Advanced foundation topics

The foundation elements that have been explored in Chapter 8 (shallow foundations) and Chapter 9 (piles and pile groups) are not always used in isolation. Indeed, Figure 8.1 shows a simple structure in which two adjacent strip footings are used to support the columns on either side of a structure. Design of the individual foundation elements will still require the use of the techniques described in Chapters 8 and 9; however, the load-sharing between the different elements of the system must also be considered, either to derive the loading conditions for design of the individual elements, or for determining any modi­fications­to­the­element­response­due­to­its­incorporation­in­the­system.­Section­10.2­will­consider­ the performance of a range of different foundation systems under vertical load. A second key factor is that while most foundations exist mainly to carry vertical load, there are certain applications­ in­which­ significant­ horizontal­ or­moment­ loading­may­ be­ applied­ to­ the­ foundation,­ in­ addition to the vertical load. Indeed, Figure 8.1 showed only vertical actions acting on the foundation system;­there­may,­however,­be­horizontal­actions­due­to­wind-­loading­acting­on­the­side­of­the­structure.­This­will­clearly­add­a­horizontal­action­to­the­foundation;­if­the­columns­of­the­structure­bend­as­a­ result of the loading, then bending moments may additionally be applied at the column-footing connection and rotation of the structure may transfer additional vertical loading onto the foundation elements. It is­ therefore­ necessary­ to­ understand­ how­ combinations­ of­ actions­ influence­ the­ stability­ (ULS)­ and­ movements­(SLS)­of­a­foundation. ­ In­most­ building­ structures­ the­ horizontal­ and­moment­ actions­ are­ small­ compared­ to­ the­ vertical­ loads,­and­do­not­need­to­be­considered­explicitly­in­design.­Examples­where­there­may­be­significant­ horizontal­and­moment­actions­include:

foundations for wind turbines and electricity distribution towers;●● coastal and offshore foundations;●● foundation elements used for anchoring (e.g. to carry the tension of a suspension bridge cable which ●● is applied at an angle to the foundation); seismic actions on any foundation.●●

Section­ 10.3­ will­ develop­ the­ limit­ analysis­ techniques­ from­ Chapter­ 8­ to­ consider­ the­ stability­ of­ shallow­foundations­under­combined­loading­(ULS),­and­present­new­elastic­solutions­for­determining­ foundation­displacements­under­such­loading­(SLS).­Section­10.4­will­then­consider­deep­foundations­at­ both­ULS­and­SLS.