GUIDE TO THE DESIGN OF BRIDGE APPROACH EMBANKMENTS SUBJECT TO INUNDATION

1. INTRODUCTION Bridge approach embankments can become saturated when subjected to the periods of inundation associated with major floods. If the water level adjacent to the embankment falls rapidly, excess pore pressures can be developed within the embankment leading to instability of 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 β)

• Stone pitching thickness (P)

• Soil permeability (k)

• Drawdown rate (Ż w)

• Factor of safety (F)

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

Soil strength φ = 30° C = 0

Angle of Internal Friction of Rock Spalls φ = 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 of k

.wZ in the range affecting stability.

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

Engineering Road Note No. 6

October 2003

P/H = stone pitching thickness embankment height

wZ = vertical distance from top of embankment to water level in river

w.Z = drawdown rate( = dzw/dt) k = coefficient of hydraulic

conductivity β = slope angle

(Note: gradient of x:1 expressed as cotan β = 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 designing proposed slopes or of checking existing slopes for stability under rapid draw- down conditions. Theoretically, the charts can be used to find a solution for any one of the six variables, given the other five. However in practice, only four of these variables are under the control of the designer. The drawdown rate and flood level height (which determines the embankment height) are primarily functions of the river hydrology and the embankment designer cannot influence 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 Żw, 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 Żw the designer can choose a stone pitching

thickness (P) to provide for an adequate factor of safety (F). Case 3 - For given values of H, Cot �, P and Żw the designer can determine what the

permeability should be of the material to be used in the embankment, for an adequate factor of safety (F).

Case 4 - For an existing embankment where H, P, Cot β, k and Żw are fixed, the designer can

determine 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 embankment height exceeds 4 m, a factor of safety against slip of not less than 1.2 should be used. Embankments of less than 4 m in height will not normally be the subject of analysis.

3.2 Estimation of Soil Permeability

The accurate determination of soil permeability is fairly complex. However in most cases it will be sufficient to obtain an estimate of the order of magnitude of permeability based on particle size distribution test results. At Figure 2, a chart is provided 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 a particle size distribution test.

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 and k

.wZ .

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

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

0.5HP == (use figure 4)

0.18k = m/hour (from figure 2)

8.118.033.0

k

.

==wZ

2.3Cotβ =∴ (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 (0.167 m/hour). The batter slopes are to be 2:1. Determine the D10 size necessary for a factor of safety of 1.2.

Calculation:

0.06258

0.5HP == (use figure 4)

4.0k

.

=∴ wZ (from figure 4)

42.00.4

0.167k ==

mm15.0D10 =∴ (from figure 2)

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

REFERENCES GOLDER ASSOCIATES PTY LTD (1983): "Report to Main Roads Department on Slope Stability Computations for Bridge Approach Embankments Subject to Rapid Drawdown Conditions". 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.