STABILIZATION OF UNSTABLEROADFILL USING EXPLOSIVES: A CASE STUDY FROM MCNAUGHTONCREEK, BRITISH COLUMBIA

by

C. Wilson Muir, Michael P.Wise,Doug Erickson, and DenisCollins

To appear in the proceedingsof:

IUFRO International MountainLogging and 10th PacificSkyline Symposium,Corvallis, OregonMarch 1999

ABSTRACT: The stabilization ofroadfills on abandoned forestroads is a key component offorest road deactivation. Deactivation requires the useof tracked excavators toretrieve potentially unstableroadfills and restorehillslope hydrology. In caseswhere access for trackedexcavators is not possible ortoo costly, explosives can beused to stabilize roadfills atisolated or intermittentlocations. A methodology forusing explosives to stabilizeroadfills at McNaughton Creek,British Columbia is discussed.

KEYWORDS-- forest roadstability, deactivation,stormproofing, explosives

INTRODUCTION

Deactivation, or the“ deconstruction” and“ stormproofing” of abandonedforest roads, is often carriedout to prevent landslides(Moore, 1994). Deactivationconsists of restoring thehillslope drainage paths topre-road conditions,stabilizing roadfill byretrieving fill which mayinitiate landslides, andrehabilitating the roadcorridor to increase siteproductivity (Baumann et al,1995).

While most roads can bedeactivated using trackedexcavators, a number of areas

that require deactivation thatare inaccessible or noteconomically feasible todeactivate using excavators. In these situations,explosives may be used todeactivate areas withcomparable cost andsatisfactory results.

Although the use of explosivesmay cause a landslide, due tocontrolled factors thelandslide is smaller and drierthan if no action is taken,and poses less risk to theenvironment. Beforedeactivation with explosivesis performed, a detailed siteinvestigation and costanalysis should be completedto ensure that the use ofexplosives is the bestgeotechnical, environmentaland financial option.

SITE CONSIDERATIONS

Logging road construction onsteep, glacially oversteepenedslopes was commonly carriedout in coastal B.C. using cutand fill methods. Such roadson landslide-prone terrain,once abandoned, have beenproven to be unstable innumerous cases. Deactivationusing tracked excavators isthe best means to stabilizethese sites, but the lack ofaccess can make the use oftracked excavators costprohibitive.

The use of explosives as analternative method for

stabilization should beconsidered if the roadrequires roadfill retrieval atisolated locations andcontains one or more of thefollowing characteristics:• the immanent failure at the

site would causesignificant damage,possible serious injury, ordeath, and time is notavailable to mobilizetracked equipment to thesite;

• drainage structures orbridges that have beenpreviously removed orwashed out, eliminatingaccess;

• remote location wheremobilization costs (i.e.barging) may exceed 25% to40% of the project cost;

• where previous deactivation(roadfill pullback) haseliminated access.

If the site conditions dictatethat blasting is a possibleoption, a cost comparisonshould be carried out todetermine if the use ofexplosives is feasible. Itemsto be included in the costestimate are:• number of crew required;• access requirements

(helicopter, ATV, or foot)and travel time;

• day rate of licensed andqualified blaster andassistants;

• costs of explosives,magazine rental (if

required), andmiscellaneous supplies;

• mobile blast shelter forcrew, if needed;

• training or monitoring, ifneeded.

METHODOLOGY

The use of explosives fordeactivation has been carriedout on a trial basis for bothstabilizing roadfills andremoval of culverts (Erickson,1994). For roadfills,explosives must be locatednear the face of the slope andburied so that the energy ofthe blast is forced down andoutwards. It is important toachieve as much depth aspossible during blastholeexcavation, since explosiveeffectiveness greatlyincreases with depth ofburial. Since access islimited, hand tools aretypically the only means ofblasthole excavation and it isoften necessary to perform twoseparate blasts.

The first blast is small andintended to create“ springholes” . The firststep is to use a large metalbar to make small, deep holesin the soil. These holes areplaced about 4ft (1.3m) apart,with a minimal depth of 2ft(0.6m), and along a line witha suitable burden for thesecond blast. Each hole isthen loaded with stick powder,approximately 1 stick forevery 1m of hole depth,

backfilled with soil, anddetonated with no delays. Itis also important to blast anykey stumps or large logs atthe toe of the roadfill.

The result is loose soilaround each barred hole thatmay be excavated to a depthand diameter much larger thanexcavating using a metal baralone. This loose soil ismucked out with shovels andthe metal bar and loaded withstick powder and bulkexplosives (AMEX or ANFO) forthe second blast.Approximately 3kg of explosiveis suggested for every 0.3m ofhole depth in dry conditionsfor the second blast. Backfilling the holes withwell graded gravel is done toconfine the blast and helpdirect the energy of theexplosive laterally towardsthe roadfill. The holes aredetonated simultaneously forthe greatest amount of heave.

The intent of the second blastis to throw material down theslope in a dispersed manner. This increases stability ofthe site by unloading the topof the fill, and does notsignificantly decrease thestability of downslope areas. Note that further blasts maybe required if roadfillremains.Figure 1: Location (inset)and site map.CASE STUDY: McNaughton CreekLanding Deactivation

The McNaughton Creek watershedis located on the East coastof Vancouver Island,approximately 20km south ofCourtenay (Figure 1). Thesite is an abandoned landing,at an elevation of about 1000mand adjacent to a first ordercreek gully. These landformsare known to be prone tolandslides following clearcutharvesting (Rollerson et al,1994; Millard et al, 1998).

The landing has been isolated

by previous deactivation,situated at the end of a 650mspur road of permanentlydeactivated logging road(Figure 2, Photo 3). Accessto the landing is by foot onlyand several trips wererequired to transportexplosives, lay blasting wire,and keep a safe distance fromthe blast area.

Figure 2: Site sketch showinglanding and creek gullychannel

The surficial materials in thearea are a morainal veneerwith minor amounts ofcolluvium. Construction ofthe landing created anoversteepened fill slope,containing a mixture of soil,blasted rock, logs andsupporting stumps left inplace. As a result ofyarding, a large amount of LWD(large woody debris) hadaccumulated on top of thefillslope. The landingfillslope was measured asgreater than 50E. A smalllandslide, approximately 7 to10 ft (2 to 3m) wide, occurredat the north end of thelanding and terminated in thegully below. This small eventoccurred some time after thesite was deactivated. A largetension fracture, located

about 15ft (5m) from theeastern edge of the roadfilland approximately 30ft (10m)in length, was also found,indicating that a much largerfailure was likely.

The slopes below the landingare oriented directly abovethe first order gully. Iffailure were to take place,debris would flow directly inthe gully and sediment wouldbe transported to Rosewallcreek, causing a significantimpact on fish habitat. Ifthe debris slide from thelanding were to initiate alarger debris flow event, theresidential development andhighway could be impacted.

Following a detailedinvestigation of the landing,blasting was deemed to themost cost effective method tostabilize the slope. Threeblasts were required tocomplete the project,requiring two field days. Thefirst blast removed the LWD,while the second blast createdspringholes for the thirdblast. The final blastproduced a controlled debrisslide to remove fill material,and stabilized the slope.

Blasts A and B: Springholesand LWD Removal - The groundnear the outer edge of thelanding was compacted andcontained many boulders. Seven springholes were located

as close to the LWD aspossible, approximately 4.3ft(1.3m) apart and bored with aheavy iron bar and shovelsuitable for stick explosives(Fig 3). A generic 1” by 8”emulsion stick was used tocreate a high energy, lowheave blast to loosen thesoil. After the sticks wereplaced in the bottom of eachhole, the holes werebackfilled with dirt tooriginal ground level anddetonated simultaneously. Thelinear length of roadfillstabilized was about 33ft(10m). Table 1 presents thedata for the springholes.

Figure 3: Using a metal barto excavate springholes forBlast A.

Table1: Springhole DataSpringho

le #Depthft (m)

# ofSticks

1 3 (1)

2/3

2 4.3(1.3)

1

3 3 (1)

1

4 2 (0.6)

3/4

5 3 (1)

1

6 4.3(1.3)

1

7 4.3(1.3)

3/2

After this blast, thespringholes were mucked outwith shovel and heavy steelbar, filled with plastic bagscontaining AMEX II bulkexplosive (ANFO), andbackfilled to the groundsurface. A single stick wasused a primer in the bottom ofeach springhole. Data for theLWD blastholes are given inTable 2.

Table 2: LWD Blasthole DataHole#

Depthft (m)

AMEXlb

(kg)

# ofSticks

1 4.3(1.3)

22(10)

1

2 3.6 22 1

(1.1) (10)3 2.5

(0.8)22(10)

1

4 3 (1)

22(10)

1

5 4.3(1.3)

22(10)

1

6 3 (1)

22(10)

1

7 4(1.2)

22(10)

1

Figure 4: Excavation ofspringholes using a handshovel.

The blasthole charges weredetonated simultaneously tocreate the most heave,projecting material as far aspossible. The amount of AMEXII used per hole reflected thedry moisture conditions.Immediately to the north ofthe 7th hole, two large stumpswere located approximately10ft (3m) downslope from theedge of the landing. A chargeof 80lbs (35kg) of was placedat the base of these stumps inan attempt to remove them andinitiate the movement of theLWD amongst, and below thesestumps. This charge weredetonated in conjunction withthe holes.

The results from this blastwere satisfactory. Asignificant amount of LWD wasdislodged from the landing anda 10 to 13ft (3 to 4m) widebench was created as a stagingarea for the second blast.

Blast C: Fill Removal -Preparation of the fillremoval blast began on thesecond day. Adequate grounddisturbance from the LWD blastallowed holes to be dug forthe fill removal blast withoutthe need of springholes. Theblastholes were positionedwith a burden of 10 to 13ft (3to 4m) from the edge of the

landing, at the break in slopeof the bench created by theLWD blast. Six blastholes,spaced 4.3ft (1.3m) apart,were detonated simultaneouslyusing 1 stick per blasthole asa primer. Proportionally morebulk explosive was added toeach blasthole due to theincreased moisture content ofthe soil from overnightrainfall (Table 3).

Table 3: Fill RemovalBlasthole Data

Hole#

Depthft (m)

AMEXlb(kg)

# ofSticks

1 2.5(0.8)

33(15)

1

2 2.5(0.8)

33(15)

1

3 2.5(0.8)

33(15)

1

4 2.5(0.8)

33(15)

1

5 2.5(0.8)

33(15)

1

6 2.5(0.8)

33(15)

1

7 1.5(0.5)

22(10)

1

There was evidence of materialblown back towards the

western, stable area of thelanding. This debris is mostlikely from blasthole #6,where the short depth of 1.5ft(0.5m) was likely inadequateto confine the charge andforce the blast energydownwards.

Time and Cost Estimates - Thecosts for this project wererecorded to determine unitprices for individual tasks. Table 4 presents time and costestimates. The unit cost ofthis project was $7.70/m3

($70/lin m) of materialremoved, not including ofmobilization costs.

Table 4: LWD Blast Time and Cost Estimate (excludestransportation costs)

Task / Product Man Hours/ Total

Unit Cost Total Cost ($)

dig springholes 3.5hr $45.00/hr 160.50load springholes 0.5hr $45.00/hr 22.50dig LWD holes 3.5hr $45.00/hr 160.50load LWD holes 0.5hr $45.00/hr 22.50dig Fill removalholes

3.0hr $45.00/hr 135.00

load Fill Removalholes

0.5hr $45.00/hr 22.50

sticks 20 sticks $.72/stick 14.40AMEX II, lbs (kg) 310 (140) $0.43/lb

($0.20/kg)133.30

CORDTEX 25, ft (m) 100(30.5)

$0.16/ft($0.52/m)

16.00

safety fuses 3 $2.20/fuse 6.60Total* 693.80

Figure 5: Cross sections of landing and roadifill slope beforeand after blasting.

Conventional deactivationmethods using a large trackedexcavator range between 3 to$10/yd3 , independent ofmobilization, access, anddemobilization costs. In thisexample, these costs would beextremely high, taking intoaccount the time required totransport the excavator to thesite and reconstructing 650mof fully deactivated loggingroad to access the landing. It should also be noted thatsome decreased costs would berealized if a larger scaleproject was carried out.

SUMMARY

The use of explosives for roaddeactivation may be the bestoption in remote areas,locations where access hasbeen lost, or regions wherework is required for isolatedlocations. A detailed siteinvestigation and costanalysis must be carried outto ensure that blastingtechniques will provide asound geotechnical,environmental and costeffective solution. Asdemonstrated at McNaughtonCreek, the use of explosivescan be an adequate and costeffective stabilizationtechnique to reduce thelikelihood of landslides fromunstable roadfills.

REFERENCES CITED

Baumann, F.W., Wise, M.P., andYonin, D. 1995. GeotechnicalEngineering for Logging RoadDeactivation. Proceedings fromthe 48th Canadian GeotechnicalConference, Vancouver, B.C.: Pages 763-770

Erickson, D. 1994. BlastingAgents and Techniques -Applications for Forest RoadDeactivation. Unpublishedreport to Ministry of Forests,Bella Coola Forest District.

Moore, G.M., 1994. ResourceRoad Rehabilitation Handbook: Planning and ImplementationGuidelines (Interm Methods). Watershed RestorationTechnical Circular No. 3,Ministry of Forests andMinistry of Environment, Landsand Parks, Government ofBritish Columbia.

Millard, T.H., Wise, M.P.,Rollerson, T.P., Hogan, D.,and Chatwin, S.C. 1998. Gully system hazards, risksand forestry operations incoastal British Columbia. Proceedings, 8th InternationalAssociation of EngineeringGeologists, Vancouver. Editedby Moore, D.P. and Hungr, O.

Rollerson, T.P.; Thomson, B.;and Millard, T. 1997. Identification of coastalBritish Columbia terrainsusceptible to debris flows. First International Symposiumon Debris Flows. United

States Geological Survey /ASCE.

THE AUTHORS

Note: This project wascarried out while all of theauthors were employed by theB.C. Ministry of Forests.

C. Wilson Muir is currently ageological project engineerwith Knight-Piesold Ltd.Contact address:1400 - 750 West Pender StreetVancouver, B.C. Canada V6C2T8

Mike Wise is currently ageotechnical / geologicalengineer International ForestProducts. Contact Address:P.O. Box 49114, Tower 4,Bentall CentreVancouver, B.C. Canada V7X1H7e-mail: mike_wise@interfor.com

Doug Erickson is the DistrictEngineering Officer withMinistry of Forests.Contact address:South Island Forest District4227 6th AvenuePort Alberni, B.C. Canada V9Y 4N1

Denis Collins is a WatershedRestoration Specialist withthe Ministry of Forests. Contact Address:Ministry of Forests, VancouverRegion2100 Labieux Road

Nanaimo, B.C., Canada V9T6E9e-mail: denis.collins@gems4.gov.bc.ca