Answers to your questions about design & construction: FAQ

At our community meetings, you asked questions for our coastal/structural engineers and for the Chicago Division of the U.S. Army Corps of Engineers (USACE). Here are their responses to your questions.

Question. The McLaren Condition Study rates the limestone revetment as pretty good for the Corps' purpose of erosion control. And McLaren's Vulnerability and Alternatives Design reports use the Corps' research to conclude that the revetment needs repair but is safe as is. Has the Corps considered doing nothing and just letting the revetment erode like a natural shoreline? Much like a natural shore on Lake Superior or in Maine? Has the Conservancy considered that alternative?

Answer. Brian Moody, McLaren: The intent of our Alternatives Design Study was to restore and preserve the existing, historic fabric of the structure at the Point. Allowing the revetment to continue to fall into disrepair would allow for the continued loss of upland park area, reduce safety for park patrons, and erode the connection and integrity of the historic structure. Repair and maintenance of the limestone block structure are called for.

Answer. Mike Padilla, Chicago USACE: The Corps studies always include the “do nothing” alternative.  In the case of the 1994 Chicago Shoreline study, that alternative was not selected, probably because it was recognized that the revetment “needs repair” as stated in the question.
###

Question. The original Burnham Plan for the lakefront had islands acting as breakwaters to protect the Chicago shoreline. This seems like a very good solution for minimizing wave forces and erosion damage to the City’s lakefront parks and spaces. There are effective breakwaters left at Steelworkers Park. Has the Corps and the City considered breakwaters offshore these South Side parks like Morgan Shoal and the Point? Has the Conservancy considered this alternative? If not, why?

Answer. Brian Moody, McLaren: Offshore breakwaters could be an effective option at the Point, although not required. If the Corps and the City decided to build a breakwater, the limestone block waterfront structure would still require repair and rehabilitation as well as maintenance.
​
Answer. Mike Padilla, Chicago USACE: The offshore islands were, indeed, part of the Burnham Plan.  However, in the 1994 study, whole islands were not evaluated.  Standard “High Crest” rubble mounds were evaluated in Plan II of the study but were not part of the selected plan.  Those type of rubble mounds, while being overall smaller than the islands of the Burnham Plan, are very large and were not seen to be cost effective.  The team knew by extension that the islands would not be cost effective either. 
###


Question: If the U.S. Army Corps expects its plan to take 5 years for construction, how long would it take to complete repair and rehabilitation work at the Point?

Answer: Brian Moody, McLaren Engineering Group: A lot depends on how you define the start and end of the project. Assuming you’re only talking about the revetment, our plan presented in the Alternatives Design Study, reusing most of the stones in a rebuilt structure, could easily get between five and ten linear feet a day of production completed. (Maybe more, once the work gets rolling.) With 3,050 linear feet around the perimeter of the Point, that could be as quick as 305 working days. (The total linear feet at the Point is 3,150. One hundred linear feet is the concrete ADA ramp at the south most end of the Point at the 57th Street Beach.) So less than one year to complete repair and rehabilitation per our plan if you’re able to work year round. If work must be seasonal, then no more than two seasons. Also, it could be reasonable to perform the work in phases, allowing as much as 80% of the park to be open at any time.
###


Question. In your design plans, please add an easy way for folks to enter the water on both the north and south sides of the Point. I know people aren't supposed to swim there, but we all know that many people do and access is limited for those who aren't overly athletic.

Answer. Promontory Point Conservancy: People have been swimming at the Point since the 1920s, before it was a park: that’s nearly 100 years of people deep water swimming from the north and south sides of Promontory Point. Lake Michigan continues to be Chicago’s best cooling station.

In our Alternatives Design Study, on pages 25-32, we offer Universal Design Access (ADA compliance) upgrades at three locations along the perimeter of the Point. We worked with our coastal and structural engineers McLaren Engineering Group and with Universal Access architect Frank Heitzman on these options.

At the south edge of the Point at 56th Street, the City has already put in an ADA ramp. It’s not particularly easy to get to and the slope is steeper than the preferred 1:20” grade but it does provide access now to the limestone promenade on the south side and the concrete at 57th St Beach. We are proposing some upgrades there to make it more useful for access to the promenade and the water.

At the east end where the coffin concrete platform is in place, we are proposing multiple options for accessibility with graceful ramps down to the promenade as well as for riser-long tread stairs (stramps) for those with walkers and canes. These ramps/stramps would be provide easy access for wheelchairs but also for strollers and children to the promenade and the water.

At the north edge of the limestone revetment, where it connects with the concrete revetment north of the Point, we again are proposing 1:20’, graceful ramps and stramps from the Lakefront trail down to/up from the concrete and limestone revetments. Again, these would be for those in wheelchairs, with canes and walkers, but also strollers and children. The ramps would make it easy for kids, dogs and their families to get down to the shallow swimming area on the north side of the Point.

Finally, in the Alternative Design Study, for example pages 8, 18, 22 and 23, show riprap toe scour protection and alternative toe protection of limestone block steps. These recommendations for toe scour protection in the water protect and strengthen the water facing edge of the revetment. The rubble mound restores the original 1937-1939 structure. But, for access into the water, more limestone block steps down into the water at various places might accommodate.

Right now, we are working with an illustrator to create visitations and renderings to make dry engineering drawings more comprehensible so stay tuned for these. I am attaching renderings the Conservancy did for its 2003 engineering study of the Point which are still viable options to give you some clues to what we’re talking about.

From what we know about the USACE/CDOT plan, access to the water would be prohibited. The concrete revetment would provide ladders into and out of the water but are really for someone who falls in, not for swimmers.

###

Question. What are the advantages and disadvantages of constructing a revetment with limestone versus concrete?
Answer. Brian Moody, McLaren:
Advantages of limestone

• Limestone is the material on site which is a huge advantage. It is the preferred historic material and avoids demolition and sourcing and delivery costs for construction.
• The service life of limestone block revetment is typically 70 years whereas the service life of concrete is typically 40 years. Limestone seawalls have a proven record of use. We estimate that the use of stone material will provide almost double the useful life as compared to reinforced concrete. Current technology, such as micro textiles and marine mattresses, also allows for proper drainage of the subgrade, prevents the displacement of stones which is beginning to happen now due to erosion of the subgrade material and could extend the service life to 100 years.
• Limestone is more durable than concrete over time. Limestone blocks make a more flexible structure that expands and contracts with temperature changes and absorbs wave energy, thereby sparing the parkland the brunt of storms.
• Using small, pivoting cranes to reposition limestone blocks minimizes damage and removal of trees in the park.
• Limestone breakwater blocks have a more natural appearance making the Point look more like the coast of Maine or Lake Superior.
• Repair and rehabilitation of the existing limestone block structure ranges from $55m-$93m and is overall cheaper than demolition and replacement with concrete. And maintenance of the limestone revetment is cheaper than concrete.

Disadvantages of limestone

• There are a few dozen limestone blocks that need replacing with new in-kind materials. Although it is not known what inventory exists off the east coffins or at other locations in the City, sourcing new blocks and transporting them would be an additional cost. New blocks do NOT have to be quarried because the limestone breakwater blocks used at the Point are waste materials from limestone quarries in Southern Indiana. (The breakwater blocks are not the uniform, buff color blocks desired for buildings which constitutes about 20% of the stone quarried. Therefore, 80% are classed as breakwater blocks or for other uses.) These blocks can be sourced for as cheap as $240/block and transported for about $500/block, bringing the total cost per block under $1,000. (A typical buff block runs $2,000 before transport.)

Advantages of concrete

• Reinforced concrete as a building material can be constructed into almost any shape. This is an advantage for new construction were the limestone block revetment to be demolished.
• Near-universal availability and familiarity of the workforce with concrete.

Disadvantages of concrete

• The steel reinforcing in the marine environment will eventually corrode. The corrosion process of steel results in expansion of the corroded material and ultimately causes cracking and breaking of the surrounding concrete. Apparently, this is what has happened to the concrete revetments on the Northside where the concrete unraveled after the 2020 storms at half life.
• The service life of concrete is typically 40 years whereas the service life of limestone block revetment is typically 70 years.
• Necessity of building an exposed structure having many contact points between steel and concrete. If limestone cladding veneer is added to the concrete revetment, then it adds more points of contact for erosions and deterioration which more quickly compromise the concrete and require greater maintenance expenses.
• Concrete is a CO2 polluter and contributes to urban heat index. Concrete will discourage the fish life that currently thrives off the Point and, without fish, will drive away the night herons.
• Requires clearcutting of the trees along the revetment for demolition and construction, and some in the meadow for staging.
• The City estimates the cost of demolition, new construction with concrete with limestone cladding at $100m. This design is considerably more expensive than repair and rehabilitation of the existing limestone block structure.
• Monolithic form that cannot be modified. Uniformity of design and surface. Unnatural appearance.
• As evidenced by major repairs made to the Northside’s concrete revetments after the 2020 storm, mid-term and long-term maintenance and replacement costs are specific to the concrete design, and are greater than maintenance of the limestone revetment.

###

Question. With water levels low, you can hear waves splashing underneath the concrete coffins. Is Promontory Point going to fall into the water? Is it going to wash away?
Answer. Brian Moody, McLaren: The coffin structure is massive (approximately 5 feet thick) and spans between rows of continuous timber piles. This reinforced concrete structure exhibits only minor deterioration and is stable even with the voids beneath the platform.We do recommend that these voids be filled. In our Alternatives Design Study, we offer a grout bag solution to fill the erosion cavity beneath the coffin platform. But Promontory Point is not going to fall into the water anytime in the foreseeable future.
###
Question. Why did the existing structure fail? Is it the result of poor design, poor maintenance or some other factor?
Answer. Brian Moody, McLaren: The structure has not failed. McLaren and Wiss, Janney, Elstner Associates did extensive above water and below water analysis of the existing structure to determine that, in fact, it functions in overall good condition. Deferred maintenance has resulted in some erosion of the subgrade material under the promenade causing the settling of the limestone blocks. And the erosion in the parkland is caused by lack of adequate drainage of water from the blown spray of storm waves. Over time, this erosion of the parkland has not been replaced with new topsoil and grass seed. The revetment and promenade can be economically repaired using current technology and techniques which will increase the resiliency and useful life of the structure.
###

Question. Is the settling of the stone structure related to wave action or to compaction?
Answer. Brian Moody, McLaren: Where settling of the individual stones is observed on the promenade portion of the structure and at the top of the stepped stones in the parkland, wave activity and stormwater runoff has made its way into the soil, under and behind the stones. As water returns to Lake Michigan, it carries fill material — whether the subgrade material under the promenade blocks or parkland — out slowly over time. This erosion process leads to displacement of the stones. Using filter fabric and properly sized gravel subgrade can allow for water to pass through the soil, under the stones, and back into the lake without causing similar erosion and undermining of the stones. Our Alternatives Design Study recommends use of these materials to stabilize and lengthen the life of the structure.
###

Question. Is it possible to build a limestone revetment that will last?
Answer. Brian Moody, McLaren: The limestone revetment has lasted nearly 100 years with little or no maintenance. That’s pretty good. Any structure built on the waterfront will require maintenance and upkeep over time. Rebuilding the revetment as described in our Alternatives Design Study will provide a more resilient structure, which can last indefinitely with periodic maintenance.
###

Question. Can the limestone revetment be structurally supported without the use of steel sheet pile?
Answer. Brian Moody, McLaren: There are several ways to reconstruct the Point that will provide the same look as was provided 86 years ago. The purpose of the piling, whether timber or steel sheet, is to keep the subgrade material in place under the promenade and protect it from wave force scour. With the piling protecting the subgrade material, the block promenade stays stable and interlocks into a strong structure. The Alternatives Design Study explores several options for the water facing edge of the promenade including replacing the timber piling, which is a perfectly good solution when protected, and reinforcing the existing timber crib with concrete. Another alternative in the Study uses steel sheet pile rolled out along the existing crib structure. All of our alternatives maintain the historic fabric of the limestone block revetment and promenade.
###

​Question. Is there danger of flooding DuSable Lake Shore Drive from overtopping waves at the Point? Climate change would seem to predict bigger waves and bigger storms.
Answer. Brian Moody, McLaren: No. There is no evidence that overtopping waves have damaged the Drive or the Point. There is erosion damage from loss of subgrade material beneath the promenade and lack of adequate drainage in the parkland. From the publicly available materials from the Army Corps and CDOT, there is no evidence that these agencies see flooding danger or damage at the Point. Our Coastal Vulnerability Analysis of a repaired limestone block structure shows that the structure meets the USACE and the City’s extreme wave conditions of +6.8 (2020 storm) and a non-historic +7.7 design wave for coastal resilience.
Promontory Point Conservancy: It’s hard to predict the effects of climate change at the Point. Right now, water levels are below average and falling due to increased evaporation, possibly caused by warmer winters and less ice coverage. It’s difficult to predict.