*3.2. Testing Procedure*

The evaluation of saturated hydraulic conductivity (ksat) was performed in accordance with ASTM D5084—Method A [27], using a flexible wall constant head permeameter. A schematic diagram of the test setup used in the present study is shown in Figure 2. At the end of each curing period, the prepared specimens were transferred to the cell. Porous stone and filter paper were kept on the top and bottom end of the specimen. A rubber membrane was used to confine the specimen; two O-rings placed at both ends provide a complete seal against any water leakage. The cell was filled with distilled water, and the drainage line at the bottom and top of the cell was flushed until no more air bubbles were observed inside the cell. The various stages involved in the testing phase are described below.

**Figure 2.** Schematic diagram of a flexible wall constant head permeameter.

I. Back-Pressure Saturation:

This stage involved a simultaneous increase of both the cell pressure (CP) and the back pressure (BP) to reduce air bubbles or voids within the test sample. In this study, the effective confining pressure (defined as cell pressure minus back pressure, CP-BP) was kept at approximately 10 kPa throughout the saturation process for all specimens. This effective

confining pressure was selected to maintain sample stability without significantly affecting the stress history of the specimen. The (CP-BP) was maintained for one day. Specimen saturation was verified by measuring the B coefficient (defined as the difference in porewater pressure (Δu), divided by the difference in pressure of the cells (ΔCP) of the porous material). A saturation check involved increasing the pressure of the cell on the specimen and monitoring the pore pressure response using a pore pressure transducer connected at the top and bottom of the specimen. The theoretical B value for a fully saturated specimen reaches 1. However, in fluid flow experiments, specimens were considered saturated with the assurance of B values ≥ 0.95. If the B value is less than 0.95, the above procedure of increasing CP and BP and B value checking was repeated until the B value is >0.95.

II. Consolidation:

The specimens were consolidated under effective confining pressure (CP-BP) of 50, 100, 200, and 400 kPa. Effective confining pressure was applied by increasing the cell pressure to the level necessary to develop the desired effective confining pressure while maintaining a constant back pressure. Drainage was allowed from the base of the specimen. The outflow volumes were recorded to confirm that primary consolidation has been completed before the initiation of the next stage.

III. Permeation:

This stage involved inducing flow-through test specimens by applying a differential pressure between the top and bottom of the specimens. The differential pressure was applied by reducing the top pressure and increasing the bottom pressure such that the difference was equal to the pressure head corresponding to the desired hydraulic gradient. To speed up the test, the hydraulic gradient was fixed at 30 [27]. The water inflow and outflow were continuously monitored until a steady-state condition was established as defined by the inflow rate being equal to the outflow rate.

#### **4. Results and Discussion**

#### *4.1. Effect of Confining Pressure*

Figures 3–6 show the variation in saturated hydraulic conductivity (ksat) values for lime-treated (at 6%) expansive soil with fiber inclusion (0.2% and 0.6%) at various effective confining pressures at the end of each curing period.

In general, the ksat values reduced with an increase in confining pressure for all the tested specimens irrespective of fiber type, dosage, and curing period. A noticeable reduction in ksat values is observed when the confining pressure is increased from 50 to 200 kPa. The flow of water through the compacted specimen depends on the availability and connectivity of inter and intra-aggregate flow channels, and the k value is directly related with inter-aggregate flow paths [28,29]. Increased confining pressure contributes to a significant reduction in inter-aggregate flow paths compared to intra-aggregate flow paths. Due to this, with the increase in pressure from 50 to 200 kPa, a significant reduction in inter-aggregate flow channels causes a decrease in ksat values. A further increment in confining pressure from 200 to 400 kPa has less effect in reducing these flow paths and leads to a marginal reduction in ksat values for all the tested specimens.

For any type of soil, the higher the confining pressure, the lower the ksat values, irrespective of the permeating liquid [23]. Increased confinement causes a reduction in pore spaces and increases the unit weight, thus reducing the hydraulic conductivity [30]. Similar results were reported by de Brito Galvão et al. [31] and Shaker and Elkady [32].

The boundary condition adopted in the present study is highly correlated with the field conditions for the case of a subbase for pavements material in which the subbase material will be subjected to surcharge load coming on it.

**Figure 3.** Variations in saturated hydraulic conductivity (ksat) with confining pressure (**a**) FC (**b**) FM without lime treatment (without curing).

**Figure 4.** Variations in saturated hydraulic conductivity (ksat) with confining pressure (**a**) FC (**b**) FM with lime treatment (without curing).

**Figure 5.** Variations in saturated hydraulic conductivity (ksat) with confining pressure (**a**) FC (**b**) FM with lime treatment (after 7-day curing period).

**Figure 6.** Variation of saturated hydraulic conductivity with confining pressure (**a**) FC (**b**) FM with lime treatment (after 28-day curing period).
