How to Build Walls Longer Than an Aircraft Carrier

One runs longer than an aircraft carrier and the other rises higher than a five-story building. Every day they watch the I-405 highway curve away from them below while they work to hold back a hillside.

Without them, the northbound high-occupancy vehicle lane could not be added as part of the I-405 Sepulveda Pass Improvements Project.

First, let's get through the numbers. The two retaining walls east of the I-405 highway, near the Getty Trail Head, are named for their location on the project. The northernmost wall, 1852, runs 359 feet, longer than a football field.

The southern wall, 1836, twists and turns 1,313 feet, longer than the United States nuclear aircraft carrier the U.S.S. Nimitz. This wall climbs to a maximum height of 86 feet.

The two walls are called "soil nail walls" because nails drilled into the hillside they support, keeping the façade of the wall in place. But these are not the kind of nails a carpenter would store in her work belt.

For wall 1836, for example, 15 rows of soil nails are required to support the wall. These nails run from 14 feet to 85 feet long. The two walls are separated by approximately 186 feet.

The shorter 1852 wall requires only nine rows of soil nails. In comparison with the nails of its longer brother, these nails are dainty, varying from 14 to 59 feet long.

The walls allow the expansion of the freeway to the east, providing room for the addition of the new high-occupancy vehicle lane to the northbound lanes.

These photographs were taken in September. Since then shotcrete has been sprayed over the walls, hiding the supporting rebar.

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North of the 1852 wall, a worker mixes water and cement to create grout, which will be injected into spaces in the wall at high pressure to seal the wall.

 

A worker injects grout into the spaces around the “nails” drilled into the hillside east of the wall. The soil nails support the wall face, which retains the hillside. The northern wall, referred to as 1852, stands 360 feet long and 30 feet high at its tallest point.

 

Perched above the northbound lanes of the I-405, a worker grouts the wall to prevent water seepage around the new soil nails. These two retaining walls will allow addition of the high-occupancy vehicle lane to the northbound lanes.

 

Slightly above this worker rests a vertical down-drain pipe which will remove water from the top of the finished wall. He cuts the rebar on the face of the wall to clear an opening. The opening will allow installation of the lower length of the drain.

 

The drill pierces the wall to allow installation of a soil nail. The space between the nails depends on the contour of the wall. Retaining walls are commonly soil-nail walls.

 

While a worker prepares the wall for its shotcrete finish, the drill inserts another soil nail. Some of the nail are 85-feet long.

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