Introduction

As coal mining extends deeper, dynamic disasters induced by roof fracturing, such as rock bursts and face crushing, pose severe threats to mine safety. Periodic weighting, a core manifestation of roof instability, is dynamically controlled by the coupling effect of geological conditions and mining parameters, particularly the advancement speed. Current research primarily focuses on static geological factors:

Key stratum theory research demonstrates that the alternating occurrence of large and small periodic weightings in shallow coal seams fundamentally stems from the instability mechanism of overlying key stratum cantilever structures. This mechanistic behavior has been systematically investigated through physical modeling and numerical simulation1,2,3,4. Building on this foundation, the developed key stratum structural model successfully deciphers the strata behavior induced by the coordinated fracturing of the overlying strata. The model integrates key stratum geomechanical parameters with stope geometrical characteristics, providing theoretical support for support design5,6,7,8,9. Concurrently, through integrated rock mechanics testing and numerical modeling, the fracture patterns and underlying mechanical mechanisms of the key strata have been comprehensively elucidated10,11,12,13,14,15,16.

In ground pressure monitoring technology, advanced approaches incorporating microseismic energy temporal analysis, microseismic-stress coupling modeling, and fiber-optic sensing applications have enabled: Precursory warning of roof fracturing; Identification of clustered event patterns in high-stress zones༛Real-time tracking of overburden fracture propagation17,18,19,20.

However, the quantitative characterization of the dynamic effects of the advancing rate on overburden structural evolution—particularly regarding periodic weighting characteristics (e.g., interval, intensity, and microseismic activity)—remains insufficient. Although recent studies have proposed novel methods for determining hydraulic support resistance and confirmed that rapid advancement increases front abutment pressure peaks by 25% to 40%21,22, while emphasizing the critical role of advancement rate-pressure response models in smart mining23,24,25, systematic field validation and in-depth mechanical mechanism analysis require further reinforcement.

Based on the 22,116 working face, this study comprehensively utilizes online ground pressure monitoring (support resistance), high-precision microseismic monitoring, and key stratum theory to deeply analyze the evolution of overburden fracture structures and ground pressure behavior under different advancement speeds. It focuses on elucidating the dynamic influence mechanism of advancement speed on the periodic weighting interval and microseismic activity characteristics. The aim is to provide direct guidance for safe production at the 22,116 face and theoretical support for optimizing intelligent mining parameters under similar conditions. As shown in Fig. 1 below, the spatial layout of Working Face 22,116 is presented along with its associated monitoring systems.

Fig. 1
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Spatial layout of the 22,116 working face and deployed monitoring systems.

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Analysis of overlying strata structure at working face 22,116

Physical and mechanical properties of overlying strata

Based on mine-provided data and references26, the physical and mechanical parameters of the No. 2 coal seam and the main overlying strata of the Datong Formation at working face 22,116 are detailed in Table 1. These parameters provide a basis for subsequent rock beam fracture analysis and key stratum identification.

Table 1 Physical and mechanical parameters of No. 2 coal seam and overlying strata.
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Identification of immediate roof and main roof

The fracturing and caving behavior of roof strata under mining influence determines whether they belong to the immediate roof (caving concurrently with mining) or the main roof (capable of forming periodic fracture structures). The primary criteria are the beam’s fracture span and its post-fracture stability.

Treating the stratum as a beam fixed at both ends, its initial fracture span Lc is given by the material mechanics formula:

$${L_{\text{c}}}{\text{=}}h\sqrt {\frac{{2{R_T}}}{q}}$$
(1)

Where: ℎ is beam thickness; RT​ is tensile strength of the beam; q is the load borne by the beam (MPa), typically its self-weight and effective overburden load.

In comparison, the FGS (feldspar-dominated) and Ss adjacent to the coal seam show little difference in elastic modulus and uniaxial tensile strength. According to the curvature calculation formula (k = M/(EI), where M is the bending moment and I is the sectional moment of inertia), for a rectangular cross-sectional beam, I is proportional to h³. The thickness of FGS (feldspar-dominated) is approximately 0.52 times that of Ss; thus, the moment of inertia of FGS (feldspar-dominated) is roughly (0.52)³ ≈ 0.14 times that of Ss. Under the conditions of similar bending moments M (bearing similar loads) and similar elastic moduli E, the curvature k of FGS (feldspar-dominated) will be significantly greater than that of Ss, meaning that FGS (feldspar-dominated) is more prone to bending deformation.

When substituted into Eq. (1), in a preliminary analysis that ignores differences in overlying loads (mainly considering self-weight), due to the smaller thickness h and slightly lower tensile strength RT of FGS (feldspar-dominated), the calculated initial fracture distance Lc is also smaller than that of Ss. Therefore, FGS (feldspar-dominated) will fracture before the overlying Ss. Together with the overlying Sandy mudstone, these three strata—FGS (feldspar-dominated), Ss, and Sandy mudstone—will fracture and collapse after the coal seam is mined over a certain distance, exhibiting the characteristic of immediate caving with mining.

Based on field measurements from the adjacent 22,105 working face, the initial caving span of the 4 m thick Ss is 17 m. Using proportional calculation with Eq. (1), the initial caving span Lc​ of the 5 m thick Ss in this working face is approximately:

$${L_{{c_{{\mathbf{Ss}}}}}}=17m \times \sqrt {\frac{{5m}}{{4m}}} \approx 19m$$

The periodic fracture spacing (step distance of periodic fracturing after the initial fracture) is approximately half of the initial fracture span, i.e., \({L_{{{\text{d}}_{{\mathbf{Ss}}}}}} \approx 19/2=9.5{\text{m}}\). Based on ground pressure monitoring data, the average unsupported roof span in this context is taken as approximately 7.8 m.

The length of rock blocks formed after Ss fracturing averages approximately half of the unsupported roof span, i.e., \({L_{block}} \approx 7.8m/2=3.9m\). The key indicator for its stability is the ratio of block length to thickness \({L_{block}}/{h_{Ss}}=3.9m/5.0m=0.78\).

Classical “Voussoir Beam” stability theory indicates that to avoid sliding instability, a rock block must satisfy \({L_{block}}/h<0.5tan\phi\) (where ϕ is the friction angle between blocks, typically 30°~35°). Even adopting a higher friction angle of ϕ = 35°, the critical value is calculated as 0.5tan35°≈0.5 × 0.7 = 0.35. With the computed value of 0.78 > 0.35, this indicates that the rock blocks formed after Ss fracturing are highly prone to sliding instability, preventing the development of stable voussoir beam structures to support overlying strata. The overlying MGS, with its limited thickness and strength comparable to Ss, displays fracturing behavior governed by the Ss; consequently, both strata undergo synchronized fracturing and immediate caving.

Based on this analysis, the FGS (feldspar-dominated), sandy mudstone, Ss and MGS collectively constitute the immediate roof, exhibiting immediate caving behavior upon excavation. The total thickness of the immediate roof is calculated as 2.6 m + 3.0 m + 5.0 m + 2.6 m = 13.2 m. Overlying this assemblage, a 23.0 m-thick FGS (quartz-dominated) stratum, owing to its substantial thickness and superior mechanical properties (uniaxial compressive strength = 90 MPa, elastic modulus = 18 GPa), demonstrates the capacity to form periodic fracturing structures and is thus classified as the main roof.

Key stratum analysis of roof

Identification of main key stratum

The grey medium-grained feldspathic quartz sandstone at the base of the upper section of the Yungang Formation possesses characteristics of large thickness and high strength (referencing the strength of MGS and FGS (quartz-dominated), its uniaxial compressive strength is estimated > 80–90 MPa, elastic modulus > 15–18 GPa). The overlying strata of the upper Yungang Formation (predominantly mudstone interbedded with sandstone) and the entire Tianchihe Formation (except for the basal conglomeratic sandstone, mainly interbedded medium-strength sandstone and weak mudstone, with poor integrity in the upper section) are significantly weaker in overall thickness and strength compared to this basal sandstone layer. According to the definition of key strata, this grey medium-grained feldspathic quartz sandstone layer dominantly controls the movement of all overlying strata up to the surface and is therefore identified as the main key stratum.

Identification of sub-key stratum

The main roof FGS (quartz-dominated) itself has high strength (90 MPa), large thickness (23.0 m), and high elastic modulus (18 GPa). Its immediate overburden is the lower section of the Yungang Formation, averaging 70 m thick. This section primarily consists of weak mudstone interbedded with unstable sandstone, with an overall strength far lower than FGS (quartz-dominated). Above the lower Yungang Formation lies the main key stratum (grey medium-grained feldspathic quartz sandstone).

To verify whether FGS (quartz-dominated) bears part of the load of its overlying lower Yungang Formation strata (i.e., whether it acts as a sub-key stratum), its actual bearing load q is calculated:

$$q={q_1}+{q_{1{\text{sup}}}}={E_1}h_{1}^{2}\left( {\sum\limits_{{i=1}}^{n} {{\gamma _i}{h_i}} } \right)/\left( {\sum\limits_{{i=1}}^{n} {{E_{i}}h_{i}^{3}} } \right)$$
(2)

Where:

  • \({q_1}\)-Self-weight load of FGS (quartz-dominated) (MPa), \({q_1}\)​=γ6​h6;

  • \({q_{1{\text{sup}}}}\)-Load imposed on FGS (quartz-dominated) through load transfer from overlying strata (MPa);

\({E_1}\), \({h_1}\)-Elastic modulus (GPa) and thickness (m) of FGS (quartz-dominated);

\({\gamma _i}\), \({h_i}\), \({E_{i}}\)-Unit weight (MN/m³), thickness (m), and elastic modulus (GPa) of the i-th overlying stratum;

n-Number of strata from above FGS (quartz-dominated) to below the main key stratum (i.e., the lower Yungang Formation).

Based on the mine-measured mechanical parameters and the database of typical rock mechanical parameters for coal measure strata, the key parameters were substituted for estimation as follows:

Self-weight load of FGS (quartz-dominated)\({q_{1}}={\gamma _6}{h_6}=0.026MN/m3 \times 23.0m \approx 0.598MPa\)。Assuming the average unit weight of the 70 m lower Yungang Formation \({\gamma _{avg}}\)≈ 0.025 MN/m³, and its average elastic modulus is significantly less than E6​ (taken as a representative value \({E_{avg}}\)≈ 5–10 GPa). Due to the large \({E_6}h_{6}^{3}\) value (E₆ = 18 GPa, h₆ = 23.0 m), while the denominator \(\sum {E_{i}}h_{i}^{3}\)is relatively small (overlying strata Ei​ small and hi​ dispersed), the load transfer coefficient in Eq. (2) will be significantly greater than 1, resulting in a large \({q_{1{\text{sup}}}}\)​.

Based on the measured fracture span in the text, the total load q ≈ 0.728 MPa > q1​=0.598 MPa, conclusively proving that FGS (quartz-dominated) bears part of the weight of its upper strata averaging about 70 m thick (i.e., the lower Yungang Formation). Relative to the main key stratum controlling all overlying strata, FGS (quartz-dominated) controls the movement of its overlying strata averaging 70 m thick (up to the base of the main key stratum) and is therefore identified as the sub-key stratum.

Through mechanical analysis and key stratum theory, the overburden structure of the 22,116 working face is determined: the grey medium-grained feldspathic quartz sandstone at the base of the upper Yungang Formation is the main key stratum, controlling the movement of all overlying strata; the 23.0 m quartz-rich fine sandstone of the Datong Formation is the sub-key stratum, controlling the movement of its overlying strata averaging about 70 m thick. The fracture movement of the sub-key stratum will decisively influence the ground pressure behavior at the working face.

Consequently, the fracture behavior of the sub-key stratum constitutes the fundamental driving mechanism behind the ground pressure manifestations at Panel 22,116. The timing, mode, and induced energy release of fracturing directly govern the interval, intensity of periodic weighting, and corresponding microseismic activity characteristics. Given that the fracturing behavior of the sub-key stratum is significantly governed by mining-induced disturbances, particularly the face advance rate, this study systematically examines the dynamic characteristics of periodic weighting reflected in support resistance under different advance rates based on the aforementioned key stratum structural identification. Furthermore, we will integrate microseismic monitoring and numerical simulations to elucidate the advance-rate-dependent characteristics of microseismic energy release and roof stress distribution.

Analysis of periodic weighting characteristics based on ground pressure monitoring

To accurately grasp the periodic weighting pattern of the 22,116 working face—particularly the influence of advance rate on weighting intervals—this chapter employs an advanced online ground pressure monitoring system for field measurements. Starting from the first support at the upper end of the working face, sensors are installed every eighth support from top to bottom, with each unit positioned on both the front and rear columns of the hydraulic supports. A total of 15 sensor units are deployed for data collection and transmission. The monitoring data are then subjected to in-depth analysis combined with theoretical mechanical models。.

Determination of weighting criteria

Drawing on years of practical experience, the criterion for main roof weighting is defined as27:

$${P_{LY}}=\bar {P}+K\sigma$$
(3)

Where:

  • \({P_{LY}}\)- Weighting judgment indicator value;

  • \(\bar {P}\)-Average working resistance of front and rear hydraulic supports during the observation period, KN;

  • K- Coefficient of variation, taken as 0.8;

  • \(\sigma\)-Root mean square, kN。.

Taking typical data from 2 representative supports each in the upper, middle, and lower sections of the face for ground pressure analysis, the main roof weighting criterion derived from Eq. (3) is shown in Table 2.

Table 2 Working face support weighting criterion table.
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Working face weighting judgment and analysis

To reasonably determine and understand the periodic weighting interval at the 22,116 working face, the monitoring period started at 00:00 on April 4, 2025, spanning 21 days. Statistics on hydraulic support working resistance versus the face’s average cumulative advance are shown in Table 3; the face accumulated an advance of 133.05 m. During the observation period, three periodic weighting events occurred, characterized by a general increase in hydraulic support working resistance during weighting. Figure 2 shows the variation of hydraulic support working resistance with face advance.

Table 3 Statistics of accumulated footage of 22,116 working face from April 4th.
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Fig. 2
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Working resistance of hydraulic support changes with working face advance.

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As shown in Fig. 2, during the initial monitoring period, the face advancement speed was relatively slow (average daily rate ~ 5.0 m). When the cumulative advance reached approximately 35.4 m (April 9th), the working resistance of supports in the upper, middle, and lower sections all showed significant increases, reaching or exceeding the criterion value \({P_{LY}}\), marking the first periodic weighting event. Microseismic activity during this stage was relatively weak (see Fig. 3).

Starting April 10th, supports 10# and 36# in the upper section experienced periodic weighting again when the cumulative advance reached 57.05 m; supports 63# and 81# in the middle section experienced weighting at cumulative advances of 43.85 m and 49.85 m respectively, with a mean weighting interval of 46.85 m; supports 99# and 117# in the lower section experienced weighting at cumulative advances of 38.65 m and 43.85 m respectively, with a mean interval of 41.85 m. Due to the face inclination angle and differences in the spatial position of the upper and lower goaf areas, variations occurred in the weighting intervals across the face sections. However, from April 10th to the next weighting event, the average weighting interval was 48.38 m, showing a significant increase compared to the previous interval.

The analysis attributes this to: The advancement speed during this stage exhibited alternating slow-fast-slow characteristics. After rock beam fracture on April 9th, advancement slowed from April 10th to 12th (average 3.2 m/d). This slower speed allowed sufficient time for the fractured rock blocks to undergo gradual bending subsidence. This slow deformation promoted readjustment of contact relationships between fractured blocks, tending to form a relatively stable three-hinged arch structure together with earlier fractured blocks.

Advancement accelerated from April 13th to 15th (average 7.4 m/d). Rapid advancement induced intense disturbance in the overburden (especially the sub-key stratum), resulting in strong microseismic activity (see Fig. 3 in Chap. 3). Under rapid unloading, the articulated or frictional structures between already fractured blocks became prone to instability.

Advancement remained relatively fast from April 16th to 17th (average 6.4 m/d), accompanied by strong microseismicity. Consequently, the rock beam fracturing on April 17th behaved as a “beam fixed at one end and elastically supported at the other end” before fracture. Its fracture span fell between that of a “beam fixed at one end and sliding at the other” and a “beam fixed at one end and simply supported at the other”. This transformation process from arch structure instability towards beam/arch structure stability allowed the bending moment borne by subsequent strata (sub-key stratum) before fracture to be redistributed or partially transferred. A larger accumulated bending moment was thus required to reach the fracture condition, ultimately manifesting as a significantly increased weighting interval.

Following the rock beam fracture on April 17th, the beam fracturing on April 22nd behaved as a cantilever beam before fracture. Its average weighting interval was 35.3 m, consistent with the period before April 4th. After the sub-key stratum fractured (April 17th weighting), the face maintained a sustained high advancement speed (> 6.8 m/d average). Under this sustained rapid mining influence, the stress concentration zone at the root of the cantilever beam structure (ahead of the coal wall) advanced rapidly. The cantilevered portion lacked time to form a more stable structure and rapidly underwent the next fracture under its own weight and overburden load, causing intense microseismicity. Using the fracture span of the cantilever beam (34.8 m) as a reference, the fracture span of a “beam fixed at one end and sliding at the other” can be deduced as 42.6 m, and that of a “beam fixed at one end and simply supported at the other” as 69.6 m. The observed interval of 48.38 m falls within 42.6 m < 48.38 m < 69.6 m, indicating that the sub-key stratum before fracture during this stage was closer to the “fixed at one end, sliding at the other” boundary condition, validating the above analysis.

In summary: Alternating fast-slow advancement tends to promote a transformation of the overburden structure from an unstable arch towards a stable beam/arch configuration, increasing the weighting interval. Sustained rapid advancement maintains the continuous fracture mode of the cantilever beam structure, keeping the weighting interval within its inherent smaller range. Slow advancement promotes the formation of a more stable arch structure. Therefore, the advancement speed should be strictly controlled to reduce the weighting interval and mitigate intense microseismic activity caused by roof fracturing.

Synchronization analysis of microseismic monitoring and ground pressure manifestation

Based on the aforementioned ground pressure measurements and theoretical analysis, the variation in weighting interval was explained and the sub-key stratum identified. To gain a clearer understanding of the impact of sub-key stratum fracture on ground pressure, long-term real-time monitoring was conducted in the 22,116 tailgate (adjacent to the 22118 goaf) using a high-precision integrated acoustic emission-microseismic and electromagnetic coupling monitoring system. Figure 3 shows the time-domain distribution of microseismic events under different advancement speeds.

Fig. 3
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Time-domain distribution of microseismic events at different advancement speeds.

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Figure 3 shows that as the advancement speed gradually decreased from April 9th to 15th, the manifested microseismic events also decreased. However, after three consecutive days of rapid advancement (April 13th-15th), the 22,116 working face exhibited intense microseismicity. This indicates that more of the rock beam bending energy was released in the form of microseismic events. Concurrently, Fig. 2 shows that hydraulic support resistance was rapidly increasing during this period to cope with the intense roof weighting.

Although advancement slowed from April 16th-17th, the intense roof activity triggered by the prior rapid advancement led to roof fracture around the 17th, characterized by increased support resistance and frequent microseismic events. After April 17th, the face maintained a sustained high advancement speed, resulting in frequent and intense microseismic events on April 21 st and 22nd. This indicates significant disturbance to the overlying roof during face advance, leading to progressive fracturing and frequent microseismic signals. The high average advancement rate during this phase confirms the direct link between microseismic intensity and face advancement speed, further validating the accuracy and correlation of the content described in Sect. 2. Therefore, to prevent roof instability caused by excessive microseismic intensity, strict control of the face advancement rate is essential.

Analysis of microseismic data revealed significant differences in the frequency characteristics of microseismic sources among the 2.6 m feldspar-rich fine sandstone, 3.0 m sandy mudstone, 5 m siltstone, and the 23.0 m quartz-rich fine sandstone after mining. Analyzing the collected microseismic signals, the waveform of a typical microseismic event from the 23.0 m fine sandstone is shown in Fig. 4 (using Channel 3 signal as an example), and the occurrence times of strong microseismic signals are listed in Table 4 (partial data).

Fig. 4
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Microseismic signal pattern of fine-grained sandstone of 23.0 m.

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Table 4 Daily strong microseismic signal statistics (Sub-key stratum FGS (quartz-dominated)).
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Table 4 shows that faster sustained advancement correlates with stronger microseismicity from the bending/fracture of the 23.0 m fine sandstone. Microseismicity was weaker before weighting and stronger immediately preceding weighting. According to the definition of periodic weighting, the phenomenon where periodic fracture of the main roof strata causes periodic instability of the “Voussoir Beam” structure is termed periodic weighting. The 23.0 m fine sandstone exhibited high microseismic frequency on April 17th and 22nd, displaying clear periodic weighting characteristics consistent with ground pressure monitoring results. This further demonstrates the correlative impact of advancement speed on hydraulic support resistance and microseismic events, providing data support and guidance for mine production practice.

Numerical simulation of advancement speed influence on roof stress distribution

As evidenced by the preceding analysis, the advance rate directly influences roof fracturing. To further investigate the effects of varying advance rates on roof weighting behavior and stress distribution characteristics, a UDEC-based geomechanical model of Panel 22,116 was developed. The model incorporated fixed lateral and basal boundaries, with the top surface remaining stress-free. Initial equilibrium was achieved under gravitational stress conditions.

Progressive excavation simulations were conducted until goaf stabilization, encompassing four distinct working conditions with constant advance rates of 5, 6, 7, and 8 m/d. Monitoring lines were deployed along the panel strike to capture vertical stress distribution profiles, thereby simulating the impact of daily advance rates on roof vertical stress and abutment pressure evolution. The simulation outcomes are presented in Figs. 5 and 6.

Fig. 5
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Advance-rate-dependent stress distribution characteristics. (a) At an Advance Rate of 5 m/d, (b) At an Advance Rate of 6 m/d, (c) At an Advance Rate of 7 m/d, (d) At an Advance Rate of 8 m/d.

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Fig. 6
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Stress distribution characteristics under conditions of different daily advancement.

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The monitoring results in Fig. 6 show: At a daily advancement of 5 m, the peak front abutment pressure was 10.6 MPa; at 6 m/d, 11.4 MPa; at 7 m/d, 12.9 MPa; at 8 m/d, 14.1 MPa. Clearly, as the advancement speed increases, the peak front abutment pressure gradually increases. This imposes higher demands on hydraulic support working resistance, increasing the risk of support damage and potential safety hazards.

Figure 5 show a direct link between roof fracture/subsidence in the goaf and advancement speed. At 5 m/d, the roof subsided slowly, weighting was less pronounced, and the interval was smaller. As the daily speed increased, the roof lacked sufficient time to fracture completely before the next mining cycle. Continuous disturbance caused roof weighting to increase sharply, the weighting interval increased, and fracture-induced microseismic events intensified, posing significant safety risks.

This simulation study shows high consistency with hydraulic support resistance monitoring and microseismic monitoring results. Based on this research, strict control of daily advancement is necessary. It is recommended to control it around 6 m. If considering production capacity, the speed should not exceed 6.5 m/d to avoid widespread intense roof weighting increasing safety risks at the face.

Conclusions

By integrating field ground pressure and microseismic monitoring, key stratum theory analysis, and UDEC numerical simulation, this study systematically elucidated the regulatory mechanism of advancement speed on the dynamic characteristics of periodic weighting at the 22,116 working face. The following innovative conclusions were obtained:

  1. 1)

    Based on theoretical analysis and measured inversion, the sandstone at the base of the Yungang Formation was identified as the main key stratum, controlling the movement of all its overlying strata. The 23.0 m thick fine-grained sandstone of the Datong Formation was identified as the sub-key stratum, controlling the movement of its overlying strata approximately 70 m thick. The fracture movement of the sub-key stratum dominates the ground pressure manifestation at the working face.

  2. 2)

    Advancement speed dynamically regulates overburden structure: Alternating fast-slow advancement (3.2 m/d → 7.4 m/d) promoted a transformation of the overburden structure from an unstable arch shape to a relatively stable beam/arch configuration, resulting in a significant increase in the periodic weighting interval to 48.38 m, representing a 37.0% increase. In contrast, sustained rapid advancement maintained the continuous fracture mode of the cantilever beam structure, keeping the weighting interval stable within its inherent smaller range.

  3. 3)

    Sustained rapid advancement significantly intensifies microseismic energy release. Microseismic monitoring indicates that microseismic event frequency induced by sub-key stratum fracture under this mode increased by approximately 2.1 times compared to baseline levels (Table 4). UDEC numerical simulation further confirmed that the peak front abutment pressure increased by 33.0% when daily advancement speed rose from 5 m to 8 m. Strong microseismic activity was also observed during the rapid advancement phases within the alternating speed stage.

  4. 4)

    Integrating field monitoring and numerical simulation results, a tiered control strategy for advancement speed is proposed to effectively manage roof weighting intensity and microseismic activity: During stages requiring roof disaster risk prevention and control, the daily advancement speed should be maintained below approximately 6.5 m/d. During stable production stages, speeds should generally not exceed 7.0 m/d to avoid significantly increased risks.