Acceptable Pore Pressure Value during High Embankment Construction 2

[studio13]

I was wondering if anyone knew of a guideline or Spec that sets an allowable amount of pore water pressure generation during embankment construction as read with VW piezometers. In other words, we would want to set an upper bound during filling of XX% of the effective overburden to prevent the soil from reaching "liquefaction". I'm being asked to set a standard that someone (with an unknown amount of knowledge on the subject) can use to monitor the safe staged construction of a 33 foot high embankment. So I have to set some guidelines that won't get them over their head, or better yet, put an excessive amount of liability on my firm because they were watching purely the VWP and didn't notice the deformation occurring.

 

[ntschwanz]

I don't know of a guide or spec that sets PP limits in staged construction but if I did know of one I wouldn't use it without completing the actual stability analysis myself. I may be missing something with your project but staged construction is typically a slope stability and consolidation problem. This is an analysis that the geotechnical engineer should be doing and setting construction criteria with a goal of completing the project without excessive deformation or failure and in the end achieve a tolerable performance risk. He should also be intricately involved with the project during the construction process to review data (VW; settlement gages; other survey points) for comparing to his analysis. Suggest the geotech become friends with the on-site inspector and they talk and share data freely.

 

[dgillette]

Liquefaction? If that's an issue (as the term is conventionally defined), something is very wrong here. If your foundation is so loose that there is potential for "static" liquefaction due to construction, I hope the embankment isn't a dam.

Is it the foundation or the embankment materials you are concerned with? If it's a dam, who designed it, your firm or someone else? Is it zoned? What materials? Is the fill going in significantly wet of optimum? What climate?

The fact that the embankment is to be only 33 feet high PROBABLY solves the problem automatically ASSUMING THAT the concern is for high pore-water pressure in conventional fill placement just wet of to just dry of optimum. (Don't expect that if the issue is the strength of a saturated clay foundation.) Recall that, right after compaction, the material is partially saturated and is held together by capillarity (negative pore-water pressure). It takes a lot of overburden pressure to compress the material enough to get it close to saturation and high positive pore-water pressure. Under a very high fill, the increase in confining stress compresses the material closer to saturation (decrease in void ratio without decrease %w), causing positive and then very high positive values. 33 feet of fill may not be enough.

Only stability analysis (or sometimes a practiced, expert eye) can tell you how much pore-water pressure is tolerable. However, assuming that it is the fill you are concerned about, proper moisture control in placement may well keep it from ever being an issue.

Remember also that there is some lag time with even with VW piezos in saturated sand.

 

[studio13]

Thanks for the responses, I guess I should clarify a couple things. We are concerned with the failure of the foundation soils which are predominantly soft clays, so bearing capacity type failures and maybe even mudwaving may be something to be concerned with. Slope stability analysis has been run, the issue is a FS below 1 for the embankment at full height (assuming the instantaneous placement of 33 feet of fill). Through the analysis we determined that we can safely build the embankment up to about 26 to 28 feet. Now I understand that the instantaneous solution is conservative and as such we recommended an instrumentation program be implemented in order to assist in determining actual start and stopping times in the field. This is a state job and because of the way they are done here, we will not be involved with the monitoring of the instrumentation or the construction so we've been asked to provide guidelines that an unknown entity with an unknown level of experience on the matter can use to determine when to start and stop fill placement. As much as I'd like to just write "this can only be defined by experience and combined observations of all available data in the field and not by a value so hire us or at least an experienced Geotechnical" I wanted to exhaust some information paths to see if I could provide what they are requesting. Every other job I've worked on like this we were involved with the instrumentation program so I've never had to quantitatively define what the values should be.

 

[dgillette]

If the clay is at all sensitive, small movements can rapidly turn into big ones. Given all the little details that govern undrained strength*, you probably need a higher FS to be comfortable than if you were dealing with dry material or rolled fill. (In good material, FS=1.1 can mean very low probability of failure. In others, FS=1.5 shouldn't make you feel warm and fuzzy.)

I'd sure hate to rely on piezos to be the main trip wire, especially with their slow response in clay, with the slightest bit of air in the porous stone the space between stone and diaphragm, even with VW piezos. At minimum, I would probably ask for stakes at the toe of the slope, to be shot periodically (both vertical and horizontal). Any significant heave or horiz displacement starts a rest period (plus frequent visual monitoring). Inclinometers would be good also, although a state DOT may balk.

You've seen Chuck Ladd's Terzaghi Lecture, "Stability Evaluation During Staged Construction"? If not, see ASCE JGE April 1991. It is right up your alley. It is the best single reference I know for undrained strength of clays.

I would also be inclined to put in disclaimers writ large, emphasizing the uncertain nature of subsurface conditions, particularly undrained shear strength, and the importance of monitoring to make certain that designs assumptions are met...Unfortunately, those things are usually managed by bean counters, not by people that understand there is a difference between geotechnical engineering and doing QC on toner cartridges.

The crack and slump in the photo appeared one morning. All was fine the night before.

*OCR
Rate effects
Mineralogy
Small changes in void ratio
Degree of saturation
Rotation of principal stresses
Assumed Nkt for CPT
Adjustment factor for VST

 

[oldestguy]

Studio13: Your firm had better be very firm on what sort of inspection is needed. If that fill slips out, I'd bet your boss will be in big do-do and government departments (and neighboring properties) usually are not very forgiving about stuff like this.

If they (the state) don't agree to a good program of engineers watching the job, I'd walk away and not risk it. A SF of 1.1 sounds real dumb under these circumstances and maybe 1.5 is also.

In my prior DOT experience two of these kinds of things happened and all hell broke loose. It ain't worth sticking your neck out.

dgillette got it right.

 

[DRC1]

I have done some embankmanets on soft clay when I worked for a highway contractor. It was several years ago, but as I recall this is what happened: The DOT had set out settlement monitering platforms and piezometers. The concern was not the fills themselves but the underlying clay. We brought the fill up in 1 foot lifts. When we brought the fill up 5 feet, the platforms were checked and so were the piezometers. If everything was okay, we continued on. If not we waited. (Actaully there was always other parts of the job to work on, so we were not idled.) As I recall we had to wait several weeks at on point.
The five feet was probably critical in the middle, as we could have gone higher at the bottom because the total load was lower, and more at the top as the incermental load was less, but having one lift thickness at which the instrumentation was checked made things simple. Simple is good sometimes.
However, the pore pressures are controlled by permiability and so is its disipation. Even with some lab testing (which I would wager you did not get a heck of a lot of, if any, from the sound of the DOT's KISS requirement), it is really hard to predicit what will happen. The range of permiability for typical soil is analogous to the change in your pocket to the size of the national debt. You can not accurately predict the exact preformance of the clay and the DOT should realize it. Their requirement that an unknown entity with unkown experience matter can use to start and stop fill placement is rediculous and they know it. They can have anybody read the instrumentation (and that is an issue by itself) and if poor descions are made and there is a problem, its your fault. You may be able to aurgue that directing the contractor to start or stop the fill is engineering. If you are not going to be involved in construction, then you do not have responsible charge and the contractor should be directed by another PE who does have responsible charge. Or you could do both the design and the monitering. Good luck. Personally, I think it is attitudes like this (by orginizations that should know better that cheapen engineering services)

 

[BigH]

If, as implied, the clay is soft (or firm) and the loadings will result in unacceptable stresses - requiring stage loading, do yourself a favour and install pvc wick drains on a 1.2 to 1.5 m rectangular spacing. This will hasten drastically the time for any pore pressure dissipation. We had a job in India where we put in wick drains - had 6 m of very soft (N = 1, Su = 20 kPa max) clay overlying firm to stiff clay. The project had to support 11 m high RECO walls - of course, only 4 m or so could be put on before reaching near failure. We put in wick drains, and had to wait only about 45 days before the next lift of 3 to 4 m could be placed. We waited again . . . The consolidation, as you know, increases the undrained shear strength (i.e., dissipates the porewater pressures (to the drained path - which is to the right of the applied stress path) - this allows more load to be put on. Simple stage loading conceps. We ended up with more than 900 mm of settlement during construction. No failures occurred when outside forces didn't get involved; had one when the penultimate load wasn't allowed to be on long enough. Works fine. Need to monitor with settlement plates and efficient piezometers (quick acting - not standpipes). Analyses using various porewater pressure heights in effective stress analysis would be beneficial to determine the maximum porewater pressure rise permitted. Otherwise, use Su analysis with FS of 1.15 or so . . . (stage loading). WOuld be a good idea to have the opportunity to put down boreholes to confirm expected increases in Su. There was a Tech Note in ASCE GE Journal back in 1975 (I think) by someone and Dr. Som on this subject of increase in Su with such stage loading. Anyway - the above is for what it is worth - while soaking up the sun in Hua Hin . . .

 

[Thamae]

My input is not a reply but I have a question almost similar to the one under discussion.

I need to decide on the most appropriate instrument to be used to monitor possible deep seated shear failure in a 150m hign mine waste rock dump. The dump is constructed on weak residuum and steep topography (>25deg in places).

We had inclinometers in mind to be installed on holes drilled through the dump as construction progresses. Due to size of boulders however, and the expected settlement, inclinoometer casings will be out of profile within a few yrs after installations.

Excess pore presure built up must also be prevented or limited so we will also have to limit the rate of rise of the dump while monitoring pore pressure. What would be the most sensitive type of piesometers that we can use?

 

[dgillette]

Hello Thamae.

I think the most sensitive piezometers (at least in terms of response time) are vibrating wires, because very little water has to move in or out for a given change of pressure. They are not much trouble to read, compared to lowering, raising, and recoiling 100m of MScope cable. To get the fast response, VWs have to be sealed well in the drill hole or casing, and properly saturated before they get put in, but they should be far faster than standpipes. They go bad eventually, however. When they do, they are more likely to read too high, falsely indicating a problem.

Don't have a good answer about the movement measurements. I have no experience with inclinometers in bouldery material. Shear strips are a possibility, but they wouldn't let you measure small movements before they tear, and they might give "false positives" due to the same settlement and deformation of bouldery material that would mess up the inclinometer casings. Can't say for sure.

Bon Chance!
DRG

 

[ntschwanz]

Agree with dgillette that vw's are sensitive and we routinely grout them in place with a permeable grout. Of course they are now disposable and there is no backup reading or redundancy since there is no standpipe. But this is one aspect that makes them so sensitive when grouted in place as you don't need the volume change to raise the water level in the standpipe. When they go bad, we typically notice right away as changes in head can be significant, like 100 ft.

Can you put inclinometers at a small distance from the toe of waste dump? An expensive solution might be to install Shape Accel Array's in holes beneath the dump site prior to placing waste fill but unfortunately these would not be recovered. SAA's were used on a lateral load test in New Orleans with great success (very precise xyz data). You may have several options depending on the criticality of data and available budget.