AEG Annual Meeting – Kona, Hawaii, Sept. 22, 2016Technical Session #6

William Godwin, CEG – Lettis Consultants Intl Mike Turner, CEG - AECOMJustin Pearce, CEG – Infraterra (now USACE)Holly Nichols, CEG - CA Dept Water Resources

Perris Dam Seismic Remediation –Geologic Observations from CDSM Foundation Treatment

Perris Dam-Presentation Topics Introduction History of Perris Dam Physical and Geologic Setting Seismic Remediation Design Elements Cement Deep Soil Mixing Venting Observations Questions

INTRODUCTIONOwner

California Dept of Water Resources

QC Laboratory

InspectorsAECOM

LCIInfraTerra

DWR

Specialty SubJensen

Drilling Co.Specialty

SubJAFEC USA

General ContractorPulice, Inc.

Bd of Consultants

INTRODUCTION

HISTORY OF DAM

HISTORY OF DAM • Terminal end of

California Aqueduct• Zoned Earth

Embankment, L= 2.2mi., H = 128ft

• Partial core and foundation over-ex (<6ft)

• Construction Completed in 1973

• Capacity – 131,450af• Currently lowered 25ft

to allow 37ft freeboard

Perris Dam Geologic Setting

USGS OFR 99-172, 2004

San Jacinto FZ

Geologic Profile – Left Reach

ReservoirA

YA

CPT transect, upstream

SPT transect, upstream

Profile location

Approx Bdrk

?

Sample Cross-Section, Left Reach

(Friesen and Balakrishan, 2012)

Seismic Remediation Design Elements

Seismic Remediation Design ElementsFoundation Treatment and Stability Berm

Cement Deep Soil Mixing - CDSM Objective: Strengthen the foundation and

along with the stability berm to limit the deformation of the dam to the acceptable level during a strong seismic event

DWR decided against removal and replacement because of shallow groundwater in granular soils… judged they couldn’t dewater sufficiently.

Seismic Remediation Design ElementsFoundation Treatment

Typical Cell Layout Down

stream

Longitudinal Wall Segments, Typ.

Transverse Walls, Typ.

Deep Cell PlanShallow Cell Plan

A A Section A-A

Transverse Walls

CDSM Mixing Rig

CDSM Predrilling Homogenize alluvial soils by loosening dense

zones, cemented layers and prepare for optimal mixing

Use same equipment as that used in mixing Inject water and air to facilitate advance Advance rate <8ft/min Withdrawal rate <20ft/min Primary/Secondary Installation Sequence

CDSM – Grout Mixing Introduce cementitious grout to soils at a mix

dosage of 210 kg/m3 and W:C ration of 0.9 Advance rate <4ft/min Withdrawal rate <9ft/min Primary/Secondary Installation Sequence Remix bottom 10ft and other zones

Required Work Flow – Pre-drill/Mixing

CDSM – Verification CoringThe recovered samples are checked for recovery and uniformity. • Recovery Criteria: – The recovery from each 5 foot core run should be at least 85 percent. – Continuous core recovery should be at least 90 percent when averaged over all core runs within a single boring. • Uniformity Criteria: – Within a single core run, the sum length of unmixed or poorly mixed soil regions or lumps that extend entirely across the cross section of the core sample (minimum of 2.5 inches) is not to exceed 15% of the core length. – All lengths of unrecovered core shall be assumed to be unimproved soil.

Verification Coring

Inspectors Role Serve as DWR representatives Observe CDSM activities, noting geologic

anomalies or variations. Document whether or not construction is

proceeding according to specifications Log element coreholes Choose core samples for cutting Prepare Daily Inspection Reports

Design Validation Using Geologic Data Review Databases from Contractor and Inspectors and analyze using GIS Preliminary Analysis

Venting – Preferential channel, influence of dewatering Analysis in Progress

Element Advance RatesSpeed with DepthDrilling Resistance – Correlation to Young Alluvium or lower SPT blow CountsRefusals & Restrokes – Correlation with Higher Bedrock and Alluvial Stiffness

Venting Where did it occur?

– In relief wells upstream and downstream of CDSM pad

– On pad surface– Adjacent to and up to 75ft from predrilling

Preferential alignment upstream and downstream. Distances of venting from predrill were generally less in the cross-valley direction. Possibly occurring along channels

Mostly Venting Less VentingNo Data

Blue = No Venting Noted

Venting When did it occur?

– During and after Predrilling; not mixing– Could linger after predrilling paddles removed

Why did it occur?– Excessive air and/or water pressure– Bottom line is that we don’t really understand

exactly how the pressures were being transmitted from the column being predrilled

Ground Venting – Monitoring Wells (video)

Ground Venting – Ground Surface (video)

Venting WHAT was the concern?

– Possible hydrofracturing of foundation soils– Possible grout migration. Would be a

particular problem if it contaminated the blanket drain for dam.

– Possible hydrofracturing of CDSM columns as they were curing

Venting Mitigation – RFI 117 Working air pressures during predrilling will be

monitored and regulated to remain <120 psi. Given the concern that air may damage the dam,

no isolated elements will be predrilled within the length of two (2) elements of the upstream side of the CDSM working platform.

For this purpose, isolated elements are those elements which are more than 12 feet from the nearest predrilled element. This is based on the observation that neighboring predrilled elements will serve as vents.

Venting – Conditional Acceptance RFI117 Provided DWR and DSOD are able to monitor air

pressures during predrilling Contractor has the ability to effectively control

and adjust air pressures as required during pre-drilling

Significant venting conditions (such as soil erosion) caused by water and pressurized air are avoided during pre-drilling

Engineers has right to halt work if significant venting is observed

Venting Analysis: Compare with Channel Structure

Venting Analysis: Overall pattern may correlate with large

channel structure Prominent ‘No Venting’ patches (Cells 34 and

45) within ‘Mostly Venting’ central part of CDSM layout

Blue = No Venting NotedGreen = Venting Noted

Cell 34 Cell 45

Green venting noted Blue venting not noted

Venting Analysis: Dewatering Observations Observed at Construction Observation Wells COW-15 through COW-25 are located along CDSM

footprint

Dewatering Observations

1450145514601465147014751480148514901495

Magnitude of Lowering During This Time PeriodFor gray well (COW 22)

=21 ft

Maximum Dewatering Moderate Dewatering

21 ft

Venting: During Dewatering Period Larger channel structure may contribute to

venting locations Dewatering time periods may correlate with

lack of venting We are still refining the venting/no venting

dataset

Conclusions CDSM operation completed on May 24, 2016 All cores passed the acceptance criteria and the

percentage of recovery and uniformity rates Venting was most common in the center of the

paleo valley where– Groundwater was shallowest– Highest concentration of coarse grained

deposits exist Venting was much less common in cells

constructed towards the end of the project where groundwater was deeper (south end of the dam)

Acknowledgements Michael Driller, GE and Grace Chen, PE –

DWR Geotechnical Engineers Joan Weber, PE – DWR Resident Engineer Hans Abrahamson-Ward, CEG – LCI Principal

Geologist Stephanie Briggs, PhD, PG – LCI Sr. Project

Geologist

Questions