6585844998

209

South Africa

E* = 358(N + 5) kN/ra²    (5)

in saturated clayey fine sands.

Ratios between settlements calculated from Equations (4) and (5), using the Boussi-nesq stress distribution, and measured settlements have ranged between 0,70 and 1,35 for shallow column bases, Steel tanks and earth fills on normally Consolidated estuarine sands and clayey sands.

FOOTING WIDTH, B , METRES

Fig. 6. Allowable bearing pressure on sand, after Peck, Hanson and Thombum (1963).

Taking into account the possible sources of error in standard penetration tests, it is preferred, where possible, to esti-matę settlements from the results of Dutch static cone penetration tests, which are not only morę economical but also less dependent on operator technique. In some cases when q^values are not directly available they are assessed from SPT N values using previously determined corre-lations for the area. Comparison of Equations (4) and (13) for example reflect an average ratio of q_ę = 214N kN/m² in the fine to medium normally Consolidated estuarine sands in Durban. As an expediert Schmertmann (1970) recoramends the ratios in Column 2 of Table 2 which are regarded as independent of depth, relative density and water conditions.

Eguations (4) and (5) and (13) and (14) were derived for normally Consolidated sediments. It is apparent, from the re-latively high q_c or N values often obser-ved within the penetration graphs or bore-hole logs, however, that some subsoil strata may be overconsolidatęd in places.

This is consistent with the effects of periodic desiccation and movement of sand dunes during the depositional history of the estuarine sediments. As pointed out by Schmertmann (1970) a statically loaded footing would settle less in an overcon-solidated sand than in a normally Consolidated sand of the same density and rela-tive density at the same depth because the greater lateral stress results in a grea-ter value of E_s, the equivalent Young's modulus for static compression of sand. Thus q_c or N values at least change in the proper direction as the result of overcoir solidation. It is not practicable to determine the magnitude of the insitu lateral stresses in the estuarine sediments, nor easily to assess the effects of such stresses either on cone resistance or on foundation settlement. It is assumed that both q_c and N for overconsolidated sands and clayey sands are quantatively correct in predicting settlements in overconsolidated sediments, although this approach may we11 be too conservative.

Usually Conservative q_c/N Ratios, After Schmertmann (1970)

TABLE 2

Type of Soil	qc/N kg/cm^2	qc/N kN/ra^2
Silts, sandy silts, slightly cohesive silt-sand mixtures.	2,0	approximately 200
Clean, fine to med. sands and slightly silty sands.	3,5	350
Coarse sands and sands with little gravel.	5	500
Sandy gravels and gravel.	8	800

Dynamie Cone Penetrometer Test.

The DCPT blow count is frequently em-ployed, in a qualitative way, to describe the consistency of subsoils on the basis of correlations obtained in conjunction with standard penetration tests or by carrying out dynamie cone penetration tests close to large diameter auger holes in which the soil profile is examined. Generally the dynamie cone penetration test is carried out in soils which cannot be penetrated to sufficient depth by the Dutch static cone penetrometer. Being relatively inexpensive the dynamie cone penetration test can be carried out in volume and the results eroployed to supp-lement Information obtained from standard penetration tests, or inspection shafts, in establishing the distribution and con-tinuity of different subsoil strata over a site.