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Lake Houston Dam Lake Houston Dam Comprehensive Evaluation of Comprehensive Evaluation of
an Ambursen Daman Ambursen Dam
ASDSOASDSOSeptember 10, 2008September 10, 2008
John Rutledge John Rutledge --
Freese & Nichols, Inc.Freese & Nichols, Inc.Chuck Easton Chuck Easton --
Freese & Nichols, Inc.Freese & Nichols, Inc.Janis Murphy Janis Murphy --
Freese & Nichols, Inc.Freese & Nichols, Inc.Bob Ringholz Bob Ringholz ––
Fugro ConsultantsFugro ConsultantsRon WatersRon Waters
Lake Houston DamLake Houston Dam
Designed by Ambursen, completed in 1954Designed by Ambursen, completed in 1954Flat slab and buttress spillwayFlat slab and buttress spillwayMaximum height 66 feetMaximum height 66 feet12,10012,100’’ longlongUncontrolled spillway of 3,077Uncontrolled spillway of 3,077’’Four gates Four gates –– two radial ( 18 x 20) and two flashboards (18 x two radial ( 18 x 20) and two flashboards (18 x 5)5)Two earthen embankmentsTwo earthen embankments
San Jacinto River northeast of Houston, TexasSan Jacinto River northeast of Houston, TexasOwned By City of Houston, operated by Coastal Water Owned By City of Houston, operated by Coastal Water Authority (CWA)Authority (CWA)
Typical Buttress SectionTypical Buttress Section
158 Spillway buttresses158 Spillway buttresses2020’’ on centerson centers3 rows of sheetpiles3 rows of sheetpiles
Comprehensive EvaluationComprehensive Evaluation
Review of DocumentsReview of DocumentsSite Inspection Site Inspection Update of Probable Maximum Flood Update of Probable Maximum Flood Alignment Survey Alignment Survey Identification of Dam Safety IssuesIdentification of Dam Safety IssuesAnalysis/Resolution of Dam Safety IssuesAnalysis/Resolution of Dam Safety Issues
Update of PMF Update of PMF
Update was performed to include:Update was performed to include:New Texas Design Storm GuidelinesNew Texas Design Storm GuidelinesUpdate methodology to new hydraulic unsteady flow Update methodology to new hydraulic unsteady flow model (HECmodel (HEC--RAS) to improve accuracyRAS) to improve accuracy
CalibrationCalibration
PMF ResultsPMF Results
Elev. 55.3 vs. 57.9 in previous reportElev. 55.3 vs. 57.9 in previous reportOf 2.6Of 2.6’’ difference, 0.5difference, 0.5’’ due to new TCEQ Guidelinesdue to new TCEQ GuidelinesOf 2.6Of 2.6’’ difference, 2.1difference, 2.1’’ due to unsteady HECdue to unsteady HEC--RASRAS
Peak Discharge of 578,500 cfs vs. 700,000 cfsPeak Discharge of 578,500 cfs vs. 700,000 cfsCompares to 50.8 in 1994Compares to 50.8 in 1994
400 yr event400 yr event> 50% of the PMF> 50% of the PMF
Issue IdentificationIssue Identification
32 potential dam safety issues32 potential dam safety issuesIdentified in document research and site investigationIdentified in document research and site investigation
13 routine maintenance items13 routine maintenance itemsReviewed and analyzed each issueReviewed and analyzed each issue
Develop a resolution, orDevelop a resolution, orDevelop a plan to resolve in future phasesDevelop a plan to resolve in future phases
32 Dam Safety Issues32 Dam Safety Issues
2 Issues resolved with no action needed2 Issues resolved with no action needed13 Issues and 13 Maintenance Items Resolved 13 Issues and 13 Maintenance Items Resolved with remedial action needed by CWA:with remedial action needed by CWA:17 Issues addressed in second phase17 Issues addressed in second phase
Issue ResolutionIssue Resolution Remaining 17 IssuesRemaining 17 Issues
One Primary IssueOne Primary IssueSliding Stability of Spillway StructuresSliding Stability of Spillway Structures
Nine Secondary IssuesNine Secondary IssuesDependent on results of Stability AnalysisDependent on results of Stability Analysis
Seven Other Issues Seven Other Issues Unrelated to StabilityUnrelated to StabilityAttempted to resolve in second phaseAttempted to resolve in second phase
Sliding StabilitySliding StabilityNo visual evidence of stability concerns. However:No visual evidence of stability concerns. However:
History of Ambursen DamsHistory of Ambursen DamsDid not know actual uplift forces acting on hearth slabDid not know actual uplift forces acting on hearth slab
No existing instrumentationNo existing instrumentationDesign was based on undrained shear strengthsDesign was based on undrained shear strengths
Would likely show tendency for reduced strength over timeWould likely show tendency for reduced strength over timeCurrent practice would be to use drained strengthsCurrent practice would be to use drained strengths
Drained shear strengths data available from previous Drained shear strengths data available from previous analyses was limitedanalyses was limitedDid not have soil strength tests from samples directly Did not have soil strength tests from samples directly under slabsunder slabs
Sliding StabilitySliding Stability
Factors of Safety from Previous EstimatesFactors of Safety from Previous EstimatesOriginal Ambursen Design (1951) Original Ambursen Design (1951) –– 1.481.481994 Study 1994 Study –– 1.90 Used original design strength 1.90 Used original design strength parameters.parameters.2001 2001 –– 1.60 (1.35 when duplicated)1.60 (1.35 when duplicated)
Recommended ValuesRecommended Values2.0 for new construction2.0 for new constructionProbably 1.5 for existing structure with good Probably 1.5 for existing structure with good knowledge of foundationknowledge of foundation
Field ExplorationField Exploration
15 Borings and 20 piezometers in spillway area15 Borings and 20 piezometers in spillway area5 borings, 10 piezometers in hearth5 borings, 10 piezometers in hearth
““shallowshallow”” and and ““deepdeep”” piezometerspiezometers
6 borings and piezometers under spillway6 borings and piezometers under spillway4 borings and piezometers downstream from gates4 borings and piezometers downstream from gates
Samples taken from boringsSamples taken from boringsPiezometric data tracked from August 2007Piezometric data tracked from August 2007
Plan of Borings
Piezometric Data Piezometric Data ––
Buttress 66Buttress 66
0
5
10
15
20
25
30
35
40
45
50
1-A
ug-0
7
8-A
ug-0
7
15-A
ug-0
7
22-A
ug-0
7
29-A
ug-0
7
5-Se
p-07
12-S
ep-0
7
19-S
ep-0
7
26-S
ep-0
7
3-O
ct-0
7
10-O
ct-0
7
17-O
ct-0
7
24-O
ct-0
7
31-O
ct-0
7
7-N
ov-0
7
14-N
ov-0
7
21-N
ov-0
7
28-N
ov-0
7
5-D
ec-0
7
12-D
ec-0
7
19-D
ec-0
7
Wat
er E
leva
tion
(feet
msl )
-10
-8
-6
-4
-2
0
2
4
Rai
nfal
l (in
ches
)
Precipitation Lake Level A (-18.75 to -23.75) B Shallow (9.5 to 1.5) B Deep (-14.5 to -24.5)
2
0
4
Piezometric Data Piezometric Data ––
Buttress 36Buttress 36 Nov. 18 Nov. 18 ––
Dec. 20, 2007Dec. 20, 2007
10
15
20
25
30
18-N
ov-0
7
19-N
ov-0
7
20-N
ov-0
7
21-N
ov-0
7
22-N
ov-0
7
23-N
ov-0
7
24-N
ov-0
7
25-N
ov-0
7
26-N
ov-0
7
27-N
ov-0
7
28-N
ov-0
7
29-N
ov-0
7
30-N
ov-0
7
01-D
ec-0
7
02-D
ec-0
7
03-D
ec-0
7
04-D
ec-0
7
05-D
ec-0
7
06-D
ec-0
7
07-D
ec-0
7
08-D
ec-0
7
09-D
ec-0
7
10-D
ec-0
7
11-D
ec-0
7
12-D
ec-0
7
13-D
ec-0
7
14-D
ec-0
7
15-D
ec-0
7
16-D
ec-0
7
17-D
ec-0
7
18-D
ec-0
7
19-D
ec-0
7
20-D
ec-0
7
21-D
ec-0
7
22-D
ec-0
7
23-D
ec-0
7
24-D
ec-0
7
Adj
uste
d T
ailw
ater
Ele
vatio
n (fe
et m
s
38.038.338.538.839.039.339.539.840.040.340.540.841.041.341.541.842.042.342.542.843.043.343.543.844.044.3
Lak
e L
evel
(fee
t-msl)
Tailwater in Hearth Uplift Lake Level
Stability AnalysisStability Analysis
158 Spillway buttresses158 Spillway buttresses2020’’ on centerson centers3 rows of sheetpiles3 rows of sheetpiles
Typical Buttress Section:Typical Buttress Section:
Normal Pool Stability AssumptionsNormal Pool Stability Assumptions
ConsultantConsultant CohesionCohesion Friction Friction AngleAngle
Ambursen (1951)Ambursen (1951) 1370 psf1370 psf 0 deg0 deg
19941994 1370 psf1370 psf 0 deg0 deg
20012001 670 psf670 psf 22 deg22 deg
Freese and Freese and Nichols / FugroNichols / Fugro
400 psf400 psf 18 deg18 deg
Normal Pool Stability ResultsNormal Pool Stability ResultsAnalysisAnalysis Headwater/Headwater/
TailwaterTailwaterUpliftUplift(msl)(msl)
Factor of Factor of SafetySafety
Ambursen (1951)Ambursen (1951) 42.5/10.642.5/10.6 2525 1.481.48
19941994 42.5/10.642.5/10.6 2525 1.901.90
20012001 42.5/10.642.5/10.6 2525 1.351.35
Freese and Nichols / Freese and Nichols / FugroFugro
42.5/10.642.5/10.642.5/10.642.5/10.642.5/42.5/--0.60.6
141420201414
1.261.261.031.030.930.93
Maximum Uplift – To be tested whether feasibleDrained Hearth Conditions – Not recommended
Needed ImprovementsNeeded Improvements
Increase sliding resistance of both spillways to:Increase sliding resistance of both spillways to:1.50 for normal conditions1.50 for normal conditions1.25 for drained hearth conditions1.25 for drained hearth conditions
Improve drainage and pressure relief systemsImprove drainage and pressure relief systemsAddress excessive uplift under slab downstream Address excessive uplift under slab downstream from gated spillwayfrom gated spillwayFocus of next phase should be to focus on tailwater Focus of next phase should be to focus on tailwater and uplift relationship to confirm or reduce the and uplift relationship to confirm or reduce the maximum uplift assumption.maximum uplift assumption.
Potential AlternativesPotential Alternatives
Uncontrolled Spillway:Uncontrolled Spillway: (Gated similar)(Gated similar)Anchors not viableAnchors not viableConcrete OverlayConcrete Overlay
22’’ in spillway, 5in spillway, 5’’ on hearthon hearth
Improved Drainage Improved Drainage –– Reduce Uplift PressuresReduce Uplift PressuresHave to keep uplift below elevation 9.0Have to keep uplift below elevation 9.0
Permanently Increase TailwaterPermanently Increase TailwaterIncrease to elevation 19.1Increase to elevation 19.1Will not help with drained conditionsWill not help with drained conditions
Potential AlternativesPotential Alternatives
Combination Example: Combination Example: (Gated similar)(Gated similar)Concrete Overlay Concrete Overlay -- 11’’ in spillway, 2in spillway, 2’’ on hearthon hearthImproved Drainage Improved Drainage –– Reduce max. uplift to elev. 15.5Reduce max. uplift to elev. 15.5Permanently Increase Tailwater to elev. 14.0 Permanently Increase Tailwater to elev. 14.0 Will have Factor of Safety of 1.5Will have Factor of Safety of 1.5Reduce uplift to 9.5 to keep Factor of Safety to 1.25 for a fullReduce uplift to 9.5 to keep Factor of Safety to 1.25 for a fully y drained hearth.drained hearth.Combination not optimized Combination not optimized –– to be done as part of next phase to be done as part of next phase $8 to $12 Million$8 to $12 Million
Final Design of ImprovementsFinal Design of Improvements
Design of Stability ImprovementsDesign of Stability ImprovementsLikely a combination of concrete overlay, permanently Likely a combination of concrete overlay, permanently increased tailwater, and improved drainage systemincreased tailwater, and improved drainage systemOverlay and drains installed in sections as hearth is Overlay and drains installed in sections as hearth is draineddrainedWhile drained:While drained:
Clear debrisClear debrisVisual inspectionVisual inspectionGeophysicalGeophysical testing for voids testing for voids -- RepairRepairImprove protection for existing piezometersImprove protection for existing piezometers
Design Repairs to slab downstream from gated Design Repairs to slab downstream from gated spillwayspillway
Questions?Questions?
