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Date: October 18, 1993
Author: Thomas E. Jewett, Vice President Engineering
Subject: Tahoe Keys Resort Sheet pile bulkhead restoration.
Project Site: St. Moritz Isle, Tahoe Keys Resort (Lake Tahoe),
California (U.S.A.)
Description of Problem: Homeowners of the exclusive St. Moritz
Isle in the environmentally sensitive Tahoe Keys Resort were faced with an immediate concern: Over 100 homes and
properties were being threatened by progressive structural failure of a sheet pile bulkhead that separated their
residential properties from Lake Tahoe.
Site / Design Conditions: Several sections of the seawall bulkhead
failed during the winter of 1993. It was determined that steel cable tie-backs between the seawall bulkhead and
concrete deadmans had failed, causing a progressive overload of the sheet pile bulkhead and remaining tie-backs.
Another winter with heavy snowfall could cause more catastrophic failures.
Proposed Solution / Selected Solution: In a 28 working day time period,
the failing bulkhead underwent complete renovation that included strict installation and proof testing of 868 MANTA
RAY® earth anchors. Other options proved to be prohibitively expensive or environmentally undesirable. MANTA
RAY® anchors were installed from the front side of the bulkhead, through a "window" cut in the sheet
pile, causing no damage or soil disturbance. The anchors were immediately proof tested to ensure required holding
capacity was achieved. Time was of the essence to avoid the impending Winter snows, which precipitated the previous
failures.
Installation Details: The contractor used the MANTA RAY® Anchor
Driver custom mounted to a mini-excavator, which was positioned on the narrow walkway above the bulkhead. The excavator
"reached" over the edge of the bulkhead and aligned the MANTA RAY® Anchor Driver with a "window"
previously cut in the sheet pile. Alignment was critical in order to drive the anchors past numerous underground
obstructions, including existing concrete deadmen, fence posts, storm drain culverts and deck support posts - with
no disturbance to the homeowner's property.
Anchors: MANTA RAY® MRSR321703: MR-SR anchor w/ 1" x 17'
rod.
Proof Test Requirement: 18,000 lbs. for 5 minutes, maximum allowable
deflection .125 in.
Installation Equipment: MANTA RAY® Anchor Driver "custom
mounted" to a mini-excavator.
Actual Anchors Installed:
| Actual Anchors Installed: |
Number |
Percent |
Recorded Anchors in Log
"Omitted Anchor Location" |
868
7
|
-
|
| Actual Anchors Placed into Service |
861
|
100.0%
|
| "Abandon Anchors" |
7
|
0.8%
|
| "Contractor Re-Drove" |
17
|
2.0%
|
| Total Anchors Driven |
885
|
102.8%
|
| Failed Load Test - Deadman Poured |
14
|
1.6%
|
| Not Tested - Deadman Poured |
3
|
0.4%
|
Conclusion / Results: The total anchor installation time was 28 working
days for 868 anchors. This included re-driving of anchors that did meet with unavoidable obstructions and proof
loading of all anchors. The entire contract was completed in under the 45 day limit, entitling the construction
contractor to collect a bonus for rapid completion of the project.
The MANTA RAY® system provided an environmentally clean, fast, secure
and cost effective project.
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