Stability Issues

Kentucky Department for Kentucky Department for Natural ResourcesNatural Resources

Division of Mine PermitsDivision of Mine PermitsSept. 6 and 14, Oct. 4 and 18, Sept. 6 and 14, Oct. 4 and 18,

20122012

Fills trending more and more toward upland areas (non-jurisdictional)

Tending towards steeper ground slopes

As DMP reviews more of these type fills, and in particular those considering as-built conditions, we’ve become aware of that in many cases a COMPOSITE failure analysis is most appropriate.

Stability Issues

Stability Issues

The regulatory standard for stability of excess spoil fills is “a minimum long-term static safety factor of one and five-tenths (1.5)”. (405 KAR 16:130, Section 1)

Historically the industry has provided and the Cabinet has accepted circular and sliding failure analysis only, dating back to the early 1980’s (or earlier)

It appears that in many cases that the minimum factor of safety in a hollow fill lies along a composite failure surface, involving a circular failure that interests a softer soils resulting in a slip surface in the lower portion of the fill.

The REAME software, which is most commonly used in demonstrating the safety factor for hollow fills, performs the composite analysis, as do other available software systems. As such the technology is readily available.

Stability Issues

Stability Issues

Typical Failure Surface-Cylindrical

Stability Issues

Composite Failure Surface

Typical CylindricalFailure Slice

Typical Failure Surface:Non-Cylindrical

Stability Issues

Composite Failure Surface

Typical Non-CylindricalFailure Surface

Typical Failure Composite

Stability Issues

Composite Failure Surface

The composite surface allowthe slice (cylindrical) to intersect the subsoil, then following a sliding surface

The composite analysis should never be higher than the corresponding cylindrical or plane failure analysis, but it often results in a lower factor of safety. The regulations require that we test for the minimum factor of safety (using the best available technology).

The degree of difference, and indeed the applicability of the composite surface, depends largely on the difference between the material characteristic of the excess spoil and the sub-soil.

It has been common practice to use very conservative assumed values for these sub-soils for many years in doing the analysis. However, there is concern these values may be to low for all soils that may be affected.

Stability Issues

Composite Failure Surface

The composite surface allowthe slice (cylindrical) to intersect the subsoil, then following a sliding surface

Typical Failure Surface-Cylindrical

Stability Issues

Composite Failure Surface

Typical CylindricalFailure Slice

Factor of Safety- 1.470

Typical Failure Surface:Non-Cylindrical

Stability Issues

Composite Failure Surface

Typical Non-CylindricalFailure Surface

Factor of Safety- +/- 1.6

Typical Failure Composite

Stability Issues

Composite Failure Surface

The composite surface allowthe slice (cylindrical) to intersect the subsoil, then following a sliding surface

Factor of Safety- About 1.32

In order to address this issue, DMP held a series of internal meetings as well as discussions with various industry representatives.

With the assistance of several consulting firms, a data set of actual sub-soil analyses was compiled.

We have formulated some preliminary conclusions based on our review of that data.

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

A total of 30 soil tests were compiled from eight E. Ky. Coalfield counties. The initial macro analysis revealed two significant, and generally anticipated,

trends were detected;• Significant cohesion values were detected only in soils with somewhat higher

densities and relatively low friction angles. A review of the data found these were noted as being clayey soils.

• Cohesions lowered rapidly as the friction angle rose.

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

Plotted against Ascending FA

y = -0.0002x6 + 0.016x5 - 0.4813x4 + 5.0879x3 + 16.999x2 - 640.47x + 3048.9

R2 = 0.8096

y = 0.0003x4 - 0.0307x3 + 0.9578x2 - 11.997x + 167.36

R2 = 0.1099

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Friction Angle Values

Co

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Density Cohesion Poly. (Cohesion) Poly. (Density)

The data was plotted for both the friction angles (FA) and cohesion against each other (FA ascending). • The data points are shown by symbols, while the actual line

shown is a developed trendline based on the data (so as to minimize the effect of anomalies).

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

FA-Cohesion: All Data

y = -0.0002x6 + 0.016x5 - 0.4813x4 + 5.0879x3 + 16.999x2 - 640.47x + 3048.9

R2 = 0.8096

y = 22.172x0.1421

R2 = 0.9532

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Friction Angle Data Cohesion Data Poly. (Cohesion Data) Power (Friction Angle Data)

Looking closely at this plot, you might note that the data fit is much better towards the right of the chart with more “scatter” towards the left.• This was reflected in some relatively high standard deviations, particularly so

for cohesion.

• The curve fit was somewhat forced as well, again with the emphasis on cohesion.

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

FA-Cohesion: All Data

y = -0.0002x6 + 0.016x5 - 0.4813x4 + 5.0879x3 + 16.999x2 - 640.47x + 3048.9

R2 = 0.8096

y = 22.172x0.1421

R2 = 0.9532

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d)

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Friction Angle Data Cohesion Data Poly. (Cohesion Data) Power (Friction Angle Data)

After some further review of the data, and the overall objective, it was noted that the more troublesome data lay in the higher density, lower friction angle, higher cohesion areas.

Based on the descriptions in the lab reports as well as the indications from the material testing, an effort was made to separate the coarser, somewhat lighter “Sandy” soils from the finer, denser," Clayey” soils.

In order to do this, soils with a FA of 30 degrees and larger were isolated.

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

FA-Cohesion: All Data

y = -0.0002x6 + 0.016x5 - 0.4813x4 + 5.0879x3 + 16.999x2 - 640.47x + 3048.9

R2 = 0.8096

y = 22.172x0.1421

R2 = 0.9532

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10152025303540

Co. of Sample

Fri

c. A

ng

le (

d)

-500

0

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1500

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2500

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Co

hes

ion

(p

cf)

Friction Angle Data Cohesion Data Poly. (Cohesion Data) Power (Friction Angle Data)

This segregation is particularly appropriate given the nature of the soils formation. Specifically, the finer soils will tend to lie in lower areas with much gentler slopes.

The original concern of this exercise was the increasing incidence of fills in steep, upland areas. This is where the coarser, sandy type soils are more apt to be found.

So, for our purposes, the “right side” of the chart represents the data we are in fact seeking.

Here is the chart for the coarser soils only.

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

FA-Cohesion: Coarse

y = -0.0013x6 + 0.0966x5 - 2.867x4 + 41.176x3 - 290.39x2 + 878.5x - 651.37

R2 = 0.3053

y = -0.0001x4 + 0.0083x3 - 0.1616x2 + 1.3713x + 29.093

R2 = 0.963

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Friction Angle Data Friction-Coarse Cohesion DataCohesion-Coarse Poly. (Cohesion Data) Poly. (Friction Angle Data)

Here we find an extremely good curve fit, and appropriate standard deviations, for friction angle values.

The cohesion values remain a poor fit, poorer in fact than before. This stems from the high incidence of zero values, rendering any reading something of an anomaly.

FA-Cohesion: Coarse

y = -0.0013x6 + 0.0966x5 - 2.867x4 + 41.176x3 - 290.39x2 + 878.5x - 651.37

R2 = 0.3053

y = -0.0001x4 + 0.0083x3 - 0.1616x2 + 1.3713x + 29.093

R2 = 0.963

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Co. of Sample

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Friction Angle Data Friction-Coarse Cohesion DataCohesion-Coarse Poly. (Cohesion Data) Poly. (Friction Angle Data)

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

Previously Accepted Value

24.00 160.00 125.00

Average Values 33.50 42.61 114.76

Adj. using Std. Dev.- 31.13 -273.52 92.13

Accepted Value- 31.00 0.00 115.00

The “adjustment” referred to here involves reduction of the average by two Std. Devs. As can be seen on the chart, this allows the “Accepted Value” to encompass about 95% of anticipated data points.

Due to uncertainty, the cohesion is conservatively set to zero.

The “accepted” density was set nearer the mean due to the scarcity of data, resulting in a high Std. Dev., and it’s relatively low influence on the F.S.

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

Previously Accepted Value

24.00 160.00 125.00

Average Values 33.50 42.61 114.76

Adj. using Std. Dev.- 31.13 -273.52 92.13

Accepted Value- 31.00 0.00 115.00

The composite surface allowthe slice (cylindrical) to intersect the subsoil, then following a sliding surface

Previously Accepted Values-F.S.- 1.32

Modified Accepted Values-F.S.- 1.46

Summary: We will continue to look at this issue, as we obtain more data. The accepted values of the past will remain applicable to lower, gentler slopes and valley floors. This analysis has resulted in a second set of parameters deemed acceptable for sub-soils on the

upper, and steeper, slope areas. Note that these are assumed values, with a high degree of confidence. As such they are

conservative. If they appear too conservative for your design, you are invited and encouraged to obtain and test site specific samples.

Stability Issues

Composite Failure SurfaceSub-Soil Strength Characteristics.

Sub-Soil Values: Accepted Parameters

Soil Description Friction Angle

Cohesion Density

Upland Upper hill slopes 31 0 115

Valley Valley bottom, more gently sloping

24 160 125

The composite analysis can be run by most available software systems available and in use by the industry.

It is easily accessible on the REAME software, which is most commonly used for hollow fill assessment (based on DMP’s experience).

DMP will begin asking for the composite analysis for fills designed with sub-soils in place, and especially for upland areas.

Stability Issues

Composite Failure SurfaceComposite Analysis

The composite surface allowthe slice (cylindrical) to intersect the subsoil, then following a sliding surface

REAME Composite Surface

Depending upon the version you are using, the composite analysis should include;

-designation of the “soft soil” (SSN), i.e., the sub-soil

-we prefer that that the sub-soil be modeled with pore pressure ratio of 0.1.

Stability Issues

Stability Issues

QAny Wauestions or coments?Any Questions or Comments?