Slope Creep 

Tshien Ma, SE

Synopsis: 

This article proposes a practical method to determine the lateral load on a caisson due to slope creep. The method takes into account slope angle, caisson location, caisson diameter, soil type and depth of creep zone.   

Slope Creep and Caisson

Some building foundations are founded on slopes over silty soils. When wet, silty soils are prone to moving slowly down hill by gravity, a phenomenon called slope creep. Slope creep may cause fences to tilt, foundations to settle, and in some cases, buildings to split.

Caisson embedded in competent soil is a common method to mitigate the slope creep damage. To the author’s knowledge, no practical method is available to allow a systematic determination of creep load on the caisson.  Some prescriptive creep loads  exist (See 2^nd paragraph, page 2), but little information is available for engineers to quantify the lateral slope creep load on the caisson (creep load thereafter) for a particular situation.

The proposed method will serve as a practical tool for engineers to quantify the creep load  and to encourage further studies on the subject. 

Telltale Signs of Slope Creep   

Creep movement is virtually imperceptible. Soil testing can identify soils that are prone to creep. Such soils are silt, clayey silt, sandy silt etc. 

The presence of creep can be observed from some telltales signs:

·        Presence of step-like bulges on the slope

·        Single roll-like bulge at the bottom of slope

·        Tilted fence parallel to  the top of slope

·        Vertical, v-shaped cracks on masonry fence walls perpendicular to the slope

·        Diagonal cracks emanating from corners of wall openings

·        Slab on grade pulling away from the building foundation

·        Slab on grade cracks parallel to the top of slope

·        Foundation settlement near slope

·        Dog leg tree trunks that point up

·        Parallel soil cracks on top of slope

·        Houses that split in two

Parameters Contributing to Creep Load on Caisson

After 1994 Northridge earthquake, many building foundations constructed on silty slopes failed. Often caissons were used to repair the earthquake damaged slope and to stabilize the building foundation.

For caisson design, County of Los Angeles promulgated a creep load of  1000 lbs per lineal foot for the top 10 ft of caisson. This prescriptive creep load has since been widely adopted for caisson design. Recently, some geotechnical engineers recommend 55 pcf equivalent fluid pressure acting on a tributary width of twice the diameter on top 3 ft of  caisson. These prescriptive creep loads are independent of such variables as steepness of slope, location of caissons with respect to slope, soil type etc. They have resulted in widely different caisson designs.

The proposed method that allows the determination of creep load on caisson systematically by taking into account slope angle, caisson position on the slope, caisson diameter, soil type and depth of creep.

 Slope Angle

The magnitude of creep load depends on the slope angle. It increases with the steepness of slope angle. The common slope angles α (Figure 1) are 33.69˚ (1.5/1), 26.56˚ (2/1), 18.44˚ (3/1) and 14.05˚ (4/1).

Caisson Position on the Slope

The closer the location of caisson is to the top of slope, the lesser the creep load it receives. It drops to zero when it falls, at a distance of D/tan α from the top of slope (Figure 1). However, in a rare situation where x is very large in relation to d, the creep load on caisson becomes very small. The situation is similar to the flow resistance of a bolder in a small stream that would be insignificant when it finds itself in a river.      

   Caisson Diameter

The magnitude of creep load is approximately proportional to the caisson diameter. 

For single caisson, the tributary area is likely to have the shape of a slender bell  curve with the ordinates on each side dropping sharply from the center. This is because the flow of creep forces prefer the paths of least resistance on the sides of the narrow center band behind the caisson. For design purpose, the shape of the tributary area may be approximated by a rectangle that has a width of 1.5 times the caisson diameter and a length of from caisson to D/tan α where creep zone vanishes (Figure 1).

For multiple caissons spaced at 2 to 6 times the diameter on centers, soil arching can effectively direct the entire creep load within a band of 2 to 5 times the diameter, to the caissons (Figure 2). Soil with higher shear strength permits wider spacing than  soil with lower shear strength.     

  Soil Type

Silty soil is prone to creep when wet due to the decrease of its effective stress. When dry,  silty soils exhibit high shear strength.

A slope’s tendency to creep varies with its soil’s plasticity index PI. Soil with smaller PI is indicative of its high content of silt and its readiness to transform  from solid to liquid as its moisture content increases. Silty soils that have PI less than 8, in author’s opinion, is more prone to slope creep. The tendency of the slope to creep is negligible when PI exceeds 16.

Creep load on caisson is the resultant of down slope creep forces deducted by up slope resisting forces due to soil’s  boundary and internal soil shear strengths. In silty soils, shear strength is derived primarily from internal friction. The contribution to the shear strength from cohesion is small compared with friction. For design purpose, we may conservatively estimate the shear strength using friction only.

For silt the effective friction angle f’ ranges from 24˚ to 35˚. Creep load is more pronounced when f’ is small. Unless a precise f’ is available from laboratory test, a f’ of 24˚  will be used for common cases. To determine the influence of f’ on creep load, we will define a lateral load factor k: k= (1- sin f’) / (1+ sin f’).   k factor for f’= 24˚ is 0.37. This means that the lateral load applied to the caisson is equal to 37% of the soil weight.

Creep Load Distribution

For the convenience of discussion, we will call creep forces parallel to slope, pressure and horizontal components of pressure, load.

We know of no published information about the distribution of lateral creep pressure. At some distance above the boundary between creep zone and non-creep zone, influence of boundary friction fades. In its stead, is the soil internal friction. The patterns of creep pressure distribution shown on Figure 3a.  

To simplify caisson design, we idealize the creep load distribution for soil mass below the top of slope by a rectangle (Figure 3b) and the creep load distribution for soil mass between the top of slope and where creep vanishes, by a triangle.      

  Depth of Creep

The depth of creep commonly used for caisson design ranges from 2 ft to 6 ft.  Depths of creep can be determined by soil test or given in soil report. In its absence,  a depth of creep of 4 ft may be used (Figure 1).

Proposed Method to Determinate Creep Load on Caisson

Nomenclature: (See Figures 1, 2, 3)

α (degrees)    Slope angle

f’(degrees)  Effective soil internal friction angle     

g (lb/cu ft) Soil specific weight    

s₁  (lb/ft)     Unit creep load from soil mass W₁  (See equations 4 and 5)  

s₂ (lb/ft)    Unit creep load from soil mass W₂

d (ft)             Caisson diameter

k                   Active lateral load factor due to boundary and internal friction forces

x (ft)             Distance from caisson center to top of slope

y (ft)            Point of application of creep load resultant P, measured from ground z (ft)             Creep zone tributary width behind caisson

D (ft)            Creep zone depth below ground

D / tan α (ft) Distance from top of slope to end of slope zone

P (lbs)           Creep load resultant perpendicular to caisson

W₁ (lbs)        Weight of soil mass between caisson and top of slope

W₂ (lbs)        Weight of soil mass between top of slope and end of creep zone

The hatched areas in Figures 1, 2 , represent the soil masses that contribute to the creep load on caisson. Referring to these figures, equations below have been derived.

P, the horizontal resultant of creep pressure acting on caisson, depends on W₁, weight of mass between the caisson and the top of slope and W₂  weight of soil mass between top of slope and where the creep zone vanishes (Figure 1).   

W₁ = (x /cos α) z D g                                  (1a)

W₂ =  z ((D² / tan α ) / 2) g                          (1b)

P = k (W₁ + W₂ ) sin α cos α                        (2)

k value depends on soil shear strength and friction at the boundary between the creep zone and the non creep zone. As explained in page 2,  k value of 0.37 corresponding to f’=24˚  may be used for design if an accurate k is not available.

It is clear from equation (2) that P is sensitive to its position on the slope. P is maximum when caisson is located at toe of slope and reduces as the location of the caisson  moves up the slope. It becomes minimum at a distance of (D / tan α) from top of slope where creep zone ends.

P is applied to the caisson, at a distance y from the ground surface (Figure 3).

y =  (W₁  / 2 + 2W₂  / 3) D / (W₁ + W₂)        (3)

         The unit creep loads s₁, s₂ can be computed equations (4) and (5).

         s₁=(P/D -s₂ /2)              (4)

s₂=(12P/D) ((y/D) –0.5)         (5)  

       s_1, and  s₂ are useful for the computation of bending moment of the caisson, in the

         creep zone.  

Example:                       

Given: α = 26.565˚ (2:1 slope), D = 4 ft, d= 2 ft, g= 110 pcf, x = 3 ft, k = 0.37,

z = 1.5d = 3 ft.

Find: P, y

W₁ = (3) (cos 26.565˚) (3) (4) (110) = 4427 lbs. Soil mass behind caisson, below top of slope.

W₂ = (3) (4² / tan 26.565˚) / 2) (110) = 5280 lbs. Soil mass between  top of    slope and end of creep zone.    

 P = (0.37) (4427 + 5280) (sin 26.565˚) (cos 26.565˚) = 1437 lbs

 y = (4427) (4) /(2) + (5280) (4) (2 / 3) / (4427 + 13200) = 2.36 ft, point of 

 application of  resultant P, measured from ground surface down.

   s₂=(12) (1437/4) ((2.36)/(4) –0.5) = 377 lb/ft

         s₁=(1437/4 -377/2) = 171 lb/ft                        

Concluding Remarks:                                                                                                               

The proposed method will provide the engineers a tool to quantify the slope creep load on caisson. This method takes into account, slope angle, caisson position, caisson diameter, soil type and depth of creep.

The creep load magnitude, as a function of caisson position,  reduction factor k , depth of creep D, tributary width z, pressure and load distributions, have not been verified by experiment or research. The author sincerely hopes that this paper may serve as a starting point for further study of this useful subject for the determination of caisson creep load .

Disclaimer:

The method proposed above has not been verified by experiment. It is mainly based on the author’s own experience in designing caisson to resist slope creep forces. Application of this method to any particular project is at user’s risk. No liability is assumed by the author. 

Figures:

Figure 1,  Slope Section

Figure 2,  Active Creep Zone, Plan Views

Figure 3,  Creep load on caisson

(T. Ma,  August 22, 2006)

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