With in situ technology, it is expected that it would be feasible to analyze the soil status instantly. It will take some time to collect data and make conclusions from the laboratory testing method in the case of the soil sampling technique. We have recently used a variety of in situ testing procedures depending on the site and the type of structure [1-3].

Static penetration tests (Fig. 1) are used in the field to gauge a cone's resistance to piercing under the influence of static force. Hydraulic presses are used to apply indentation force, with the drawbacks of heavier loads requiring more anchors. These involve a freely moving steel rod with a diameter of 15 mm, a steel cone on top, and a hollow tube with a diameter of 36 mm. The 36 mm diameter cone is the most popular; however, the 45mm cone is also utilized. The tube and cone penetrometer's cross-section measurements are depicted in the bridge.

In the first step of the static penetration tests, only cones were crushed at a depth of 10 cm using rods. On this occasion, we measured the resistance to cone penetration. When assessed for lateral friction, just enough of the second phase was injected to allow the pipe to join the cone. To achieve a full sinking of about 20 centimeters, the third stage involved pressing the cone and pipe together by an additional 10 cm. In Fig. (2), the phase of the field testing during the static penetration tests represents the overall penetration resistance.

The following equation indicates the resistance to penetration:

(1)

Where,

A is the cross-sectional area of the cone and

P is an impressive force.

The soil surrounding the cone remained flexible as it penetrated the earth; the resistance to cone penetration can be illustrated as follows:

(2)

(3)

Where,

σ ₀ is the internal soil friction angle

The observed depth of the higher soil layers has been found to be under effective stress. The angle of internal friction ϕ can be computed using resistance to penetration and these relationships. However, this cannot be calculated based on the friction angle for coherent layers due to the effect of pore pressure that occurs during the process of penetrating coherent materials. The correlation between internal friction coefficient V_bd and angle ϕ is depicted in Figs. (3 and 4).

According to Buisman, there is a direct correlation between the compressibility index C_c and the resistance to cone R_p penetration:

(4)

Fig. (2). Static penetration tests' testing phase.

Fig. (3). Relationship between the coefficient Vbd and angle of internal friction φ.

Fig. (4). Safety factor (SF) variation with depth.

It should be underlined that only sparingly should this particular value of the coefficient Cc be employed. Some tables show how the relative density, consistency index, and uniaxial strength relate to the resistance to cone penetration that may be found in the pertinent research.

Cone penetration resistance (Vbd) and non-cohesive material compression modulus (Mv) were found to be correlated by Buisma [9, 10].

(5)

De Beer (1948) coined the following formula while researching the predicted and observed settling of bridges on sand:

(6)

According to Vesi, the relative density affects the compressibility modulus, and a correlation between the two has been identified [11]:

(7)

Where, for completely loose sand stems M_v=2V_bd and for compacted ones, M_v=4V_bd.

The following correlations for sand materials were used in Belgium to establish a parallel assessment of the static penetration test and the SPT:

(8)

Where,

N is the number of blows at SPT.

Suklje proposed the following expression based on information from the literature:

(9)

Where,

2000 < C₁ < 4000 and 400 < C₂ < 800.

The main objective of this test is to assess the relative densities of the penetrating cohesionless soils. To determine the in situ characteristics of the soil, the standard penetration test (SPT) is carried out according to the standards required for the soil where the loads act [12-14]. The test is particularly suitable for obtaining results by drilling into soils to agitate a moving sample and to provide a rough assessment of the soil's dynamic resistance. The Raymond Pile Company developed the test in 1902 and it is now widely used for in situ testing worldwide. ASTM D 1586 and AASHTO T-206 both provide thorough instructions for the SPT. A hollow, thick-walled tube is driven into the ground, and the sampler is moved 300 mm (1 foot) vertically by counting the number of blows. A 63.5 kg hammer is repeatedly lowered from 0.76 m (30 inches) using a drop-weight mechanism to create three sequential increments of 150 mm (6 inches).

The N-value (blow count) or SPT-resistance is the total number of blow counts necessary to advance the second and third increments, while the first increment is referred to as ”seating” (measured in blows per 0.3 m). If the sampler cannot be pushed 450 mm, the number of blows for each 150 mm increment and each half increase is noted on the boring log. For partial increments, the depth of penetration is indicated along with the total number of blows.

The test may be used to determine several types of soil and weak rocks; however, it is not particularly helpful in determining soft clays or gravel deposits. The test's ability to offer both a sample and a number is helpful, but it is troublesome because it is impossible to accomplish two things well at once. The SPT is carried out at the bottom of a soil boring that has been prepared using either flight augers or rotary wash drilling techniques.

The drilling operation is halted periodically to carry out the SPT. At depths shallower than 7 meters, tests are typically done every 1.5 meters, and every 2.0 meters after that. The water head of the borehole must be kept at or above the ambient groundwater level to avoid water intrusion and borehole instability. The effective overburden pressure ('v), drilling rod length, borehole diameter, sample technique, and hammer type are some of the variables that affect the SPT below counts [15]. Therefore, many modifications to the fundamental number of blows acquired in SPT tests have been suggested by researchers to determine a reasonable index for measuring soil density and resistance. The NCEER-98 study for granular soils by Roberts and Wride has provided the most thorough relations [16]. By converting the measured N to N60 as follows, the discrepancies in testing processes may be at least adjusted for Skempton [17, 18]. The adjusted SPT below the 60% energy level is calculated using this method using the following relation:

(10)

Where,

N - counted SPT below measured

C_E - depending on the type of hammer, the energy effect coefficient

C_B - borehole diameter correction factor;

C_R - corrected rod length factor

C_S - modification factor for sampler type (with or without liners)

C_N - the equation used to calculate the effective overburden pressure coefficient:

(11)

Where,

Pa - atmospheric pressure

σ'_v0 - actual vertical pressure at the depth being considered

The optimum correction factors according to NCEER guidelines are shown in Tables 1-3.

It is not essential to provide soil parameters during the classification of soils [19]; the program completes this phase automatically by assigning them to the geological profile (Table 4). As a result, the CPT evaluation is completed quickly and with great clarity. According to Robertson (2010), the classification of soils is based on the measurement of penetration resistance (qc), local skin friction (fs), and pore pressure (u₂). Based on the graphs below, the software determines the soil behavior type (SBT) based on the corrected results of the friction ratio Rf = fs/qt.pa and the cone resistance qt = qc + u₂ * (1 - a), or percentage ratio qc/pa, when atmospheric pressure equals 100 kPa (or 1tsf).

According to Robertson, the more recent categorization of soils has fewer distinct soil groups than the initial classification (Table 5). However, according to Robertson, soil classification is now more accurate and widely used around the world. Soil classification according to Robertson [4-6] has been depicted in Figs. (5 and 6).

According to Robertson, the more recent categorization of soils has fewer distinct soil groups than the initial classification from 1986 [10-12]. Nevertheless, Robertson's classification of soils is based on their measured penetration resistance (qc), local skin friction (fs), and pore pressure (u₂) values [5]. The following figures (Figs. 5 and 6) show how the algorithm automatically calculates the soil behavior type (SBT) based on the corrected value of cone resistance [q_t = q_c + u₂ (1 - a)] or the percentage ratio qc/pa, and the friction ratio Rf = f_s/q_t, where atmospheric pressure is pa (100 kPa).

Narrow test pits are routinely dug as part of a foundation study in the northern area of Prizren town (up to 100 cm across). To define and test the soil profile, including relative density, such holes are lowered while trained persons are inside. Even at considerable depths, routine on-site testing of gravels is possible due to the enormous diameter of the boreholes. With equipment that could be set up inside a 100 cm diameter borehole, vertical plate stress tests were carried out at the bottom of shallow boreholes. For a small additional fee, these tests can be carried out during routine foundation inspections. Due to the wide range of gravel quality, periodic testing is preferred at each location. The equipment's size and mass were kept to a minimum due to this and the constrained working space in the borehole. The tool comprised a small hydraulic jack with connectors to link a 305 mm diameter circular plate (Fig. 7). The testing technique adhered to ASTM D1194. The plate was positioned in the hole's center. By using a jack, incremental loads of one-fourth to one-fifth of the anticipated ultimate load have been delivered to the plate [18]. Dial gauges were used to track the plate's settlement with each step load application. Each load application stage must be separated by at least an hour. The test was run until failure, or at the very least until the plate had settled by 25 mm. It is not surprising that the concept of modeling soil as an elastic medium was first put forth by Winkler and is now known as the Winkler soil model. By using working examples, like the settlement analysis of shallow flexible foundations and laterally loaded piles, several applications of this idea have been addressed [19, 20]. The most well-known example is that of a horizontal, continuous beam or footing resting on an elastic subgrade. At every given point along the beam, it is believed that the subgrade response is inversely in proportion to the beam's vertical displacement. To put it another way, Hook's law is applied to the soil. Since P is the ground bearing pressure at a particular location along the beam and is the vertical displacement of the beam at that point, Ks is the modulus of subgrade response (Ks) for the soil, which is defined as Ks=P.

Fig. (5). Non-normalized CPT soil behavior type (SBT) chart according to Robertson (source: Robertson et al., 1986; https://www.finesoftware.eu/help/geo5/en/classification-of-soils-according-to-robertson-01/ January 2023).

Fig. (6). Non-normalized CPT soil behavior type (SBT) chart according to Robertson, 2010 (source: Fig. (21), pp. 26 [4]; https://www.finesoftware.eu/help/geo5/en/classification-of-soils-according-to-robertson-01/ January 2023).

Fig. (7). Plate bearing test apparatus.

Fig. (8). Ks measurement of plate load test results.

Since soil can be a very variable substance, the key challenge in implementing the Winkler soil model is quantifying the modulus of subgrade reaction (Ks) to be employed in the analysis. Only by conducting in situ plate bearing tests or by somehow connecting it to the intrinsic deformation properties of the soil can Ks be effectively and realistically estimated. In ASTM D1194, the plate bearing test is completely detailed and is often utilized. As shown in Fig. (8), the value of Ks in foundation design is the graph's secant modulus across the anticipated operating range of bearing pressure (p'), as opposed to pavement design. Due to a lack of plate load testing data, only empirical and theoretical correlations are considered. Numerous Ks relation-influencing factors that depend on the shape of the footing and parameter, the least lateral dimension of the footing, the non-dimensional soil mass per unit length, the coefficient of subgrade reaction for a plate 30.5 cm wide, and others, have all been proposed.

3.4.1. Calculating the Modulus of Subgrade Reaction

The following equation must be used to determine the modulus of subgrade reaction, Ks, in MN/m³ [20-22] (Fig. 8).

(12)

Where,

σ ₀ is the average normal stress in MN/m²;

s is the settlement of the loading plate in m.

To construct the pavements for roads and airports, the modulus of subgrade reaction, which corresponds to an average settlement of 1.25 mm, must be derived from the normal stress (Fig. 7).

Fig. (9). Foundation static penetration diagrams no. 7 and 8.

Static penetration tests have been performed on sand; the results are shown in the Rp-z diagram. Underneath the circular foundation, having a diameter of D = 10 m, at a depth of D_f = 1.0 m, the layer of sand has shown a significant thickness. Groundwater levels were discovered by a field investigation at a depth of 4.0 m. Below the water's surface γ' = 10.0kN/m³, there was sand γ= 17.15kN/m³, bulkier and denser than the groundwater above it. Fig. (9) displays the findings of static piercing tests conducted while looking at test piles.

The circular foundation's settling can be calculated using the static penetration value. The Busman-De Beer's expression [23, 24] was applied to determine the settlement of the foundation's center point:

(13)

Where,

C is the constant of compressibility,

(14)

Subsidence s = 7.82 cm according to the above equation.

Regular penetration tests have been conducted for each site, and the SPT below numbers have been calculated at corresponding depths [25, 26, 27]. The NCCER-97 (1997) technique has then been used to analyze the adjusted SPT values (N1)60 (Fig. 9). The comparison of SPT results with other geotechnical properties of gravel, such as Ks, would be very helpful given that the standard penetration test has the most impact on geotechnical projects in Kosovo. The information regarding the modulus of subgrade reaction (Ks) and the subsequent corrected SPT values (N1)60 is shown in Table 6.

Fig. (10). Typical (p-δ) curve obtained from the plate load test.

The modulus of subgrade reaction Ks from the Bowles relationship was used to evaluate the permitted bearing capacity:

(15)

Where,

q_a is the maximum permissible load per square meter (KN/m²) and Fs is the safety factor, which is set at 3. In Table 7, the permitted bearing capacity is displayed.

Where,

Ks =160 × 3× q_all. (field) for max. ΔH= 6 mm (according to equation 15)

Ten vertical plate load investigations on medium- to dense-textured soils have beenconducted in the Prizren alluvium. Fig. (10) shows the measurements of vertical settlement (s) and contact pressure (p) for each test. The secant modulus (Ks) of each graph has then been determined.