Tract 25318 is located at the northern end of Las Flores Mesa Drive and includes Eagle Pass Drive. It lies on a part of the western slope of the first ravine east of Las Flores Canyon which I choose to call Eagle Pass Ravine. The tract includes roughly 60 acres and was subdivided early in the 1960s into nineteen residential lots. Within a few years, beginning about 1963, eighteen had been developed. Some fifteen years later, a landslide involving all of those properties and an area adjacent to the south began.

LANDSLIDING
The private consulting company that provided the original geotechnical reports for the tract considered the area to be underlain by bedrock. This was consistent with the generally authoritative works of Yerkes and Campbell (1980) and Dibblee (1993). However, their field work was done prior to when the slide began which, according to Lung (1986, p. 1), was early in 1978. Lung has produced what probably is the only published study of the Tract 25318 landslide which he, however, calls the "Las Flores Drive - Eagle Pass Road Landslide." He considers the local area to have been underlain by one or more masses of "ancient" landside debris (op. cit., p. 1-2). This, together with fills placed during subdivision grading, renders the area rather difficult to interpret geologically.

The storm seasons of 1977-78 and 1979-80 in southern California produced rains in such quantities and intensities that they caused many landslides in the Santa Monica Mountains and elsewhere. Lung indicates (ibid.) that evidence of landsliding was first noted early in 1978 as incipient movements began to cause damage to houses in the tract and requiring the demolition of one, presumably by 1986. Probably, that was one of three others along the eastern side of Eagle Pass Road that also were later demolished. The greater damage to these four house was due to their location near the upper edge of the steep ravine slope where the movement has been greatest. Furthermore, a house at 20966 Las Flores Mesa Drive, just south of the tract also was undergoing damage about that time.

Lung reports (ibid.) that in 1981, ground-water levels initially encountered in borings at depths between 30 and 45 feet rose to within 6 to 24 feet of the surface. This artesian condition demonstrates that at the time a hydraulic head was acting at depth, but whether it was confined at the base of the debris or some impermeable bedrock section is unclear. Furthermore, the source of the ground water is uncertain. It likely was in part that of septic-system effluent injected during the previous 15 years and in part infiltrated rain. However, it also is possible that the local area is being pressurized through the casing of an oil well originally located just north of the Eagle Pass Drive cul-de-sac. If the pump still is in place, or if it was removed without properly abandoning the well, such pressurization easily is possible. However, it seems clear ground water acted to reduce effective stress to such an extent that a shear landslide developed.

Some years ago, roads in the tract show were repaved. Now they show numerous fractures that have been patched indicating that movement is continuing at least periodically. I understand from a recent casual conversation with one local resident that slope indicators installed in the area are showing only small increments of movement.

FIRE DESTRUCTION AND REDEVELOPMENT
The so-called "Topanga Fire" that began on November 3, 1993 destroyed many houses in the Las Flores Mesa area. In Tract 25318, only those at 20607, 20629, and 20635 Eagle Pass Drive, and that at 20734 Las Flores Mesa Drive survived the fire. It appears that at the time, thirteen houses had been inhabited despite landslide damage. It is uncertain whether the house at 20966, not part of the tract, had been abandoned due to landslide movement or was destroyed by the fire.

Phoenix-like today, Tract 25318 has risen from the ashes of the Topanga fire. With the exception of five properties along the downslope side of Eagle Pass Drive, most others in the tract have been redeveloped or are in the redevelopment process. There are thirteen properties between Eagle Pass Drive and Las Flores Mesa Drive. Of the four that escaped serious fire damage, three have been extensively remodeled. Of the nine that were destroyed, six have been redeveloped, two are in the redevelopment process, and one remains vacant land. This means that of the original seventeen developed properties, there will soon be in operation twelve of the original nineteen septic systems. From this, it is reasonable to conclude that the normal rate of ground-water recharge will be soon at least 63 percent. of what it was originally and possibly higher because of the up-graded character of the redevelopments.

REMEDIAL WORK
Lung (op. cit., pp. 99 -101) correctly observed that the only feasible means to stabilize the landsliding affecting Tract 25318 is by buttressing in the ravine supplemented by dewatering. He estimates that a million cubic yards of fill would be required. There are three obstacles to this: [i] fill importation along Las Flores Mesa Drive, certain to be opposed by local residents because of its environmental impact; [ii] the cost of such importation; [iii] the environmental impacts associated with filling in the ravine. Obstacles [i] and [ii] renders as unfeasible any remedial work that relies upon Las Flores Mesa Drive for access. Obstacle [iii] brings into conflict concerns about the natural environment of the ravine with the fundamental pragmatism of land development and improvement.

However, a nearby source of fill for buttressing could be developed by grading the crest of the high ridge along the eastern side of the ravine. The topography there is such that much of it could be imported simply by dumping. That ridge is reached by an unimproved road from the intersection of Big Rock Drive and Cool Oak Way in the upper Big Rock Mesa area. Its value for residential sites presently is marginal because of limited level area. Such a plan would: [i] stabilize the Tract 25318 landslide; [ii] render the ridge crest west of Las Flores Mesa Drive safe for development; [iii] greatly increase the value of the east ridge for residential sites; [iv] introduce an additional fire-evacuation route for both the Big Rock and Las Flores Mesa areas; [v] provide a public recreational area in the filled ravine. In purely economic terms, such a plan seems well worth study.

On the other hand, if the Tract 25318 landslide is not stabilized, it will continue to move because of increasingly high ground-water levels. Like it or not, someday it will rain again in Malibu. At the very least, a dewatering program should be introduced because that could raise the present very low safety factor significantly. Furthermore, conditions at the oil well site should be investigated. And as a last word, it is fair to say, simply as an observation rather than a prediction: of all the landslides in Malibu, I believe that affecting Tract 25318 is the most likely to move in the future catastrophically.

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