Specialty geotechnical contractors such as Hayward Baker, Moretrench – now a Hayward Baker Company – and others have long been key players in advancing the state of the groundwater control practice in the United States. The authors, both industry specialists, discuss the realm of groundwater control methods currently in use.
Anybody who has been fortunate enough to work with renowned geotechnical consultant Ron Heuer has been blessed with his colorful word pictures that bring geotechnical concepts to life for us common folk. “Grout is dumb and slow, but groundwater is smart and tenacious” is a good example. We may not have stated it quite like that, but his point is crystal clear– groundwater is a fierce and unrelenting enemy. Whenever you let your guard down, it will act, and often with swiftness and power.
As long as the properties of soil and water remain constant (which we can probably count on for a long time) groundwater will remain the No. 1 source of problems in underground construction. Could this be due to the Ron Heuer groundwater tenacity or the fact that there is a very wide range of factors and conditions that aggregate successful or unsuccessful groundwater control? It’s probably a combination of the two.
With tunneling work particularly, we put our methods to the test. We go deep. Sometimes really deep. We traverse variable ground conditions many times along uncompromising alignments which cannot be varied to avoid unfavorable conditions. In tunnels, shafts, adits, breakouts, cross passages, etc., we never have to space to accommodate “additional measures.” It is an unforgiving environment and tunneling rarely allows a “work around” when there is a problem. We typically cannot afford a “hiccup” in the work. That’s why the groundwater control approach on a project is so critical.
Before the common use of the pressurized face TBM, dewatering was a huge component of most soft-ground tunneling projects. The open face digger shield, for example, would struggle when perched or residual water was encountered in potentially running ground conditions, and catastrophic soil run-ins would occur when an isolated pocket of undrained sandy soil would be encountered by surprise.
In easily dewaterable ground like thick deposits of sand and gravel, it was like a tunneler’s dream. If there was little risk of off-site adverse effects (consolidation of compressible soils or moving plumes) or the ground could be dewatered without concern about difficult changes in geology, perching layers, or recharge from open water or even utilities, a dewatering approach was the lowest risk option. The greatest risk hinged on the effectiveness of the dewatering program. That risk remains today with sequentially excavated tunnels; however, the third-party impacts are less likely because the ground is invariably of lower permeability or “tighter” as that condition is a prerequisite for the stand-up time that goes hand-in-hand with the sequential excavation. It seems as if today there are more concerns about groundwater lowering perhaps because we are driving tunnels through some of the most congested urban areas, there is contaminated groundwater everywhere, and there is always the perception that dewatering is just going to cause problems.
The typical tunneling project today is a sequence of “bathtub excavations” connected by tunnels. The bathtubs are relied upon even when the ground conditions are highly favorable for dewatering. The vertical walls of the bathtub are constructed with slurry walls, secant piles, soil mixing, and steel sheeting. Ideally, a natural, low permeability cut-off stratum exists to act as a bottom of the bathtub, but when that doesn’t exist, we must make the bottom with jet or permeation grouting. The bathtub approach is not risk-free by any means. A slight imperfection in a deep cut-off in the right (wrong) soil conditions can be catastrophic. Good craftsmanship is more important the deeper one goes. Soil/structure interaction and compatibility of dissimilar cut-off methods cannot be overlooked. There are a lot of elements of the work that must be executed flawlessly.
The recipe for a bathtub disaster is a deep excavation, a slight defect in a cut-off, plus a non-cohesive soil. Looking back on some of the more horrific ground loss events that we have been called in to, there is a common theme – excavation well below the water table, some kind of an unanticipated gap in a cut-off, and surprisingly, a low permeability, but non-cohesive silt. Many people disregard the unstable potential of a non-cohesive silt (like Bull’s Liver) because they recognize it as a low permeability material. Low permeability material is just assumed to be less susceptible to ground loss. Experience has shown just the opposite. We let our guard down and the tenacity of the groundwater wins.
Typically in tunneling, there is little we can do to accommodate leakage and groundwater intrusion without ground loss. A bottom seal can be configured as a deep blanket with soil between subgrade and the deep blanket to accommodate some leakage through properly constructed wells or wellpoints. But this option doesn’t apply to vertical cut-off walls. When there are defects in cut-offs, unfortunately we aren’t usually aware of them until they reveal themselves with a vengeance. The desired response is usually chemical grouting and/or jet grouting, and compaction grouting to replace lost ground. But a high degree of ground disturbance is a potential game changer. This is when we call on ground freezing.
We usually rely on ground freezing when there is absolutely no room for error, when we are deep and in ground conditions that are not amenable to displacement or erosion, when we have disturbed conditions or we are attempting to work in and amongst the grout of previous attempts. This is where ground freezing excels. It fulfills a need that is difficult or sometimes impossible to fill through other geotechnical methods. But in underground construction below the water table nothing is ever 100% guaranteed and even ground freezing has its Achilles’ heel – moving groundwater. Excessive groundwater velocity can hinder the formation of a freeze, leading to windows in the frozen wall. We have learned how to diagnose the potential for this and address it early on with localized grouting to reduce the permeability of the soil and achieve closure. It’s all in reading the groundwater behavior and responding quickly and precisely.
As history has shown in the many, many instances of projects significantly impacted or even abandoned because of groundwater inundation, this is often the riskiest and most challenging element of a project. It is hard enough to do it flawlessly when you have the right approach. An inappropriate or ineffective groundwater control approach is a recipe for disaster. It’s critical, therefore, that groundwater should always be addressed early in the project cycle and the most appropriate groundwater control method or methods determined. Striking the right balance between cut-off and dewatering is a key factor, and for this, experienced practitioners are essential.
Paul C. Schmall, Ph.D, P.E., is a Senior Vice President and Chief Engineer for Moretrench, a Hayward Baker Company. Gary E. Taylor, P.E., LEG, is a Vice President with Hayward Baker’s Western Division.