Subsurface Mapping for a Slope Stability Slide along I40 ...stgec.org/presentations/STGEC_2019/Talk 6.pdf · Subsurface Mapping for a Slope Stability Slide along I40 near Ozark, Arkansas

Subsurface Mapping for a Slope Stability Slide along I40 near Ozark, Arkansas using Noninvasive Geophysical Techniques

1

Clinton M. Wooda, PhD PESalman Rahimi, Vanessa Lebow, Michelle Barry

aDepartment of Civil Engineering, The University of Arkansas, Fayetteville, USA.

Chattanooga, TN; 5 Nov., 2019

Geophysical Subsurface Mapping 2

Project Background and Goals

• Work on the project by the University of Arkansas was conducted as part of ARDOT TRC1803 Mapping Subsurface Conditions for Transportation Applications.

• The goals of the research project are to identify, evaluate, and validate the use of geophysical methods for use on transportation projects with specific applications to slope stability and bedrock mapping problems.

• This particular project is part of the validation phase where we apply the methods to current or future ARDOT project.

• The goals of this testing are to provide detailed information on subsurface conditions (particularly bedrock depth and water conditions) for a slope which is experiencing stability issues.

Ozark, AR


Project Background

• Stability issues along a 400-500 ft section of the west bound (lower) lane of interstate 40 have been observed since the 1970’s.

• Numerous failures occurred prior to 1980. Failures were typically redressed to temporarily repair the slope

• From 1979-1983, changes to the drainage system including redirecting water to other sections of the highway and geotechnical borings were undertaken to better understand the failure mechanisms.

• In 1983, clay materials were excavated along the toe of the slope and replaced with granular material to buttress the slope and provide drainage.

Interstate 40

Observed Cracking

b-Section 1

Section 1

Section 2

c-Section 235% Grade


Project Background• Temporary repairs continued

periodically until 2016 when a more comprehensive subsurface investigation was undertaken using drilling and SPT at 18 locations.

• Borings generally indicate moist clay to sandy clay with rock fragments. The N values varied significantly with both depth and location from 30-40 to 5-15. The site is underlined by shale bedrock with depth between 35-60 ft.

• Four inclinometers were install at the bottom and top of the slope to determine the location and depth of the slip plane. An average N value of approximately 10 were observed along the slip plane at the bottom of the slope.

No Information on Water or Bedrock

Bedrock

Bedrock


Project Repair• In 2018, 100 rock anchors were

installed on the project to repair the slope. Anchors were installed near the top of the slope in 3 rows with the majority of the anchors in rows 1 and 2. The anchors were bonded into the shale bedrock below.

• Less than a year after the repair additional movement was observed in the slope with cracks forming along the top of the slope and crossing through the roadway.

• We were invited as part of TRC1803 to provide more detailed subsurface information for the slope using geophysical methods.

In 2018, 100 rock anchors were installed to repair the slope.


Methods used for the project

1. Multichannel Analysis of Surface Waves (MASW)

2. P‐wave Refraction 3. Electrical Resistivity

Tomography (ERT)4. Horizontal‐to‐Vertical

Spectral Ratio (HVSR)

• Linear array of 48 4.5‐Hz vertical geophones • Equal geophone spacing of 1 or 2m (48‐94 m long array)• Source locations off each end of the array and every 4 m within the

array• Data is analyzed as a 24 geophone set which is moved along the array







• Frequency domain beamformer (FDBF) dispersion analysis• 24 geophone sub arrays analyzed to generate up to 24 Vs profiles per 48 channel array. • Geopsy software used for inversion• Neighborhood search algorithm (Wathelet et al. 2004)• Joint inversion of:

– Rayleigh wave dispersion data– H/V peak (theoretical Rayleigh wave ellipiticity)

• Model parametrization based on boring information• > 400,000 velocity models for each analysis• Median of top 1000 Vs profiles from inversion selected as “best”/most likely Vs profile

0 20 40 60 80

0

0.2

0.4

0.6

0.8

1

Receiver offset (m)

Tim

e (s

)

2D Transformation

Time (t) Frequency (F)Space (x) Wavenumber (K)

VR = F (2/K) VS (ft/s)







• Used same array as MASW data.• Used primarily to determine depth to saturated material (i.e., 1500 m/s or 5000

ft/s). Given the shale rock in the area, detection of bedrock was unlikely.

General reciprocal method and critical distance methods used to determine depth to layers.







• Data collected and processed by Dr. Stacy Kulesza research group at Kansas State • SuperSting R8 system with 56 electrodes• Electrode spacing of 1.5‐2.0 meters (82.5 m and 110 m)• Strong gradient array (combination of Wenner and Dipole‐Dipole arrays)• EarthImager 2D used to invert apparent resistivity measurements







• This method uses the resonant frequency of the soil to estimate the depth to stiffness boundaries (i.e., bedrock) at a site.

• Data was collected with 3 component broadband Trillium Compact sensors and Centaur Digitizers

• Microtremors were recorded for 15‐20 mins at each location. • Data was processed according to SESAME Guidelines

Low

freq

uenc

y pe

ak

Hig

hfr

eque

ncy

peak

b

Vertical

North

East

N

E

V

Soil

Rock

Height


Testing locations

• Initial investigation • 5 MASW and P-wave refraction

lines. 4 lines along the slope and 1 line down the slope.

• HVSR testing primarily along the MASW lines with some additional locations.

• Secondary investigation• 8 ERT lines. 5 lines along the slope

and 3 lines down the slope.• Dense array of HVSR tests along

the toe of the slope.

BoringsHVSR

MASWERT

HVSR


MASW Results

• Clayey sand to sandy clay from boings with some gravel and shale fragments.

• Shale bedrock is resolved with good accuracy. 050100150200250300350400

Distance (ft)

600

650

Elev

atio

n (f

t)

0

650

1970

6000

Soil

Rock

Vs (ft/s)

Sandy clay

Shale

BH5

Boulders

Sand

Shale

BH9

Sandy clay

Gravel or sand

Shale

BH15

Sandstone cobblesClay


MASW Results

• Clayey sand to sandy clay from boings with some gravel and shale fragments.

• Shale bedrock is resolved with good accuracy.

• Generally consistent bedrock depth with shallower bedrock to the west.

050100150200250300350400Distance (ft)

600

650

Elev

atio

n (f

t)

0

650

1970

6000

Vs (ft/s)

Soil

RockSandy claywith gravel

Shale

Sandy claywith gravel

Shale

BH7BH10

Sandy clayor gravel with clay

Shale

BH13

Sandy clayor clay with gravel

Shale

BH16

Sandy clay

Shale

BH18Clayey sand


MASW Results


• Generally consistent bedrock depth with shallower bedrock to the west.

Soil

Rock

Vs (ft/s)


MASW Results

• Sandy clay from boings with some gravel and shale fragments.


• Generally consistent bedrock depth with shallower bedrock to the East

050100150200250Distance (ft)

550

600

0

650

1970

6000

Soil

Rock

Vs (ft/s)

Sandy clay

Shale

BH4

Shale

BH8

Shale

BH12


MASW Results

• Bedrock depth is consistent until just below the lower line of borings.

• The increase in bedrock elevation causes a bowel in the subsurface.

Large increase in bedrock elevation

Soil

Rock

Vs (ft/s)

Shale

BH12

Bowl in bedrock


HVSR Results

• HVSR measurements were made in a grid pattern. The fundamental resonant frequencies was identified for each measurement location.

• The fundamental frequencies and average shear wave velocity (estimated based on the MASW results and calibrated using boreholes with known soil thickness) were used with the equation below to estimate depth to bedrock.

Fundamental Frequencies, F0


HVSR Results

• Contour map of soil thickness created based on the HVSRmeasurements.

• Large variations in soil thickness are observed at the bottom of the slope.

Large decrease in soil thickness

Area of deeper soil

Soil Thickness (ft)


HVSR Results

• Contour map of surface elevation based on on LiDAR and surveying.

Surface Elevation (ft)


HVSR Results

• Contour map of bedrock elevation. Surface elevation minus soil thickness.

• Significant changes in bedrock elevation are observed at the bottom of the slope. Significant bedrock changes

Bedrock Elevation (ft)


HVSR Results

• Including additional site observations, the springs (where water is exiting the subsurface) are observed near depressions in the bedrock.

• The failure scarp is observed in the vicinity of the springs and where the bedrock elevation is more consistent.

Failure Scarp

Spring

Spring


HVSR Results

• 3D picture of bedrock elevation at the site.

a-North-South view

b-West-East view c-East-West view

Notice numerous depressions in the bedrock


ERT Results

• Plot includes bedrock elevation from MASW and HVSR results

• A saturated zone is observed above bedrock. However, the shale bedrock can not be distinguished from the clay layer. Bedrock

elevation from MASW/HVSR

Clay (potentially saturated)


ERT Results

• Plot includes bedrock elevation from MASW, and HVSRresults and saturation line from P-wave refraction.

• Similar observations where a unsaturated or more resistivity zone is observed at the surface. A conductivity zone is observed above bedrock.

• The P-wave refraction results (Vs=5000 ft/s) confirms the clay layer above bedrock is saturated.

050100150200250300350Distance (ft)

600

650

Saturation line from P-wave refraction

Bedrock elevation from MAW/HVSR


ERT Results


• Very resistive zone is observed in the resistivity results, which may be due to concrete anchor blocks in the area and potential fill material.

• The P-wave refraction results (Vs=5000 ft/s) show the area is still saturated.

50100150200250300350Distance (ft)

600



Surface water from horizontal drains


ERT Results


• A resistive zone is observed at the surface followed by a conductive area.

• The P-wave refraction results (Vs=5000 ft/s) indicate the conductive area is saturated.

• Borehole water table measurements indicate similar water levels as observed in the P-wave refraction results.

Saturation line from P-wave refraction.

Bedrock Elevation from MASW/HVSR

Water table depth in Boring 17 at time of testing.


ERT Results

• Plot includes bedrock elevation from MASW, and HVSRresults

• A resistive zone is observed near the top of the slope with a saturated layer near the bottom of the slope



ERT Results


• A resistive zone is observed near the top of the slope with a saturated layer near the bottom of the slope.

• P-wave refraction results indicate similar saturation elevation as the ERT.

• Water is being trapped in the bowl at the bottom of the slope.


Bedrock Elevation from MASW/HVSR

Possibly fill or concrete .

Water saturated area.


ERT Results


• A resistive zone is observed near the top of the slope with a saturated layer near the bottom of the slope.

Elev

atio

n (f

t)El

evat

ion

(ft)



ERT Results


• A highly saturated zone is observed at the bottom of the slope.

Water saturated area Bedrock

Elevation from MASW/HVSR


Considering all Results

• Depressions and peaks in the bedrock are trapping water at the bottom of the slope.

• The failure scarp is observed in the vicinity of the springs and where the bedrock elevation is more consistent.

Spring

Spring

Head Scarp of slide


Current Work

• We are working on slope stability models and repair solutions which incorporate this information.

• Currently we are modeling the slope with using borings only and later will incorporate the geophysical results in the model to assess any improvements.

Cracking observed in the field

Highly saturated zone


Conclusions and Final Thoughts

• Geophysical results can provide an excellent means of exploring a much larger potion of the subsurface than drilling and sampling.

• Geophysical results can reveal subsurface issues which would often be difficult to fully visually using only drilling and sampling

• Not all geophysical methods are good for all issues. Particularly, ERT was not able to resolve the bedrock depth at most locations across the site.

• HVSR is a viable method for mapping bedrock depth which is utilized for geotechnical earthquake engineering applications, but is under utilized for transportation applications.

• The geophysical results provide a significant amount of additional subsurface information, which is currently being utilized to develop alternative solutions to the slope stability issues.


Questions?

AcknowledgementsThis work supported by the Arkansas Department of Transportation (ARDOT) under TRC1803. Any opinion, findings, and conclusions or recommendations expressed in this presentation are those of the authors and do not necessarily reflect the view of the ARDOT.

This research would not have been possible without the hard work of graduate students Salman Rahimi, Michael Deschenes, Ashraf Himel, and Landon Woodfield. A special thanks also goes out to Paul Tinsley, Paul Campbell, Matt Green, and many others at ARDOT. Their advice and assistance are instrumental in the success of the work.

Documents

Subsurface Mapping for a Slope Stability Slide along I40 ...stgec.org/presentations/STGEC_2019/Talk 6.pdf · Subsurface Mapping for a Slope Stability Slide along I40 near Ozark, Arkansas