ALA Soil Spring Effective Unit Weight

[mecheng1t0]

I would really appreciate if someone could please shed light on what exactly does "effective soil unit weight" mean for soil spring calculations in Annex B of ALA Guidlines for Buried Steel Pipe. Also, why does the Annex B vertical bearing capacity equation has two separate unit weights "effective unit weight" and "total unit weight". Thanks everyone; following is the link to the ALA Guideline:

http://www.americanlifelinesalliance.com/pdf/Update061305.pdf

 

[saplanti]

Appendix B second paragraph gives a proper explanation. I think it is very clear, you do not need interpretation.

 

[mecheng1t0]

The reason why I am confused is because I talked to someone and they were of the opinion that in these soil spring equations the term "effective unit weight" means submerged unit weight regardless of whether the soil is actually submerged or not, which to me does not make sense. For example in case of a location where the water table is no where near the pipe and the total unit density was 1800 kg/m3. In this case they are using a submerged density of 800kg/m3 for axial soil spring (eqation B1), lateral soil spring (equation B2) and vertical uplift soil spring (equation B3). But when it comes to the vertical bearing soil spring (equation B-4) then they use 800kg/m3 for effective unit weight and 1800kg/m3 for total unit weight.

I hope this clarifies my question. Does someone know the background of these soil spring equations ie how did these equations came about? Thanks

 

[BigInch]

Vertical Bearing spring strength (downward loads) is not dependent on whether the soil is submerged or not. Why should soil bearing strength change. Dry compact sand is strong. Ever pounded wet sand? Also extremely hard. That wet sand's bearing strength would not change, or only get harder with even more compaction, until the sand became liquified. Bearing strength is not affected by the submerged weight of soil below the pipeline.

Vertical Uplift springs however are heavily dependent on the weight of soil above the pipeline, because weight is the only thing that provides the uplift spring. Less weight above means lots of uplift movement from relatively small buoyant uplift forces.

 

[saplanti]

Note from CAESAR II pipe stress analysis software for GAMAbar (the effective density) in the equations:

The effective density of the soil may differ from the dry density if the soil is wet (and thus buoyant), in which case the effective density of the soil will be less than the dry density of the soil. If it is expected that the water table may engulf the pipe even for a short time, then it is probably appropriate to enter a wet effective density. If the soil is expected to remain dry, then the dry soil density should be entered.

Some typical soil dry densities are: Clay – 1200 kg/m3, Very Loose Sand -1606 kg/m3, Loose sand – 1686 kg/m3, Medium Sand – 1797 kg/m3, Dense Sand – 1847 kg/m3, Very Dense Sand – 1928 kg/m3

Some typical wet (buoyant) densities for soils are: Clay – 757 kg/m3, Very loose Sand – 1005 kg/m3, Loose sand – 1055 kg/m3, Medium Sand – 1123 kg/m3, Dense Sand – 1155 kg/m3, Very Dense Sand – 1206 kg/m3.

I would like to warn you that the formula matrix for Nch and Nqh under Appendix B2 does not give the curve values somehow. I suggest you to use the curve values only from Fig. B3. And Alpha unit formula KPa/100 should be read KPa/50. If you make a GOOGLE search you can find this correction through ALA.

Unfortunately, somehow ALA became lazy about issuing the corrected final document. I do not even know who to talk about the corrections.

I trust this may help you to understand the effect of the wet soil. Good luck.

 

[BigInch]

IMO ALA method is not very realistic. Sand can actually supply axial friction strength of about 2X what ALA predicts.

US Army Corps of Engineers have much better soil-pipe frictional force determination methods.
Search for COE Engineering Manual EM-1110