Techniques to reduce the environmental impacts and costs of road construction a results based study
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Techniques to Reduce the Environmental Impacts and Costs of Road Construction A Results Based Study. Other authors: Ron Neden, P.Eng. & Freeman Smith, P.Geo. of Terratech Consulting Ltd. James Schwab, R.P.F., P.Geo. Of B.C. Ministry of Forests. Acknowledgements. Skeena Cellulous

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Techniques to Reduce the Environmental Impacts and Costs of Road Construction A Results Based Study

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Techniques to Reduce the Environmental Impacts and Costs of Road Construction A Results Based Study


Other authors:Ron Neden, P.Eng. & Freeman Smith, P.Geo. of Terratech Consulting Ltd.James Schwab, R.P.F., P.Geo. Of B.C. Ministry of Forests


Acknowledgements

Skeena Cellulous

West Fraser Timber

FRBC and FII

BC Ministry of Forests

Robert Balshaw

Silvicon Services Ltd

Silvatech Consulting Ltd

BGC Engineering Ltd


QUESTIONS

  • Can the incidence of road fillslope landslides be reduced?

  • Can forest road construction practises be improved and/or economized?

  • Can both be done at the same time?


LOOKING BACKAT PAST RESULTS

  • Past road practises -- What did not work ?

  • Why?

  • What worked?

  • How can we build upon it?


STUDY DESIGN

  • Focused on road fillslope landslides

  • Existing roads constructed across slopes greater than 50% (based on TRIM mapping)

  • Past road construction and management techniques


HOW

  • Collect terrain and road attributes at sites where fillslope landslides occurred; and

  • Collect the same attributes at similar sites where landslides had not occurred.

  • Compare the data sets statistically

  • Determine what combinations of terrain and road construction attributes contribute to fillslope landslides; and by default

  • What combination of terrain and road construction attributes do not contribute to fillslope landslides


DATA COLLECTED INCLUDED

  • Existing topographic, road, bedrock and surficial geology data;

  • Interpreted information from aerial photographs; and

  • Field data


Terrain Attributes

Slope (up and down)

Surficial Material

Aspect

Drainage

Bedrock Type

Slope Profile (Shape)

Etc.

Road Attributes

Fill Width

Fill Slope Length & Angle

Fill Type (R, SM, GP, etc)

Ditch Condition

Wood in fill

Configuration of wood in fill

Cracks in Road

Deactivation?

Etc.

FIELD DATA INCLUDED


STUDY AREA STATISTICS

Kalum Forest District

  • Coastal Western Hemlock Biogeoclimatic Zone

  • 158,000 hectares

  • 1079 km of forest roads

  • 196 km or 18% located on moderately steep to steep slopes (based on TRIM data)

  • Williams; West Copper; Kleanza; Legate-Chimdemash; and West Kalum


STUDY AREA Kalum Forest District


Road lengths


RESULTS OF STUDY

  • Field data collected at 40 landslide sites and 89 null site (non landslide sites)

  • Distribution of terrain slopes where data was collected is as follows:


Natural slope down


STATISTICAL ANALYSIS

  • Bivariate Analysis

  • Logistic Regression Model

    Statistical analysis conducted by Dr. Robert Balshaw, Ph.D.


Exploratory Classification Tree from Logistic Regression Model

Natural Landslides

No (113)

Yes 2N/14L

Concave or Convex 55N/6L

Slope Profile

Escarpment or Straight (52)

<2.15 m (42)

Perch Height

>2.15 m 2N/8L

Good or Acceptable 10N/0L

Ditch Condition

Poor or None (32)

Rapid or Well 18N/7L

Drainage Class

Moderate or imperfect 2N/5L


Natural Instability


Slope profile


Perch height


Ditch condition


Drainage class


WHAT DID NOT WORK?

Airphoto 60 Kleanza River


Although there can be many factors that give rise to landslide activity, there is only one trigger (Wieczorek, 1996).

This means that although many factors may contribute to a landslide, only one factor causes the slope to fail.


Landslide triggers can be grouped into one of four categories:

  • Increased loading on the slope

  • Removal of material from the toe of the slope

  • Vibration loading (such as earth-quake or man-caused vibration)

  • Increased pore water pressure


Slide trigger


Blocked Seepage

Creek flowing out of bedrock near original toe of fill


WHAT DID NOT WORK?

Road Drainage Systems


WHY?

  • Inappropriate location of culverts

  • Inadequate number and in some cases size of culverts

  • Inadequate culverts maintenance

  • Lack of maintenance

  • Lack of deactivation

  • Concentration of surface and seepage water flows

  • Inadequate ditching and ditch maintenance

  • Inadequate control of seepage water


CAN THIS BE IMPROVED?

YES

Existing legislation requires the maintenance of natural surface water flow paths. This has gone a long way to reducing the incidence of all landslide activity within the forest land base.


HOWEVER

Detailed assessments and planning of road drainage systems is typically limited to terrain class IV and V (potentially unstable and unstable terrain)

and

Drainage issues on moderate to gentle terrain and on non-status roads and trails continue to contribute to landslide activity downslope of these areas


THEREFORE:

Detailed assessments of development related impacts on natural site drainage should be conducted upslope of all moderately steep to steep slopes or potentially unstable and unstable slopes


WHAT DID WORK?

  • Over 80% of all sites (landslide and null site) incorporated wood material into the road construction

  • Statistically, there is a 70% probability that the simple presence or absence of wood in fill has no influence on fillslope landslide activity


Forest Roads: A Synthesis of Scientific Information

“Little is documented about the potential for increased mass failures from roads resulting from decay of buried organic material that has been incorporated into road fills or landings during road building. Anecdotal evidence is abundant that failures occur predictably after decay of the organic material.”Gucinski et al, (2001) states:


Is this really an issue?


Wood in fill


Is this an Issue?


Cracks


Observation

The failure plane of all the landslides noted in this study was either within the C horizon soils or along the bedrock surface.

No failure planes were noted within the fill materials

In other words, the native soils beneath the fill failed.


Failure Plane in C horizon soils not in fill

Sample cross-section 1


Fillslope Stability Analysis

  • Parametric Study to look at:

    • Influence of location of perch fill

    • Height of perch fill

    • Influence of pore water pressures

    • Weight (density) of fill

    • Influence of soil matrix suction


Base Case Model

Base Case Model


Comparative Model Used For Existing Fillslope Stability

Upper slope perched

Lower slope perched

Mid slope perched


RESULTS OF LIMIT EQUILIBRIUM ANALYSES OF SLOPE STABILITYInfluence of the location of the perch fill and height of perch fill


Conclusion

  • Location of perch on slope has little influence on the stability for shallow fills

  • Location of perch on slope has greater influence on stability for deeper fills

  • Influence of water has an order of magnitude greater influence


Light Weight Fill Stability Model

115% fillslope angle


RESULTS OF LIMIT EQUILIBRIUM ANALYSES OF SLOPE STABILITYInfluence of Density of Fill on Slope Stability

Bulk density of lightweight fill varied from about 15.5kN/m3 to 8.5kN/m3


Conclusion

  • Reducing the density of the fill has little influence on the stability of the slope


Base Case Model

Base Case Model Used For Soil Suction Analysis


RESULTS OF LIMIT EQUILIBRIUM ANALYSES OF SLOPE STABILITYInfluence of Soil Suction


Conclusion

  • Soil suction can have a significant increase in the stability of the slope


Summary of Stability Analysis

  • Modest perch fill height are not a significant factor

  • Perch location on slope not an issue unless the perch height is high

  • Pore water pressures are a significant factor in stability

  • Density of fill material has little influence on stability

  • Soil suction is a significant factor in the stability of unsaturated slopes


Reinforced Soil Structures

The past use of wood in forest road can be considered as a reinforced soil structure as woody material was often included in the road fills

In some cases, the woody material was included as layers known as puncheon


Our study looked back several decades

Take a brief look back a millennium or more


HISTORICALLY

  • Dykes throughout the Netherlands and England were built using reeds to reinforce the soil

  • Portions of the Great Wall of China were built using fine woody debris (twigs) to reinforce sand and gravel fill (200 B.C.)

  • The ancient Mesopotamians (modern day Iraq) built ziggurats (towers up to 100m high, over 3000 years ago), using mats constructed of woven layers of palm fronds to reinforce granular soils


Great Wall

Section of Great Wall of China built 200 B.C.


CURRENT REINFORCED SOIL STRUCTURES

Currently, steel and plastics (geotextiles and geogrids) are most commonly used to constructed reinforced soil structures


5 m High GRS-WW Retaining Wall Constructed Across Large Soil Slump


4.7 m High GRS-WW Retaining Wall Constructed Across Very Steep Bedrock Slope


2.8 m High Wall Constructed Across Steeply sloping Glaciofluvial Escarpment


4 m High Retaining Wall Constructed Across Steeply Sloping Bedrock and Talus


HOW CAN WE BUILD UPON IT?

  • Use state of the art knowledge of the behaviour of Reinforced Soil;

  • Consider the past performance of wood supported forest road fills;

  • A healthy respect for the influence of surface and subsurface water on fillslope landslides;

  • Modern road building equipment capabilities; and

  • Team work approach


Design and Construction of Forest Roads Across Moderately Steep to Steep Slopes


Design Considerations

  • Surface and subsurface water control

  • Reinforced soil fills to accommodate steep fillslopes (150 to 400%)

  • Use of geosynthetics, steel and wood where applicable to reinforce the fill (focus on reinforcement not retention)

  • GLOBAL STABILITY

  • Constructability and equipment utilization

  • Design life of road


Internal Stability


External Stability


Forest Road Design Limitations

Based on Assumed Site Conditions with very limited subsurface data

therefore


Construction

  • Construct in accordance with the intent of the critical design details

  • Confirmation of actual site conditions is required during construction

  • Some design and construction details are likely to change as a result of the knowledge of the actual site conditions

  • Flexibility required in construction and design to facilitate changes required to suit actual site conditions


Possible Design Cross-Section Sketch

Horizontally Continuous Layers of wood reinforcement (puncheon)

Original ground surface

Cutslope

Geosynthetic Reinforcement

Well Compacted Mineral Soil Fill

Subdrain

Wood Reinforced Soil


Reinforced Fillslope Model

FoS = 1.25 with out consideration for soil suction


QUESTIONS

Can the incidence of road fillslope landslides be reduced?

YES

Can forest road construction practises be improved and/or economized?

YES

Can both be done at the same time?

YES


Further Studies

  • Extend similar research into other geographic areas

  • Conduct research into the effects of root generated soil matric suction


THE BEGINNING

Thank you


RESULTS OF LIMIT EQUILIBRIUM ANALYSES OF SLOPE STABILITY


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