Ŕ periodica polytechnica
Civil Engineering 57/1 (2013) 39-44
doi: 10.3311/PPci.2140 http://periodicapolytechnica.org/ci
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Correlation of undrained shear strength and CPT resistance Zsolt Rémai
Received 2012-05-16, accepted 2012-08-25
Abstract The correlation of CPT resistance and undrained shear
strength of soft Holocene clays is discussed in this paper. This soil type, covering significant part of Hungary is frequently in- volved in different geotechnical engineering problems such as building of highway embankments. Soil samples of eight dif- ferent sites have been studied, samples were taken at each site and cone penetration tests ...view middle of the document...
Another important issue is the limited strength and bearing capacity of soft soils. The shear strength of soils deter- mines the maximum allowable load which can be applied on it (e.g. maximum footing pressure, embankment height etc.). In the case of cohesive soils the undrained loading condition is the least favorable, so it is the undrained shear strength that governs the bearing capacity. Undrained shear strength (su) is a com- monly accepted and used soil parameter and there are numerous ways to obtain it. Nevertheless it is not a single soil parame- ter. The measured undrained shear strength depends on testing method (failure mode), strain rate, stress path and many other factors (Mayne et al., 2009) . Therefore it is important to clearly define which undrained shear the given data refers to.
This paper focuses on correlation of CPT results and undrained shear strength values determined by means of consolidated-undrained triaxial and unconfined compression tests. There are numerous methods available to estimate undrained shear strength of clays of different types, but there is no information about their reliability in case of soft Holocene clays located in the Carpathian Basin. A reliable correlation of the test results could provide more detailed information on the shear strength of such soils at many sites by having CPTu mea- surements at every two centimeters. The number of data could also enable a statistical analysis of the undrained strength val- ues, thereby could also provide information on the uncertainty of the strength parameter which is essential for failure probabil- ity assessment (Nagy, 2008) .
Data from 8 different sites are summarized and evaluated in this paper. At each location the upper 5-10 m thick layer was formed by soft, Holocene clays, having a CPT tip resistance (qc) value less than 2 MPa.
Correlation of undrained shear strength and CPT resistance 392013 57 1
2 Existing correlations, performed tests Cone penetration testing have been used for a long time in
soil exploration, and determining pile bearing capacity (Mahler, 2003)  and the undrained shear strength of soft soils is one of the earliest applications.
There are many theoretical solutions based on different con- siderations such as bearing capacity theory (Terzaghi, 1943 ; de Beer, 1977 ), cavity expansion theory (Skempton, 1951 ; Vesic, 1975 ), analytical and numerical approaches (Ladanyi, 1967)  or strain path theory (Teh, 1987) . A de- tailed summary of these methods is given in Lunne et al. (1997) . The undrained shear strength can be derived in a very simi- lar way in each of the theoretical proposals, the general formula can be written as:
su = (qc − σ0)/Nc (1)
where su is the undrained shear strength, Nc is the theoretical cone factor, σ0 is the total horizontal, vertical or mean stress (depending
on the theory considered). The derived theoretical expressions have pointed out that a