Fill change of property with time 9

[pelelo]

Experts,

Do you think it is possible that a fill (placed and compacted in layers) could change its properties with time?.

There is a case in which a house was built on a fill that was placed and propertly compacted, about 10 years ago. The fill (SP-SM) thickness is 10 ft, below the fill, there is natural material (loose to very loose sand), down to 35 feet. Water level is about 15 feet below surface.

We are doing some additional geotech investigation to the same property and the SPTs show N values in the "properly compacted fill" of between 0 (weight of hammer) to 9 blows per foot. These results were shown in 6 borings around the property.

My question is,

1 - Do you think fill properties can change their compaction level or field density with time?. I have never seen something like this before. I have records of the field density tests, of 10 years ago, and they show adequate compaction results.

2- Or do you think the fact that this fill material was placed on top of a thick (> 20 ft thick) layer of loose / very loose sands (SP and SM) (SPT-N < 10), might change with time?, as the fill weight might be contributing to long term settlement on the loose natural sands therefore SPT values have decreased?

Please let me know your thoughts. Thanks

 

[EireChch]

I think what OG is getting at is that for underpinning as well you will need a competent stratum

 

[pelelo]

Yes, i completely agree that at some point, I will need to reach the competent stratum.

 

[oldestguy]

Competent stratum?? Let's say you compact the zone under footings down to 6 feet. Settlements come about dependent on the ratio of added pressure to existing pressure as one factor. The deeper you load the existing ground with a given pressure, the less settlement you get. So with compaction grouting versus underpinning you start with a lower pressure (wide area) than with underpinning. Thus you can get by without a denser layer to transfer loads to. In addition compaction grouting pre-loads that lower zone and can be used to raise footings at the same time. BUT it has to be done by a competent contractor or unexpected things happen!!! Likewise piling of one type or another can be used in loose sand by adding light friction loads per foot through a deep depth.

 

[EireChch]

OG, I have reservations about the use of compaction grouting (LMG) for ground improvement. See extract below from the Ministry of Business, Innovation and Employment (MBIE)guidelines that are the standard for repairing houses that have suffered liquefaction induced settlement. I know liquefaction isnt the issue but the OP is left with the same problem of a house that has settled.

The EQC trials showed that where LMG is injected as a ground improvement method at shallow depths with little control (which can result in heave and dilation of the ground), then little or no improvement results, and in fact there may be a slight increase in the liquefaction vulnerability of the ground. However where it is injected for lifting purposes (refer Appendix A1.9 of the guidelines), then for typical house foundation loads, lifts up to about 75 - 100mm are possible without adversely affecting the ground. MBIE recommends that, in the absence of additional specialist input and analysis, lifting of houses with LMG should be limited to about 100mm. Greater lifts are possible where there is additional confinement from a heavy building, or the lifting is carried out at depths of greater than 4m.

In summary my thoughts would be that compaction grouting isnt that effective in improving ground conditions but it can be used to re-level the OPs house. Although, the OP has stated that there are walls cracked etc. If footings are also cracked then re-levelling through LMG may not be a goer and the only option may be to demo and rebuild. Or you could just re-cast the internal concrete slabs and leave the walls out of whack and learn to live with it.

At the risk of potentially opening another can of worms, would the house insurance cover the repair? If the ground conditions were never appropriate for the house/applied loads, well then the foundations were probably never fit for purpose. Surely someone has to be held responsible for the house settling? The designer of the foundations, the contractor for not compacting properly....who knows.

I know if it was my house i would be pretty pi553ed....

 

[oldestguy]

Yes there can be problems with comp0action grouting. However I've seen some great precision work re-leveling foundations by AN EXPERIENCED contractor. So if interested, get references first.

 

[aeoliantexan]

To get back to your original questions:

Yes fill properties can change with time, usually because the water content has changed. Plastic clay can shrink and settle when it dries, or swell and heave when it gets wetter. This is probably not happening with your SP-SM.

Most fill materials can soften and compress when wetted. If properly compacted and lightly loaded, there may be no significant settlement even when wetted to saturation. Well-compacted fills more than about 25 feet thick can compress significantly under their own weight when wetted. see "Review of Wetting-Induced Collapse in Compacted Soils" by Lawton, Fragaszy, and Hetherington, in ASCE Journal of Geotechnical Engineering, Vol. 118, No. 9, Sept. 1992.

It seems unlikely that a properly compacted silty sand would become loose to very loose on wetting. Like several other posters here, I would suspect poor construction (little or no compaction) combined with poor testing. I have seen cases where deep backfills were compacted in lifts, as demonstrated by photos of equipment on the backfill, and documented with tests to meet the specifications, but settled many inches when they became wet. In both cases I suspect dry soil and thick lifts. One was tested by sampling with a drill rig at 5-foot intervals. Pushing Shelby tubes in a loose fill can produce a dense sample due to friction along the sides of the tube.

The weight of the fill could have compressed the underlying very loose to loose sand, but the settlement would not be expected to loosen the fill unless the settlement was so differential as to cause cracks and voids due to arching. The sand should have compressed mostly as it was loaded, so that explanation is unlikely.

I would check the house for settlement by shooting elevations of the floor and of some perimeter feature such as a mortar joint or soffit that was probably built level. There will be some random variations, but significant settlement should show patterns.

Areas such as Texas, where the house perimeters tend to settle due to drying and shrinkage of the fat clays, have many firms that underpin homes. Many jack 3-inch tubes to refusal by reacting against the weight of the house with a bracket attached to edge of the footing. Some dig pits under the footings and jack concrete cylinders into the ground until refusal is encountered. These methods may be more economical than compaction grouting. You probably need to consult a local geotechnical engineer.

Good luck.

 

[Mad Mike]

It would seem highly unlikely for a granular fill to soften (loosen) so anomalously with time.

I think the timing of the cracking (even anecdotal evidence) could provide insight- if the SP-SM fill was compacted negligibly I would expect significant cracking to have occurred within the first year post-construction. Is there any possibility of the sandy fill having been eroded internally (piping) through damaged service pipes?

OG's comment regarding the SPTn = 0 values being recorded around the structure as opposed to beneath the structure is important- I have mistakenly jumped to conclusions based on offset testing on several occasions- you could have intersected an animal burrow or some similar anomaly!

The commonest means of underpinning in thick deposits of loose sands in my area is on jacked piles- hydraulically-driven small diameter steel tube piles. Though there is no "competent" stratum at depth the tube piles will pick up good friction in the sands and, because they are jacked to a set resistance with a hydraulic gauge, they are effectively load-tested during installation.

All the best.

 

[fattdad]

I eschew the notion that clay fills soften when made wet. The benchmark for strength is contingent on sample saturation. So, the only softening that could possibly occur in a properly engineered fill is the restoration of saturated strength. When actual fill materials are placed and compacted, they are not saturated. They include negative pore pressure, capillary suction, and GREATER strength then one would determine in the laboratory.

We are using the laboratory strength values, eh? Okay, let's say we are not - we are using correlations from the fathers of geotechnical engineering! Those fathers related the index properties to saturated (i.e., laboratory) strength.

When actual clay fill is placed, we can only hope there was some measure of material verification in measuring relative compaction. If the compaction did not achieve the critical void ratio, then there will be problems. The technician may have perceived dry strength, thought the fill was strong and little basis to question the finding. If the soil was actually governed by dry strength (i.e., in part), making that interval of fill saturated would negate all dry strength and the soil would behave as mud.

Step 1: reference the correct proctor.
Step 2: get proper compaction

Clay cores, embankment dams, etc. do not fail when they get saturated. The state of stresses change, but we'd have considered that in design.

Elastic compression (i.e., immediate settlement) does occur as the fill is placed (primarily). There is a long-term parameter, "Ct" that informs the likelihood of continued movement. Over a span of 30 years, that time-frame term can return 20 to 30 percent more settlement then calculated using elastic methods. So, if there is some anticipation of 9 inches elastic compression (i.e., a ridiculous amount of immediate settlement), over the next 30 years you may realize another 3 inches.

Just a few thoughts.

f-d

ípapß gordo ainÆt no madre flaca!