DECLARATION 1: PLAGIARISM
2. CHAPTER 2: LITERATURE REVIEW
2.8 Current design procedures
2.8.1 SANS 207/BS8006:1995
SANS 207:2006 covers the design of a reinforced layer at the base of an embankment situated over a subgrade void. The design process in SANS 207 is the same as the British code BS 8006:1995. The role of the basal layer of reinforcement design in SANS 207 is to limit vertical settlement and maintain serviceability until a time when permanent reinforcement measures can be put in place. This is a temporary reinforcement measure to prevent ULS (collapse) from taking place.
When designing foundations over poor soil conditions the following factors are considered in SANS 207:
i. The maximum acceptable soil surface deformations for the structures requirements i.e.
road, railway line, embankment
a
ba = [1 – 2%]. Depending on the expected traffic flow in the design area, 1% = high traffic flow, 2% = low traffic flow.
ii. Choosing a suitable soil void diameter
Survey of the ground conditions in the area and the geological history will help determine the diameter of the void that will be used. SANS 207 states that it is preferable to select a conservative value due to “uncertainties of future subsidence, and the consequent risks involved”.
iii. Determine the maximum strain allowable in the reinforcement
As shown in Figure 2.4 above, SANS 207 assumes a void of diameter D forming beneath a fill layer, resulting in a deflection bowl developing at the surface of the fill. The symbols are defined as follows:
Ds = Diameter of surface settlement trough (mm) ds = Vertical settlement at surface of fill layer (mm) H = Height of fill layer (mm)
D = Design diameter of void (mm)
d = deflection of geosynthetic reinforcement (mm) = Angle of draw/angle of void propagation
The shape of the deflected geosynthetic layer is assumed to be parabolic by SANS 207. The vertical forces the geosynthetic are due to weight of the soil and any addition surcharge loading.
All loading is regarded as vertical and uniformly distributed.
Figure 2.4: Upward propagation of void as assumed by SANS 207
The parabolic shape of the deflected geosynthetic is shown in Figure 2.5 below:
The parabolic equation describing the vertical deflection of the geosynthetic then becomes:
( = 4B'4
>4 − B
Using the extended length of the geosynthetic, the reinforcement strain is derived in Steinman (1957):
= dfbee (Steinman, 1957)
SANS 207 assumes that no volumetric change takes place in the soil mass, hence the soil volume displaced at geosynthetic level = the volume of the surface settlement trough. The volume of the settlement trough subsequently depends on the shape of the void that forms. Two void shapes are considered, an axisymmetric void (circular in shape) and a longitudinal void (in the shape of a long trench).
The volume of soil displaced due to these void shapes is as follows:
1) Longitudinal void: V = 2 B>
3 = 2B >
3
2) Circular void: V = B>4
3 = B >4 3
Figure 2.5: Shape of deflected geosynthetic fabric as per SANS 207 specifications
The mass of soil effected by the formation of the void is in a funnel-like shape which is governed by the angle of void propagation .
Given this information, an expression can be written for the settlement trough diameter:
> = > + 2h ijN
Substituting the expressions for d and D in the equation the following is obtained for circular voids:
This is done using the formulae εmax:
k dlbaae
em(bn 4op Cqr)s fbs
iv. Determine the tensile properties of the reinforcement needed for design
t is the tensile load in extensible geosynthetic fabric. This is calculated by apply safety factors to the maximum stress in a cable as per Steinman (1957):
t = 0.5λ (wR h + wY ) D x1 + zVy
t = Tensile load in the reinforcement per m run
λ = Load distribution coefficient. (Axisymmetric = 0.67; plane strain longitudinal voids = 1) wR = Partial load factor for soil unit weight, ffs = fq = 1.3 (ULS), ffs = fq =1.0 (SLS).
= Soil unit weight H = Embankment height
wY = Partial load factor for embankment loading = Embankment surcharge
D = Selected void diameter
= Reinforcement strain (< εmax)
For inextensible reinforcement SANS 207 states that alternate measures should be considered.
v. Determine the reinforcement bond length
The reinforcement bond length is the length of geogrid required for anchorage to take place, ensuring that no slippage takes place at the base of the embankment. This is based on activating soil friction and requires interlocking between the soil particles and the geogrid mesh. The minimum bond length required to carry the load of t is:
≥ wC wZ t h (|yian φHcv1
w + |4tan φHcv2
w )
fn = Partial factor governing the economic ramifications of failure (fn = 1 for embankments and
soil, fn = 2 for structures of larger monetary value)
fp = Partial factor applied to the pull-out resistance of the reinforcement.
H = Average height of fill over the bond length of the reinforcement;
γ = Unit weight of the embankment fill;
α1 = Interaction coefficient relating the soil/reinforcement bond angle to tan φ'cv on one side of the reinforcement;
α2 = Interaction coefficient relating the soil/reinforcement bond angle to tan φ'cv on the opposite side of the reinforcement; fms
fms = Partial material factor applied to ian φH (fms = 1 for both ULS and SLS)
vi. SANS 207 design assumptions
SANS 207 calculates the expected soil surface deflections and geotextile strain by using volumetric change calculations. It assumes that the volume of soil contained in the basal layer when a loss of support takes place is the same as the volume of soil in the surface
displacement trough. No volumetric change is accounted for, hence SANS 207 does not account for any soil dilation due to shearing. The soil is also assumed to be cohesion less.
When tensile forces are activated in the geosynthetic layer after soil collapse has taken place, SANS 207 assumes that the geogrid layers deflected shape is parabolic.
The second analytical method considered was developed by the RAFAEL team (Renforcement des Assises Ferroviaires et Autoroutières contre les Effrondrements Localisés – reinforcement of railway and motorway foundations against localised subsidence) (Blivet, et al., 2002).