# Behaviour of Uniform Drava River Sand in Drained Condition--A Critical State Approach
## 3 Tests Equipment and Procedures
### Sample Preparation  
As it is specified in Table 2, specimens at three different densities were tested in this research. To achieve comparable results, the undercompaction method in specimen's preparation was employed. The method was developed by [PERSON] [49] and provides continuous density throughout the sample height and it was used in laboratory tests very often. The method ensures repeatable test results, minimizes particle segregation and offers a preparation of reconstituted sand specimen, [49]. Figure 4 presents the specimen preparation using undercompaction method (Figure 4a) and prepared specimen for drained compression test (Figure 4b). The diameter of a compaction rod was 1 cm and was made of a hard wood. This ensured the best control of energy used for compaction and reduced the possibility of grain crushing during compaction process.  
The authors are well aware that the method can produce some effects on the initial state, which in turn can have influence on critical state, [9; 50; 51; 52]. Nevertheless, the method is used mainly because of the repeatability of the sample preparation. Before the start of main test stage, several tests were performed under different undercompaction degree, as suggested by several authors [1; 46; 49]. It has been found that the optimal value of undercompaction degree is between 4 and 5%. The authors used the value of 5% for the undercompaction degree in this research. Besides initial tests performed to establish a degree of undercompaction, several tests were also performed to define the optimum number of the undercompaction layer. During the initial tests, it has been found out that the optimum  
Figure 3: Scheme of static triaxial system: (a) triaxial cell, (b) sample, (c) load cell, (d) Linear Variable Differential Transformer, (e) cell pressure transducer, (f) pore water pressure transducer, (g) back pressure transducer, (h) volume controller, (i) pneumatic pressure system, (j) deaired water tank, (k) servo-flow, (l) RTC module, (m) ATD data logger, (n) personal computer, (o) compressed air distributor.  
number of layers is around 15 layers. Several tests were performed in order to take into account the different initial water content during undercompaction preparation. The water content used for preparation was found to be the best at 3.5%. At lower water content the sand was relatively dry, thus limiting the grain-to-grain sliding, while at higher water content the sand grains were able to easily slide of one another due to water film developed around soil particles [42], making it harder to finish the top layer during specimen preparation. This showed some issues during initial testing, especially in extension tests, in the view of \"necking\" in the upper part of the specimen (near the top cap), which can in turn lead to nonuniform density distribution throughout a specimen. The height of each layer was controlled using a standard laboratory caliper. Tamping of each layer was performed using wood stopper with the edges slightly rounded in order to reduce possible damages to the latex membrane. Each layer was compacted by 10 blows.