Large Penetration Test Research Articles

A method for correlating large penetration test (LPT) to standard penetration test (SPT) blow counts

The standard penetration test (SPT) split-spoon sampler is too small for investigations in gravelly soils. For this reason, several researchers have developed scaled-up versions of the SPT, commonly referred to as "large penetration tests" (LPT), and attempted to correlate the measured blow counts to SPT blow counts. Several LPTs have been in use worldwide; each with different drill rods, sampler dimensions, and hammer energies; hence existing published LPT–SPT correlations differ for each system. This paper summarizes the LPT data in the literature and presents a fundamental method for predicting LPT–SPT correlations. The proposed method is based on wave equation analyses of SPT and LPT and considers variations in test equipment, penetration resistance, and energy. It is shown that the method provides a unified approach for assimilating the various published LPT–SPT correlations. Additional SPT and LPT data were collected at a sand research site to check the proposed method and to expand the LPT database. The observed and predicted correlation factors are shown to be in good agreement. The proposed correlation method will be useful as a logical framework for the comparison of LPT data collected throughout the world. Additional research will be required to extend this procedure to gravel sites.Key words: large penetration test, LPT, fundamental correlation method, wave equation analysis, grain size effects.

Correlation between Strengths and Penetration Resistances for Gravelly Soils

High-quality undisturbed gravel samples, 30 cm in diameter and 60 cm high, are obtained by the in-situ freezing method at four sites. Both the undrained cyclic shear strength and drained shear strength of these samples are measured by using a large scale triaxial test apparatus. The standard penetration test (SPT) and large penetration test (LPT) are conducted at each site to obtain possible correlations between their penetration resistances and the in-situ shear strengths. The data of gravels obtained by other investigators are also reviewed and discussed. The laboratory test results and the review of previous studies lead to the following conclusions. (1) The liquefaction resistance (the shear stress ratio required to cause 2 or 2.5% double amplitude axial strain in 15 load cycles) of the high-quality undisturbed gravel samples shows a good correlation with the value of N L 1 (normalized for the effective vertical stress of 1 kgf/cm 2 ) over a range of N L 1 between 20 and 50 in such a way that the liquefaction resistance increases with an increase in N L 1 . The correlation appears to be independent of effective vertical stress. (2) The liquefaction resistance of the high-quality undisturbed gravel samples is overestimated particularly for N 1 ,-values larger than 30, if correlations between the liquefaction strength and the blowcount from the conventional SPT obtained for sands are directly applied.

Undrained Cyclic Strength of Gravelly Soil and Its Evaluation by Penetration Resistance and Shear Modulus

Undrained cyclic triaxial tests and drained triaxial compression tests were conducted on undisturbed samples of diluvial dense gravelly soils obtained by in-situ freeze sampling at four sites. These test results, together with test results by other researchers indicate that undrained cyclic strengths of gravelly soils depend not only on the modified blow count of penetration tests but also on effective confining pressure. By considering the effect of effective confining pressure on undrained cyclic strength, a method to evaluate undrained cyclic strength of natural gravelly deposits using the blow count of Large Penetration Test (LPT) is proposed. Furthermore, the possibility of evaluating undrained cyclic strength of natural gravelly deposits using initial shear modulus was investigated. It was found that although initial shear modulus can be used for this purpose, the accuracy is much less than that based on LPT penetration resistance.