|
• H5.4 Stone Columns
<br />The response of piezometers in [he stone columns can be found in graphs provided in Appendix H3.
<br />Locations of the stone columns are shown on Figure H 1.
<br />The stone columns are 36 inches in diameter, filled with 3/4 inch gravel with the upper 2 to 4 feet of the
<br />column provided with a bentonite cap. The majority of stone column depths extend through the upper and
<br />lower colluvium into the cobble layerjust above bedrock. However, some stone columns did not extend to
<br />the cobble layer, and were completed in the lower colluvium, including stone columns 20. 22 and 23.
<br />When compared to adjacent stone columns with depths that extend to the cobble layer, these stone columns
<br />provide valuable information on the effectiveness of the stone column method for collecting and removing
<br />water.
<br />Stone columns 20, 22, and 23 respond in a similar fashion to the other monitoring wells at the head of the
<br />slide. Stone column 20 shows a peak rise at March 31, 1998, a slow drawdown thereafter, and a rise once
<br />• again m the Spnng of 1999. The graphs also show that Stone columns 20, 22 and 23 were pumped with
<br />sharp drawdowns in water level. The stone columns appear to recover quickly to previous levels,
<br />indicating they aze effective in collection of water. This indicates that water is coming from the upper
<br />portion of the landslide rather than from deep sources below the landslide. Other stone columns show
<br />similar collection characteristics, including stone columns 02, 48, 49, 50, 51, 60, 61, 69, and 70. These
<br />columns should be pumped regularly to remove ground water that has collected.
<br />Stone column 18 was completed in the cobble layer. Stone column 18 shows a 45-foot increase in water
<br />level during the 1998 spring period, which it is able to drain quickly after the spring melt is completed. In
<br />1999, it shows similar response but with smaller increases in water level during the spring. Other stone
<br />columns show similaz effectiveness in draining water, including O5, 07, 15, 16, 17, 44, 47, 52, 64, and 67.
<br />Stone column 21 shows no response to spring conditions in 1998, but a large response in 1999 followed by
<br />a drop in water levels, indicating adequate drainage to the cobble layer. Other stone columns show similar
<br />effectiveness in draining water, including 19, 21, 24, 25, 26, 27, 28, 29, 30, 31, 32, and 33.
<br />• Barr Engineering Company July 71,1999
<br />PCDOCS\2082201CAK 19 Appendix H
<br />1999 Addendum to
<br />1997199 Landslide Corrective Measures Report
<br />
|