BGASCE7-10 Section 4.1


Last Revised: 11/04/2014

Loads computed from soil and/or hydrostatic pressure are designated as "H" in the load combination equations and are found in ASCE 7 Chapter 3, along with dead loads.  This chapter is relatively light on recommendations regarding soil loads and hydrostatic pressure and is specifically focused on the lateral pressures exerted on structures by those loads.  Both soil and hydrostatic loads can be treated as dead load when acting as gravity loads.

The implication of the first paragraph of ASCE 7-10 3.2.1 is that the first choice of soils load information should come from a soils report which is acceptable to the authority have jurisdiction over the design.  In the absence of a soils report, ASCE 7-10 Table 3.2-1 gives some typical values for "Design Lateral Soil Load per foot of depth".  These values are occasionally referred to as "equivalent fluid densities" as they are used in the same pressure equations used for hydrostatic pressure in place of fluid density.

Soils reports tend to provide much more site specific detail and are more accurate estimates of lateral pressures than a generic table can present.  The report will also account for site specific conditions. The computation of lateral pressures can be quite complex and, in most cases, should involve an experienced geotechnical engineer.  It is beyond the scope of the Standard and this text to go into the technical details of lateral load computations. There are many great texts on the subject and require significant study to fully grasp.

Using the simplified analysis implied by ASCE 7-10 3.2.1 should probably be limited to soil conditions which can readily be matched to those found in ASCE 7-10 Table 3.2-1 and to design situations not involving surcharge loading near the building, increasing slopes of backfill, or significant depths.

It is interesting that both Table 3.2-1 and the commentary give 'design advice' by specifically recommending against using specific soil types as backfill and advises design strategies for dealing with sites with such soils.  It is generally possible to import acceptable soils for backfill.  Pressure computations should reflect the actual soils used and their interaction with the native soils if necessary.

The commentary spends some time addressing the issue of expansive soils.  The general recommendation is remove expansive soils adjacent to the structure and back fill with non-expansive soils and provide means for preventing water from reaching the expansive soils which cannot be removed.

The Standard is silent in regards to effects of seasonally frozen soils adjacent to structures.  Well drained, non-frost susceptible soils generally do not exert addition lateral pressure on a wall, however other soils which allow for the formation of ice lenses do.  It is strongly recommended that frost susceptible soils be removed and non-frost susceptible soils be used as back fill similar to the recommendations found in the commentary regarding expansive soils.

One errata note:  In the edition of ASCE 7-10 which we have, the footnotes to Table 3.2-1 are incorrectly labeled.  The should be labeled in order from 'a' for the first and 'd' for the last ('d' should be 'Unsuitable for backfill material').