The Mystery of the Butte Sinkhole Willis D. Weight PhD PE
The Facts In July 2012 three significant precipitation events occurred (July 7, 14, and 16) July 7 th intensities as high as 1.49 in/hr. fell for 20 30 minutes July 14 light to moderate rains ~ 0.4 inches July 16 0.67 inches fell and within 15 minutes the storm drains were overwhelmed A sinkhole developed on 3210 Phillips St. also on July 16 th http://www.epa.gov/region8/superfund/mt/sbcbutte/buttepsou_arcouaomonthlyreportjul20 12.pdf
Guatemala City 2010 Taken by Willis Weight Oct 2010
Historical Background Hired by Butte School District in 1996 to Investigate why the basement of Hillcrest School was flooding each year. Investigation included drilling wells, collecting water-level data and surface water flow data. Resulted in a dewatering design and continued monitoring until 2004. Some of the same water issues have reoccurred once again at the Preston Residence.
I-15 exchange with I-90 Reese Canyon 72 Culvert Photo by Dr. Hugh Dresser 1996
There is a practice of placing nonuniform fill materials and then building Fill Materials Photo taken by Willis Weight
Fill Materials Examples: Hillcrest School The Preston residence on 3210 Phillips Both of these constructions were built upon fill materials
The 1996 Investigation Included Drilling Monitoring Wells Photo taken July 1996 by Willis Weight
Monitoring Well Distribution Flow Path of Surface Water and Groundwater HILLCREST SCHOOL 72-Inch Culvert North WELL LOCATION ID FENCE 0 200 SCALE FEET AUGUST 1, 1996
View to Southwest Surface Water Groundwater Photo taken June 1996 by Willis Weight
Relative Groundwater Elevations 8/1996 95 93 91 89 87 85 83 103 101 105 99 97 HILLCREST SCHOOL N WELL LOCATION ID FENCE SCALE Relative Elevation 0 200 FEET AUGUST 1, 1996
The Dewatering design was tested March 1997 @ 180% of Normal Precipitation Photo taken March 1997 by Willis Weight
Relative Groundwater Elevations 3/1997 With pumping Occurring 105 109 111 103 101 99 97 93 95 91 89 87 85 83 107 N SCALE HILLCREST SCHOOL WELL LOCATION ID FENCE RELATIVE ELEVATION 0 200 FEET MARCH 29, 1997
A very Wet Spring in 1997 resulted in upward pore pressures that caused surface damage Photo taken May 1997 by Willis Weight
Relative Groundwater Elevations 5/1997 (peak runoff) With pumping Occurring 107 105 111 109 103 101 87 99 97 85 83 95 93 91 89 87 HILLCREST SCHOOL N SCALE WELL LOCATION ID FENCE RELATIVE ELEVATION 0 200 FEET MAY 20, 1997
Dewatering System Designed Control boxes for wells Trenching into the basement Pump Discharge Water From well Photo taken Oct 1996 by Willis Weight
Looking West to Continental Drive June 1996 NWE Substation Photo taken June 1996 by Willis Weight
Summary of Surface and Groundwater conditions until 2004 Spring and storm-water runoff passed from Reese Canyon under I-15 via 72 culvert to west side. Flow was conveyed directly toward Hillcrest School. 50% of surface flow seeped into ground at NE corner of school property When surface flow turned west there were additional losses, very little reached the culvert under Continental Drive unless storm events were very large. GW dissipated by early summer.
Surface Conditions changed in Winter 2004
Beetle Kills of Pine Trees Allow Increased Surface-Water Runoff http://www.nytimes.com/2008/11/18/science/18trees.html?_r=1
On the other Side of Continental Drive Preston Residence Yellow House Surface Water Groundwater Taken Sept 2012 by Willis Weight
NWE bought a substation from Montana Power that was constructed with non-native gravel materials, which created a Basic French Drain. Photo taken Sept 2012 by Willis Weight
French Drain Concepts http://pixgood.com/french-drain-design-plans.html
Taken 9/7/2013 Added Material Photo taken Sept 2014 by Willis Weight
Looking East 9/2012 Photo taken Sept 2012 by Willis Weight
What s in the Sinkhole? 9/2012 Sand & Gravel Metal Sheeting Photo taken Sept 2012 by Willis Weight
Hole in Sept 2013 Photos taken Sept 2013 by Willis Weight
Taken June 1996 Taken Sept 2012 Lots more water coming down the Culvert Photos taken by Willis Weight
Contrast in Willow Growth Taken by Charlene Preston Year 2012 Year 2000
Taken by Charlene Preston July 2012
Taken by Charlene Preston July 2012
Taken by Charlene Preston July 2012
Taken by Charlene Preston July 2012
Attempts to Contain Excess Flow are evident along the current ditch. Taken Sept 2012 Blue Tarp Tire Photos Taken Sept 2012 by Willis Weight
Additional Field Work Monitoring wells installed Sept 20 th 2013 One monitoring well placed in the Preston Backyard Grain size analysis of sediment from drillholes Survey to establish gradients On Sept 23 2013 there was another significant storm and on Sept 28 th 2013 a water level measurement was observed in the Preston residence monitoring well. Additional water levels were observed in 2014.
Groundwater Study CP 2 Preston backyard Stickup 44 inches (3.67 ft) 0.0 0.5 fine to coarse sand some gravel 1.0 1.5 dark silty sand - clayey 2.0 2.5 fine to coarse sand looser 3.0 3.5 interbedded clayey and sandy zones 4.0 sand med to coarse 4.5 dark silty sand - clayey 5.0 5.5 fine to coarse sand some silty stringeres 6.0 6.5 coarse sand some fine gravel dry Cap CP 3 Over by Willows Stickup 11" (~1 ft) 0.0 Mucky carbonaceos silt 0.5 1.0 Fine to Med sand 1.5 soft sand silt 2.0 2.5 3.0 fine gravel sand matrix 3.5 4.0 CP 1 Near concrete culvert Surface stickup 2.5 feet 0.0 Mucky carbonaceos silt 0.5 1.0 Silty sand 1.5 2.0 Med to Coarse sand 2.5 probed to 4 feet all coarse snd Cap refused to drill
Monitoring well Locations
Conclusions A 72-inch culvert was installed when I-15 was constructed to convey water from Reese Canyon to the west Once water arrives to the west, little has been done to manage its affects. Beetle Kills of pine trees in 2006 have exacerbated the surface flow volumes and increased storm affects for delivering excess water.
Conclusions - cont A greater amount of flow is reaching the culvert under Continental Drive. The Preston's are the first home in the path of this increased excess flow to the west. This is enhanced by the construction of the NW energy substation using high permeability gravels thus creating a basic French drain. The sediments become saturated from larger precipitation events and entrain sediments around obstructions in the flow path. This resulted in a sinkhole developing in the fill materials of the Preston backyard.
Mystery Solved! Photo Taken Sept 2014 by Willis Weight