1

1. INTRODUCTION

A NTHS has been conducted for a natural hillside catchment (hereinafter denoted as the “site”) above a

densely populated village (Figure 1). The study shows that CDF hazard exists in the site, i.e. there had been recent persistent landslides with strong geological and geomorphological evidence suggesting CDF could occur. Residential buildings at the toe of the site would be affected. This paper presents the detailed geotechnical design of the mitigation works for the CDF hazard. The design methodology includes the development of geological models, DMM analyses and detailed design for the mitigation measures. The geological models are developed with reference to the NTHS, recommended good practice for design of NTH mitigation measures, site-specific API, site reconnaissance, detailed topographical survey and options available for the mitigation works.

2. BACKGROUND

This site, with an approximate

area of 10,000m2, has an east-trending drainage line (hereinafter denoted as “DL1”) facing a densely populated village (hereinafter denoted as the “Village”). It rises from the Village from +70mPD to an elevation of +140mPD next to an access road. Below the gentle undulating terrain above +130mPD, the site is

ABSTRACT

This paper presents the detailed design of the mitigation works for the channelized debris flow (CDF) hazard identified in a natural hillside catchment facing a densely populated village. This includes a review of a natural terrain hazard study (NTHS), specific aerial photograph interpretation (API), site reconnaissance, option assessment, establishment of geological models, debris mobility modeling (DMM) analyses and detailed geotechnical design for the barriers. The importance of developing proper geological models is highlighted.

Detailed Design of Mitigation Measures for Channelised Debris Flow Hazard

Carie L H LAM

Geotechnical Engineering Office (GEO), Civil Engineering and Development Department (CEDD), The Government of the Hong Kong Special Administrative Region (HKSARG)

Figure 1 – Location plan for the site

40m

Main road

Footbridge

The site

Access road

Past landslides

The Village

Natural terrain

Man-made slopes

N

DL1

2

characterized by steep slopes with intermittent rock outcrop extending down to the lower hill area where the Village is situated. The steepness of the upper hill area makes the site inaccessible from the crest access road. The closest vehicle access point is on the opposite side of a main road 400m southeastwards. Access to the toe of DL1 from the main road has to be via a footbridge and around >20 nos. of village houses (Figure 1).

The 1:20,000 scale geological map (GEO, 1992) shows that the site is underlain by coarse-grained granite. Nearby ground investigations (GI) reveal a 1.5m thick slope wash / colluvial material overlaying a thin layer of completely decomposed granite (CDG) above the bedrock. Design Event Approach (DEA) was adopted in the NTHS, which concludes a Design Event1 of 125m3 in oversteepened valley side-slopes (35o-45o) should be adopted. It was also concluded that CDF along the incised section of DL1 could entrain colluvium at a rate of 0.3m3/m2.

3. SITE-SPECIFIC API

Site-specific API was conducted on low flight (1800ft)

aerial photographs taken in 1964. The incised section of DL1 can be clearly seen under the stereoscopic view. No big boulders were observed along DL1. A rock cliff resulting in an oversteepened terrain was seen behind a recent landslide. The toe of DL1 was observed to be twisted around a hummocky fan structure, suggesting a possible debris lobe at this location (Figure 2).

4. SITE RECONNAISSANCE

Site reconnaissance verified the shallow rockhead profile, particularly near the toe of DL1. The toe of DL1

ends at a basin-like concrete structure, with a diameter of 6.5m, at about +64mPD. About 1m thick of fines was found deposited in the structure during the site visit which suggests possible CDF/ heavy fines washed down from DL1 during rainstorms. The southern side of the structure is an open area. The northern side is a narrow exit streamcourse (<300mm wide) that was separated from the structure by a rock outcrop. Based on an interview with a Village resident, surface water overflow from the basin-like structure would go towards the village houses to the south of the structure, in lieu of the natural exit streamcourse to the north (Plate 1).

About 10m to the east of the structure (i.e. behind the planter, Plate 1) is a 5m high (approximately) rock

cliff. The rock cliff forms a 12m wide platform (hereinafter denoted as “the +69mPD platform”) with both sides bounded by rocks. The +69mPD platform extends for about 4m eastwards and connects to the upstream drainage line. A big rock outcrop (about 2.5m wide) in the middle of the platform exaggerates the depression

1 Design event refers to the magnitude of a “Conservative” landslide hazard that is required to be mitigated against, based on the DEA framework described in GEO Report No. 138 (Ng et al, 2003 and GEO, 2014a).

Rock cliff

Potential debris lobe

Figure 2 – The 1964 aerial photograph

Plate 1 – View of the existing basin-like concrete structure at +64mPD

Exit streamcourse

Rock outcrop

6.5m dia. basin-like concrete structure

Planter

N

Staircase to village houses

+64mPD

The +69mPD platform

Open area

3

(about 1m) of DL1 near the edge of the platform. This gap will be gradually reduce to <300mm with a <2m move upstream (Figure 3).

DL1 is not incised in the lower portion of the catchment, and the streambed is rocky until it branches out at +85mPD (Figure 4). Above +85mPD, DL1 is incised with a channel width of 6.5m. Colluvium is present along this incised channel section which may be entrained at a rate of 0.3m3/m2.

Both the +64mPD and +69mPD platforms are located upstream of the debris lobe identified in API. The closest hazardous location of the CDF is 2m away from the west of the basin-like structure (Figure 3). These two platforms are considered possible barrier locations to contain the CDF hazard. Possible CDF approaching +64mPD may threaten villagers even if it is contained. Therefore the first level of defense should be used at the +69mPD platform. Based on site observations, it is estimated that about 150m3 of debris can be retained with a 5m barrier. To verify the retention volume available at both platforms, detailed topographical survey has been conducted. 5. OPTION ASSESSMENT

Common mitigation strategies include active prevention of landslides by stabilizing the oversteepened valley side-slopes using soil nails and passive mitigation measures by installing flexible and/or rigid barrier to contain the hazard. Given the dense vegetation and steep gradient in the upper and middle part of the site, it is difficult to mobilize plants, material and labour for installing soil nails in the upper hillside.

Based on site observations together with available GI results, the site has a shallow rockhead profile, especially near the toe of DL1 (Plate 1). To construct a single rigid debris barrier with a total retention volume of about 240m3, an abundant amount of excavation into rock is required. Besides, the construction of a single large rigid barrier is not aesthetically pleasing.

The most cost-effective solution to mitigate the CDF hazard in the site is therefore left with the construction of a flexible barrier. Flexible barriers are best utilized placing perpendicular to the debris flow path, with anchors fixed into rocks on both sides. The +69mPD platform fulfils these requirements and is considered a better position than the +64mPD platform, for the reason that it is further away from the Village. The remaining fines / debris after this first level of defense can be easily contained by a small-scale barrier at the +64mPD platform.

6. GEOTECHNICAL DESIGN OF THE FLEXIBLE BARRIER USING ENERGY APPROACH 6.1 Establishment of geological model

After consolidating the above-mentioned information, a hazard model for the design of the proposed flexible barrier is developed as illustrated in Figure 4. This model is critical for subsequent DMM analysis.

The most mobile CDF, if developed, will be located at landslide source S1 (Figure 4), taken into consideration recommended by the NTHS, site-specific API and topography of the site.

The longitudinal profile of the debris flow path for the CDF (DL1, Figure 4) is established based on the multi-return air-borne Light Detection and Ranging (LiDAR) data and detailed topographical survey.

The channel width along the longitudinal profile of DL1 is drawn based on site measurements. With the channel width of the incised section of the drainage line, an entrainment volume of about 110m3 is calculated based on an entrainment rate of 0.3m3/m2.

Basin-like structure at +64mPD

The +69mPD platform

2.5m wide rock outcrop

A

A

Section A-A (N.T.S.)

gap <300mm

Figure 3 – Result of detailed topographical survey

Structure

4

To minimize debris possibly escaping below the barrier, a 5m high flexible barrier is proposed to be erected in the middle of the +69mPD platform where the level difference between the bottom of the barrier and DL1 is <300mm. This corresponds to chainage (x=117m) in the geological model. Detailed topographical survey indicates that this barrier can retain 167m3 of debris, assuming a horizontal deposition profile and a 25% reduction in barrier height (Geobrugg, 2012).

6.2 Debris mobility modeling

DAN-W Release 10 (Hungr, 1995) was used as the DMM tool for the site. Following the recommendation in GEO Report No. 104 (Lo, 2000) and GEO Technical Guidance Note (TGN) No. 29 (GEO, 2011), Voellmy model with a friction angle (φa) of 11o and a turbulence coefficient (ξ) of 500m/s2 is used to model the CDF. Other key input parameters in the DMM are given in Table 1.

Table 1: Key input parameters into DAN-W for the design of the flexible barrier

Case Source volume

Source depth

Initial velocity

Shape factor#

Debris density (ρd)

Internal friction angle

Erosion depth

Voellmy model φa ξ

(i) 125m3 1.5m 0m/s 0.67 2,200kg/m3 35o 0.25m3/m 11o 500m/s2 #According to Hungr (2010), a shape factor of 0.67 should be used to model CDF.

DMM result shows that about 210m3 of debris will pass through the proposed flexible barrier at time t=18.5s with a runout length of about 137m (Figure 5a). The velocity and thickness hydrographs from the DMM analysis at x=117m is shown in Figure 5b and 5c respectively.

Figure 4 – Geological model for the design of flexible barrier

Relict landslides

Site boundary

Residential

Oversteepened valley side-slopes

Recent landslides

+64mPD

+69mPD

N

+85mPD

+130mPD

Chainage (x) (m) 117 130 132 Width (m) 10 6.5 10 15 12 6.5 Active volume (m3) 125 240 240 210 0

Ele

vatio

n, z

(m

PD

)

Nearest structure

Incised section of DL1 with entrainable

colluvium

Landslide source S1

Longitudinal profile for the CDF flow

path, DL1 Debris lobe (API)

Rock cliff (API)

Proposed flexible barrier on the

+69mPD platform

Existing basin-like structure on the

+64mPD platform

5

6.3 Determination of the energy rating for the proposed flexible barrier

With reference to the worked example supplementing

DN 1/2012 (Kwan & Cheung, 2012), the maximum of (i) the kinetic energy (KE) of the whole active debris mass when the debris front reaches the barrier location (Figure 6a) and (ii) the theoretical KE of the debris mass that travelled beyond the barrier location (Figure 6b), is taken to check against 75% of the energy rating of a flexible barrier system2. This is the upper bound KE of the whole debris mass neglecting any energy dissipation due to mixing and turbulence of debris upon impact on the flexible barrier.

6.4 Force assessment against critical loading conditions

In addition to the energy rating for the design

of a flexible barrier system, the total impact force (Eqn [1], Figure 7) with the overflow drag force ( τ ) included is calculated for the design of the anchorage system (Sun & Law, 2012). Possible debris run-up height (hr) is also checked against the designed height of the flexible barrier system using the energy conservation equation [2]. The detailed design parameters of the flexible barrier system at +69mPD are summarised in Table 2.

wdpF stotal )2

1( τ+= = 1,900 kN ..………..…… [1]

mmg

Uhr 8.32

2

2max <≈= ….……………….…… [2] and gUh or 2/2= =1.8m ..................…. Eqn [4]

Table 2: Designed parameters for the proposed intermediate flexible barrier at +69mPD

Barrier height, H

Designed height, d=3/4H

Barrier Length (w)

Retention volume#

Energy rating, E

Maximum impact force, Ftotal

5m 3.8m 12m Min. 167m3 8,000kJ 2,000kN #The designed debris retention volume is calculated using ArcGIS 3D model based on detailed topographical survey.

7. GEOTECHNICAL DESIGN OF THE TERMINAL BARRIER USING FORCE APPROACH To relief the surface runoff overflow problem claimed by the village resident, a rigid barrier with proper

drainage details is proposed to replace the existing basin-like structure on the +64mPD platform. A 2m high barrier in the dimension of the existing basin-like structure is sufficient to retain the remaining debris.

2 As the energy rating of market available flexible barrier system is calibrated against rockfall impact, GEO (2014b) specifies that only 75% of it should be used to retain debris flow deposit.

Figure 6 – Determination of energy rating (E) for the proposed flexible barrier

Figure 6a: E = max (Eki, Ek2) / 0.75 ~ 7,400 kJ

Figure 6b:

m1v1

m2v2..…

mivi Ek1=1/2ΣΣΣΣmivi

2 ~5,200kJ

At t = 18.5s

At x=117m

Ek2=1/2ΣΣΣΣt t t t mtvt2

~3,600kJ where mt = f (ht)

Note: vt & ht are obtained from Figure 5b & 5c respectively

Figure 7 – Critical loading condition for the flexible barrier Barrier with designed height, d

where K=Lateral earth pressure coefficient=1 (Sun & Law, 2012), ps=pd=2200kg/m3

where h=hmax and tanφe=tanφa+Umax/(hoξ)

Figure 5a – DMM result with observation point set at x=117m on the +69mPD platform

Volume of debris passing through the barrier, V ~210m3

Width at x=117m, w=12m

80 100 117 120 137 140 x

z (m

PD

)

+60

+80

+69mPD

Figure 5b (upper) – Velocity hydrograph at x=117m Figure 5c (lower) – Thickness hydrograph at x=117m

Debris max. velocity (Umax) ~ debris frontal velocity ~ 6m/s

Impact time ~ 18.5s

Time (s)

Time (s)

Velocity (m/s)

Thickness (m) Max. debris thk, hmax = 0.4m

6

7.1 Establishment of geological model

In the DMM model (Section 6.2), given the 167m3 of debris retained by the proposed flexible barrier is represented by mass blocks Nos. 1 to f, the maximum velocity of the remaining mass blocks (Nos. f+1 to n) would be selected as the debris launching velocity, vm (Figure 8).

With vm, the maximum projectile distance which defines the landing position of the debris front (xi) is determined by the energy conservation principle (Eqn [3]). This is for checking of the terminal barrier overshooting with the remaining debris.

mmv

zdg

g

vx

m

mi 127

)(22

2<≈+= …………………………………………………………………………..………..[3]

Considering the KE of remaining debris (KEmr=1/2Σm(f+1 to n)v(f+1 to n)

2) and the KE transferred from potential energy from the drop in height, the debris velocity just before landing (vr) is conservatively estimated using Eqn [4], ignoring any energy dissipation due to mixing and turbulence of debris upon impact on the flexible barrier.

Based on the current state of landside studies, the ratio of velocity parallel to debris trail after and before landing (R) could range from 2.3 to 7.5. As the flow dynamics of debris impact on the ground could be very complex, R=0.7 is used for robustness (Kwan et al, 2013). The reduced velocity after vector resolution is calculated to be 3m/s (Eqn [5]).

smm

zdgmKEv

r

rmr

r /13)}({2

≈++

= ……..…[4] smKE

zdgmRvv

rm

rri /3

)(tancos 1 ≈

+

= − ……..…[5]

7.2 Debris mobility modeling

The starting point of the remaining debris front (70m3) is idealized to begin at the landing position with the

length of debris extending backwards. A constant debris thickness that corresponds to hmax (Section 6.2) is adopted for robustness. Assuming all remaining debris would have an initial velocity (vi) of 3m/s, a DMM (see Table 3 for other input parameters) is conducted to evaluate the debris impact velocity at the proposed rigid barrier location (x=130m).

Table 3: Key input parameters into DAN-W for the design of the terminal rigid barrier Case Source

volume Source depth Initial

velocity Shape factor#

Debris density (ρd)

Internal friction angle

Voellmy model φa ξ

(ii) 70m3 0.4m < hmax 3m/s 1 2,200kg/m3 35o 11o 500m/s2 #As the debris flow path is on flat ground, a shape factor of 1 is adopted (Hungr, 2010).

DMM result shows that the debris would just

reach the barrier (Figure 9a). Debris reaching the barrier at various times would have different values of velocity (v(t)) and thickness (h(t)) as illustrated in Figure 9b and 9c respectively.

Figure 8 – Geological model for the design of terminal rigid barrier

DMM with vi

Structure

Existing basin-like structure to be modified to a 2m high rigid barrier

vm ~ 5m/s

vr ?

?

?

? ?

?

?

d

z

+64mPD

xi ~ 7m

+69mPD

10m 2m 2m

vi 2m

+72.8mPD

Figure 9a – DMM result with observation point set at x=130m on the +64mPD platform

Width, wr=6.5m z

(mP

D)

+60

+64mPD

+80

100 120 130 140 x

Figure 9b (upper) – Velocity hydrograph at x=130m Figure 9c (lower) – Thickness hydrograph at x=130m

Impact time ~ 2s

Debris frontal thk, ho,r ~ 0.12m

Time (s)

Time (s)

Thickness (m) Max. debris thk, hmax,r ~ 0.23m

Velocity (m/s) Debris frontal (max.) velocity, vmax,r ~ 2m/s

7

7.3 Geotechnical design of terminal barrier using force approach

Using the hydro-dynamic model, the force (Fd) acting on the terminal rigid barrier induced by debris impact can be calculated as shown in Eqn [6].

rwthtvdtdF )()(2')( ρα= …………………….…………………[6] where α’= dynamic impact coefficient for rigid barrier =2.5 (Kwan, 2012).

As the average debris thickness (have=(hmax,r+ho,r)/2=0.18m) is too thin (Figure 9c), the debris impact thickness (Hdeb) is assumed to be 0.3m. Fd calculated from Hdeb and vmax,r is obviously larger than Fd calculated from the combination of h and v in different scenarios at various times. Hence, this (Figure 10a) is compared to the stated load due to unconsolidated debris together with the drag force of overflow (Figure 10b), and the larger impact load is taken for robust design of the terminal rigid barrier.

Taking the weight of the debris into account (GEO, 2015), the terminal wall stability is checked following the procedures and partial factors recommended by Geoguide I (GEO, 1993) with rockfill (cohesion=0kPa and effective friction angle=40o) as the wall base material. The result shows that a 300mm thick rigid barrier with the configuration as shown in Figure 8 is sufficient to provide sliding resistance (critical scenario) against debris impact.

Possible debris run-up height is also checked using Eqn [2] with vmax,r. Any other structural calculations in relation to the barriers design are not within the scope of discussion in this paper.

8. OTHER DESIGN CONSIDERATIONS

Both barriers will be founded on / anchored into rocks. During construction, potentially unstable wedges or

planes will be stabilized. Both barrier locations are chosen with due consideration for ease of access, and only minor modification of existing staircases may be required subject to comment from relevant maintenance departments. Landscaping works in association with the mitigation works are needed to be considered to harmonize with the existing environment. Corrosion protection for the flexible barrier will be provided in accordance with relevant codes and specifications, also taking site environmental conditions into account. A secondary mesh will be provided to minimize the amount of fines passing through the flexible barrier. 9. DISCUSSIONS

Development of proper geological models is critical in the design of mitigation measures. The model

should be able to incorporate considerations for proper site constraints (e.g. difficult access to the toe of the drainage line, presence of sensitive receivers close to the potential hazardous location, geological and physical environment of possible barrier locations, etc.), detailed topographical survey (e.g. retention volume estimation, level difference between the bottom of the barrier and drainage line) and API results (e.g. debris lobe location) in order to suitably select and locate the appropriate mitigation works. Apart from following recommendations and guidelines, engineering judgment should be used to countercheck and review the results. To holistically solve existing site problems, site observations should be properly recorded to finalize the design with sufficient details (e.g. to design the terminal barrier with proper drainage details to divert surface water away from village houses, to widen the exit streamcourse to incorporate additional flow, etc.). Further studies to explore the use of baffles to reduce energy impact on the flexible barrier are recommended subject to the finalization and endorsement of the algorithm used in the debris mobility analyses.

Figure 10 – Design impact scenario for terminal rigid barrier

(b) Loading on barrier during debris overtoppling = 45kN/m

Hdeb=0.3m

=43kN/m

Barrier height = 2m

(a) Impact force when debris piles up behind barrier with last surge filling at the top = 38kN/m

Barrier height = 2m

No boulder impact in this site

Dynamic impact force due to Hdeb and vmax,r = 7kN/m

=0.3m

=31kN/m

2kN/m

8

10. CONCLUSION This paper presented a detailed geotechnical design of the mitigation measures for CDF hazard affecting a

densely populated village. The mitigation measures include a 5m high intermediate flexible barrier and modification of the existing basin-like structure to a 2m high terminal rigid barrier. The success of the design has been relied upon proper development of geological models with applications of recommended good practices / guidelines. ACKNOWLEDGEMENTS This paper is published with the permission of the Head of the Geotechnical Engineering Office and the Director of Civil Engineering and Development, Government of the Hong Kong Special Administrative Region. I offer my deep gratitude to my supervisor Mr Anthony Lam, who has guided me throughout the preparation of the paper and Mr Patrick Tam, who has assisted in the preparation of calculations and figures. REFERENCES GEO (1992). 1:20,000-scale Solid & Superficial Geology. Sheet 7 Sha Tin. GEO, HK. GEO (1993). Guide to Retaining Wall (Geoguide 1). GEO, HK, 258 p. GEO (2011). Guidelines on the Assessment of Debris Mobility for CDFs (GEO TGN No. 29). GEO, HK, 6 p. GEO (2012). Guidelines on Assessment of Debris Mobility for Open Hillslope Failures (GEO TGN No. 29).

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Barriers (GEO Technical Note (TN) No. 3/2013). GEO, HK, 69 p. Kwan, J.S.H. & Cheung, R.W.M. (2012). Suggestions on Design Approaches for Debris-resisting Flexible

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