This part of our ongoing series about building settlement and geotechnical engineering strategies to mitigate settlement. Mike Talbot is OTO’s geotechnical engineering practice lead.

I recently had the pleasure of presenting a talk on this topic at the Connecticut Society of Civil Engineers Spring 2017 Geotechnical Conference. This post provides some highlights of my talk. The entire presentation is available on OTO’s web site.

Connecticut River Valley Varved Clay (or CVVC) is a type of fined grained soil deposit that was laid down within ancestral Lake Hitchcock, which filled the Connecticut River Valley after the melting of the continental glaciers  approximately 14,000 years ago.  CVVC is characterized by alternating layers of silt and clay, and is similar in composition to other varved deposits that formed in meltwater lakes across the northern part of North America following the retreat of the continental glaciers.

CVVC deposits are soft and compressible, and significant building or embankment settlement may result from building dead or live loads or from the placement of new fill loads. Having practiced extensively in the Connecticut River Valley for approximately 25 years, OTO has wide-ranging experience in investigating CVVC sites and providing geotechnical engineering solutions to address settlement concerns.

In my talk, I presented two case studies where a soil preload was constructed to mitigate post-construction settlement.  One case study involved the new Easthampton (Massachusetts) High School and the second involved a new book depository in Hatfield, Massachusetts. Both project sites were similar in that they were underlain by greater than 50 feet of soft CVVC, and in that the new construction (a combination of fill placed to form the building pad, and building dead and live loads) resulted in an increase of greater than 1,000 psf (pounds per square foot) in the vertical effective stress within the soil profile– books can  be heavy! Without ground improvement, this load increase could have caused the maximum past pressure to be exceeded within portions of the soil profile, causing virgin consolidation to occur. Consolidation is the process where the water content of the soil decreases, without being replaced by air, so that an overall volume of the soil layer changes. During design, we estimated that up to 6 inches of total settlement and as much as 3 inches of differential settlement could occur at both sites, enough to cause  damage and loss of functionality to the planned buildings.

Both structures were sensitive to post-construction settlement, so it was determined that soil improvement was required to mitigate the amount of settlement. The application of a soil preload was selected as the appropriate soil improvement technique. In essence, preloading is intended to simulate the design loads of a building, in this case by stockpiling large quantities of soil on the site, so that the consolidation occurs before the building is constructed. In addition, the construction schedule was tight for both projects, so the design solution included features (wick drains, which provided additional pathways for water to be eliminated) to expedite the consolidation process. Our design solution was similar for both projects and involved the following:

• The installation of wick drains to help remove water from the soil matrix, and speed the rate of settlement (in this case, time was reduced to about three months),
• The placement of a preload fill, which varied from two to 11 feet high, and
• The monitoring of settlement during construction.

Preload settlement monitoring and the post-construction performance of both structures indicate that the preload application was successful in reducing the amount of post-construction settlement. The use of wick drains allowed the preload settlement to occur without significantly impacting the construction schedule. However, despite the similarities of the site geology and the selection of a similar design solution the amount of preload settlement varied significantly between the two sites. At the Easthampton site, the preload settlement was approximately half of what occurred at the Hatfield site. The variation is likely attributable to a high silt and sand content in the CVVC at the Easthampton site.  We have found that simple moisture content data from bulk samples (combination of both silt and clay varves) provide a good indication of this variation.

More detail is provided in the complete presentation, downloadable from the link above. If you have any questions please feel free to contact us.

In Part I of this topic, we discussed the assessment and the identification of the cause of settlement of existing structures.  Once the causes have been identified, we can then provide alternatives to prevent on-going settlement, if needed.  This post will discuss a few of the engineering solutions that are available to mitigate a settlement problem of an existing structure.

It should be mentioned that we do not always propose mitigation or remediation.  For instance, if the settlement of the foundation appears to be due to the placement of compacted engineered fill in the 1960s (during construction) over soft compressible clay, OTO likely may recommend that the client delay large and expensive repairs and mitigation, and instead monitor the rate settlement over the next couple of years. In these instances, the rate of settlement often has decreased to negligible amounts and further significant settlement may be unlikely.  At that time, we often recommend that the owner proceed with larger structural and cosmetic repairs.

If settlement concerns appear to be due to improper drainage and the introduction of large amounts of water into the soil mass, OTO will provide recommendations for correcting the drainage problems.  We often can provide local contractor names, upon request, to help repair or install new drainage systems.   Often times, these repair or maintenance tasks can be performed by the owner or facilities manager.

If the cause of the building settlement is the presence of an unsuitable bearing layer, such as loose, non-engineered fill that may continue to compress, or a thick organic peat layer that may continue to degrade, we will recommend a mitigation alternative such as a deep foundation or a soil improvement technique.

A deep foundation system transfers loads through the unsuitable layer to a firm bearing layer, such as driving pilings through a clay layer to bear on a layer of dense sand or bedrock.  Deep foundation alternatives to mitigate the settlement of existing buildings may include helical piles or mini piles.  Helical piles consist of a central steel shaft with horizontal bearing plates (8 to 14 inches in diameter) welded to the shaft at spacings on the order of 12 inches, which are augered into the soil. Mini piles are drilled, cast in place, cement grouted shafts. The piles are constructed by drilling and advancing casing (three to ten inches in diameter) to a selected depth or bearing stratum, installing a steel reinforcing bar down the center of the casing, and injecting cement grout into the casing.  The grout is pumped into the borehole at high pressure, starting at the bottom of the casing and moving upward in order to displace drilling mud or any remaining soil cuttings from the borehole. As the grout is pumped into the borehole, the casing is pulled up to a selected depth at the top of the “bond zone,” allowing contact between the grout and the surrounding soil. Helical or mini-piles are typically connected to the existing footings using an underpinning bracket.

Soil improvement techniques, which improve the existing loose soil so that it can function as a suitable bearing layer, may include pressure or compaction grouting.  In compaction grouting, the soils within the improvement zone are densified and strengthened by a systematic, pressurized injection of controlled low mobility cement grout. The goal of the process is to achieve increased strength of the soil mass.

Many factors must be considered in order to recommend the most appropriate engineered solution for settlement issues.  OTO will often discuss existing building and soil conditions and proposed mitigation techniques with specialty geotechnical contractors to evaluate possible alternatives costs.  OTO maintains relationships with most of the foundation specialty contractors in New England and often can provide two or three independent contractor contacts to the client so that competitive cost information can be obtained.  Once the mitigation alternative and contractor is chosen, OTO can assist during construction by documenting the installation and addressing any concerns that arise.

If you have other questions about building settlement, contact Ashley Sullivan at 413-276-4253 or sullivan@oto-env.com to see how OTO can help!

Large areas of the Connecticut River Valley are underlain by a type of soft soil deposit known as Connecticut Valley Varved Clay (CVVC).  This deposit formed at the bottom of a large glacial lake, commonly known as Lake Hitchcock, which once filled almost the entire valley from Vermont to near Long Island Sound. This lake formed following the retreat of the last glacier and drained approximately 14,000 years ago.  During the warm “summer” runoff season, as the ice melted, soil particles were carried into the lake.  As water drained rapidly into the lake, the sand and silt, having a larger grain size, settled out first.  Fine-grained clay particles have a tendency to remain suspended much longer, particularly in turbulent water.  As the cold “winter” months moved in, the waters calmed and allowed the clay particles to settle.  This act of differential sedimentation created alternating layers of sand, silt and clay, also known as varves.

The word varve comes from the Swedish word varv, which can be translated to revolution, round, or layer, and is used to describe a layer of semi-annual sediment deposited by a glacial lake.  In CVVC, these varves are typically composed of either dark gray or red clay, or light gray or red sand and silt.  The finer grained clay particles tend to hold moisture much longer resulting in a darker coloration, whereas the sand and silt particles tend to allow moisture to escape more easily, resulting in a lighter coloration.  The transition from one layer to the next is often abrupt, which is characteristic of the rapid deposition of sand and silt particles that occurred during the runoff season.  The individual varves are typically between 1/16 and 1/2 inches in thickness.

Lake Hitchcock existed for many thousands of years, so there can be several tens of thousands of individual varves within a soil profile.  The depth of the varved clay varies drastically throughout the valley, with the thickest sections exceeding 250 feet.  The distribution and thickness of the clay can be seen in more detail on the provided map.

The largest issue presented by varved clay is consolidation.  Fine-grained, cohesive clay restricts the movement of water, and therefore takes a significant amount of time to expel water and consolidate.  The sand lenses give the water a path to escape and significantly reduce the initial consolidation time, but maximum consolidation can typically take up to 10 years.  Depending on the size of the proposed structure, and the clay conditions at the Site, the amount of settlement can be significant and may lead to structural damages over time.

A typical characteristic of the varved silt and clay stratum is that the upper few feet tends to be desiccated, resulting in a very stiff to medium consistency.  This can be advantageous, as this layer is less prone to settlement, reducing the time to consolidate under the applied load.

Several solutions exist to reduce the effects of post-construction settlement, including wick drains, preloading, and post-construction settlement monitoring.  Preloading involves the temporary placement of material to compress the varved clay prior to the construction of a building.  As discussed above, the sand lenses significantly reduce the time rate of consolidation within the varved silt and clay, since they allow excess pore water pressures that are generated when loads are applied to dissipate relatively quickly (allowing the soil to consolidate quickly under the applied load).  Wick drains may be installed, allowing additional paths for the water to escape and further expediting the consolidation process.  Post-construction monitoring helps assess the effectiveness of the pre-load (if applied).  Additionally, if on-going settlement occurs after construction, preventative measures may be taken before significant damages occur.

For large structures such as tall buildings or bridges, it may not be possible to support the structure directly on the varved clay, and deep foundations may be required.  In addition, slope instability may be of concern where embankments are built over the clay (such as highway or railroad embankments) or if deep excavations are made into the clay.  For example, significant geotechnical studies and improvements were required for the construction of the I-91 roadway embankments.

Building on varved clay will always be a challenge.  However, with modern technology and practices it is often not a question of whether it can be done, rather a question of how much it will cost and how much time it will take.  We would love to help you evaluate projects involving CVVC soils, so feel free to contact us at 413-788-6222 or www.oto-env.com.