Matakuliah : S0522/ Aplikasi Geosintetik Dalam Teknik Sipil

Tahun : Juli 2005

Versi : 01/01

Pertemuan 12APLIKASI DAN PEMILIHAN MATERIAL GEOSINTETIK

Learning Outcomes

Pada akhir pertemuan ini, diharapkan

mahasiswa akan mampu : Mahasiswa mampu membedakan

pemakaian geosintetik sesuai kebutuhan desain di lapangan C6

Outline Materi• Pertimbangan desain• Kondisi tanah• Analisa pemilihan material • Perbandingan penggunaan

geosintetik sesuai kebutuhan dan kondisi lapangan

• Aplikasi geosintetik sesuai kondisi lapangan

Sebagian dari materi ini dikutip dari IGS Lecturer notes No. 17 of 19Geosynthetics in DamsBy Daniele CazzuffiENEL Hydro, Milan, ItalyNo. 18 of 19Geosynthetics in Asphalt PavementsBy Prof. S.F. Brown FEngUniversity of NottinghamNo. 12 of 19Geosynthetics in Erosion ProtectionBy Dr David Elton, P.EAuburn University

Geosynthetics in Sediment and Erosion Control

• Introduction and Applications

• What is it?

• Erosion control is a means of keeping a soil in place or catching a soil after it has been displaced but before it moves into surface waters.

Why Is It Needed?

There are public laws that :

• preclude the polluting of surface waters with sediments

• preserve topographic integrity

• preserve soil for farming

• preserve foundation integrity for structures founded on soil.

Rill and Gully Slope Erosion

Riverbank Erosion, IncludingSapping (Formation of Caves)

Where Is It Needed?

• It is needed on construction and agricultural sites and natural places where water causes soils to displace.

• In short: where soil and moving water interact at the ground surface

Factors Influencing Types of Erosion

• Rainfall-induced erosion factors:

– intensity and duration of rainfall– slope of land – soil type

• All affect the amount of erosion and the erosion control measures selected.

Factors Influencing Types Of Erosion (Continued)

• Shoreline erosion factors:

– soil type– wave height– beach slope

– duration and intensity of storm

• All affect the amount of erosion.

Factors Influencing Types Of Erosion (Continued)

• Scour:

– The amount of scour, around bridge piers for example, is affected by pier shape, depth of stream, storm duration and channel shape, in addition to soil type.

Hard Armor Erosion Control on Riverbank

Concrete cast in a geotextile former; geotextile filter underneath(not visible)

• Slopes and Channels

Channel erosion damage caused by inadequate filter beneath hard armor

Design Approach

Strategy

• Choose least costly erosion control measure and evaluate:– LOW cost nothing (fallow ground)

plantsdegradable RECPspermanent RECPspermanent TRMssoft armor

– HIGH cost hard armor w/geotextile filter

• In conjunction with these choices, consider:– reducing flow in any manner– flattening slopes– widening channels

Design Procedures for Erosion Control in Slopes and Channels

Slopes• There are several methods of estimating soil loss. The most

commonly used in the US is: • USLE - Universal Soil Loss Equation:

A = R K LS C Pwhere: A = computed soil loss (tons/acre or kg/hectare) for a

given storm period or time intervalR = rainfall factorK = soil erodibility valueLS = slope length and steepness factorC = vegetation or cover factorP = erosion control practice factor

• All factors, except C, do not vary more than one order of magnitude. C changes several orders of magnitudes.

• NOTE: many of these factors are described in USDA (1997)

Cover Index factor (C) for Different Ground Cover Conditions

Type of Cover Factor Percent

C Effectiveness None (fallow ground) 1.0 0.0

Temporary seedings (90% stand)

Ryegrass (perennial type) 0.05 95

Ryegrass (annuals) 0.10 90

Small grain 0.05 95

Millet or sudan grass 0.05 95

Field bromegrass 0.03 97

Permanent seedings (90% stand) 0.01 99

Sod (laid immediately) 0.01 99

Mulch

Hay, rate of application, tons/ac:

0.5 0.25 75

1.0 0.13 87

2.0 0.02 98

Small grain straw 2.0 0.02 98

Wood chips 6.0 0.06 94

Wood cellulose 1.5 0.10 90 Fiberglas 1.5 0.05 95

Source: primarily HEC-15 (1988) percent soil loss reduction as compared with fallow ground

Proposed C-Factors for RECPs

Update of RUSLE Equation

RECP Approximate Reported Proposed

Category Mass/Unit Range of C-factors Area C-factors g/m2

(oz/ yd2)

ECN 34 to 100 100% Woven 0.02 0.01(1 to 3) Polypropylene400 to 880 100% Woven 0.002 to 0.003 0.01 to 0.1(12 to 26) Coir/Jute

ECB 270 to 340 100% Straw 0.002 to 0.30 0.01(8 to 10)270 to 370 Straw/Coconut 0.002 to 0.11 0.01(8 to 11)270 to 400 100% Coconut/ 0.003 to 0.09 0.01(8 to 12) Excelsior

TRM 270 to 490 100% Synthetic 0.003 to 0.11 0.01(8 to 14)

NOTES/REFERENCES

All terminology consistent with the Erosion Control Technology Council approved terms.“ECN” indicates temporary degradable erosion control net. Values assumes slope is flatter than 2H:1V.“ECB” indicates temporary degradable erosion control blanket. Values assumes slope is flatter than 2H:1V.“TRM” indicate permanent nondegradable turf reinforcement mat. Values assumes slope is flatter than 1H:1V.

Procedure

• Calculate A with the C value of a given permanent erosion control solution (usually vegetation)

• Compare A with an acceptable A (e.g. 44 kN/ha/year (2 tons/acre/year))

• If A is acceptable*, check effectiveness of the temporary (degradable) erosion control (RECP) measure used to establish the permanent erosion control solution over the life of the RECP. Choose a temporary solution with a small enough C to satisfy regulations.

• *typically, acceptance is based on government regulations. If A is unacceptable for permanent solution (vegetation), try vegetation plus turf reinforcement mat for long term solution. C values available from test or manufacturers.

• If that combination produces a satisfactory A, check A for temporary erosion control solution used while permanent solution is taking hold.

Channel Linings

• Two Common Methods of Analysis:

– Permissible velocity in channel

– Permissible shear stress in channel

• Velocity Calculation

where:

V - velocity of flow (ft/sec)

n - Manning's roughness coefficient (see Table 3)

R - hydraulic radius ( A / wetted perimeter) (ft)

Sf - slope of channel, for uniform flow conditions.

VnR S

f

149 2 3. /

• In SI units, this equation becomes:

V - velocity of flow (m3/sec)n - Manning's roughness coefficient (see Table 3)R - hydraulic radius ( A / wetted perimeter) (m)

Sf - slope of channel, for uniform flow conditions.

• Compare calculated V with an acceptable V from standard tables or manufacturer's literature

Vn

R Sf2 3/

Table 3. Manning’s Roughness Coefficients.

n - value1

Depth Ranges 0-0.5 ft 0.5-2.0 ft >2.0 ft

Lining Lining type (0-15cm) (15-60 cm) (> 60 cm) Category Rigid Concrete 0.015 0.013 0.013

Grouted riprap 0.040 0.0300.028

Stone masonry 0.042 0.0320.030

Soil cement 0.025 0.022 0.020 Asphalt 0.018 0.016 0.016

Unlined Bare soil 0.023 0.020 0.020 Rock cut 0.045 0.035 0.025

Temporary Woven paper net 0.016 0.015 0.015 Jute net 0.028 0.022 0.019 Fiberglass roving 0.028 0.021 0.019 Straw with net 0.065 0.033

0.025 Curled wood mat 0.066 0.035 0.028 Synthetic mat 0.036 0.025

0.021

Gravel Riprap 1-inch (2.5-cm) D50 0.044 0.033 0.030

2-inch (5-cm) D50 0.066 0.041 0.034

Rock Riprap 6-inch (15-cm) D50 0.104 0.069 0.035

12-inch (30-cm) D50 -- 0.078 0.040

1Based on data primarily from HEC-15 (Chen and Cotton, 1988)

Notes:

• Values listed are representative values for the respective depth ranges.

• Manning’s roughness coefficients, n, vary with the flow depth.

• n-values for vegetative linings are found in Chen and Cotton (1988) (HEC-15) on pages 42 - 46.

• Another method is to evaluate the shear stress on the ground surface caused by the running water. (Method follows).

• maximum shear stress on channel base:

where: - shear stress

w - unit weight of waterd - depth of flowSf - gradient of channel for uniform flow

conditions

• Calculate the expected shear stress and compare with acceptable shear stress

Shear Stress Calculation

w fd S

Design Variables

• increasing the width/depth ratio of channel reduces

– channel slope: flatter reduces (note: 10% is considered a "steep Channel". Sf > 10% usually requires hard armor)

Shear stress approach :• calculate the maximum

• Note: this requires d. If Q is known, and the gradient and lining material are known, d can be found from Chart 3.

w fd S

Design Procedure for Sf < 10% Channels

If Q is unknown, use Manning's equation to get Q

where: Q = flow (cfs)n - Manning's roughness coefficient (see Table 3)A - cross-sectional area of channel (ft2)R - hydraulic radius ( A / wetted perimeter) (ft)Sf - slope of channel for uniform flow conditions.

• Then, knowing Q, use Chart 3 to get d. With d, calculate maximum shear, , and compare with tabulated values of for RECPs (Tables 1 and 2 from HEC-15) or with manufacturer's data.

Qn

A R Sf149 2 3. /

In SI units, this equation is:

where:Q = flow (m3/sec)n - Manning's roughness coefficient (see Table 3)A - cross-sectional area of channel (m2)R - hydraulic radius ( A / wetted perimeter) (m)Sf - slope of channel for uniform flow conditions.

Qn

AR Sf

2 3/

Design Chart / Nomogram

ref: Chen, Y.H. and G.K. Cotton (1988)

Table 1. Classification of Vegetal Covers as to Degree of Retardance. Retardance Cover Condition

Class A Weeping lovegrass Excellent stand, tall (average 30") (76 cm)

Yellow bluestemIschaemum Excellent stand, tall (average 36") (91 cm)

Kudzu Very dense, growth, uncutBermuda grass Good stand, tall (average 12") (30 cm)Native grass mixture.(little bluestem, bluestem,blue gamma, and other long

B and short midwest grasses) Good stand, unmowedWeeping lovegrass Good stand, tall (average 24") (61 cm)Lespedeza sericea Good stand, not woody, tall (average 19")Alfalfa (48 cm)Weeping lovegrass Good stand, uncut (average 11") (28 cm)Kudzu Good stand, unmowed (average 13") (33 cm)Blue gamma Dense growth, uncut

Good stand, uncut (average 13") (28 cm)

Table 1. Classification of Vegetal Covers as to Degree of Retardance.

Retardance Cover Condition Class

Crabgrass Fair stand, uncut (25 to 120 cm)Bermuda grass Good stand, mowed (average 15 cm)Common lespedeza Good stand, uncut (average 28 cm)Grass-legume mixture--

C summer (orchard grass,redtop, Italian ryegrass,and common lespedeza Good stand, uncut (15 to

20cm)Centipedegrass Kentucky bluegrass Very dense cover (average 15 cm)

Bermuda grass Good stand, headed 15 to 30cm Common Lespedeza Good stand, cut to 6 cm Buffalo grass Excellent stand, uncut (11 cm) Grass-legume mixture Good stand, uncut (8 to

15 cm) fall, spring (orchard grass, Good stand, uncut (10 to 13cm)

D redtop, Italian ryegrass, After cutting to (5 cm) Very good

and common lespedeza) stand before cuttingLespedeza sericea

Table 1. Classification of Vegetal Covers as to Degree of Retardance.(HEC - 15)

Retardance Cover Condition

Class

E Bermuda grass Good stand, cut 4cm heightBermuda grass Burned stubble

Note: Covers classified have been tested in experimental channels. Covers were green and generally uniform.

Table 2. Permissible Shear Stresses for Lining Materials

Permissible

Unit Shear Stress1

Lining Category Lining Type (lb/ft2)(kg/m2)

Temporary Woven Paper Net 0.15 0.73Jute Net 0.45 2.20Fiberglass Roving:Single 0.60 2.93Double 0.85 4.15Straw with Net 1.45 7.08Curled Wood Mat 1.55 7.57Synthetic Mat 2.00 9.76

Vegetative Class A 3.70 18.06Class B 2.10 10.25Class C 1.00 4.88Class D 0.60 2.93Class E 0.35 1.71

Gravel Riprap 1-inch (2.54 cm) 0.33 1.61 2-inch (5 cm) 0.67 3.22

Table 2. Permissible Shear Stresses for Lining Materials

Permissible

Unit Shear Stress1

Lining Category Lining Type (lb/ft2)(kg/m2)

Rock Riprap 6-inch (15 cm) 2.00 9.7612-inch (30 cm) 4.00 19.52

Bare Soil Non-cohesive See Chart 1Cohesive See Chart 2

1ref: HEC - 15

For fallow cohesionless soils, compare with Chart 1

note: if particle diameters are larger than 100mm (0.33 ft), use

= 25.5 D50

with D50 being the mean rock size in meters, and in kPa,

or = 4 D50

with D50 being the mean rock size in feet, and in psf.

Ref. Chen and Cotton (1988)

Erosion Control Using Geosynthetics

Applications:• Introduction• Useful in scour, surface and shoreline

protection• Geosynthetic functions include:

– filtration– containment (bags)– protection– providing a medium for plant growth

Erosion Control Using Geosynthetics

Scour Applications

– Bridge pier footing

– Canal Lining

Surface Protection

Philosophy• reduce the intensity of the raindrops impacting the soil,

reduce the speed of runoff, increase the amount of water that soaks into the soil rather than running off.

Roving• Roving is fine threads spread out on the ground surface,

tacked down with a spray that holds it in place while vegetation takes hold.

• Roving is applied manually, with a light machine. The method is slow, but useful in smaller areas with uneven surfaces.

Roving Materials

• spools of thread

• placed using air guns

Roving Being Applied

1. Roving (white) being sprayed on to the ground surface,

2. Being tacked down with asphalt spray

Rolled Erosion Control Product

Being installed in a ditch

Permanent Installations (Permanent Erosion and Revegetation Mats)

Soft PERMs

• Turf Reinforcement Mats (TRMs)

– TRMs are typically placed on the surface and then filled in

with soil. They reinforce the ground surface, making erosion

more difficult. The TERM holds the soil in place while

vegetation takes hold.

• Erosion Control and Revegetation Mats (ECRMs)

– These combine surface control and surface slope

stabilization at the same time.

Geocell Confinement Systems (GCS)

Geocellular confinement system on a slope being filled with soil

GCS – Soil Fill

GCS are an expensive, rugged way to stabilize a slope or roadbed. These cellular mats are filled with soil. They are very strong and very effective.

Hard Permeable System

• Gabions - wire baskets filled with cobble-sized rocks

• Hard armor: Gabion channel lining (geotextile filter underneath not visible)

• Loose stone – riprap. Random placement is best. There are various methods for estimating how large these rocks must be to avoid displacement.

Hard Armor

• Riprap being installed with geotextile filter underneath

• Concrete or masonry: Dolos (large concrete objects), articulated blocks (forming a mat), concrete facings (cast in place or precast)