Granular slip forecasting can conflate material state, loading progress, and the geometry of event-centered sampling. We separated these contributions in slowly sheared two-dimensional frictional disks using a compact neural score of stress, pressure, coordination, non-affine motion, and force-network observables. The model was developed on 36 trajectories and frozen efore testing on 18 new trajectories under two nested stress-drop definitions. Inspection of held-out results revealed post-event sampling asymmetry; recovery-aware analyses are therefore descriptive. With trajectories weighted equally, the compact score ranked near-slip windows above both prevalence and within-trajectory circular-phase controls under both definitions (representative average precision 0.310 versus prevalence 0.173; phase-null upper bound 0.257). Loading-history coordinates ranked more strongly, reaching 0.534 for causal elapsed strain. The recovery-aware rule retained 78.4% of activity-gated events and preferentially selected longer preceding intervals; ranking by time since the previous catalogued event remained compatible with a count-conditioned geometry null. Compact observables thus contain temporally aligned slip information, but stronger loading-history baselines and window-geometry sensitivity bound that evidence. These startup data do not isolate a state-specific short-horizon precursor beyond loading history or support a renewal interpretation of elapsed-strain ranking.
Figures & tables
Figure 1: Simulation and evaluation design. (a) Bidisperse configuration; displayed edges satisfy fn≥1.2⟨fn⟩ and periodic-crossing edges are omitted from the drawing only. (b) Instantaneous and five-frame-smoothed stress, one accepted onset, its forecast interval, and the original event-exclusion interval. (c) Schematic of the observed original post-event asymmetry for H=4 . Applying the original negative-window buffer to positive and negative windows gives a common onset-based re-entry at offset 25; the lookback-aware rule can move it later until the complete Dmin2 lookback follows trough and recovery. (d) Each (ϕ,μ) cell contributes four development and two held-out trajectories
Definition
Rule
Windows (positive)
Mean positive fraction
Compact AP
Cumulative- strain AP
Causal elapsed AP
Gated
Original
6,166 (1,230)
0.208
0.339
0.409
0.539
Gated
Common exclusion
5,935 (999)
0.177
0.311
0.388
0.519
Gated
Lookback-aware
5,742 (939)
0.173
0.310
0.398
0.534
Ungated
Original
5,945 (1,396)
0.247
0.359
0.443
0.534
Ungated
Common exclusion
5,609 (1,060)
0.202
0.326
0.406
0.499
Ungated
Lookback-aware
5,439 (1,006)
0.199
0.325
0.420
0.508
Table 1: Mean per-trajectory AP on the held-out cohort. “Common exclusion” applies the same previous-onset separation to positive and negative windows. “Lookback-aware” also requires every frame used by the four-frame Dmin2 history to follow the previous trough and recovery. Elapsed AP uses the causal prefix replay in Eq. ( 9 ).
Definition
Rule
Compact − prevalence
Compact − strain
Compact − elapsed
Gated
Original
0.131 [0.076, 0.187]
-0.070 [-0.127, -0.013]
-0.200 [-0.260, -0.140]
Gated
Common exclusion
0.133 [0.081, 0.186]
-0.077 [-0.143, -0.010]
-0.208 [-0.270, -0.146]
Gated
Lookback-aware
0.138 [0.083, 0.192]
-0.088 [-0.162, -0.013]
-0.223 [-0.288, -0.159]
Ungated
Original
0.112 [0.065, 0.158]
-0.085 [-0.133, -0.036]
-0.175 [-0.217, -0.134]
Ungated
Common exclusion
0.124 [0.081, 0.167]
-0.080 [-0.138, -0.022]
-0.173 [-0.216, -0.131]
Ungated
Lookback-aware
0.126 [0.080, 0.171]
-0.095 [-0.161, -0.029]
-0.184 [-0.224, -0.143]
Table 2: Paired mean per-trajectory AP differences. Brackets are exact 95% intervals from all 19,683 condition-stratified resampling compositions. The elapsed score is the causal prefix replay.
Figure 2: Loading-history comparison and window-rule sensitivity. (a) Lookback-aware mean per-trajectory AP and pooled-window point estimates ( × ; omitted for the phase null). Error bars are exact 95% resampling intervals except for the circular-phase null, where they show the central 95% randomization range about its mean. (b) Mean per-trajectory compact AP minus mean per-trajectory prevalence. (c) Compact minus cumulative-strain AP. (d) Compact minus causal prefix-replay elapsed-strain AP. Original denotes the initial window set; common exclusion applies the same previous-onset separation to both window types. The lookback-aware rule additionally places the complete Dmin2 history after previous-event recovery. Every comparison is paired on identical windows within a trajectory
Appendix figures & tables23 assets
Supplementary material from the paper’s appendix.
Appendix
Quantity
Definition or value
Preparation / production step
10−4 / 4×10−4
Contact-scale regime
Across 26,955,862 stored contacts, overlap divided by pair diameter had median 0.011, 97.5th percentile 0.045, and maximum 0.141. The range of kn divided by trajectory-median pressure was 38.4–286.9.
Damped contact times
Small–small / small–large / large–large restitution: 0.568/0.520/0.447; damped collision duration in production steps: 71/82/101.
Loading and drag times
1/ζ=0.500 ; one saved-frame interval =0.333 ; the forecast horizon spans 10.000 drag times.
Compression and consolidation
Compression from ϕ=0.58 in 16 stages; zero-shear convergence checks every 250 preparation steps; velocities and angular velocities multiplied by 0.5 every 500 steps.
Normalized kinetic-energy criterion
(K/N)/(kndˉ2)≤10−7 , where dˉ is mean diameter.
Appendix
Table 3: Reproducibility details for simulation, labels, inputs, and independent-test evaluation.
Quantity
Count
Activity-gated onsets
88
Activity-ungated onsets
106
Additional activity-ungated onsets
18
Activity-gated onsets matched exactly
88
Activity-gated onsets matched within tolerance
88
Trajectories with changed event list
11
Appendix
Table 4: Independent-test event counts and onset overlap. “Within tolerance” uses the same Δγ=0.010 separation used to merge nearby candidates.
Definition
History
Windows
Positive windows
Events
Activity-gated
H=4
6166
1230
88
Activity-gated
H=8
5856
1163
84
Activity-ungated
H=4
5945
1396
106
Activity-ungated
H=8
5586
1287
97
Appendix
Table 5: Independent-test window, positive-window, and distinct-event counts. Differences between H=4 and H=8 reflect complete-history availability; differences between definitions additionally reflect their event lists and exclusion regions.
Definition
Model
Seed 3407
Seed 7349
Seed 9109
Ensemble AP
Prevalence
Activity-gated
Compact MLP, H=4
0.2812
0.2792
0.2585
0.284
0.199
Activity-gated
Stress–pressure–ratio MLP, H=4
0.2725
0.2420
0.2662
0.259
0.199
Activity-gated
Compact MLP, H=8
0.2107
0.2386
0.2420
0.234
0.199
Activity-gated
Graph GNN–LSTM, H=8
0.2489
0.2451
0.2204
0.244
0.199
Activity-ungated
Compact MLP, H=4
0.3029
0.2805
0.3138
0.305
0.235
Activity-ungated
Stress–pressure–ratio MLP, H=4
0.3242
0.3361
0.3248
0.337
0.235
Appendix
Table 6: Fixed seed-level AP, probability-ensemble AP, and ensemble prevalence on the independent test set.
Difference
Common windows
Estimate
95% interval
Fraction >0
Activity-gated compact H=4− control
6166
0.025
[-0.016, 0.072]
0.873
Activity-ungated compact H=4− control
5945
-0.032
[-0.071, 0.016]
0.085
Activity-gated compact H=8− graph
5856
-0.010
[-0.046, 0.028]
0.295
Activity-ungated compact H=8− graph
5586
0.009
[-0.013, 0.031]
0.762
Appendix
Table 7: Independent-test paired AP differences. The final column is the descriptive fraction of valid condition-stratified bootstrap draws above zero.
Definition
Rule
Positive frac.
Compact
Strain
Causal elapsed
Fraction diag.
Activity-gated
Original
0.199
0.284
0.331
0.329
0.306
Activity-gated
Common exclusion
0.168
0.248
0.287
0.286
0.252
Activity-gated
Lookback-aware
0.164
0.245
0.290
0.290
0.246
Activity-ungated
Original
0.235
0.305
0.364
0.344
0.375
Activity-ungated
Common exclusion
0.189
0.245
0.296
0.284
0.301
Activity-ungated
Lookback-aware
0.185
0.241
0.301
0.286
0.297
Appendix
Table 8: Pooled-window AP under the window sensitivities. Elapsed strain is reconstructed by causal prefix replay. The positive-fraction diagnostic uses evaluation labels and is shown only as a nuisance check.
Definition
Original events
Lookback events
Coverage
Median preceding interval, original → lookback
Activity-gated
88
69
78.4%
48.0 → 78.0
Activity-ungated
106
74
69.8%
39.0 → 72.5
Appendix
Table 9: Event selection by the lookback-aware window rule. Coverage is relative to events represented by the original positive windows. Preceding intervals are onset-to-onset saved-frame intervals with every retained event weighted once.
Definition
Compact AP
Circular-phase null
Phase upper tail
Catalog-age AP
Hard-core onset null
Activity-gated
0.310
0.211 [0.171, 0.257]
0.00005
0.527
0.461 [0.383, 0.538]
Activity-ungated
0.325
0.239 [0.196, 0.287]
0.00051
0.504
0.503 [0.428, 0.572]
Appendix
Table 10: Post-test temporal-null audit. Brackets are central 95% randomization ranges, not trajectory-resampling confidence intervals. The phase null uses 100,000 cohort compositions; the hard-core onset null uses 2,000 surrogate cohorts.
Definition
Compact AP
Strain AP
Compact − strain
Time increment
Time+condition increment
Activity-gated
0.405
0.296
0.109
0.029
0.012
Activity-ungated
0.476
0.326
0.150
0.051
0.029
Appendix
Table 11: Development-test ordering and nested-stacker increments. The time-plus-condition stacker used one validation trajectory per condition for fitting.
Definition
Window set
Windows
Compact AP
Prev.
γobs AP
Compact − prev. (95% interval)
Compact −γobs (95% interval)
Activity-gated
Original
6166
0.284
0.199
0.331
0.085 [0.038, 0.152]
-0.047 [-0.101, 0.013]
Activity-gated
Post-yield
4492
0.314
0.258
0.338
0.055 [0.018, 0.102]
-0.025 [-0.065, 0.015]
Activity-ungated
Original
5945
0.305
0.235
0.364
0.070 [0.035, 0.130]
-0.059 [-0.111, -0.004]
Activity-ungated
Post-yield
4285
0.338
0.310
0.371
0.028 [-0.007, 0.082]
-0.033 [-0.079, 0.010]
Appendix
Table 12: The all-frozen compact-minus-prevalence entries reproduce the original prespecified primary checks. Cumulative-strain comparisons and all post-yield entries were specified after the independent outcomes were inspected. Intervals use 10,000 condition-stratified trajectory draws.
Figure 3: Loading-progress audit. (a) Prevalence, compact-model AP, and zero-parameter cumulative-strain AP on the original H=4 endpoints. (b) Compact AP minus prevalence on all endpoints and on the post-yield subset. The all-endpoint compact-minus-prevalence checks were prespecified; cumulative-strain and post-yield comparisons were introduced after the frozen summary was inspected. (c) AP increment obtained by adding the fixed compact score to development-fitted loading-time stackers, without and with condition indicators. Error bars are percentile 95% intervals from 10,000 condition-stratified trajectory draws. Panel (c) is post-test
Definition
Endpoint subset
Causal elapsed AP
Compact − causal elapsed (95% interval)
Activity-gated
Lookback-aware
0.534
-0.223 [-0.288, -0.159]
Activity-ungated
Lookback-aware
0.508
-0.184 [-0.224, -0.143]
Activity-gated
Post-first
0.652
-0.332 [-0.402, -0.261]
Activity-ungated
Post-first
0.593
-0.251 [-0.298, -0.204]
Appendix
Table 13: Post-test causal-availability audit on lookback-aware windows. “Post-first” removes endpoints before the prefix replay has detected its first event. Brackets are exact 95% intervals from the finite condition-stratified resampling design.
Definition
Score
AP
Compact AP minus score AP (95% interval)
Activity-gated
Final-catalog elapsed score
0.281
0.003 [-0.057, 0.068]
Activity-ungated
Final-catalog elapsed score
0.295
0.010 [-0.033, 0.067]
Activity-gated
Loading-and-event- history MLP
0.326
-0.042 [-0.112, 0.032]
Appendix
Table 14: Earlier retrospective temporal baselines on the independent set. Every paired difference is computed on exact common endpoints; intervals use 10,000 condition-stratified trajectory draws.
Definition / endpoints
Paired difference
Bootstrap estimate (95% interval)
Exact central 95% interval
Activity-gated / H=4
Compact − strain
-0.047 [-0.101, 0.013]
[-0.100, 0.012]
Activity-ungated / H=4
Compact − strain
-0.059 [-0.111, -0.004]
[-0.112, -0.004]
Activity-gated / H=8
Compact − strain
-0.100 [-0.160, -0.045]
[-0.161, -0.044]
Activity-ungated / H=8
Compact − strain
-0.075 [-0.142, -0.011]
[-0.143, -0.013]
Activity-gated / H=8
Graph − strain
-0.090 [-0.150, -0.029]
[-0.152, -0.028]
Activity-ungated / H=8
Graph − strain
-0.083 [-0.146, -0.024]
[-0.146, -0.025]
Appendix
Table 15: Loading-progress comparisons under Monte Carlo bootstrap and exact enumeration. Negative values favor cumulative strain.
Definition
Events
Median drop/ P
Median duration Δγ
Median interevent Δγ
Event qD2
Quiet qD2
Event − quiet (95% interval)
Activity-gated
88
0.028
0.010
0.048
0.108
0.178
-0.073 [-0.086, -0.060]
Activity-ungated
106
0.025
0.009
0.039
0.108
0.179
-0.073 [-0.084, -0.061]
Appendix
Table 16: Exploratory event and non-affine concentration statistics. A participation ratio qD2=1 denotes uniform activity, with smaller values denoting stronger concentration. Event and quiet columns are pooled medians; the final column is a paired mean difference formed after within-trajectory averaging and therefore is not their arithmetic difference.
Definition
Events
Matched
Coverage
Trajectories
Activity-maximum qD2 difference
Onset qD2 difference
Activity-gated
88
60
68.2%
18
-0.044 [-0.056, -0.031]
0.040 [0.018, 0.062]
Activity-ungated
106
64
60.4%
18
-0.042 [-0.053, -0.030]
0.026 [0.003, 0.049]
Appendix
Table 17: Exploratory duration- and selector-matched quiet-window control. Differences are event minus matched quiet values; brackets are condition-stratified trajectory-bootstrap 95% intervals. Before the post-test spatial contrasts were computed, the adequacy target was set to at least 80% event coverage with all 18 trajectories represented.
Figure 4: Illustrative matched pair under the activity-gated definition at ϕ=0.85 and μ=0.3 . The pair was selected before map inspection as the activity-gated match whose event-minus-quiet participation difference lay closest to the median. (a) Event onset. (b) Within-release activity maximum. (c) Matched quiet-window activity maximum. Colors show NDi2/∑jDj2 on one logarithmic scale. Smaller qD2 denotes stronger concentration. The maps illustrate the exploratory matched subset and do not enter its estimates
Figure 5: Original held-out pipeline comparisons retained as secondary analyses. (a) Event counts and exact onset overlap. (b) Pooled AP minus prevalence for the four-frame compact and stress–pressure–ratio models. (c) Pooled AP minus prevalence for the history-matched eight-frame compact and graph models. (d) Observation-time-matched AP differences. Error bars are 95% intervals from condition-stratified trajectory resampling. These pooled-window comparisons use the original endpoint rule and are not the lookback-aware mean per-trajectory analysis in the main text
Definition
Seed
Best epoch
Best validation AP
Final validation AP
Loss, first → final
Activity-gated
3407
27
0.381
0.372
0.655 → 0.074
Activity-gated
7349
19
0.358
0.339
0.662 → 0.057
Activity-gated
9109
22
0.393
0.377
0.656 → 0.056
Activity-ungated
3407
10
0.445
0.345
0.659 → 0.055
Activity-ungated
7349
13
0.382
0.315
0.660 → 0.054
Activity-ungated
9109
16
0.413
0.371
0.660 → 0.052
Appendix
Table 19: Saved graph-model training histories. Validation AP selected the checkpoint; “loss” gives the first and final training loss over 30 epochs.
Activity-gated
Activity-ungated
ϕ
μ
Windows
Events
AP
Prev.
γ AP
Windows
Events
AP
Prev.
γ AP
0.84
0.1
594
16
0.399
0.350
0.556
553
20
0.427
0.438
0.568
0.84
0.3
662
10
0.517
0.208
0.298
631
12
0.385
0.242
0.302
0.84
0.5
648
11
0.314
0.235
0.425
639
12
0.308
0.252
0.463
0.85
0.1
643
12
0.398
0.261
0.425
609
17
0.595
0.343
0.641
0.85
0.3
714
9
0.298
0.172
0.389
683
10
0.395
0.202
0.377
Appendix
Table 20: Condition-level results for the H=4 compact ensemble. Each row contains two trajectories. These values are descriptive; no subgroup inference is made.
Figure 6: Event-aligned compact observables in the development set: 162 cleanly alignable activity-gated slips out of 182 from 36 trajectories. Each event has a complete [−15,+5] frame window, is more than 20 frames after the preceding onset, and more than five frames before the next onset. Values are normalized with development-training medians and interquartile ranges. Events are averaged first within trajectory, and bands show 95% trajectory-bootstrap intervals. The dashed line denotes onset. Panel (a) shows oriented shear stress, so an accepted onset is a local maximum. ⟨Dmin2⟩i is the all-particle mean, fmaxn is evaluated over all contacts, and Smax is the largest-component particle count at αf=1.2 . The activity-gated event selection conditions the post-onset non-affine profile; it is not evidence of a pre-onset precursor. These profiles are descriptive and do not enter the independent tests
Difference
Development estimate (95% interval)
Independent-test estimate (95% interval)
Activity-gated compact H=4− control
0.020 [-0.064, 0.119]
0.025 [-0.016, 0.072]
Activity-ungated compact H=4− control
0.080 [-0.014, 0.154]
-0.032 [-0.071, 0.016]
Activity-gated compact H=8− graph
0.115 [-0.021, 0.251]
-0.010 [-0.046, 0.028]
Activity-ungated compact H=8− graph
0.160 [0.015, 0.249]
0.009 [-0.013, 0.031]
Appendix
Table 21: Selected development-stage paired AP differences versus their independent-test estimates. Development intervals used nine trajectory clusters and are exploratory.
Locomotion on granular slopes such as sand dunes remains a fundamental challenge for legged robots due to reduced shear strength and gravity-induced anisotropic yielding of granular media. Using a hexapedal robot on a tiltable granular bed, we systematically measure locomotion speed together with slope-dependent normal and shear granular resistive forces. While normal penetration resistance remains nearly unchanged with inclination, shear resistance decreases substantially as slope angle increases. Guided by these measurements, we develop a simple robot-terrain interaction model that predicts anchoring timing, step length, and resulting robot speed, as functions of terrain strength and slope angle. The model reveals that slope-induced performance loss is primarily governed by delayed anchoring and increased backward slip rather than excessive sinkage. By extending the model to generalized terrain conditions, we construct failure phase diagrams that identify sinkage- and slippage-induced failure regimes, enabling quantitative risk estimation for locomotion on granular slopes. This physics-informed framework provides predictive insight into terrain-dependent failure mechanisms and offers guidance for safer and more robust robot operation on deformable inclines.
Long Zhao, Xingjue Liao, Feifei Qian
Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, CA, USA.
Physics-informed neural networks (PINNs) provide a mesh-free framework for solving governing equations, but their application to granular avalanche dynamics over curved terrain remains largely unexplored. This study extends a depth-averaged PINN formulation based on the Savage-Hutter equations to an exponentially curved chute with spatially varying inclination and a strain-rate-dependent Mohr-Coulomb earth-pressure closure. The model is validated against measured front- and rear-edge trajectories from a laboratory granular-avalanche experiment, with selected observations withheld from training. A staged temporal curriculum proved essential for accurate prediction, reducing the held-out trajectory error by approximately two orders of magnitude compared with training over the full time domain from the outset. Sparse-data experiments further showed that observation placement was more influential than observation number within the configurations tested. Four observations bracketing the transition from acceleration to deceleration achieved nearly the same accuracy as the eight-observation reference configuration, whereas observations clustered at early or late times performed poorly. The results demonstrate the importance of both training strategy and informative data placement when applying PINNs to granular flows over curved topography.
Learned graph simulators provide an efficient alternative to high-fidelity solvers for granular dynamics. However, granular motion depends strongly on inter-granular contact history, which is difficult to preserve when particle contacts form, break, and rearrange. Existing simulators mainly store temporal information in node features or node-level memory. Here we introduce TRACE, a graph-network simulator that stores interaction history directly on contact edges. Each edge maintains a persistent memory updated by attention-based message passing and a gated recurrent unit, while an edge-identity dictionary preserves this memory as the contact graph changes. A physics-structured decoder predicts inter-granular normal and tangential contact forces, enforces the Coulomb friction limit, and applies equal-and-opposite internal forces. The model is trained with single-step pretraining followed by autoregressive rollout fine-tuning. We evaluate TRACE on 2D and 3D granular column-collapse benchmarks. In both cases, TRACE produces stable, physically consistent long-horizon rollouts, closely reproducing the final deposit geometry and the kinetic energy released during collapse. Compared with graph network simulator (GNS) and node-memory graph neural simulator (NMGNS), TRACE reduces long-rollout position error by 31-62% and final-deposit error by 58-89% across the two benchmarks, while using fewer parameters and maintaining near-zero particle interpenetration. TRACE also achieves 12.2× and 8.9× speedups over the material point method (MPM) reference solver in 2D and 3D, respectively. Our code is available at https://github.com/Data-Driven-Computational-Geotechnics/TRACE.
Changjian Zhou, Negin Yousefpour, Jie Qi +3
Faculty of Engineering and IT, The University of Melbourne · School of Computing, National University of Singapore · Norwegian Geotechnical Institute