Table 4.

Factors and levels used in the orthogonal experiments.

Levels	Factors
	$x_1$ (%)	$x_2$ (%)	$x_3$ (mm)	$x_4$ (times)	$x_5$ (°C)	$x_6$ (kPa)
1	28	96	350	9	− 5	50
2	24	93	300	7	− 10	40
3	20	90	250	5	− 15	30
4	16	87	200	3	− 20	20
5	12	84	150	1	− 25	10

where, $x_1$ is initial moisture content, %; $x_2$ is compaction degree, %; $x_3$ is reinforced spacing, mm; $x_4$ is number of FTC, times; $x_5$ is freezing temperature, ℃; $x_6$ is effective vertical stress, kPa.