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Tetra Tech will continue its work on the Mandalay Bay seawalls and perform a geotechnical investigation and seismic analysis. (File photo by Chris Frost)
Thursday, September 10, 2020

By Chris Frost


Oxnard-- The Public Works and Transportation Committee, Sept 8, approved a second amendment to its agreement with TetraTech, Inc. for geotechnical investigation and seismic analysis of seawalls at Mandalay Bay.


 The $121,319 amendment shall not exceed the contract amount of $296,801 and extends the agreement term from March 14, 2021, to March 14, 2022.


The city approved developing 740 attached and detached single-family homes and 37 greenbelts that became Mandalay Bay in the late 1960s and early 1970s.


The developer installed reinforced concrete Boise and Zurn style seawalls to create the Oxnard waterways portion of the Channel Islands Harbor and allowed residential lots by the water.


The seawalls started to degrade because of their adverse reaction to the marine environment and needed repair.


Tetra Tech recommended installing panels and waler tiebacks or installing cantilever sheet piles.


The tie-back option installs a new panel in front of the existing wall, filling the space between the wall and installing tiebacks that dig into the non-liquefiable soils.


These tiebacks are similar to the ones in place now but would extend to 80 feet deep, which is deeper.


The cantilever sheet pile installs new sheets over the existing one using a pressing method. That means less vibrations and noise for homeowners.


The new seawalls will withstand seismic activity.


City Engineer Tatiana Arnoudt presented the item to the committee, and she said Tetra Tech would do seismic work and technical analysis in the development.


The second amendment confirms the tie back method feasibility before initiating phase two of the original contract: the plans and specs for the Hemlock St repairs.


"The second amendment to the Tetra Tech agreement will allow the consultant to confirm this feasibility of the tie-back method prior to initiating phase two of the original contract, which are the plans and specs for the repairs along Hemlock St," she said. "The scope for this amendment includes due technical field exploration, four exploratory borings across the site to the depth of 40-50 feet below the ground surface. Two of the borings are planned in vacant lots, and two are planned in the street right-of-way. The final locations will be determined based on the consultant's coordination with the individual property owners."


The scope also calls for laboratory testing of the boring samples, she said, along with inspecting the existing piles to determine their current condition and completing the seismic vulnerability study of the existing seawalls and the development of the structural design of the tie-back panel. 


"With the approval of the FEMA HMDP Grant recognition, and the appropriation for $132,000 in grant funds from the FEMA staff report that goes to council on Oct. 26, there will be sufficient money for this amendment," Arnoudt said. 


Committee Member Tim Flynn said he's anxious to get the work started.


Chairman Bert Perello asked about observing the walls in low tide in the report, and he wanted to know if that happens once a year.


"It happened to land on the low tide, but we were looking at it on a quarterly basis," Arnoudt said. "We did on it the next lowest tide in the next quarter after the city manager approved the first amendment. It happened to land on the annual low tide."


Perello asked about ongoing work between 3900-3966 W Hemlock St, and he wanted to know if there is potential litigation at those addresses.


City Attorney Stephen Fischer said there'd been litigation relating to certain repairs.


"As far as those addresses, I don't have that particular case file with me to see how those relate to the plaintiffs in that matter," he said.


Perello appreciates the sheet pile press down method.


"One of the big concerns of the neighbors is not the noise, but the potential for vibration and what that would do to the foundations," he said. "On this pressing of the items into the ground, how is the pressure exerted, so it does it not cause vibrations?"


Arnoudt said the sheet pile gets pushed down very slowly.


"It's a slow, arduous process," she said. "It's a giant machine pushing this thing down, so you won't have something constantly hitting against it. It's like trying to drive a credit card through the sand."


The estimated completion date is 20 weeks, plus six months for the construction documentation.


The motion passed unanimously.