The dope on the slope


    With notice that repair of the so-called Las Flores-PCH landslide may take “into next year” [“Caltrans nixes third lane on PCH”, Aug. 27] and the continued absence of any comments from the city geologists about the real character of the slide, it is time for a few comments from the gallery. We have been told on various occasions that it was a reactivated prehistoric landslide, that it would require excavating a volume to fill 20 Rose Bowls, later changed to two Rose Bowls, and that it was caused by El Nino-related rains. None of those assertions is true.

    There was no prehistoric landslide, but rather a localized spalling section of highway road cut. El Nino rains may have added indirectly to the problem, but, in fact, the slide was caused by a relatively small area at the base of the cut saturated with ground water. The surface of the cut there had been spalling away for at least the last 10 years to my certain recollection. Finally, during one episode of spalling earlier this year, enough material fell on the side of the highway that Caltrans decided to eliminate the over-steepened section by excavating back to more stable gradients. The volume thus far excavated I estimate to be somewhere between 60,000 and 80,000 cubic yards, say with bulking a maximum of 100,000 cubic yards, which is no more than two or three tenths the volume of the Rose Bowl even on New Year’s Day.

    The slope at the base of which the road cut was made when PCH was last widened in the 1940s is about 200 feet high. The lower 150 feet or so of that slope is underlain by sandstones and shales of the bedrock Calabasas Formation. The cut itself in that section was only about 60 feet high. Overlying the bedrock is about 25-40 feet of terrace deposits composed mostly of consolidated silty sands and gravels, up to the level of Sierks Way cul-de-sac. The two houses that recently had to be razed were built upon these terrace deposits.

    Caltrans decided to cut back the slope to about 34 degrees from horizontal in the bedrock and about 26 degrees in the terrace deposits. In the process, Calabasas bedrock supporting an outer section of the terrace deposits was removed. The terrace deposits then failed, in the process initiating movement that eventually would have destroyed the two houses at the end of Sierks Way.

    It was wrong to cut back the entire slope, especially without considering the effect on the terrace deposits. Nearby sections of the Calabasas Formation are historically stable in slopes quite a bit steeper than 34 degrees, as is apparent, for example, in the cut for Rambla Pacifico just north of PCH. A better approach would have been to cut away the spalling section of road cut to a gradient no steeper than about 45 degrees, install hydrauger drains, and erect a barrier fence to protect against future spalling failures — in other words, live with the condition much as we do at many other places along PCH. There was no reason to fear a deep-seated failure in the Calabasas Formation and hence no reason to engage in the massive regrading operation that has so badly disrupted traffic this summer.

    I do not see why the current repairs cannot be completed in a few weeks. The existing newly graded slope should be at least as stable now as it was historically for many years at a much steeper gradient. A bench, apparently now under construction, is needed at the base of the terrace deposits to prevent dislodgement of basal conglomerate boulders such as those that rolled down the slope some weeks ago during initial grading operations. That, and removal of loose debris still on the slope, should finish the job.

    It would be a mistake to go to the time and expense of an impact wall such as that recently erected near Moonshadows where a true avalanche risk extends to the top of the slope. It would be an even worse mistake to erect a “250-foot buttress wall” as reported in the Surfside News [Aug. 27]. Perhaps the term “buttress” was used in error. A buttress provides a supporting reaction force. No such support is needed because the regraded slope clearly has a high safety factor and cannot produce a force to react against. The major effort which causes traffic delays, that of hauling, should now be over within a few days. I am interested to know what else Caltrans feels the job requires that would run it into next year, and I think city officials should be similarly interested.

    E.D. Michael