## 1 Introduction  
Granular matter is widely distributed in the earth's lithosphere and surface. Under long-term complex geological action, faults and joints on the earth are usually filled by grained gougeus (quartz, clay, and phyllosilicates, etc.). The convex bodies on the ordinary fault surfaces will break under shear and compression. This can cause the fracture zone of the fault to be filled by small-sized grains, forming a fault gouge with a thickness of several meters [1, 2, 3]. When the gouge reaches a certain thickness, the shear strength of the structural surface will depend entirely on the characteristics of the filler [2, 3], making the particle-filled structure more prone to damage under critical loading conditions.  
The granular materials acting as gougees in joints or faults are often subjected to long-term stable shear forces. Human activities (e.g., reservoir drainage or excavation works), earth's surface processes (river cutting or glacial ablation, etc.) or tectonic movements [4, 5, 6, 7] can change the stress state of these earth masses in the disturbed areas. Mostly, a pre-existing fault zone subjected to normal and shear stresses was initially in a frictional equilibrium state. The normal stress was larger than the shear stress. The reduced normal stress applied to the fault zone causes the deformation of the fault gauges in the fault segment. Specifically, if excavating a tunnel near a fault zone, the normal stress on the fault will be reduced [5]. Also, for example, when an earthquake wave from a distance is transmitted to a sheared fault, the normal stress applied will also drop sharply. Consequently, intensive unloading and stress redistributions can lead to the fast sliding of earth, causing landslides, ground surface cracking, and earthquakes [4, 6, 7]. Many former studies have focused on baldly contacted rock joints [8, 9, 10] or a small block sample with a thin layer of Gouge [11, 12, 13, 14, 15, 16] under the conditions of normal turbulence [8, 9, 10, 11, 12, 13, 14, 15, 16]. The effects of the characteristics of fault gouge, including humidity, mineral compositions, particle radii, and shapes, on the slip patterns have been studied mechanically and microscopically [17, 18, 19, 20]. In particular, tests involving velocity stepping and slide-hold-slide have shown that the frictional weakening phenomenon is strongly related to instability. However, the mechanical behavior of granular layers in the thick-filled faults in nature have not attracted enough academic attention.  
The shear strength of dry granular matters mainly depends on the normal force, shear rate, and the physical properties of the grains [21, 22, 23]. [PERSON] et al. [24] investigated the particle crushing of isotropically consolidated silica sand specimens. [PERSON] et al. [25] studied the fundamental role of particle size and shear speed in the frictional instabilities of locally sheared granular materials. [PERSON] and [PERSON] [26] investigated the dynamic triggering of frictional slip on simulated granular gouges. Shear experiments of geomaterials with complex stress routines were widely performed, and some of them also considered hydraulic effects [27, 28, 29, 30, 31, 32].  
Because of the un-prescribed sliding velocity, the shear properties of granular materials under normal unloading conditions can be quite different from the conventional experiments. However, related topics have not been explored before. Therefore, in this study, we intend to reveal the mechanical patterns of dry sands under invariable shear forces during the normal unloading process. This work is useful for better comprehending the movement patterns of natural granular layers and the prevention of corresponding geohazards.