Sand Boils Observed During Excavation For Segmental Wall
Sand Boils Observed During Excavation For Segmental Wall
(OP)
Here's the scenario; there is a site with a 10 foot +/- tall earth embankment. Although the embankment has been stable for the last 30 years, numerous seeps were observed a few years ago. Soil borings were completed in the area at the time the seeps were observed. The geology in the area is complex, with alternating silty clay and water-bearing sandy strata to depth. It is believed that several discrete aquifers exist within these soils. Some of these aquifers are artesian, while others represent the regional groundwater flow towards the bay about 800 feet away.
Now, to provide room for a new development, the embankment will be replaced with a new reinforced segmental retaining wall.
A contractor came in and installed a sheet pile wall (believed to be about 20 feet in length), along with 2 inch PVC well points (believed to be about 20 feet long, screened on the bottom 5 feet) on the side of the sheet wall opposite of the proposed excavation for the new segmental wall. The location of the sheet wall was near the crest of the existing embankment at the top. The wells were installed by jetting in under high water pressure on approximate 3 to 4 feet centers. Prior to the pump being turned on, some artesian flow was observed in about 50% of the wells. This was not surprising based on the original soil boring information.
After the pump had been started and was running for a few days, the contractor excavated the area adjacent to the sheet pile wall down approximately 8 to 10 feet. They noticed that even with the pumps running, there were sand boils at some areas near the wall, and the area of the proposed retaining wall was generally wet.
We were called out to the site to help them understand why their dewatering system was not working. First of all, it was obvious to me that the well points were not doing the job for which they were intended. I believe the well points are probably pulling water from deeper aquifers that probably do not need to be dewatered in order to dry the site up. There are more shallow aquifers making it through and up and around the sheet wall that are not affected by the well points.
My opinion about the sand boils is this; while they may be result of natural discontinuities in the soil strata that are now exposed to air for the first time, I believe that the pressure of the jets may have aggravated the problem by weakening certain strata and ultimately causing more sand boils to form during excavation in that area than would have existed otherwise. What do you think of that? In your experience, have you seen the act of jetting well points in cause damage in a highly stratified soil system? This is an important question to answer since long term dewatering and water management of the segmental wall will be dependent in part on how much water is rising upwards from depth after construction.
My concern is that if the contractor installs additional well points (at varying tip depths) in an attempt to dry the site up for construction, additional damage to the soil strata may occur and more upward flows will be the end result. My thought at this point is that the area should be dewatered using a more gentle approach; namely shallow sump pits along that area to avoid breaching these deeper artesian layers.
What do you think about the dewatering and sand boil theory? Thanks for your comments about this!
P.S. There are other concerns that we have been addressing on the fly for this site too; namely a cantilever sheet pile wall that was installed by the contractor with no design or analysis...
Now, to provide room for a new development, the embankment will be replaced with a new reinforced segmental retaining wall.
A contractor came in and installed a sheet pile wall (believed to be about 20 feet in length), along with 2 inch PVC well points (believed to be about 20 feet long, screened on the bottom 5 feet) on the side of the sheet wall opposite of the proposed excavation for the new segmental wall. The location of the sheet wall was near the crest of the existing embankment at the top. The wells were installed by jetting in under high water pressure on approximate 3 to 4 feet centers. Prior to the pump being turned on, some artesian flow was observed in about 50% of the wells. This was not surprising based on the original soil boring information.
After the pump had been started and was running for a few days, the contractor excavated the area adjacent to the sheet pile wall down approximately 8 to 10 feet. They noticed that even with the pumps running, there were sand boils at some areas near the wall, and the area of the proposed retaining wall was generally wet.
We were called out to the site to help them understand why their dewatering system was not working. First of all, it was obvious to me that the well points were not doing the job for which they were intended. I believe the well points are probably pulling water from deeper aquifers that probably do not need to be dewatered in order to dry the site up. There are more shallow aquifers making it through and up and around the sheet wall that are not affected by the well points.
My opinion about the sand boils is this; while they may be result of natural discontinuities in the soil strata that are now exposed to air for the first time, I believe that the pressure of the jets may have aggravated the problem by weakening certain strata and ultimately causing more sand boils to form during excavation in that area than would have existed otherwise. What do you think of that? In your experience, have you seen the act of jetting well points in cause damage in a highly stratified soil system? This is an important question to answer since long term dewatering and water management of the segmental wall will be dependent in part on how much water is rising upwards from depth after construction.
My concern is that if the contractor installs additional well points (at varying tip depths) in an attempt to dry the site up for construction, additional damage to the soil strata may occur and more upward flows will be the end result. My thought at this point is that the area should be dewatered using a more gentle approach; namely shallow sump pits along that area to avoid breaching these deeper artesian layers.
What do you think about the dewatering and sand boil theory? Thanks for your comments about this!
P.S. There are other concerns that we have been addressing on the fly for this site too; namely a cantilever sheet pile wall that was installed by the contractor with no design or analysis...





RE: Sand Boils Observed During Excavation For Segmental Wall
I'm not sure I completly understand what is being constructed. It seams that the land side of the existing embankment/dike is being replaced by a segmental wall and that the dike has had seepage problems in the past. I would question the wisdom of this design. Has anyone looked at the new seepage path through the dike since the path will now be shorter? If long term seepage will be a problem, it may make since to install a permanent cut off wall along the crest of the dike. Given the soil conditions the wall would likely need to be a slurry wall with a depth dependent on the specific geometry and soils.
RE: Sand Boils Observed During Excavation For Segmental Wall
You may need to install a french drain along the base of the wall - and direct the artesian flow away from the site - hopefully you have some ground contour. I had something like this happen too in Vancouver. No rain for months and the french drain was flowing like a river.
And, you would be quite right to know who designed what and who authorized the method of support (sheet-pile walls) etc. Was a specialist dewatering contractor used? . . . or just a makeshift attempt by a general contractor?
RE: Sand Boils Observed During Excavation For Segmental Wall
My guess is that the sheet piling has cut off the flow of at least one of the aquifers. With an impermiable layer above and below and an impermiable wall downstream, head pressure at the wall will rise, causing the boils as excavation relies overburden pressure.
Although good in sand, vaccum well points don't function well in low permiability soils. They also will have little effect through the sheetpile, so if boils are occuring in the inside of the wall, ponts should be installed on the inside to the depth of the layer that has the boils. I suspect you are correct that the wellpoints go too deep and do not drain the layer of concern. Generally jetting produces only localized effects which quickly dissapate. I would not worry about a jetting operation, as long as it performed with proper care.
It seems that the sheeting and dewatering systems were installed with little or no engineering. This should be remidied.
Note that OSHA requires a PE stamp the sheeting drawings for the excavation, and if you have sand boils the soil below the dredge line may be liquifing, which will dangerously reduce the capacity of the sheeting. This should be examined before anyone renters the excavation.
RE: Sand Boils Observed During Excavation For Segmental Wall
Based on a soil exploration completed about 5 years ago, we knew that there would be a lot of water to contend with after construction. Our site plan has a fairly robust drainage system for the new wall as well as an underdrain system for the new building to be constructed. Although we anticipated a large amount of water approaching the wall horizontally (regional groundwater, etc.) and vertically (via artesian pressure, communication between horizontal strata, etc.), if the act of installing well points using high pressure damages those confining layers, we may have more water than ever expected. That's my theory, at least.
GeoPaveTraffic, you seemed to agree that the idea of jetting wells in and damaging the natural stratification allowing for greater communication between those strata is plausible. DRC, you seemed to think that it would not be cause for any unreasonable concern. This is good as it gives me both sides of the story. I would be interested to hear further why each tends to lean one way or the other on the well point jetting issue.
We're also in the process of looking further at the stability of the wall itself. One thing the contractor did do before calling us, is stop excavation near the wall when the boils were discovered!
RE: Sand Boils Observed During Excavation For Segmental Wall
RE: Sand Boils Observed During Excavation For Segmental Wall
RE: Sand Boils Observed During Excavation For Segmental Wall
RE: Sand Boils Observed During Excavation For Segmental Wall
RE: Sand Boils Observed During Excavation For Segmental Wall
BigH, when you say "french drain," a near-surface drainage structure made of stone and geotextile in some configuration comes to mind. When you say "deep french drain," I think of what we may have already designed. We've got a system that includes clean sand backfill behind the wall with a horizontal drain near the base. This drain is near bottom of wall level and contains angular, uniformly graded stone. This horizontal drainage layer is intended to intercept upward groundwater flow (the wild card, if you will), and to help lower groundwater approaching the wall from higher elevations within the backfill area of the wall. Then there is a drainage layer immediately behind the wall, as many walls of this type have, to help keep water away from the immediate back-of-wall area. The water is collected in a perforated 6 inch line near the base of the footing where it travels to daylight, away from the wall.
Is this what you have in mind?
RE: Sand Boils Observed During Excavation For Segmental Wall
RE: Sand Boils Observed During Excavation For Segmental Wall