ft:
Volume of Fill:
Height of Dam:
Length of Dam:
Top of Dam (effective)
Upstream Embankment Slope:
Downstream Embankment Slope:
Principal Spillway Crest:

5,376 acres (as-built 03/16/1966)
EarthenFlood Retarding
North – South896 (as-built 03/16/1966)
2,867 (as-built 03/16/1966)
203,540 yds3
40ft (as-built 03/16/1966)
2,923ft (Sta. 10+17 CL Dam to Sta. 39+40 CL Dam
642.5ft (as-built 03/16/1966)
2.5H to 1V
2.5H to 1V
638.5ft (as-built 03/16/1966)

Pipe Invert:
Auxiliary Spillway Location:
Width Auxiliary Spillway:
Auxiliary Spillway Crest:
Auxiliary Spillway Capacity, ft 3 /sec:
PP

607.55ft Sta.3+75 PS (as-built 03/16/1966)
Left Abutment
350ft
638.5ft6257 (as-built 03/16/1966)

Surface Geology and Physiography
Physiographic Description:
Mineral Land Unit:
Topography:
Physical Properties:

Attitude of Beds (Regional):

Blackland Prairie
C-1 Expansive Clay
Flat to Gently Rolling Prairie
Low Slope Stability, Low Foundation Strength,
Highly Plastic and Expansive Clays, HighCorrosion Potential
Strike: Approximately N 24° EDip: 0.5° N 114° EGeneral Geology

Two horizons are present within the Plum Creek Site 6 location. The horizons consist of Upper
Cretaceous Pecan gap (Kpg) and Recent Quaternary Alluvium (Qal).
Pecan GapUThe claystone of the Kpg occurs at depth throughout the site. It outcrops on both abutments andthe outside cut and floor of the Auxiliary Spillway. The Kpg is generally described as very softrock (hard soil-like material on outcrop), silty, thinly bedded to laminated claystone. Thematerial is light brown mottled gray when weathered and dark gray when slight to nonweathered. Iron stained and gypsum filled fractures and a few carbonate concretions are present.
Quaternary AlluviumU

Qal material outcrops in the Plunge Basin, along portions of the Downstream Toe of the Damand the Exit Slope of the Auxiliary Spillway. The material is generally composed of two broadhorizons. The upper horizon is generally described as silty, stiff, highly plastic brown clay, CH.
The lower horizon is generally described as stiff, highly plastic, silty clay with some fine tocoarse grain sand and gravel CH. The lower horizon by Gamma Log analyses and drillingcharacteristics indicates some thin streaks of GC to GM material may be present.
SeismicU

Probabilistic ground motion values as measured by %g for this site indicates low seismicity(Note: the term “low” is used as a descriptive term and not a classification term). The attached
Texas Seismic Hazard Map and Deaggregation plots (USGS) indicate the main seismic
contribution to the site is from magnitude 4 to 6 earthquakes within a 200 km radius of the site.
Additional contributions are from 5.5 to 7.0 magnitude earthquakes in a 500 km radius (Meers2Fault north of Lawton OK and Rio Grande Rift Zone Trans-Pecos Texas Northern Mexico) and7.5 to 8.0 magnitude earthquakes 900 plus km away. Peak Horizontal Ground Acceleration at
this site is 0.03177, 0.04956 and 0.07580 g with a Mean Return Time of 2475, 4950 and 9950
years with an Annual Exceedance Rate of 0.405 x 10 -3 , 0.202 x 10 -3 and 0.101 x 10 -3
respectively (Values from USGS Earthquake Hazard Custom Mapping and Analysis Tools,
http://earthquake.usgs.gov/hazards/apps/#deaggint . Mw verses distance liquefaction plotsverses the Papadopoulos and Lefkopoulos e – e’ curve indicate “low” potential for occurrence ofliquefaction. Further, as outlined in FEMA 65, Section IV Part 3.C.a:
P

31TU

PP

PP

P

U31T

••
•
•

Dam and foundation materials are not subject to liquefaction and do not include loose
soils or sensitive clays.
Embankment was built with a minimum dry density, percent of field density test,
maximum dry density of 85
Phreatic Line is “well” below the slope of the embankment (projected from water levels
Hole 301, 802 and lake level at time of drilling.
Maximum peak ground acceleration at the base of the embankment is less than 0.2g (site0.07580g, Exceedance Rate 0.101x10 -3 , Mean Return Time 9,900 years).
Freeboard of the embankment is greater than 3% to 5% embankment height and greaterthan 3 feet. Embankment height 40 feet, freeboard TOD 643.4 – PS Crest 626.4 = 17 feetor 42.5%. (Data from as-built 03/18/1966)
P•

PGeneral
BackgroundUConstruction of Plum Creek 6 was completed on 03/01/1967 (as-built 03/18/1966). The structure
is located on Porter Creek. Porter Creek flows into the Bunton Branch 2.6 miles south southeastof the site. Bunton Branch flows into Plum Creek 0.48 miles further southeast.
The structure was built as a “low” hazard classification and has been reclassified to “high”.
County Road 157, Goforth Road, is parallel and downstream of the dam’s alignment, crosses
between the Principal Spillway pipe and toe of the structure and crosses the exit slope of theAuxiliary Spillway at an angle. Several small businesses are on the southwest side of GoforthRoad downstream of the dam.
There are several homes built along the downstream toe of the dam beginning at approximately
Sta. 26+00 CL Dam 70’ DS and continuing to the right abutment. A quadplex is locateddownstream near Sta. 31+50 CL Dam. The septic system for this dwelling is at or on the toe ofthe dam beginning at approximately Sta. 29+15 CL Dam 63’ DS and ending at approximately
Sta. 30+30 CL Dam 63’ DS. Several homes and outbuildings are built northwest of thepermanent pool area. Some of these structures appear by visual observation to be built within theflood pool.
Centerline of the Dam

UNo holes were drilled along the Centerline of the Dam during the 2010 investigation.
3Auxiliary Spillway

USix holes were drilled along the inside cut of the Auxiliary Spillway for erodibility. Nine
samples were taken within the Auxiliary Spillway. A list of the samples taken is outlined in thefollowing table:
Hole Depth

U

201
201
201
202
202
202
202
202
203
203

Type

0’ – 2’3” Push
10’ – 11’ 3” Push
30’ – 35’ Small
5’ – 7’3” Push
10’ – 11’ 3” Push
15’ – 16’ 3” Push
19’ – 20’ Small
20’ – 20.5’ 3” Push
17’ – 19’ Small
24’ – 25’ Small

Description Remarks___________________________________CH Topsoil and CS, hi/wea, breaks down to hi/pl clay CHCS, fe stain and gyp, hi/wea, breaks down to hi/pl clay CH
CS gyp filled frac, sl-non/wea, breaks down to hi/pl clay CH
CS gyp filled frac, hi/wea, breaks down to hi/pl clay CH
CS gyp, frac, hi/wea, breaks down to hi/pl clay CH, refusal 11’CS gyp, frac, hi/wea, breaks down to hi/pl clay CH, refusal 16’
CS gyp filled frac, hi/wea, breaks down to hi/pl clay CH, fe stain
CS gyp, frac, wea, breaks down to hi/pl clay CH, refusal 20.5’
CS gyp filled frac, hi/wea, breaks down to hi/pl clay CH, fe stain
CS gyp filled frac, hi/wea, breaks down to hi/pl clay CH, fe stain

A Labyrinth Weir over the crest of the dam is planned to replace the Auxiliary Spillway. Thisdesign change requires plugging of the existing Auxiliary Spillway in favor of a structural
spillway located on the embankment negates the erodibility assessment normally provided for.
Preliminary material characterization indicated very weak to weak material existed to a depth ofapproximately 10 feet in the control section with the material then becoming stronger with depth.
It also appears all the material may be extremely sensitive to water and may be more erosive thanmight be expected of a soil of this type. Any assessment would have required sensitivity
analyses utilizing SITES with results yielding a high probability that engineered structural
measures may be necessary to control erosion.
Principal Spillway

U

Two holes were drilled to investigate design for a new Principal Spillway pipe and Impact Basin.
A list of the samples taken is outlined in the following table:

Hole Depth __Type

Description Remarks___________________________________

301
802

Clay, silty, sl/fine grain gravel, hi/pl, sat., CH
Clay, silty, sl/fine grain gravel, hi/pl, sat., CHU

14’ – 16’ 5” Push
7’ – 8.5’ 3” Push

The existing Principal Spillway pipe is to be filled with grout and replaced with a new 42”
diameter pipe and a new intake tower constructed. In Hole 301 and 802 on the Downstream Toe
there is 15ft to 16ft of CH, slight gravel alluvial material on top of weathered claystone. The
alluvial material was removed to weathered claystone and replaced with compacted fill at thecore trench under the current spillway. Under the slopes of the dam, the pipe is placed within CH
alluvial material and compacted fill. The procedure to remove and replace material to claystone4

at the centerline of the dam and thru alluvial CH under the slopes of the dam provided anadequate foundation with desired support for the existing pipe.
Toe Drain

U11 holes were drilled to investigate placement of a Toe Drain. All the holes were drilled to nonweathered claystone to project core trench material removed during original construction andinvestigate potential water movement thru fractures. Recovery tests from holes 801 and 803
indicate permeability between 0.001799 to 0.004126 ft/day. Water levels ranged from 1.5ft (hole802) to 9.1ft (hole 807) on 08/13/10. Holes 808 – 811 encountered no water during drilling
operations in August 2010.
A profile of the downstream toe was constructed. The profile indicates that there is a secondary
buried alluvial channel north and west of 807. No water was observed in the holes drilled
through this buried channel.
No holes were drilled north and west of Hole 811 because of septic systems and development
downstream of the dams toe. There is a septic leach field and tanks known to exist approximately
90ft northwest of Hole 811. It is unknown if the home between Sta.27+00 and 28+00 CL Dam ison a septic system.
Conclusions and Recommendations
DamU

The as-built diagrams indicate zonation construction within the dam. Claystone material from
Auxiliary Spillway was used in construction of the dam. It is suggested that 5” Undisturbed
samples of foundation material be taken within the footprint of the Labyrinth Weir. It is alsosuggested that SPT be performed through the fill material and into the claystone material under
the dam to assure material integrity.
Principal Spillway

U

It is suggested that 5” undisturbed samples be taken at the foundation level of the Impact
Structure / pipe support. An additional hole is suggested on the centerline of the dam for pipe
foundation support information.
Auxiliary Spillway

UThe dam is to be extended filling in the Auxiliary Spillway. It is suggested to drill 2 investigation
borings along the dam extension in the spillway to identify depth of desiccation. Hole depth isestimated to be 10ft
Toe Drain

UNo additional holes are recommended for Toe Drain investigation. Placement of the toe Drain
5should take into account septic systems in the area to prevent a direct path for effluent to enter
the creek. No water was observed past approximately Sta. 22+65. It is suggested that the Toe
Drain not be installed northwest of this approximate location. Abundant iron staining wasobserved in drill cuttings. It is suggested that the upstream end of the drain be day lighted with acleanout port.
Borrow

UNo borrow work was performed in 2010. It is suggested that a borrow area be designated and aminimum of 2 times the cubic yards needed for construction be confirmed and sampled.

Bryan S. Moffatt, PG 2887
State Geologist
USDA/NRCS
Fort Worth, Texas

6

Attachments
Seismic1a)
1b)
1c)
1d)
1e)
1f)
1g)

Earthquake Hazard map
Probability of Exceedance, Magnitude 5.0, 50 years
Probability of Exceedance, Magnitude 5.0, 5,000 years
Probability of Exceedance, Magnitude 5.0, 10,000 years
Deaggregation Plot 2% Probability of Exceedance in 50 Years
Deaggregation Plot 2% Probability of Exceedance in 100 Years
Deaggregation Plot 2% Probability of Exceedance in 200 Years

Sample Logs
2a)
2b)
2c)

Auxiliary Spillway
Principal Spillway
Toe Drain

3a)
3b)
3c)

Geologic Outcrop map
Aerial Photo
Hole Location Plat

MapsProfiles
4a)4b)Auxiliary Spillway
Downstream Toe

Sample Data5a)5b)5c)Sample List
Index Tests
Shear Tests Page 1 and 2

Rising Head Tests
6a)
6b)
6c)
6d)
6e)

Hole 801
Hole 803
Hole 804
Hole 805
Hole 806

7

Attachment 1a
Earthquake Hazard Map8

Attachment 1b
Probability of Exceedance, Magnitude 5.0, 50 years

Attachment 1c
Probability of Exceedance, Magnitude 5.0, 5,000 years

9Attachment 1d
Probability of Exceedance, Magnitude 5.0, 10,000 years

10Attachment 1ePlum Creek 6 Deaggregation Plot
2% Probability of Exceedance in 50 Years

11

Attachment 1f
Plum Creek 6 Deaggregation Plot
2% Probability of Exceedance in 100 Years

12

Attachment 1g
Plum Creek 6 Deaggregation Plot
2% Probability of Exceedance in 200 Years

Attachments 1a-1f from http://earthquake.usgs.gov/hazards/apps/#deaggint Pa padopoulos and Lefkopoulos
data from Magnitude-Distance Relations for Liquefaction in Soil from Earthquakes, Bulletin of theSeismological Society of America, Vol. 83, June 1993
31TUU

31T13Attachment 2aHole 201

14

Attachment 2a
Hole 202

15

Attachment 2a
Hole 203

16

Attachment 2a
Hole 20417

Attachment 2a
Hole 205

18

Attachment 2a
Hole 206

19

Attachment 2b
Hole 301

20

Attachment 2c
Hole 801

21

Attachment 2c
Hole 80222

Attachment 2c
Hole 803

23

Attachment 2c
Hole 80424

Attachment 2c
Hole 805

25

Attachment 2c
Hole 806

26

Attachment 2c
Hole 807

27

Attachment 2c
Hole 808

28

Attachment 2c
Hole 809

29

Attachment 2c
Hole 810

30

Attachment 2c
Hole 811

31

Attachment 3aGeologic Outcrop map32Attachment 3b
Aerial Photo33

Attachment 3c
Hole Location Map34

Attachment 4a
Auxiliary Spillway Profile

35

Attachment 4b
Downstream Toe Profile

36

Attachment 5a
Sample List37

Attachment 5b
Index Tests

38

Attachment 5c
Shear Tests Page 1

39

Attachment 5c
Shear Tests Page 240

Attachment 6a
Hole 801

Field Permeability Testing by Rising Head Test
Hole 801
Definition Sketch
D2

D1
Well Dimensions

H2
H1

Radius of boring, inches
Length of Test Section, feet2
29.6

Depth to Water Table

3.5
LReadings

Depth 1
Depth 2
Time, minutes

2511.4863

H1H221.5
7.9

Depth 1
Depth 2Time, minutes

000

H1
H200

Depth 1
Depth 2Time, minutes

000H1
H200K1, ft/day

0.00406

41

Attachment 6b
Hole 803

Field Permeability Testing by Rising Head Test
Hole 803
Definition Sketch
D2

D1
Well Dimensions

H2
H1

Radius of boring, inches
Length of Test Section, feet

243.8

Depth to Water Table

4.9
LReadings

Depth 1
Depth 2Time, minutes

43.9
38.9200

H1H23934

Depth 1
Depth 2Time, minutes

000H1H218.5
13

Depth 1
Depth 2
Time, minutes

000H1
H200K1, ft/day

0.001745

42Attachment 6c
Hole 804

Field Permeability Testing by Rising Head Test
Hole 804
Definition Sketch
D2

D1Well Dimensions

H2
H1

Radius of boring, inches
Length of Test Section, feet2
22.5

Depth to Water Table

5.5
LReadings

Depth 1
Depth 2
Time, minutes

2824320

H1
H2

22.5
18.5

Depth 1
Depth 2Time, minutes

2418.5884

H1
H218.513

Depth 1
Depth 2Time, minutes

000H1
H200K1, ft/day
K2, ft.day

0.002667
0.0017443

Attachment 6d
Hole 805

Field Permeability Testing by Rising Head Test
Hole 805
Definition SketchD2D1
Well Dimensions

H2H1

Radius of boring, inches
Length of Test Section, feet2
29.5

Depth to Water Table

6.9
LReadings

Depth 1
Depth 2Time, minutes

2924.3857

H1
H222.1
17.4

Depth 1
Depth 2Time, minutes

000

H1
H200

Depth 1
Depth 2Time, minutes

000

H1
H200K1, ft/day

0.000979

44

Attachment 6e
Hole 806

Field Permeability Testing by Rising Head Test
Hole 806
Definition Sketch
D2D1
Well Dimensions

H2H1

Radius of boring, inches
Length of Test Section, feet

225.6

Depth to Water Table

5.6
LReadings

Depth 1
Depth 2Time, minutes

2918877

H1
H2

23.4
12.4

Depth 1
Depth 2Time, minutes

000

H1
H200

Depth 1
Depth 2Time, minutes

000

H1
H200K1, ft/day

0.00284845