MUD spots, a common issue in ballasted track, are often associated with increased track geometry deterioration and sleeper pumping, which not only affects the geometry but can also result in shorter sleeper life and increased failures in the track structure. While full ballast replacement from undercutting is ideal, drainage improvement methods such as spot shoulder ballast cleaning (SBC) may slow track structure deterioration and extend the time before undercutting is required.

Recognising the need to address mud spots to maintain a healthy track structure and reduce future track issues, MxV Rail performed SBC on a severely fine-filled ballast location to evaluate how improved shoulder drainage affects track performance (Figure 1). The investigation shows that, while SBC was effective in areas of highly degraded upper ballast layers, the results showed that the method’s overall utility will vary by location.

SBC conducted more than a year of testing on the “rainy section” of the Facility for Accelerated Service Testing (Fast) in Pueblo, Colorado. This focused specifically on surface mud pumping, settlement, drainage, and fine migration, to determine whether SBC “cleans out” the centre ballast section.

This work was jointly funded by the Association of American Railroads (AAR) Strategic Research Initiatives program (SRI) and the Federal Railroad Administration (FRA) Heavy Axle Load Research In-Track Testing Program.

The Fast rainy section has served as a testbed for mud pumping and fine-filled ballast research since 2017. The section is 6.09m long, has a Selig’s Fouling Index (FI) of 40 (where the majority of ballast voids are filled with fines), and can create controlled artificial rainfall using a manual irrigation system.

Phase 1 testing between 2017 and 2019 involved quantifying the influence of moisture on track performance. Rainfall events were simulated by manually wetting the track and comparing wet and dry performance. A key finding from Phase 1 testing indicated that wetting the track until surface mud pumping occurs (around one hour of rainfall at 10mm/hour) can increase settlement rates by a factor of up to 15, can reduce track stiffness, and can extend drainage times from about a day to more than a week. A second finding showed that when water is applied from above the track (as with rainfall), the mud slurry travels only a few cm below the bottom of the sleeper. Thus, the drainage process should focus on opening up drainage paths near the bottom of the sleeper.

Phase 2 testing

Phase 2 testing between 2020 and 2022 involved performing spot SBC and comparing the performance results with the poorly drained Phase 1 results. For the spot SBC, the shoulders were stripped manually from the sleeper end outward to a depth of 150mm below the bottom of the sleeper. A scarifier, a bar used to remove material underneath the sleeper end, was replicated using heavy equipment to further open up drainage paths underneath the sleeper, and new American Railway Engineering and Maintenance-of-Way Association (Arema) Grade 4 ballast replaced the shoulders. Figure 2 illustrates the shoulder cleaning set-up.

Figure 2: diagram of shoulder ballast cleaning.

Three wetting tests were conducted to simulate heavy rainfall events. Over the course of the test period, Fast experienced slightly higher than average natural precipitation, and the rainfall varied considerably from month to month. For Tests 1 and 3, the combined natural and artificial rainfall in April 2021 and 2022 (around 33mm) was slightly drier than the 38mm average. Test 2, however, occurred during a very wet period, with May 2021 experiencing a combined 146mm of natural and artificial rainfall compared with the 38mm average. This differing natural precipitation allowed the research team to use the different tests as proxies for various climate conditions.


SBC performance was assessed in three ways: surface mud pumping, drainage, and track settlement.

Surface Mud Pumping

Because mud spot locations are typically identified through visual inspection, it is important to note these visual changes along with the underlying changes in drainage. No surface mud pumping was observed after each of the three tests. While surface fines were moist, they did not result in the slurry formation that was common in Phase 1. This lack of mud pumping shows improved performance and indicates that the drainage paths remained open over the 13-month test, including seven months of train operation. These results show SBC can provide improved drainage to prevent mud pumping.


Excess water from rainfall should drain through and away from the track, ideally keeping the track in a drier state. Drainage was calculated by monitoring moisture levels in the track centre in the crib. To assess drainage, the “days-to-drain below saturation” (15% from moisture sensors) was used. Although this is not a perfect indicator, it is a simple metric that represents complicated behaviour in a reasonable manner, based on past experience. Figure 3 compares the number of days it took for the section to drain below saturation moisture levels for two Phase 1 tests (tests A and B in red), the two drier SBC tests (tests 1 and 3 in green), and the wetter SBC (Test 2 in blue).

Figure 3: change in moisture content at crib centre after wetting.

Results show that the Phase 1 testing (red) took upwards of five days for moisture to drop below saturation; this agrees with visual observation of mud pumping and ponding of surface water for days after wetting. Tests 1 and 3 (green) showed the quickest drainage, about three to six hours. This quick drainage was attributed to surface runoff and to the initial dryness. Test 2 (blue) took longer to drain (around two days), but the pre-wetting moisture was already near saturation at about 14.3%, and it took about two days for the test section to return to its pre-wetting moisture level. The drainage difference suggests that SBC will improve drainage in both dry and wet climates, but it will be most effective at draining in drier climates where the track is able to dry out between wetting events.


Settlement indicates track deformation. When the settlement is localised in a dip, as it was for this test, settlement has a strong relationship with surface profile. Track settlement was measured with unloaded top-of-rail survey elevations. Figure 4 compares the track settlement at the centre of the dip of the rainy section after tamping to the anticipated settlement with no SBC using Phase 1 settlement rates.

Tamping of the rainy section occurred immediately after SBC. The initial 12mm of settlement resulted from ballast compaction, which is common after tamping. The three shaded regions show the wetting tests. Tests 1 and 3 showed minimal settlement. These low settlement rates match well with the Phase 1 dry settlement (no wetting). Test 2 showed much greater settlement rates, which was similar to the wet settlement rate in Phase 1, suggesting that SBC effectiveness will be reduced in consistently wet environments. If no SBC had occurred, high settlement rates would be anticipated for all three tests, based on past experience.

Figure 4: track settlement of Phase 2 rainy section over entire test cycle compared with nearby track.

In arid climates that experience sporadic rainstorms, SBC can reduce track geometry degradation because the excess water drains out from the ballast shoulders instead of collecting underneath the sleeper, and the dry climate allows any trapped moisture underneath the sleeper to dry. In wetter climates, locations may still experience greater settlement rates if the fines become wet to a near-saturated state from repeated rainfall events. These locations still drain more quickly than locations without shoulder drainage, thereby preventing surface mud pumping.


As a spot maintenance method, cutting the ballast shoulder at a severely degraded ballast location that is experiencing mud pumping, and opening up the drainage for a few inches below the bottom of the sleeper should provide intermediate benefits. Tamping with a higher lift (38mm-50mm) may also help raise the track out of the mud. Spot SBC can be used as a temporary measure until the location is undercut or renewed. Spot SBC will not clean or fix the root cause of the mud pumping issue, but it is likely to provide stability and avoid surface mud until enough rainfall induces settlement or fines fill the shoulder again. Success will also depend on the ballast profile underneath the sleeper. If the ballast has multiple low spots, ponding and mud pumping can still occur locally underneath the sleeper even if the shoulders are draining freely.