# Application of Direct Shear Test to Analysis of the Rate of Soil Improvement with Polyester Fibres
## 3 Results
### Influence of the Specim Number on Test Results  
An analysis of the influence of specimen number on the test results was also carried out. Values of the shear strength parameters of the soils and those reinforced by fibres evaluated from various combinations of specimens (all four tested specimens and chosen combinations of only three specimens) are introduced in Table 4 (for soil CI), Table 5 (for soil CS1), and Table 6 (for soil CS2). Values of the shear strength parameters were evaluated based on normal stresses and shear stresses, posted in Table 3, where specimen No. 1 was loaded by a normal stress of about 50 kPa, specimen No. 2--by a normal stress of about 100 kPa, specimen No. 3--by a normal stress of about 200 kPa, and specimen No. 4--by a normal stress of about 300 kPa. The reason for introducing these combinations of specimens No. 1, 2, and 3 and No. 2, 3, and 4 was that these two combinations had a common interval of normal stress from 100 kPa to 200 kPa; therefore the shear strength parameters obtained from them would be valid for the mentioned interval of normal stress (but they were very different), as can be seen later. Differences in shear strength parameters, obtained by various combinations, are introduced in Table 8 (for soil CI), Table 9 (for soil CS1), and Table 10 (for soil CS2). As can be seen in Tables 4-6, the values of the correlation coefficient R between shear stresses and normal stresses were high and the test results could be accepted since tests with only three specimens were in accordance with [12; 13; 14; 15; 16], where no prescribed value of the correlation coefficient was posted. As has been mentioned, many analyses in existing \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Differences in Strength Parameters**} \\ \cline{3-6}  & **Fibre Amount** & & \(\varphi\) & & **c** \\ \cline{2-6}  & **(\%)** & **(\({}^{\circ}\))** & **(\%)** & **(kPa)** & **(\%)** \\ \hline Case No. 1 (4 specimens: No. 1, 2, 3, & 0.0 & 1.161 & 3.76 & \(-2.663\) & \(-4.45\) \\ and 4)—Case No. 2 (3 specimens: No. 1, 2, and 3) (\({}^{\circ}\) or kPa, \%) & **1.5** & **\(-\)3.706** & **\(-\)13.92** & **10.819** & **12.48** \\ \hline Case No. 1 (4 specimens: No. 1, 2, 3, & 0.0 & 0.88 & 2.83 & \(-5.18\) & \(-8.31\) \\ and 4)—Case No. 3 (3 specimens: No. 1.0 & 0.5 & \(-0.004\) & \(-0.01\) & 0.019 & 0.03 \\ 2, 4, and 4) (\({}^{\circ}\) or kPa, \%) & **1.0** & **2.752** & **8.32** & **\(-\)17.294** & **\(-\)21.18** \\ \hline Case No. 2 (3 specimens: No. 1, 2, & 0.0 & \(-0.281\) & \(-0.90\) & \(-2.517\) & \(-4.04\) \\ and 3)—Case No. 3 (3 specimens: No. 1.0 & 0.5 & 0.247 & 0.81 & \(-0.544\) & \(-0.84\) \\ 2, 4, and 4) (\({}^{\circ}\) or kPa, \%) & **1.0** & **6.668** & **20.15** & **\(-27.769\)** & **\(-\)34.01** \\ \hline \hline \end{tabular} Note: in Tables 8–10, underlined red numbers show the worst case (extremely large differences in shear strength parameters, caused by different combinations of 3 specimens); no underlined red numbers show more favorable case, but there are still very large differences in shear strength parameters.  
\end{table} 
Table 8: Differences in shear strength parameters between various specimen combinations (soil CI). research are carried out from data obtained from DST of only three specimens. However, various combinations of three specimens provided different results. For example, for soil CI, see Table 4 (case No. 2, 1.5% fibres), based on the test results of the combination of specimens No. 1, 2, and 3, one can state that the angle of internal friction of soil with an amount of fibres of 1.5% was 33.803\({}^{\circ}\) and cohesion was 86.688 kPa. But based on the results from the combination of specimens No. 2, 3, and 4 (case No. 3), these values were 23.833\({}^{\circ}\) and 125.349 kPa. The differences were very high, up to 9.920\({}^{\circ}\) (41.54%) and \(-\)38.661 kPa (\(-\)30.84%), see underlined red number in Table 8 (Case No. 2 (three specimens: No. 1, 2, and 3)--Case No. 3 (three specimens: No. 2, 4, and 4)).