2007 Assessing Slope Protection Methods for Weak Rock Slopes in Southwestern Taiwan Engineering Geology

(2007) 100–116www.elsevier.com/locate/enggeo

Engineering Geology 91

Assessing slope protection methods for weak rockslopes in Southwestern Taiwan

Der-Her Lee a,b, Yi-En Yang a,c,⁎, Hung-Ming Lin d

a Department of Civil Engineering, National Cheng Kung University, Tainan, Taiwanb Sustainable Environment Research Center, National Cheng Kung University, Tainan, Taiwanc Department of Construction Engineering, Nan-Jeon Institute of Technology, Tainan, Taiwan

d Department of Construction Technology, Leader University, Tainan, Taiwan

Received 3 July 2006; received in revised form 15 December 2006; accepted 20 December 2006Available online 10 January 2007

Abstract

Failures of weak-rock slopes adjacent to roadsides in southwestern Taiwan most often occurred during or immediately afterheavy rainfall. Field survey of weak rock slopes along the national South-2 Freeway conducted in this study showed that the slopeprotection methods employed in the study areas mainly included vegetation (82.0%), prestressed rock anchors with vegetation(6.8%), grille beam (5.9%), rock anchors with grille beam (3.6%). The highest failure rate occurred in the slopes that wereprotected by the vegetation method. The most frequently encountered weak rock formation along the South-2 Freeway is thealternating sandstone–shale formation (36.3%), followed in sequence by sandstone (24.4%), conglomerates (21.9%) and mudstone(17.4%). The field survey also found that the mudstone slopes present the highest failure rate among all rock types, and the mostcommonly encountered modes of failure were surface erosion and shallow slides. Factors affecting slope failure include inadequatedrainage of storm water runoff, disparate rock types and vegetation on slope surfaces, slope angles and heights. This paper presentsresults of the field survey of the rate of failures of weak rock slopes in Southwestern Taiwan and examines the attributes of slopefailures and the effectiveness of commonly used slope protection methods in the region. Requirements or essential features of aneffective slope protection method are then presented along with the preliminary results of its field implementation.© 2007 Elsevier B.V. All rights reserved.

Keywords: Weak rock; Mudstone; Slope failure; Slope characteristics; Slope protection methods

1. Introduction

Weak rock or soft rock is a geological material harderand stronger than engineering soils, but not behaving asa hard rock. Whether a rock is a weak rock is commonlyjudged with the rheological properties of the rock,namely, the deformability, the uniaxial compressive

⁎ Corresponding author. Department of Construction Engineering,Nan-Jeon Institute of Technology, Tainan, Taiwan.

E-mail address: [email protected] (Y.-E. Yang).

0013-7952/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2006.12.005

strength and shear strength and its time dependency(Oliveira, 1993).

Mudstone is a poorly indurated weak rock having thetexture and composition of shale, but lacking its finelamination and fissility. According to Goodman (1993),mudstone is the preferred name for a silt/clay sedimen-tary rock that lacks lamination or fissility, although thename of mudrock is popular in many parts of the world(Cripps and Taylor, 1981; Dick and Shakoor, 1992).Durability is the most important property of mudrocksin projects that involve exposure of mudrocks to

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http://dx.doi.org/10.1016/j.enggeo.2006.12.005

101D.-H. Lee et al. / Engineering Geology 91 (2007) 100–116

weathering. The presence of nondurable mudrocks in anenvironment of severe weathering is generally recog-nized as the main reason for the instability problems ofmudrock slopes (Dick and Shakoor, 1992). Physical andengineering properties of mudstones have been studiedand reported in many parts of the world (Dick andShakoor, 1992; Chang et al., 1996).

This paper presents the results of field study in-volving the collection of geological data along theSouth-2 (S-2) Freeway in the southwestern region ofTaiwan detailing the effectiveness of existing slope pro-tections. Because of the scarcity of land for economic

Fig. 1. Geological map along S-2 Freeway Ce

development in southwestern Taiwan, the S-2 Freewayand its network highways were routed through unstableweak rock foothills, characterized by geological depos-its consisting mainly of Neogene debris sedimentaryrock (Fig. 1). The rock types along the S-2 Freeway,to the south of Meishan, mainly consist of sandstone,alternating sandstone–shale formation, and mudstone.These weak rocks pose a significant challenge in termsof hillside roadway maintenance. Sandstones are looselycemented, mudstone can easily be softened and slakedwhen it gets wet, and alternating sandstone–shale for-mation is geologically unstable (Lee et al., 1996). The

ntral Geological Survey, MOEA (2000).

102 D.-H. Lee et al. / Engineering Geology 91 (2007) 100–116

adverse characteristics of weak rocks, coupled withthe steep, high and long slopes created during the con-struction of the freeway, have exacerbated the alreadyunstable nature of these foothills in southwesternTaiwan. Consequently, slope failures regularly occurredboth during and following the construction of the S-2Freeway.

Among the weak rock slopes in the study area,mudstone slopes are particularly susceptible to erosion.Distinct wet and dry seasons characterize the weather inthis area. Heavy rains during the summer and dryweather during the fall and winter severely impacts localterrain, resulting in hillsides nearly devoid of vegetation.

Fig. 2. Geological map of the Southwestern

Consequently, severe erosion coupled with poor vege-tation results in extremely unstable mudstone slopes.Large amounts of rainfall during the typhoon seasoncompound these erosion difficulties, resulting in nearconstant slope failures and debris flows. For instance,a slide failure occurring on steep slopes in a rock-anchored area at mileage post-300 km+600 of thenorthbound South-2 Freeway in July 1997 was inducedby heavy rainfall into the mudstone slopes. Similarincidents occurred on the northbound Freeway as well,between markers 362 km+200 to 364 km+800 in 1999and 2000, and markers 373 km+200 and 373 km+700during July to August 2001. Another slide occurred near

mudstone area near Tainan, Taiwan.

Table 2Results of the swelling pressure tests in the study area

Test method Swelling pressure (kgf/cm2)

CPC Kaohsiung base Tianliao

Fresh mudstone Remolded mudstone Freshmudstone

Different pressuremethod

2.86 0.90 4.0

Strain-controlledswelling test

1.39 0.57 –


the south Jhongliao Tunnel portal, at 380 km+700 to381 km+000 of the southbound S-2 Freeway involvinga grille beam as a result of torrential rains loosening thealready unstable soil (CEC, 1999; Chen, 2002).

The attributes of slope failures and the effectivenessof commonly used slope protection methods in theregion will be first examined followed by a subsequenttreatment of the requirements and essential featuresnecessary for effective slope protection design. Theultimate goal of the study was to develop an effectiveand economic slope protection method based on thecharacteristics of mudstone and climatic conditions inthe Southwestern Taiwan.

2. Characteristics of mudstone and other weakrocks in Southwestern Taiwan

2.1. Distribution of mudstone in Southwestern Taiwan

In Taiwan, exposed mudstone formations largelypredominate in the foothills of the southwestern regionof the country, the southern part of the East CoastMountain Range, and the Hengchun Peninsula. Thesouthwestern region has the largest area of mudstoneexposure, ranging from the city of Chiayi in the north toKaohsiung in the south and covering an area over1000 m2. Fig. 2 shows the distribution of mudstone inthis region (Lee et al., 1996; Wang and Huang, 2002).This mudstone range was largely formed in theMiocene, Pliocene and Pleistocene, and as such, it isprone to swelling and slaking with water, and has weakerosion resistance to stream flows, surface runoffs andrainfall. Severe erosion often results in bald landformswith gullies and exposed surfaces (Chang et al., 1996).

2.2. Physical and chemical properties of the mudstone

Chemical composition of this mudstones includesSiO2 (63.49%), Al2O3.Fe2O3 (21.53%) and CaO(2.71%). Primary mineral components of the mudstone,based on X-ray diffraction testing results of Tsai (1984),include illite (30.54%), chlorite (28.70%) and quartz(28.45%). Secondary mineral constituents includefeldspar, calcite and kaolinite. The particle size distri-

Table 1Results of the slake durability test in the study area

Tianliao mudstone Id1 (%) Id2 (%)

Fresh mudstone 73.6 51.7Fresh mudstone 81.3 65.6Surface weathered mudstone 68.5 44.5Surface weathered mudstone 69.2 51.1

bution of the weathered mudstone shows the averageconsistency is 49% of silt-size particles, 29% of clay-size particles, and 22% of sand-size particles. The indexproperties of the mudstone are: LL (liquid limit)=38, PL (plastic limit)=24, Gs (specific gravity)=2.74,γd (dry unit weight)=1.77 gf/cm3 (17.4 kN/m3), ande (void ratio) =0.51. The weathered mudstone insouthwestern Taiwan is a low-plasticity clay, classifiedas CL according to the Unified Soil ClassificationSystem (USCS) (Lee et al., 1996).

2.3. Slaking and swelling characteristics of themudstone

While possessing relatively high retention strengthin dry conditions, the strength of mudstone can bedrastically reduced when it absorbs too much water. Aswater infiltrates the mudstone, the diagenetic bondsbetween mudstone grains are gradually destroyed. Asthese bonds are destroyed, the recoverable strain energystored in the compressed and deformed mudstone grainsis released (Bjerrum, 1967). As the space between theseparticles is widened, an associated swelling of the clayminerals also occurs, resulting in swelling and slakingphenomena of the mudstone (Lee et al., 1994).

Table 1 shows results of the slake durability test(Franklin et al., 1979) of the mudstone taken fromTianliao, Kaohsiung County. The first-cycle slakedurability index values of fresh mudstone (i.e., Id1),are 73.6% and 81.3% respectively, falling within therange of 60 to 85% under the category of low durabilityas suggested by Gamble (Goodman, 1980). The second-cycle slake durability index values (Id2) are 51.7% and65.6%, which are within the ranges of 30 to 60% and 60to 85% respectively, so they fall under low to mediumdurability categories. The slake durability index valuesof surface weathered mudstone in both cycles fall withinthe range of low durability.

If an external force is imposed to constrain theswelling of the mudstone, pressure resulting from this

Table 3Comparison of slope protection methods in the study area

Stabilization methods Functions Suitability

Erosionprotection

Geologicalstability

Ecologicalconsiderations

Natural materialconsiderations

Inclinationlimit

Slope protection methodsVegetation 1 3 1 2 3 b35° slopeGrille beam 1 3 2 3 2 b45° ~60° slopeFabric form 1 3 2 3 2 b45° ~60° slopeShotcrete 1 3 3 3 1 Preventing water from entering the slopeGreen coating 1 3 3 3 1 Preventing water from entering the slopeStone masonry 1 3 3 3 2 b45° ~60° slope

Geological stability methodsGabion 3 2 2 2 2 Stabilize slope toesPrestressed rock anchor 3 1 3 3 1 Deal with geological instabilityConcrete retaining wall 3 2 3 3 2 Increase slope stability

Rating: 1-good, 2-acceptable, 3-bad.


swelling can be measured. Table 2 shows results ofthe swelling pressure. The mudstone taken from CPCKaohsiung base with the different pressure method andthe strain-controlled swelling test method (Proter andNelson, 1980) show that the swelling pressure is in therange of 1.4 to 2.9 kgf/cm2 (137 kN/m2 to 284 kN/m2).The swelling pressure of the remolded mudstone ismuch lower than that of fresh mudstone (Lee et al.,1994, 1996). Furthermore, as with the different pressuremethod, the swelling pressure of fresh mudstoneobtained from Tianliao in Kaohsiung County isapproximately 4.0 kgf/cm2 (392 kN/m2).

2.4. Characteristics of other weak rocks

2.4.1. SandstoneIn weak rock areas of the southwestern sections of

Taiwan, sandstone formation is primarily exhibited inthe exposure points of the Hsiangshan facies of theToukoshan, Liushuang and Nanshihlun Sandstoneformations. Sandstone within the Hsiangshan facies ofthe Toukoshan formation is cemented by silty sedi-ments. Younger sandstone is poorly cemented, prone toslaking in water with an extremely weak weatheringresistance. Rock types in the Liushuang Formationmainly consist of three variants; thick-bedded massivesandstone, thick sandstone interbedded with thin layersof mudstone, and interbedded sandstone and mudstoneformations. The peak shear strength parameters (cp andϕp) are 3.8 kPa and 29.7°, respectively as derived fromthe direct shear tests which the specimens are sub-merged in the shear box for 48 h before the test starts,while the parameters from residual shear strength tests,cr and ϕr, are 0 and 26.3°, respectively (Lin et al., 2005).

2.4.2. ShaleShale of southwestern Taiwan is primarily exhibited in

exposed Kaitzuliao Shale formations. Shale is a fine-grained argillaceous rock with foliations that are usuallygrayish black or black, forming planes of fissility. Becausethis foliation constitutes a weak plane with little shearstrength that is further reduced due to the effect of waterinfiltration and slides easily occur along foliation planeson dip shale slopes. For interbedded sandstone–shaleformations consisting of sandstone and shale, waterinfiltrating into these formations accumulates betweenpermeable sandstone and almost impermeable shalebedding surfaces. Thus the sandstone–shale interfacetends to become a weak plane of extreme instability.

3. Erosion, vegetation and slope protection

3.1. Erosion characteristics of the mudstone

In the study of slope erosion in the Nanhua mudstonearea, Chen et al. (1984) and Chang et al. (1996) ob-served that south-facing slopes are more susceptible toerosion than those facing north and that the percentageof bald slopes facing south was also correspondinglyhigher. The angles on these denuded slopes range from36° to 55°, and it was observed that the longer the slopelengths, the more serious the erosion rate, with steeperslopes experiencing lower rates of erosion. The erosiondepth of the bald mudstone slopes often exceeds 10 cm,while the infiltration depth of ground water may reachapproximately 20 to 40 cm vertically below surfaces(Chang et al., 1996).

Compared to bald slopes, slopes covered withvegetation generally experienced less erosion depth


and land loss. Comparison between different vegetativeplants indicates that herbaceous plants (e.g., Paspalumnotatum and Cynodon dactylon) or herbaceous plantsmixed with woody plants (e.g., Rhus semialataroxburgiana and Psidium guajava) can better protectslopes than woody plants alone (Chiu, 1999). In order toexplore the effects of mudstone soil conservation with

Fig. 3. Slope protection methods used along S-2 Freeway: (a) vegetation, (anchors with grille beam, (e) concrete stairs with shotcrete, (f) precast ceme

the vegetation method in cut slopes, Kuo and Lin (1998)used erosion pins to conduct erosion experiments onmudstone slopes. They concluded that the erosion onmudstone slopes was largely affected by the rainfall, andthat the erosion depth of bald mudstone slopes (at aslope angle of 33° with a southwest aspect) averagedabout 7.94 cm.

b) prestressed rock anchors with vegetation, (c) grille beam, (d) rocknt frame, (g) gabion, (h) fabric form (upper).


3.2. Vegetation on mudstone slopes

Several investigators have previously studied theissue of using vegetation to prevent erosion of mudstoneslopes in the south of Meishan region (e.g., Lee et al.,1996; Chen, 1999). A summary of events possiblyleading to severe erosion is presented herein. In dryseasons, plant roots are prone to damage from sig-nificant shrinkages and cracks that generate exfoliationinterfaces on the mudstone surfaces. Since the mudstoneis loosely cemented, and is subjected to weatheringwith water, exfoliation interfaces may cause immediateslaking and loss of mudstone surfaces, which leads tofurther runoff erosion and exposure of plant roots.Mudstone is structured so closely that water cannoteasily infiltrate. Since these mudstone material surfacescontain no geologic aquifers, existing plants usually diefrom lack of water. Future growth is also limited becauseplant roots are unable to penetrate these denselycompacted surfaces. Large amounts of soluble salts inthe soil make the ground nearly inhospitable to growthfrom all but the hardiest plants, thus increasing theincidents of more surface slides.

According to Lin (1997), the annual dry season insouthwestern Taiwan which lasts nearly 6 months, is themajor obstacle preventing vegetation growth in thesemudstone areas. Therefore, the species selected to “re-vegetate” these mudstone hillsides should be selectedfrom creeping grasses (e.g., C. dactylon) that are bothalkali and drought resistant and convenient in terms ofboth seed production or propagation. Close monitoringand maintenance of nascent plantings in this mudstoneregion is critical to the success of a re-vegetationprocess. As these seedlings take hold and the sites

Table 4Lengths of sections protected by various methods along S-2 Freeway includ

Road section Length orpercentage

Slope protection methods

Vegetation Rock anchor+vegetation

Grillebeam

Rg

Meishan to Sinhua(279 K to 347 K)

Length(m)

13,910 0 0

Percentage 91.9% 0 0Sinhua to Jiouru(347 K to 391 K)

Length(m)

12,750 2670 1260

Percentage 74.1% 15.5% 7.3%Kaohsiung to Cishan(0 K to 33 K)

Length(m)

5720 0 1090

Percentage 79.9% 0 15.2%Total Total

length (m)32,380 2670 2350 1

Percentage 82.0% 6.8% 5.9%

become more hospitable to plant growth, more speciesmay be introduced into the region.

3.3. Existing mudstone slope protection methods

Rainfall, slope angle and slope height are consideredthe main factors for controlling the failure of mudstoneslopes, while the slope aspect, or slant is the factormost affecting the successful growth of vegetation onthese hillsides. Using the knowledge of mudstone slopefailure characteristics, Lee (1992) recommended thatboth slope angle and slope height be considered in anymudstone slope protection and re-vegetation initiative.His research called for mudstone slopes (especiallyslopes with very steep and lengthy grades) to beredesigned into a “terraced” system. This new landscapedesign would involve cutting the larger slope into asmany smaller terraced slopes as possible, each with aheight of 5 m. For slopes with angles greater than 40°,sophisticated civil engineering slope protection methodsmust be used to ensure slope viability.

The slope protection methods commonly used by theNational Expressway Engineering Bureau and otherpublic agencies and private engineering firms includevegetation, grille beam, fabric form, shotcrete, greencoating, stone masonry, gabion, prestressed rock anchorand concrete retainingwall. The following factors shouldbe considered when selecting proper slope protectionmethods: the objectives and extent of protection to beachieved; the geological structure of the slope; the angle,height and length of the slope; and the constraints andadvantages of various slope protection methods in termsof erosion protection, geological stability, ecologicalconsiderations and natural material considerations.

ing branch line, No. 10 Highway (279 K to 391 K and 0 K to 33 K)

ock anchor+rille beam

Concrete stairs+shotcrete

Precastcementframe

Sectiontotal

Total lengthpercentage

910 0 320 15,140 38.3%

6.0% 0 2.1% 100%520 0 0 17,200 43.6%

3.1% 0 0 100%0 350 0 7160 18.1%

0 4.9% 0 100%430 350 320 39,500 100%

3.6% 0.9% 0.8% 100%


Table 3 provides a comparison of the commonlyemployed slope protection methods.

4. Field investigation of mudstone slope problemsalong South-2 (S-2) Freeway

4.1. Slope protection methods employed in S-2 Freeway

The field survey shows that the slope protectionmethods used in road sections of S-2 Freeway to thesouth of Meishan include vegetation, prestressed rockanchors with vegetation, grille beam, rock anchors withgrille beam, concrete stairs with shotcrete, precastcement frame, gabion and fabric form designs (Fig. 3).In 91.9% of the slopes, between Meishan and Sinhua,vegetation has been the primary method of holding thesoil in place, followed by 6.0% of the slopes being

Fig. 4. Slope protection problems along S-2 Freeway: (a) drainage effectivenethe interface of the vertical ditch and the slope surface resulting in a large hospace slope damage due to surface runoff overflows below the platform,; (e) sof slope; and (f) severe slope erosion.

secured in place using the rock anchor with grille beammethod and 2.1% of the slopes using other methods.Along the road section between the cities of Sinhua andChiuju, 71.4% of the slopes have been secured using thevegetation method, followed by 15.5% with rock anchorwith vegetation method, 7.3% with grille beams and3.1%with rock anchor with grille beams. Of those slopesadjacent to the Kaohsiung beltway and Cishan branchline, 79.9% are secured with vegetation, 15.2% withgrille beams, and 4.9% with concrete stairs and plusshotcrete. Along the freeway in the south of Meishan, onaverage, 82.0% of the slopes are secured using thevegetation, followed by 6.8%, 5.9% and 3.6% of slopesanchored with prestressed rock, vegetation, grille beam,and rock anchor with grille beam, respectively. Thesurvey results detailing the use of various slopeprotection methods are summarized in Table 4.

ss of reduced due to a weedy transverse ditch; (b) serious erosion due tole; (c) weedy and silt choked transverse ditch; (d) erosion and hollowagging of both platform and slope due to water accumulation at the top

Fig. 5. Slope protection problems along S-2 Freeway: (a) serious erosion of the vegetation in mudstone slope, (b) the occurrence of mudflow in themudstone slope, (c) and (d) severe grille beam slides due to steep slope.


4.2. Slope failure modes

Along the S-2 Freeway to the south of Meishan,slope failures occurred mainly in the form of surfaceerosion, shallow slides and side ditch cracks. Surfacerunoff caused surface erosion of the slopes; gullies couldform first (Fig. 4e) and then developed into more severeerosion (Fig. 4f). Also, vegetation layers could slideeasily due to severe erosion, which often occurred in themudstone and the alternating sandstone–shale forma-tion slopes (Figs. 4d and 5a). Shallow slides oftenoccurred during the season of typhoons and heavy rains(Fig. 3a), especially for mudstone slopes. The depth of

Table 5Quantity of failures along S-2 Freeway including branch line, No. 10 Highw

Road section Percentageof totallength

Failure modes

Surfaceerosion

Shallowslide

Meishan to Sinhua (279 K to347 K)

38.3% Number 14 2Percentage 87.5% 12.5%

Sinhua to Jiouru (347 K to391 K)


Kaohsiung to Cishan (0 K to33 K)


Total 100% Number 82 31Percentage 70.1% 26.5%

Note: Destruction ratio is the ratio of the percentage of total number of failu

slides was generally shallow (about 50 cm), and the sizeof the slides ranged from 5 m×4 m to 60 m×10 m.

Based on field surveys along the road sectionbetween Meishan and Sinhua, there are 14 sites wheresurface erosion is prevalent and two sites where shallowslides have occurred. Thus, the percentages of the failuremodes of “surface erosion” and “shallow slide” in thissection of freeway are 87.5% and 12.5%, respectively.Along the road section between Sinhua and Chiuju,approximately 38 sites have surface erosion, 23 sites areshallow slide areas and 4 sites have side ditch cracks,accounting for 58.5%, 35.4% and 6.1% of total slopefailures, respectively. Along the Kaohsiung beltway and

ay (279 K to 391 K and 0 K to 33 K) — comparison of failure modes

Destructionratio

Side ditchcrack

Total Percentage of total number offailures

0 16 13.7% 0.360 100%4 65 55.5% 1.276.1% 100%0 36 30.8% 1.700 100%4 117 100% –3.4% 100%

res over the percentage of total road length.

Table 6Quantity of failures along S-2 Freeway including branch line, No. 10 Highway (279 K to 391 K and 0 K to 33 K)— comparison of slope protectionmethods

Slope protectionmethod

Totallength(m)

Totallengthpercentage(%)

Failure modes Destructionratio

Surface erosion Shallow slide Side ditch crack Total Total failure percentage (%)

Vegetation 32,380 82.0 72 28 1 101 86.3 1.05Rock anchor

+vegetation2670 6.8 10 3 2 15 12.8 1.88

Grille beam 2350 5.9 0 0 0 0 0 0Rock anchor+grille

beam1430 3.6 0 0 1 1 0.9 0.25

Concrete stairs+Shotcrete

350 0.9 0 0 0 0 0 0

Precast cement frame 320 0.8 0 0 0 0 0 0Total 39,500 100 82 31 4 117 100 –

Note: Destruction ratio is the ratio of the percentage of total number of failures over the percentage of total road length.


Cishan branch line, 83.3% (30 sites) of the failuresshowed evidence of surface erosion and 16.7% (6 sites)were considered shallow slide areas. Of the roadsections investigated along S-2 Freeway to the southof Meishan, 70.1% (82/117) of the slope failures weredue to surface erosion, 26.5% (31/117) of the slopefailures were due to shallow slides, and 3.4% (4/117) ofthe slope failures were due to side ditch cracks. Surfaceerosion was determined to be the most common slopefailure mode along the S-2 Freeway.

In terms of the ratio of destruction, defined as the ratioof failure percentage to length percentage, the slopesalong the Kaohsiung beltway and the Cishan branch linehave the highest ratio of destruction of the three roadsections investigated, while the section between Meishanand Sinhua has the lowest ratio of destruction. The modesand numbers of slope failures are summarized in Table 5.

4.2.1. Distribution of slope failure modes by protectionmethods

Field investigations along the S-2 Freeway to thesouth of Meishan showed that the largest number of

Table 7Quantity of failures along S-2 Freeway including branch line, No. 10 Highw

Rock types Totallength(m)

Totallengthpercentage(%)

Failure modes

Surfaceerosion

Shallowslide

Sandstone 9630 24.4 18 8Alternating

sandstone–shale formation14,330 36.3 34 10

Mudstone 6890 17.4 20 12Conglomerate 8650 21.9 10 1Total 39,500 100 82 31

Note: Destruction ratio is the ratio of the percentage of total number of failu

slope failures occurred on vegetation-protected slopes at101 sites, which accounted for approximately 86.3% ofall recorded slope failures. Of these, surface erosion wasprevalent at 72 sites, shallow slides were prevalent at 28sites and 1 site was the result of a side ditch crack. Therock anchor with vegetation securing method hadthe second highest number of failures, accounting for12.8% (15 sites) of all failures recorded. Of these 15failures, 10 were due to surface erosion, two failureswere due to shallow slides and two failures were due toside ditch cracks. A summary of the effectiveness of theslope protection methods based on the data collectedfrom field investigation of the three road sections alongthe S-2 Freeway is presented in Table 6.

Comparing all methods in terms of the destructionratio, the ratio of destruction was highest for thoseslopes protected using rock anchor plus vegetation andusing just vegetation (1.88 and 1.05, respectively). Theratio of destruction was lowest for those slopes securedwith the grille beam method and that of concrete stairscombined shotcrete (both ratios are essentially zero).These results suggest the inevitability of slope failures

ay (279 K to 391 K and 0 K to 33 K) — comparison of rock types

Destructionratio

Side ditchcrack

Total Total failure percentage(%)

2 28 23.9 0.980 44 37.6 1.04

2 34 29.1 1.670 11 9.4 0.434 117 100 –

res over the percentage of total road length.

Table 8List of possible effects of water on slope surface

Cause offailure

Adverseeffect

Countermeasures Essentialfeature ofeffectiveprotectionmethod

Impact byrainfall

Acceleratethe erosionof mudstoneslope.

Prevent rainfrom directlyimpacting themudstoneslope

Uses filteron themudstoneslope;placementof baselayer forvegetation onthe filter.

Slope runoff Surface runoffcauses theslope erosion;accelerate theweathering ofmudstone.

Reducesurfacerunoff toprotectthe slopesurface.

Uses filter onthe mudstoneslope;placementbaselayer forvegetation onthe filterand setstransverseditch.

Transverseditchfacilitiesimproperlyconstructedor areineffectivedue toweedyvegetation

Transverseditch unable toquickly andeffectivelydrain; surfacerunoffoverflows to theslope belowplatform,causing erosion.

Remove soilweeds fromtransverse ditchto prevent slopeerosion belowplatform.

Increasescross-sectionalarea of thetransverseditch;extend thefilter to theplatform andslope belowthe platform.

Interval ofvertical(longitudinal)ditch is toowide (large)

Transverseditchunable toquickly andeffectively drain;surface runoffoverflows tothe slopebelowplatform,causingerosion.

Properdesign ofverticalditchintervalto preventslopeerosionbelowplatform.

Extends filterto theplatform andslope belowthe platform;Shorten theinterval ofvertical ditch.

Leakagefrom thevertical ditch

Serious erosionof the interfaceof the verticalditch and slope;penetration tothe slope belowthe platform.

Raiseslopesurfaceslightlyhigher thanvertical ditch;avoids lossof soil at theinterface.

Extends filterto the bottomof the verticalditch.


when vegetation is exclusively used as protection forsoft-rock slopes along the S-2 Freeway.

The survey results also show that of the slopes along theS-2 Freeway exhibiting the most severe erosion, the grillebeam slope protection method was the most commonlyemployed remedialmeasure. Thismethod involves the useof grille beams filled with soil bags or vegetative bags forplants as the vegetative base layer. Good stability cangenerally be achieved when slopes initially treated withvegetation are reinforced with grille beam protection.

4.2.2. Distribution of slope failure modes by rock typesThe rock types along the S-2 Freeway to the south of

Meishan are very complex. The S-2 Freeway was builtthrough formations of sandstone, sandstone interbeddedwith mudstone, alternate layers of sandstone and mud-stone, sandstone interbedded with shale or alternatelayers of sandstone and shale, mudstone, mudstoneinterbedded with sandstone, and conglomerate soilsinterbedded with sandstone. To facilitate the analysis, therocks adjacent to S-2 are summarized into four majortypes; sandstone, the alternating sandstone–shale for-mation (also referred to as alternation of sandstone andshale), mudstone, and conglomerate. Among all rocktypes along S-2, the alternating sandstone–shale forma-tion predominates, accounting for 36.6%; sandstone andconglomerate accounts for 24.4% and 21.9%, respec-tively; while mudstone is the least prevalent, accountingfor 17.4%. The proportion of both rock types and slopefailure modes and quantity are summarized in Table 7.Among the four rock types, slopes composed ofalternating sandstone–shale formation have the largestnumber of failures, accounting for 37.6% of the total.This is sequentially followed by slopes comprisingmudstone, sandstone, and conglomerate, accounting for29.1%, 23.9%, and 9.4% of total number of respectiveslope failures. However, in terms of destruction ratios,mudstone slopes have the highest ratio (1.67), followedby the slopes comprising alternating sandstone–shaleformation (1.04), slopes comprising in sandstone (0.98),and slopes composed of conglomerate (0.43).

All previously presented data indicate that the primaryproblem of slope failures along S-2 Freeway lies in slopesurface erosion and shallow slides in mudstone slopes,and that the secondary problem involves slope surfaceerosion in the alternating sandstone–shale formation.

5. Attributes of slope failures and possible remedialmeasures

Results of the field investigation indicate that slopefailures in weak rock areas in southwestern Taiwan is

Table 10Problems related to slope angle and height on mudstone slopes

Cause offailure

Adverse effect Countermeasures Essential featureof an effectiveprotection method

Vegetatedslope istoosteep.

Vegetation isdifficult tomaintain on asteep slope;vegetation layerscan slide easily.

Reduce slopeangle or rebuildvegetation layeron the slope.

Place a base layerfor vegetation inthe grid-frame.

Grillebeamslidedue tosteepslopeangle.

Sliding of thegrille beam andvegetation layer islikely to affecttraffic; extremelydifficult to repairgrille beam.

Reduce weightof grid-frame.

Use lightweightgrid-frame toincrease stabilityand to facilitaterapid repair, ifnecessary.

Table 9Problems to vegetation along mudstone slopes

Cause offailure

Adverse effect Countermeasures Essentialfeature of aneffectiveprotectionmethod

Slopecrackingdue toevaporationof mudstone

Forms apeeledinterface,likelyoccurrenceof bothvegetationslide andweatheringlayer slidein rainyseason.

Reduceevaporation;prevent slideof vegetationand weatheringlayer.

Use filters onmudstoneslope;placementof baselayer forvegetationon filter.

Slopeabsorbingwater andslaking

Accelerateserosion ofthe mudstoneslope; weatheringpenetrates deeperinto mudstone.

Preventmudstoneparticle lossand weathering.

Use filter onmudstoneslope;placement ofbase layerfor vegetationon filter.

Slopeabsorbingwater andswelling

Pressure fromswelling maydestroy slopeprotectionfacilities.

Enhance slopeprotectionfacility tosustain swellingpressure.

Slopeprotectionfacility ableto toleratesomedeformation.


due to water erosion and drainage, the use of onlyvegetation on slopes to protect and secure them in place,and slope angle and slope height. These attributes arefurther discussed and a new slope protection method isthen presented.

5.1. Water and drainage

Water is the most important and destructive elementcausing both slope instability and slope erosion along theS-2 Freeway. Table 8 lists all the possible effects of wateron slopes and the recommended countermeasures.Because the study area has distinct dry and rainy seasons,including heavy rains brought about by typhoons, slopeprotection measures are destined to fail, if the surface andsubsurface water cannot be effectively drained away.Fig. 4 shows examples of slope failures due to improper orinadequate drainage.

5.2. Vegetation on weak rock slopes

Based upon the presented data, slopes along the S-2Freeway protected by only vegetation are susceptible to

failure. In fact, under unfavorable geological, topo-graphical and climatic conditions, which are prevalent inthe southwestern region, vegetation protection methodsare not effective at all. For example, on mudstone slopes,erosion-related problems are severe even with substan-tial vegetation cover. Mudstone is prone to weatheringand runoff erosion can facilitate the infiltration of waterinto the interface between weathered mudstone and freshmudstone, which destabilize the entire mudstone slope.Once the mudstone slope becomes unstable, thevegetation and weathered layers migrate over theinterface to induce further vegetation layer slides. Thepotential problems of mudstone slopes with vegetationprotection are summarized in Table 9. Examples ofmudstone slope failures, including those augmentedwith vegetation protection are shown in Fig. 5(a) and (b).From these results, it becomes unequivocally apparentthat the use of vegetation without proper engineeringmeasures is totally ineffective in protecting mudstoneslopes.

5.3. Slope angles and slope height

Excessive angles and lengths of slopes increase thepossibility of surface erosion and shallow slides. Amongthose road sections surveyed, the roadside slope grade of1:1.2 (approximately 40°) was designed for the sectionbetween Sinhua and Chiuju, the Kaohsiung beltway andthe Cishan branch line. The survey results show thatvegetation slope angles generally ranged from 39° to41°, which is greater than 35° that is considered theupper bound of the angles that allow for natural invasionand propagation of plants. Moreover, the slope heightwas generally in the range of 7 to 9 m. The designed

Fig. 6. Sketch of a new slope protection method (Soil-Tire–Vegetation Method) that has been tested during this study.


slope grade along the section between Meishan andSinhua measured mostly within the range of 1:1.5(approximately 34°), with vegetative slope anglesbetween 33° to 36°. The survey also indicated asubstantial ratio of slope destruction along the roadsection between the Kaohsiung beltway and the Cishanbranch line. Indeed this destruction ratio was 1.70,

Fig. 7. Plan view of the new slope protection method (Soil-Tire–

higher than any other road section studied. Thedestruction ratio of slopes next in sequence to theKaohsiung/Cishan section was between Sinhua andChiuju with 1.27, while the section with the leastamount of measured destruction was between Meishanand Sinhua, with 0.36. This data infers that slopes withgreater angles are more likely to fail, as summarized in

Vegetation Method) that has been tested during this study.


Table 10. Fig. 5(c) and (d) provide photographicevidence for this data.

5.4. Requirements for effective slope protectionmethods

Based on the characteristics of the mudstone andthe results of the field survey, the following recommen-dations are essential for effective slope protectioncountermeasures:

Fig. 8. The field implementation of the Soil-Tire–VegetationMethod: (a) slopelaying the fabric drains; (d) laying the nonwoven geotextile sheet; (e) constructhe Soil-Tire and vegetative bags; (h) placing the vegetative mat.

1) A slope protection method must be able to resisterosion, prevent fine-grained soil loss, and facilitateadequate drainage,

2) A surface water drainage system is required to effec-tively control the rainwater,

3) The slope protection facilities must have adequatestrength to resist the swelling pressure of the mud-stone, and

4) The slope protection facilities must increase slopestability and facilitate vegetation growth.

cut to the design shape; (b) forming multi-stage slope and platform; (c)ting transverse ditch; (f) laying the separator of the Soil-Tires; (g) laying

Fig. 9. The arrangement of test slopes.

Table 11Types of vegetation and slope stabilization methods at test slopes

No. Attitude Area (m2) Vegetation treatment

Originalslope

A1 N40°W/42°SW

69.38 Original slope vegetation

Contrastslope

B1 N40°W/36°SW

112.01 Vegetative mat

B2 N40°W/36°SW


Soil-Tireslopes

C1 N40°W/33°SW

31.84 Soil-tire+Vativeriazizanioides

C2 N40°W/33°SW

18.43 fertilized Soil-Tire+V.zizanioides

D1 N40°W/33°SW

35.30 Soil-tire+vegetative mat

D2 N40°W/33°SW

20.70 Fertilized Soil-Tire+vegetative mat

E1 N40°W/34°SW


E2 N40°W/34°SW


F1 N40°W/34°SW


F2 N40°W/34°SW


Contrast G1 N40°W/36°SW


Slope G2 N41°W/35°SW



Anew slope protection method, Soil-Tire–VegetationMethod, implementing the requirements previouslydescribed, was developed for this weak-rock mudstoneregion (see Figs. 6 and 7 for implementation sketches).To determine its effectiveness, a field experiment hasbeen conducted at a mudstone slope near the proposedTungshan Rest Station of National HighwayNo. 3. Fig. 8shows the steps taken for field implementation of theSoil-Tire–Vegetation Method. The construction of thetest slopes began on 17 September, 2004 and ended on27 November, 2004. The original slope in the experi-mental zone consists of the upper and lower slopesurfaces. As shown in Fig. 9, the test slopes are dividedinto original slopes (A1), contrast slopes (B1, B2 andG1,G2) and Soil-Tire slopes (C1, C2 andD1, D2; E1, E2 andF1, F2). The attitude, area and vegetative treatment ofslopes of various groups are shown in Table 11. Thestrike of slope surfaces in the experimental zone isN40°W with a SW dip direction. The dip angles of theoriginal slopes are about 42°, while those of contrastslopes and Soil-Tire slopes are approximately 36° and34°, respectively. The width of the slopes of variousgroups is about 15 m, and the slope lengths graduallylessen from right to left. With regard to vegetativetreatment for slopes of various groups, the originalslopes are covered with originally vegetative species,while the contrast slopes are laid with vegetative matsafter slope cut to the design specification, and the Soil-Tire slopes are placed with Soil-Tires as the base layer forvegetation. As for the vegetation on Soil-Tire slopes,four Vativeria zizanioides are planted in the central spaceof Soil-Tire on the slopes of C1 and C2, and theremaining slopes (D1, D2, E1, E2, F1, and F2) arecovered with vegetative mats which consist of non-woven and straw vegetative mats. In addition, the seedsof C. dactylon and P. notatum are sowed on these slopes.The total project cost was about 35,000 U.S. dollars.

Table 12 shows the erosion of test slopes in the firstyear of experimentation. During this period, there hadbeen several typhoons and heavy rainstorms and theaccumulated rainfall reached 3553 mm. As shown inTable 12, the original slopes suffered the most seriouserosion (with erosion of 27665 g/m2 on Slope A1). The“contrast” slopes showed significant improvement as faras the erosion is concerned. Finally, the slopes protectedwith the Soil-Tire–Vegetation Method drastically re-duced the erosion and the mud slide potential. This

Table 12Soil erosion as measured at test sites

1

Note: ⁎original slope, ⁎⁎contrast slope (vegetative mat); all others: slopes protected with the Soil-Tire–Vegetation Method.


preliminary result shows the promise of the Soil-Tire–Vegetation Method in the protection of the mudstoneslopes in the southwestern Taiwan region.

6. Concluding remarks

Along the S-2 Freeway in southwestern Taiwan,vegetation protection was the primary slope protectionin 70% to 90% of all cases. Other methods used in-cluded the prestressed rock anchor plus vegetation,grille beams, rock anchor with grille beam, concretestairs with shotcrete, precast cement frame, gabion andfabric form methods. Field surveys indicated that thevegetation protection method was ineffective, particu-larly on mudstone slopes. The most commonly observedmode of failure was surface erosion, accounting forapproximately 70% of all slope failures, followed byshallow slides accounting for approximately 27%. Thelargest number of failures (86%) occurred on vegeta-tion-protected slopes, followed by slope failures whererock anchor plus vegetation was the preferred method(13%). In terms of the destruction ratio, slopes securedwith rock anchor and vegetation had the highest ratio(1.88) and the vegetation-protected slopes had thesecond highest ratio (1.05).

Among the rock types encountered in the S-2Freeway in southwestern Taiwan, the most commonlyseen was the alternating sandstone–shale formation,which accounted for 36.3%, followed in sequence bysandstone (24.4%), conglomerate (21.9%), and mud-

stone (17.4%). In terms of the number of slope failures,slopes composed of alternating sandstone–shale forma-tion had the highest number of failures, accounting for37.6% of all failures. The mudstone slopes had thesecond highest number of failures (29.1%), followed bythe sandstone slopes (23.9%), and the conglomerateslopes (9.4%). In terms of slope destruction ratios, themudstone slopes had the highest ratio (1.67), followedin sequence by slopes composed of the alternatingsandstone–shale formation (1.04), slopes composed ofsandstone (0.98), and slopes composed of conglomerate(0.43).

For mudstone slopes in Southwestern Taiwan,specifically those with high angles and length, thevegetation protection method alone was ineffective.Therefore, additional engineering protection methodsmust be employed along with this method. Consideringthe slope failure characteristics of the study area, thefollowing attributes were deemed essential in designingeffective slope protection methods: the ability to resisterosion, fine-grained soil loss prevention, proper surfacewater drainage systems to control stormwater runoff; theability to resist the swelling pressure of the mudstone;and proper features to increase the stability of the slopeand facilitate vegetation growth. A new slope protectionmethod, called the Soil-Tire–Vegetation Method, imple-menting these requirements, was developed and thepreliminary field test results demonstrated its effective-ness in protecting the mudstone slopes in the south-western Taiwan region. This field experimentation


project showcases a successful application of engineer-ing geology.

Acknowledgments

The study on which this paper is based was supportedby National Science Council through Grant No. NSC91-2211-E-006-047. This financial support is greatlyappreciated and acknowledged.

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2007 Assessing Slope Protection Methods for Weak Rock Slopes in Southwestern Taiwan Engineering Geology