Loading...
Section 4.0 Onshore Investigations 1960050-03 • 4.0 ONSHORE INVESTIGATIONS Puruose In March and October of 1996, Leighton drilled three borings at the shoreline for the City of Rancho Palos Verdes. This shoreline investigation was conducted for several reasons: to determine the subsurface bedrock structure and stratigraphy, to determine the feasibility of constructing a revetment at the shoreline, and to link the offshore studies with previous onshore investigations.The study of the bedrock structure included determining the seaward extent of the Portuguese Bend Landslide, and the thickness and structure of the Portuguese Tuff. • Shoreline Borings Three borings were drilled by Leighton (1996, 1996b) along the shoreline at Portuguese Bend (Figure 11). The logs of these three borings appear in Appendix B. These borings were located by Ehlig near the suspected toe of the active Portuguese Bend Landslide. Proposed boring locations were reviewed by geologists from the USACE. Temporary drill pads constructed of rock-filled wire gabions were built in the surf zone at each boring location. The tops of these pads were 5 to 7 feet above sea level. The pad constructed for LC-1 used a 12-inch diameter • plastic pipe (temporary casing) placed in the rock fill to prevent beach cobbles and rocks from caving. The collar of the plastic pipe was at an elevation of+9.7 (MLLW). A 24-inch diameter corrugated metal pipe (casing) was placed in the pads for LC-2 and LC-3 for the same purpose. The drilling depths recorded for LC-1 were taken from the top of the surface casing (+9.7 MLLW) and from the bottom of the casing (natural ground) for LC-2 and LC-3 (-3.7 and -3.5 MLLW, respectively). After drilling was completed, the pads and gabions were removed, and the beach was restored to its original state. LC-1 was drilled and cased using a tricone bit to a measured depth of-22 MLLW, where firm siltstone/tuff bedrock was encountered. A diamond-core bit replaced the tricone bit, and continuous coring commenced. Casing was advanced behind the tricone bit to -42 MLLW. At this depth deflection of the borehole was observed to be occurring at -28 feet (MLLW).This was apparently due to movement of the active landslide and movement (sagging) of the drill pad due to wave action and settlement. Coring was resumed after casing and continued to -53.3 feet (MLLW). As drilling progressed, the rapid and continued deflection of the boring caused the drill bit to cut through the casing. This boring was abandoned, the drill rig moved, and the boring was redrilled. The second attempt to drill the boring (LC-1A) was drilled within the same plastic pipe. The boring was cased from the surface to -32 feet (MLLW) behind the tricone bit, then continuously cored to -90 feet (MLLW). At this depth, the boring was again deflected by movement of the landslide or settlement of the drill pad: Deflection of the boring made core retrieval difficult,and ellr77111111. SA - 13 - •anurnu ALM•MnnurM run 1960050-03 drilling was terminated at -149 feet (MLLW). Geophysical logging, using a dipmeter tool, was unsuccessful due to deflection of the boring at-28 feet (MLLW). A third borehole was drilled at a location 32 feet north of LC-1 and LC-1A. This boring was drilled using an air-rotary rig with a 5-inch diameter drill bit. Casing was advanced to -4 feet (MLLW). At -87 feet (MLLW), the borehole was again deflected, and drilling was terminated. The drill rig was removed and geophysical logging using the dipmeter, caliper, and natural gamma tool was attempted. Caving at -7 feet (MLLW) was successfully removed, but additional caving from -7 to -17 feet(MLLW) prevented logging. Casing was advanced to -26 feet (MLLW), and the boring was successfully logged to the total depth. LC-2 was started with a hollow-stem auger rig that provided continuous sampling through the loose, unconsolidated material from the surface. At -11 feet (MLLW) the drill bit was changed upon reaching firm bedrock, and the boring was continuously cored using an diamond-core bit to -132 feet (MLLW). The boring was then reamed using an air-rotary rig to a diameter of 8 inches specifically for the 6-arm dipmeter. As a result of caving at -6 feet (MLLW), 8-inch diameter steel casing was advanced to -9 feet (MLLW). The boring was then geophysically logged using a 6-arm dipmeter,caliper, and natural gamma tool. LC-3 was drilled through unconsolidated material using a hollow stem auger rig to -14 feet (MLLW) and continuously sampled from the ground surface. Casing was driven to -9 feet (MLLW), and then drilling resumed at -14 feet (MLLW) in firm landslide debris using a • diamond-core bit. The boring was continuously cored to a total depth of 151 feet (-155 MLLW), �` and then reamed to a diameter of 8 inches by the air-rotary rig. As a result of caving, casing was advanced to -59 feet (MLLW), before geophysical logging was attempted. After one successful run, the boring caved at-71 feet (MLLW), and was subsequently abandoned. All the borings encountered beach deposits at the surface. Borings LC-1 and LC-1A were drilled through debris of the active Portuguese Bend Landslide. Continued deflection of LC-1 at a depth of 38 feet (-28 MLLW) verified the location of the rupture surface of the active landslide. Neither LC-2 nor LC-3 were deflected by movement of the active landslide. Together the three borings bracket the toe of the active landslide and demonstrate that the location of the toe is nearly at the shoreline. Boring LC-2, located farthest east of the three borings,was drilled east of Portuguese Canyon. It was drilled into bedrock of the Monterey Formation that was found immediately beneath the beach deposits. LC-3 encountered landslide debris at-8.5 feet(MLLW) that extended to -40.1 feet (MLLW). During the week the boring was open, it was not deflected, even though the seacliff behind the drill pad was observed to be moving. The landslide debris was therefore assumed to be inactive. The inactive landslide debris in LC-3 is not underlain by a rupture surface. The inactive landslide debris represents old landslide debris that ran out over bedrock and was subsequently partially eroded by wave erosion. Beneath the active landslide surface in LC-1 and LC-1A, unoxidized siltstone and tuffaceous siltstone of the Monterey Formation were encountered at -22 feet (MLL`ti). Portuguese Tuff 733 BA 14- LEIGHTON AND ASSOCIATES,INC. 1960050-03 underlies the siltstone at -23.7 feet (MLLW). The base of the Portuguese Tuff is at -84.7 feet(MLLW). The Portuguese Tuff has a stratigraphic thickness, corrected for dip (15 degrees), of 58 feet. Beneath the Portuguese Tuff, interbedded siltstone and tuffaceous siltstone were encountered to the bottom of the boring. Basalt was encountered at -87.3 feet down to -111.3 feet (MLLW), underlain by the interbedded siltstone and tuffaceous siltstone described above. LC-2 consisted of about 3 feet of beach deposits overlying bedrock. The Portuguese Tuff was encountered at -12.4 feet down to -73.4 feet (MLLW), giving it a.stratigraphic thickness of 58 feet. Beneath the Portuguese Tuff, interbedded siltstone, tuff, and tuffaceous siltstone continue to the total depth of the boring. Sandstone is at -101.4 feet (MLLW), with a thickness of 10.3 feet. Basalt was not encountered in LC-2. Geophysical Loss The geophysical logs from the three borings were compared with the boring logs_to verify lithologic changes and identify the base of the landslide. The logs contain caliper, gamma ray, and dipmeter data. The caliper logs were used to help identify the rupture surface; the gamma ray logs were used to correlate marker beds such as sandstone, basalt and tuff; and the dipmeter logs were used to determine subsurface structure. Borings LC-2 and LC-3 were logged using a different geophysical tool than LC-1, but this tool provided the same suite of geophysical logs. i Caliper The caliper tool uses its tensioned mechanical arms to provide a continuous record of the chances in boring diameter. The 4-arm tool used to log LC-1 utilizes 3 of its 4 arms to measure the boring diameter. The 6-arm tool used in LC-2 and LC-3 utilizes all 6 of its arms. The recorded data can be used to identify caving or washouts that may occur in the borings. The caliper has been successfully used in many other onshore borings to locate and record the caving_ zones and borehole offsets that characterize the active Portuguese Bend Landslide rupture surface. The caliper logs for LC-1 show a distinct change in boring diameter at approximately -28 feet (MLLW). We interpret this zone as an indicator of the rupture surface of the active Portuguese Bend Landslide. The logs for borings LC-2 and LC-3 do not record significant variation in the boring diameter. Gamma Ray The gamma ray tool reads the amount of natural gamma radiation emitted by the different lithologies in the boring. The gamma ray signature for a particular rock type can vary depending on the regional geology. In general at Rancho Palos Verdes, high gamma signals were found to �- i $p, - 15 - LEIGHTONANDASSOCIATES,INC. 1960050-03 be emitted by lithologies such as organic rich siltstones, claystones or shale. Basalt, tuff, and sandstone generally emit low signatures. Gamma ray logs for LC-1 recorded a high signature at the rupture surface. This high reading. is interpreted to be caused by an organic claystone located adjacent to the rupture surface. Within the bedrock, the gamma ray log does not contain any distinct signatures that could be readily correlated with distinct stratigraphic units. The gamma ray log for LC-2 shows a distinct signature of the sandstone layer with a low reading that is easily distinguished from the higher readings of the interbedded siltstone. Other than this, the gamma ray data does not show any distinct signatures that could be correlated with bedding. The gamma ray logs for LC-3 show a low signature for the sandstone as compared to the interbedded siltstone above and below it. Aside from this distinct marker, the logs are generally nondescript, and do not correlate with the boring logs. Dipmeter The dipmeter tool functions by recording the resistivity of lithologies in contact with each of its mechanical arms. These arms are equipped with electrode pads that interpret the continuity of units with similar resistivity levels across the boring and calculating the dip magnitude and direction, as well as the vertical deflection of the boring. LC-1 was logged to a depth of 94 feet, which is near the base of the Portuguese Tuff. The stratigraphic units consist of landslide debris and Portuguese Tuff. Within the landslide debris, blocks with bedding are jumbled and chaotically oriented. Below the rupture surface, the bedding dips range from 0 to 60 degrees, with an average of about 20 degrees. However, most of the bedrock logged is the Portuguese Tuff, which is composed of massively bedded bentonitic tuff and bentonite. The Portuguese Tuff generally does not provide sufficiently contrasting layers to yield reliabledipmeter data. The dipmeter results from boring LC-1 yield Iittle useful data. The dipmeter data for LC-2 shows bedding dips that range from 0 to 30 degrees, with the majority of the dips between 10 and 20 degrees to the north. The dips trend from about 10 degrees at a depth of 50 feet to 20 degrees at 130 feet. A gradual increase in dip angle is also seen in the core samples, as noted in the boring logs. The logs for LC-3 show the bottom 20 feet of the Portuguese Tuff down into interbedded siltstone and tuffaceous siltstone. The bedding dips within the Portuguese Tuff are randomly oriented. Beneath the Portuguese Tuff, the bedding dip varies from 0 to 25 degrees, with the average of about 5 to 10 degrees to the southeast. Within the massive sandstone layer, dips are unusually high due to the lack of bedding. Therefore, the dipmeter data from the sandstone interval are considered unreliable. p&p: B sok A- 16 - ` LEIGHTON AND ASSOCIATES,.INC. 1960050-03 111 Correlation with Other Onshore Borings and Offshore Seismic Surveys Three borings were drilled alone the shoreline to link the offshore and onshore studies. These data and the bedrock data from the other onshore borings shows that the bedrock beneath the landslide probably forms the southern limb of the Palos Verdes Anticline (boring logs drilled by others appear in Appendix C). The axis of the anticline roughly parallels the topographic crest of the Palos Verdes Hills (Woodring, et al; 1946). The south limb of the anticline has several smaller scale superimposed folds that interrupt the smooth dip of the bedding planes to the south. These folds generally have trends that parallel the trend of the main fold that is responsible for the anticline. Data from the offshore seismic surveys are interpreted by Dill and Norall (1996) to demonstrate that the.bedrock continues offshore with a gentle dip to the south. Summary of Findings of Shoreline Drilling The subsurface bedding attitudes from the recently drilled shoreline borings closely matched the bedrock dips obtained from core, dipmeter and downhole logging of the borings previously drilled further inland. All of the recent shoreline borings intercepted the Portuguese Tuff. The Portuguese Tuff provided a stratigraphic marker which allowed the borings to be correlated with previous onshore borings. Basalt in a sill exposed at Inspiration Point is not found in the shoreline borings. No conclusive evidence was found demonstrating_ that basalt is continuous between Inspiration Point and the beach outcrops to the east. The basalt found in Boring LC-1 is a different basalt, statigraphically below the basalt sill exposed at Inspiration Point. Active landslide debris and a rupture surface was found in the middle shoreline boring LC-1, but in neither of the shoreline borings to the east or west. Bedding generally dips seaward at gentle angles (5 to 15 degrees). Bedding in boring LC-2 dips northward at moderate angles of 20 to 30 degrees. This reversal in the .general bedding direction results from smaller scale folding of the bedrock superimposed on the regional dip. The landslide toe is at or very close to the beach (Figure 17). rmirumv. - 17 - I CIBIUTflM Aim ACCfeltIM IMI