Drilling Against a Quake : Roads: Steel cables threaded into holes bored in the interchange of the Glendale and Ventura freeways will reinforce the thoroughfares against seismic damage.

For the past few weeks, workers have been hacking with saws at the soaring bridges that lace between the Glendale and Ventura freeways in Glendale.

It would seem that they are cutting miniature trapdoors into the freeway.

And, indeed, they are.

For the next 10 months, workers will be squeezing in and out of those doors into the honeycombed concrete chambers that make up the freeway bridges.

With electric rigs and tubular augers, they will drill holes into the concrete inner structures. When they are done, there will be a mile of holes, each as much as 7 feet long and 4 inches in diameter.

Because of those holes, the interchange will be much stronger.

The solution to this apparent paradox is cable.

Sewn through the holes like thread, the cable will attach to brackets on the inner walls, binding together the mammoth stepped expansion joints, called hinges, where one segment of the elevated road joins another.

The 1971 Sylmar earthquake showed that those joints can come unhinged, causing freeway ramps to fall.

Since then, the California Department of Transportation has been sewing its freeways together, one at a time, on a schedule based on the thoroughfares’ vulnerability.

The $4.3-million reinforcement of the freeway interchange is one of seven seismic retrofit projects now under way in Los Angeles County.

The contractor, McCarthy of St. Louis, will strengthen many of the concrete columns that support the freeway, using steel casings, enlarged footings and tie-down anchors.