Total & differential liquefaction settlement

[alusih]

A paragraph out of the California SP117 (Martin & Lew, 1999) states: “Based on the above observations, it can be concluded that the differential settlements at level ground sites with natural soils are expected to be small even if the total settlement is large compared to the total settlement for conditions that typically exist in southern California. However, in the absence of extensive site investigation, it is suggested that the minimum differential settlement on the order of one-half of the total settlement be used in the design. The actual differential settlement value used is dependent upon factors such as the type of structure, bearing elevation of the foundation, subsurface conditions (relatively uniform versus highly variable laterally), number of borings/CPTs, etc.”

Review agencies seem to be hung up on the part that mentions the “rule of thumb” minimum differential settlement on the order of one-half of the total settlement. It seems to being applied in all cases, no matter how deep and thick is the liquefiable layer and no matter how far the liquefible area is from the building site.

The following sentence implies the use of engineering judgment, but I’m coming up short on finding information on quantification of this in everyday practice. I think that whatever judgments are made are presented in investigation reports, for the most part, do not have document based backup and have any sort of uniformity in practice.

I would expect that uniform application of the “rule-of-thumb” would not be the realistic. Excluding the surface manifestations of liquefaction, where a deeper layer indicates the same amount of total settlement as a shallow layer, the localized differential settlement would be less.

Is there something specific that group members are aware of that addresses this issue and gives a more analytic approach?

 

[EireChch]

Hi Alusih,

I work in New Zealand and have done a lot of work in Christchurch. Given the relatively recent earthquake events (within the last 4years) we are apparently becoming one of the world leaders now with analysis and assessment of liquefaction so I would think our guidelines would be up there with the best approach.

Foundation requirements are based on the severity of potential luquefaction. The residential land is categorised into either Technical Categorey (TC)1 TC2 or TC3. The amount of total settlement dictates which category. TC1 is 0-50mm, TC2 is 50-100 and TC3 is 100mm. The differential settlement is measured by floor survey and we apply the rule of different settlement is half to two thirds of total. Therefore is a floor suffered 70mm differential, total would be 105-140mm which would be TC3.

Our guideline recommend the half to two thirds approach, I haven't seen any specific assessment that provides back up for it. However our total settlement from a luquefaction analysis often matches to the observes.

 

[alusih]

Thanks for the response. Is the amount of differential settlement as compared to total settlement that you are talking about true if the liquefaction is occurring shallow or deep? Have you included consideration for when the structure was designed/built and what building code requirements were in effect?