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The A STM D 4546 method using consolidation test equipm ent also can be used to obtainswell estimates. It should be used instead of D 2435 where possible since the data obtainedis more directly related to swell. Basically we can obtain a swell curve by confining thesample in the ring using a very small confining pressure of about 7 kPa (1 psi) and allowingit to absorb water and swell. If we m easure the volum e chang e in these conditions, we havea free swell test. If we apply sufficient consolidation pressure to prevent the sample fromexpand ing, we can me asure the swell pressure required to maintain the zero volume ch ange.These data can be directly extrapolated to the expected heave or to the footing/overburdenpressure required to eliminate, or at least reduce , the swell m ovem ents.

The resulting estimates improve with sample quality and careful attention to test details.The estimate also improves if the current natural water content wo, and the degree of sat-uration for the laboratory volume change, are representative of the long-term in situ value.Th e latter is a very im portant conside ration since the laboratory sam ple is thin and has a ccessto sufficient free water to obtain S = 100 percent in a short time; this may never occur inthe field, at least through the full depth id of the zone (Fig. 7-6) with potential to expand orshrink.

7-10 FOUN DATIONS ON CLAYS AN D CLAYEY SILTSClays and clayey silts may range from very soft, normally consolidated, to very stiff, highlyoverconsolidated deposits. Major problems are often associated with the very soft to soft, de-posits from both bearing-capacity8 considerations and consolidation settlements. We shouldnote that "soft" implies that the soil is very wet to saturated. Consolidation settlements occurin these deposits with high water contents as found along lake and ocean fronts, as well asin beds of former lakes and old streams w here channe ls have becom e relocated but the watertable remains high.

Silts with a large Ip and/or Wi may be called plastic silts. These silts exhibit nearly thesame characteristics as those of soft clays. The plasticity results from contamination of the

8Note: Bearing capacity is a measure of soil shear strength. Settlement is a measure of soil stiffness or E5.

Fig ure 7-9 Belled piers in expansivesoils. The pier shaft should be as small aspractical to reduce shaft load from soil ex-pansion.

Load carriedhere

ShaftOversize shaft holewith straw, saw-dust, etc., as filler

Probable expansiondepth

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mass with clay minerals and/or organic material. Inorganic silts and silts with little clay con-tent may be loose, but their behavior is more that of sand, and procedures for design anddensification are similar as previously noted in Sec. 7-7. Few pure silt deposits are found innature. Most deposits contain some clay particles (with the resulting plasticity/cohesion) orquantities of fine to medium sand. In passing, note that as little as 5 percent clay can give asilt "cohesion"; 10 to 25 percent clay particles may result in the deposit being a "clay."In both these types of soil it is necessary to make a best estimate of the allowable bearingcapacity to control a shear failure w ith a suitable factor of safety and to estim ate the p robab leconsolidation settlements. The bearing capacity is most often determined using the undrainedshear strength as obtained from quality tube samples or from samples obtained from routineSPT. If the soil is highly sensitive (remoulded shear strength one-fourth or less of undis-turbed ), consideration should be given to in situ strength testing such as the van e shear or thecone penetration test.Consolidation tests should be made to determine the expected settlement if the structurehas a relatively h igh cost per unit area. For sma ller or less importan t structure s, som e type ofsettlement estimate based on the index properties might be justified.Precon solidated clay s often co ntain shrinka ge crac ks and joints (fractured into a quantityof small blocks). The presence of structural defects makes it somewhat difficult to determinethe unconfined compression strength. In many cases, and especially above the water table(S