## 1 Introduction  
In order to determine the geotechnical parameters the cone penetration tests and the standard penetration test (SPT) have wide applications. From in situ tests the cone penetration tests (cone penetration test CPT, cone penetration test with pore pressure measurement CPTU, seismic cone penetration test SCPT) have the widest use for estimating geotechnical parameters over a wide range of materials from very soft soils to weak rock. The flat dilatometer tests (DMT, SDMT) have become popular in many countries for estimating geotechnical parameters in non-cohesive and cohesive soils, however, their application is low in gravels and soft rocks [1, 2]. The flat dilatometer was developed by [PERSON] [3, 4]. The detailed procedure for conducting the standard dilatometer test (DMT) and methodology of its interpretation were presented by [PERSON] [5, 6] and [PERSON] et al. [7]. Comprehensive studies have been performed to improve some of the original correlations proposed by [PERSON]. Numerous investigations have been made to expand the application of DMT in geotechnical engineering [8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. In most cases, the relationships used to evaluate geotechnical parameters from the dilatometer test such as the corrected first pressure reading \(p_{o}\) or the corrected second pressure reading \(p_{1}\), or index parameters such as the material index \(I_{D}\), horizontal stress index \(K_{D}\), and dilatometer modulus \(E_{D}\), are commonly applied. Supplementing the instrumentation used to perform the dilatometer test with two geophones in the seismic dilatometer test (SDMT) has expanded possibilities of interpreting the test results [18; 19]. The use of geophones enabled making measurements of an additional parameter in the form of shear wave velocity \(V_{s}\). In this way, the SDMT, similarly to the SCPT, provides an additional measurement enabling the seismic assessment of the studied area and a more precise determination of many geotechnical parameters. Although the seismic dilatometer is used just for slightly more than twenty years, there is already extensive literature discussing the use of SDMT in soil characterization [20; 21; 22; 23; 24; 25], assessment of geotechnical parameters [26; 27; 28; 29; 30; 31], and design of geotechnical structures [32; 33]. The shear wave velocity \(V_{s}\) with dilatometer measurements have been used so far to evaluate the initial shear modulus \(G_{0}\)[19; 26; 28], decay of the shear modulus \(G\) with the shear strain \(\gamma\)[19; 29], interrelationship between \(G_{0}\) and operative modulus [27], detecting the presence of cementation [5], and cyclic resistance ratio \(CRR\)[30]. Undrained shear strength \(\tau_{flu}\) is the basic parameter in the geotechnical design of different structures [34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47]. Evaluation of this parameter using dilatometer tests is usually based on empirical relationships. In single-factor relationship proposed by [PERSON] [3] the horizontal stress index \(K_{D}\) is used as the independent variable. [PERSON] et al. [38] proposed an approach for estimating undrained shear strength using the corrected second pressure reading \(p_{1}\). A different relationship was proposed by [PERSON] and [PERSON] [39], in which undrained shear strength was a function of the corrected first pressure reading \(p_{o}\). For normally consolidated marine clays, [PERSON] and [PERSON] [40] proposed a relationship, in which the dilatometer modulus \(E_{D}\) is as independent variable. A multi-factor relationship was proposed by [PERSON] [41] to evaluate undrained shear strength \(\tau_{flu}\), in which as independent variables were the in situ effective vertical stress \(\sigma^{{}^{\prime}}v\), as well as the net value of the corrected first pressure reading (\(p_{o}-u_{o}\)) and the net value of a corrected second pressure reading (\(p_{1}-u_{o}\)).