Outside' Clearance Ratio, C„ = 



D, 



x 100 = 



2 to 3% 



(for cohesive soils) 



where the dimensions are as shown in Figure 12. In addition, for samples of 

 inside diameter of 2 to 3 inches, the length to diameter ratio (L/D) for stiff to 

 very soft cohesive soils should be in the range 



— = 10 to 20 



As suggested by Hvorslev, the above values are tentative. 



The length to diameter ratio 

 limitation requires that incremen- 

 tal sampling techniques be used. 

 This has also been concluded by 

 Richards and Parker 21 in their 

 study of core sampling for shear 

 strength measurements. They 

 concluded that for good-quality 

 samples from sediment depths 

 of 30 to 60 feet, this technique 

 is the only feasible method. 



The number, locations, and 

 lengths of cores that should be 

 obtained in the detail survey are 

 questions that arise prior to each 

 survey. Tentative guidelines 

 (dependent on the requirements 

 of the structure, the geographical 

 location of the site, and charac- 

 teristics of the site) could be set 

 for these quantities, but the final 

 decision should be based on one's 

 own engineering judgement. The 

 number and locations of cores to 

 be taken are functions of the 

 importance and the size of the 

 structure. For example, NCEL'S 

 Submersible Test Units could be 

 placed with a fair degree of confi- 

 dence on the basis of one core 



Figure 12. Dimensions of corer for use in 

 computing area, inside, and 

 outside ratios. 



23 



