Is Pore Pressure a problem on Stabilized Subgrades?
The nature of subgrade remediation , 1/4 of the way through the 21st century, is something, still, of an enigma. In many places globally it has become standard to use lime or cement mix (depending on chemical composition of soils), as alternative to cohesive or poor/unsuitable soil removal and replacement with structural fill or other remediation methods. But what are the affects of this industry-wide shift on actual results? My theory is there will be some reckoning regarding this in the coming decade as many of these roads and projects that utilize these techniques near the back half of their design life
Techniques and Methods regarding chemical stabilization of fine grained and cohesive/un-draining subgrades vary mildly but in most part are similar across the developed and developing world. However, the impetus and precedent for the utilization of the method is still widely mis-understood across the industry, particularly in the construction sector and even in many consulting practices, unfortunately.
What has been introduced regarding this amelioration of the risk, regarding the poor subgrade in the first place? Well, let us consider three alternatives. The first is the use, on a typical temperate climate roadway, in a typical region with typical soils, if we can consider this, and if we may, and to consider the use simply of a 8-inch rock base as-subgrade with no geogrid or fabric stabilization (just for clarification).
Now, What is the purpose of this 8-inch layer of rock? Well, the first is the consideration of the introduction of an ameliorated surficial nature of the subgrade, in which an 8-inch moisture condition and re-compact could not accommodate. IF WE DO NOT CONSIDER THIS as primary, then there is no circumstance to use rock (besides the following, which I will introduce shortly). This being, mainly, the a) Bearing Strength and b) Long Term Draining Capacity of the Soil-after completion. Comparing an 8-inch rock base to a typified section of in-place granular soils, the nature of the drained capacity and strength, can in large part be considered mildly deviant, and in some parts a nominal design decision, and PERHAPS one that can be foregone based on cost considerations and/or regional distinction.
It should be clarified, that winter conditions and frost heave, in a perfectly drained and non-fine containing and granular subgrade, is in some large part not-of a consideration. Again in many parts of the northern climates additional layers of Sand or gravel are asked for by City engineers to deal with this. In many of those climates, this issue is of some nominal concern, at least for mid range soils or SC-SM mixes that sit marginally on a strength or drainage condition but would run afoul of a design engineer if encountered on the condition of frost heave.
Now, the analysis in this point now comes to a head - if the conditions of soil-improvement can be considered solely from a condition of the 8-inches below the paving section, where does Chemical stabilization come into the mix? Well, the reality is that it cant. IF YOU ARE CONSIDERING strength capacity or drainage considerations as -primary concerns in design - Why use Stabilization Techniques? The second question I want answered by the Industry - HOW DOES the drainage capacity of the stabilized subgrade, ie its ability to horizontally shed water along its grade, compare to the sub-rock in-situ or prepared natural subgrades on a typical project using rock -base. Has this been studied? What do we know about Project Life in this capacity? And why is the trigger pulled in one way or another in many project?
Heres the reality - A cost analysis will in most conditions, utilize a rock base or-natural soils, of course, if possible or available, or feasible depending on climate or geography etc, as Primary. WE COME TO THE NOTION, here, that ONLY a deep-subgrade -being-problematic (in-nature) consideration can be approached regarding the usage of Lime Stabilization. Any other use of the method are superfluous or of-poor analysis, lazy and un-intentional as-design. You CANNOT stabilize a subgrade to 9-inches to ameliorate frost heave. It is NOT possible. The usage of this method in northern climates , unless the soils are draining, which, if you are using this method, how could they be(?), Is short -sighted, poorly considered, and purely bad engineering
If I see Chemical Stabilization taking place in New-England - I know you have a bad engineer. How could you call for that? How are you considering for frost heave? The pavement will still crack - and you didnt get away with not using the rock. PEOPLE - the problem IS NOT the nature of your subgrade. It is the nature of the climate. And you cannot fight climate with cement - Unless you do 4 feet of it and in that case - We are just bleeding money
In Texas, the conditions are so mild that they routinely call for 6-inch lime stabilize over natural clays in the delta region. There is maybe 40+ feet of fat clays in many places. There is no fighting that. But how are you fixing the strength capacity? What about the drainage? Does lime-stabilized 6-inches really adequately model the same as rock? And how does this prepared surface drain once paved over? Again these life cycles will be coming due around 2040 on many decisions. We will see what the circumstances are at that time. For Texas, I wonder if we are simply considering a means-and-methods based engineering approach (better for muddy subgrades, problems with losing rock , etc) and calling it a soil mechanics solution - Which it is not.
The issue here is that northern engineers, especially in the east northeast, are seduced by this and attempt to institute a nominally engineered solution that really has most of its place in a constructability consideration into real deal soil mechanics circumstances, with little consideration. At the end of the day, if you blow in 6-inches of lime onto the top crust of 4 feet of bad clay, the paving will still crack in the northeast, and depending where you are. Why not just use rock. You dont need the 3 feet of sand section they use in Minnesota. But I question the decisions of civil engineers spec-ing this in northern climates, and also in southern. Yes its a neat solution, and the cost-engineering IS INTERESTING I will give you that. But beyond it? Into the actually performance engineering? I really question its reliability. And this bill will come due in the coming decades.
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