For the second method, equipment that can perform cone penetration 

 tests to 40 feet from bottom-resting platforms, are available but are 

 very heavy — 40,000 pounds; Seacalf and Stingray are two examples. 

 Seacalf can operate independently of a drilling platform but requires a 

 special handling system and heave compensators. Stingray is used to 

 control a drill string and, therefore, is much like testing out of a 

 borehole. Smaller bottom-resting equipment are available such as MITS 

 (Multipurpose In-situ Testing System) but are not capable of 40-foot 

 penetrations. 



Coring of cohesionless soils to 40-foot soil depths is possible 

 with vibracorers weighing only a few tons. However, the highly dis- 

 turbed samples are not suitable for measuring strength properties. 

 Consequently, the properties must be guessed or estimated by rough rules 

 of thumb which are less than desirable procedures and will not lead to 

 confidence in a design. 



To achieve a suitable way of evaluating cohesionless soils, the 

 Naval Civil Engineering Laboratory (NCEL) first considered use of an 

 instrumented vibracorer barrel to measure cohesionless sediment prop- 

 erties. A special instrumented barrel was fabricated that measured 

 driving force, soil resistivity, and skin friction at several points 

 along the barrel while taking a core. The idea was to use these in-situ 

 measurements to control laboratory reconstitution of samples that would 

 be extensively tested. Unfortunately, when the barrel was tested, the 

 sensors were found to be easily damaged. Also, the data acquired were 

 difficult to analyze (Lee, 1979). At this point, after a re-evaluation 

 of candidate exploration techniques, it was decided that the use of 

 established in-situ sensors was preferable even if a special bottom- 

 resting platform needed to be developed to conduct the test (Lee, 1979). 



Two possibilities existed: the standard penetration test (SPT) and 

 the cone penetration test (CPT). The SPT is performed by dropping a 

 140-pound weight 30 inches (in air) onto a split-spoon sampler and 

 counting the blows required to drive the spoon 1 foot into the bottom of 

 a drilled hole after 6 inches of initial penetration. Because of the 

 empirical nature of the SPT any change from this procedure negates the 

 wealth of data that has been developed correlating blow count to soil 

 properties. The SPT is designed to be performed in boreholes; therefore 

 the objections that apply to drilling operations apply to the SPT. 



The CPT, on the other hand, does not require a borehole. In this 

 test, a cylindrical probe with a conical tip is pushed into the soil at 

 a uniform rate of 0.02 m/sec or less. The probe is instrumented to 

 measure the force on the tip of the cone and the friction on the side 

 wall of the probe. The CPT provides detailed, continuous, and repeat- 

 able information on a site and is well -suited to solving many geotech- 

 nical design problems. Two disadvantages are that a large force is 

 required to push the probe to desired depths, and no sample is obtained 

 for inspection. However, the advantages outweighed the disadvantages, 

 and the CPT was chosen as the testing device to be developed to satisfy 

 the Navy's need for reliable data on cohesionless soils. 



