1. AEG Annual Meeting – Kona, Hawaii, Sept. 22, 2016
Technical Session #6
William Godwin, CEG – Lettis Consultants Intl
Mike Turner, CEG - AECOM
Justin Pearce, CEG – Infraterra (now USACE)
Holly Nichols, CEG - CA Dept Water Resources
Perris Dam Seismic Remediation –
Geologic Observations from
CDSM Foundation Treatment
2. Perris Dam-Presentation Topics
Introduction
History of Perris Dam
Physical and Geologic Setting
Seismic Remediation Design Elements
Cement Deep Soil Mixing
Venting Observations
Questions
3. INTRODUCTION
Owner
California Dept of
Water Resources
QC
Laboratory
Inspectors
AECOM
LCI
InfraTerra
DWR
Specialty Sub
Jensen
Drilling Co.
Specialty
Sub
JAFEC USA
General
Contractor
Pulice, Inc.
Bd of
Consultants
6. 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
12. 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.
16. 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
17. 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
19. CDSM – Verification Coring
The 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.
22. 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
23. 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 Rates
Speed with Depth
Drilling Resistance – Correlation to Young Alluvium or lower
SPT blow Counts
Refusals & Restrokes – Correlation with Higher Bedrock and
Alluvial Stiffness
24. 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
25. 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
28. 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
29. 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.
30. 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
32. 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
35. 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
36. 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)
37. 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
