EngineeringRoadNote6final_24102003_9999991.pdf

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GUIDE TO THE DESIGN OF BRIDGE APPROACH EMBANKMENTS SUBJECTTO INUNDATION

1. INTRODUCTION

Bridge approach embankments can become saturated when subjected to the periods ofinundation associated with major floods. If the water level adjacent to the embankment fallsrapidly, excess pore pressures can be developed within the embankment leading to instabilityof the batter slope. This phenomenon is referred to as rapid drawdown.

This note presents a set of charts for designing bridge approach embankments to limit the risk

of slip failures due to rapid drawdown.

2. FACTORS AFFECTING EMBANKMENT SLOPE STABILITY

The following variables are provided for in the design charts.

Embankment height (H)

Embankment slope (Cot )

Stone pitching thickness (P)

Soil permeability (k)

Drawdown rate (w)

Factor of safety (F)

The following factors were held constant in the analyses used to produce the charts:

Soil strength = 30 C = 0

Angle of Internal Friction of Rock Spalls = 35

Material Density (soil and rock) 2.0 t/m3

wet 2.2 t/m3

saturated

Specific Yield 0.1

The charts are for embankment crest widths of 5H or greater; narrower embankments will

have high factors of safety for values ofk

.

wZin the range affecting stability.

The charts are based on the assumption that the embankment foundation material has thesame properties as the embankment itself. For embankments founded on soft soil, whereuse of the charts is not appropriate, an analysis should be carried out using a suitable slopestability computer program such as STABL.

Engineering Road Note No. 6

October 2003


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P/H = stone pitching thicknessembankment height

wZ = vertical distance from top of

embankment to water level inriver

w.Z = drawdown rate( = dzw/dt)

k = coefficient of hydraulicconductivity

= slope angle(Note: gradient of x:1 expressed

as cotan = x)

FIGURE 1: Definitions of Symbols

3. USE OF STABILITY CHARTS

The stability charts presented as figures 3 - 6 provide a simple and rapid method of designingproposed slopes or of checking existing slopes for stability under rapid drawdownconditions.

Theoretically, the charts can be used to find a solution for any one of the six variables, giventhe other five. However in practice, only four of these variables are under the control of thedesigner. The drawdown rate and flood level height (which determines the embankmentheight) are primarily functions of the river hydrology and the embankment designer cannotinfluence these in any significant way.

The four cases for which it is expected the charts will be used are as follows:

Case I - For given values of H, P, k and w, the designer can choose a batter slope (Cot )to provide for an adequate factor of safety (F).

Case 2 - For given values of H, Cot , k and w the designer can choose a stone pitchingthickness (P) to provide for an adequate factor of safety (F).

Case 3 - For given values of H, Cot , P and w the designer can determine what thepermeability should be of the material to be used in the embankment, for anadequate factor of safety (F).

Case 4 - For an existing embankment where H, P, Cot , k and w are fixed, the designer candetermine the factor of safety (F) against slip under flooded conditions.

3.1 Choice of Factor of Safety

It is recommended that for all new bridge and roadworks where the embankmentheight exceeds 4 m, a factor of safety against slip of not less than 1.2 should beused. Embankments of less than 4 m in height will not normally be the subject ofanalysis.

3.2 Estimation of Soil Permeability

The accurate determination of soil permeability is fairly complex. However in mostcases it will be sufficient to obtain an estimate of the order of magnitude ofpermeability based on particle size distribution test results. At Figure 2, a chart isprovided for estimating permeability based on effective size (D10). The D10 size is

defined as the size (mm) corresponding to 10% passing and is assessed from aparticle size distribution test.


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FIGURE 2: Estimating Permeability from particle size distribution test results

3.3 Stability Charts

In the analyses leading to the development of the charts, Golder Associates

established that the factor of safety could be related to dimensionless parameters

P/H andk

.

wZ.

Four charts are presented for stone pitching thickness (P) of O, 0.0625 H, 0.125 Hand 0.25H. For intermediate values of P interpolation should be used.


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4. EXAMPLE 1

Problem:

A bridge approach embankment is to be constructed 8 m high with stone pitching 0.5

m thick. The drawdown rate after peak flood level is estimated to be 8 m in 24 hours(0.33m/hour). The available fill has a D10 size of 0.10 mm.

Determine the batter slope for a factor of safety of 1.2.

Calculation:

0.06258

0.5

H

P== (use figure 4)

0.18k = m/hour (from figure 2)

8.118.033.0

k

.

==wZ

2.3Cot = (from figure 4)

Solution: The batter slope should not be steeper than 2.3:1 (horiz:vert)

EXAMPLE 2

Problem:

A bridge approach is to be constructed 8 m high with stone pitching 0.5 m thick. The

drawdown rate after peak flood level is estimated to be 8 m in 48 hours (0.167m/hour). The batter slopes are to be 2:1. Determine the D10 size necessary for afactor of safety of 1.2.

Calculation:

0.06258

0.5

H

P== (use figure 4)

4.0k

.

=wZ

(from figure 4)

42.0

0.4

0.167k ==

mm15.0D10 = (from figure 2)

Solution:The fill material should have not more than 10% passing the 0.15 mm sieve.


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REFERENCES

GOLDER ASSOCIATES PTY LTD (1983): "Report to Main Roads Department on SlopeStability Computations for Bridge Approach Embankments Subject to Rapid DrawdownConditions". Ref. No. 8364 0008.

HOUGH, B.K. (1957): "Basic Soils Engineering" Ronald Press Co p. 69.

SIEGAL, R.A. (1978): "STABL Users Manual" JHRP, Purdue University Indiana.


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EngineeringRoadNote6final_24102003_9999991.pdf