Slope Stability Report (rec. 16.04.25) · MCU25/0042

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If footings are located adjacent to an underground service or other obstruction that is not homogenous with the
founding strata, the footing should be extended a minimum 0.3 m below an imaginary line projected at a 45 degree
angle from the lowest point of the service/obstruction. This is depicted graphically for reference:




Where footings found into medium dense silty sand, they may be dimensioned for a maximum allowable bearing
pressure of 100 kPa. Based on the borelogs, the above material is encountered at approximately 1.1 m to 1.3 m
depth below existing surface levels at the time of the investigation.
The ultimate geotechnical end bearing and shaft adhesion parameters are presented in Table 2 for deep level
footings where high level footings are not suitable. The values given will need to reduced by applying a suitable
geotechnical strength reduction factor (Φg) for limit state or divided by a suitable factor of safety for working stress
methods. The Φg should be selected by the designer.
TABLE 2: DEEP LEVEL FOOTING – ULTIMATE GEOTECHNICAL PARAMETERS (LONG PILE)

                  Material                          Bored               Bored            Screw               Screw

                                                  Fb (kPa)             Fs (kPa)        Fb (kPa)             Fs (kPa)
           Medium dense silty sand                    450                 5                450                  NR
 Notes: Fb = Base Bearing (Ultimate); Fs = Shaft Adhesion (Ultimate)


The upper 1 m of soil profile should be ignored in shaft calculations for bored piles to allow for shrinkage cracks. Due
to installation disturbance, shaft adhesion should be ignored for steel screw piles. The values presented in Table 2
assume that the material equal to four pile diameters below the base of any footings is as strong or stronger.
Settlement values are subjected to a variety of factors; however, a properly designed and constructed pile should
have a serviceability settlement between 1% and 2% of its’ diameter.
Lateral capacity of steel screw piles could be increased by constructing concrete pile caps or by using proprietary
head attachments which are dragged into the soil providing additional lateral resistance at the pile head. It should
be noted that the lateral support is generally limited and is generally suited to structures that can accommodate some
lateral movement.




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