Extreme Building Damage Caused by Yazoo Clay


Every time I look at this picture I can't help but compare it with earthquake damage. The end result can be very similar. Geologic deposits like the Yazoo clay are often called "highly active" and the movement is not sudden like an earthquake. You get a little bit every day and some days you get a retraction where cracks close, a door that would not close last month suddenly works fine. But when you get heaving (or uplift) in high-rise construction, the second floor tries to resist the movement and the first floor behaves like it's being squeezed in a giant vice.

Extreme damage to structures can result from expansive soil movements. This photo is a reproduction of a Polaroid that I took before I went to the University of Texas. It shows the first floor of a high-rise building in Jackson Mississippi. This might be the most dramatic photograph of interior building damage from expansive soils ever taken. Each floor of the building was a flat plate structural slab (self-supporting). The first-floor slab was constructed with an undrained 6-inch open void underneath. However, by the time the building was 8 years old, the void had closed and the first floor had heaved as much as 8 inches. The problem was that the expansion potential (uplift) of the Yazoo clay was seriously underestimated. 


Expansive soil damage to the first floor of a high rise
building in Jackson Mississippi.

The floor of this room on the 1st floor was a structural slab that was supported by the same concrete columns that supported every floor. When it was built this slab had a 6" inch void underneath. But 10 years later heaving of the slab caused the structural slab to disconnect from the columns in an odd reverse punching shear failure. A punching shear failure is when a structural slab shears away from a supporting column. Normally the slab falls down; but in this case, the slab moved up (in reverse) from the expansion of the clay. The columns were supported by deep drilled piers and were not damaged. Fortunately, this part of the building structure was stable.

Damage seen in this photo is the result of crushing metal stud partitions (in compression) that were constructed from the first floor to the bottom of the second floor slab. The damage was so severe that most interior doors had been removed on the first floor. In the left side of the photo is a door that has been replaced by tapping sheets of brown wrapping paper over the opening (a common solution). The door near the center of the photo and the one on the right has been removed. In the room in the foreground partitions have failed by rupturing the wallboard. In the background the wallboard buckled and disconnected from the studs.

In Central Mississippi, this kind of construction represented the first efforts by engineers to isolate the bottom floor (using structural self-supporting slabs) from the expansive soils in the 70s, and early 80s. Numerous buildings were built just like this. I doubt that any of them still exist like they were originally built. We now know that a typical rate of heave for a normal soil profile with Yazoo clay is about ¾ inch per year. So a 6-inch void like this building couldn't last more than 8 years in a typical setting. In this particular case, the expansive clay was exposed at the surface, there was poor drainage, and it potentially heaved at a rate of more than double the amount from a typical condition. That is the highest heave rate that I know of in Central Mississippi.  One end of the building was built several feet below the surrounding natural grade. So construction required the removal of natural inactive soils above the clay. This created an unloading condition that promoted rebound of the expansive clay. This construction also created a condition of poor drainage around the structure.  Surface drainage that was constructed rapidly deteriorated from heaving that occurred in the ground around the building.

To repair the building, the entire first floor, and the slab was completely removed and replaced when this building was about 9 years old. As a result, this building is still in service today. The repair involved increasing the void space size to 30 inches. However, clearances under plumbing in the crawl space had to be recently dug out again in part of the crawl space. At the other end of the building, the new crawl space was still intact and some shrinkage had actually increased the crawl space by a few inches.

Unfortunately, buildings on drilled piers like this one today are still having issues with Yazoo clay in Central Mississippi. My studies of these newer buildings indicate these primary issues:

1. Inadequate clearances specified for utilities or specific elements of the foundation.

2. Inadequate retainage to prevent soil flow into the crawl space or voids under grade beams.

3.  Inadequate drainage of the void spaces or crawl spaces.

4.  Inadequate construction review of void or crawl spaces.


All rights reserved by Britt Maxwell P.E.

Historical Meteor Shockwave Events That Destroyed Structures

This blog is Part II of a three-part series about possible meteor shock wave damages to structures. It was updated on 8/18/2019. Part I is about "Google Earth Photos taken the same day as the Chelyabinsk meteor event on February 15, 2013. This blog (Part II) is about structures that have been destroyed (as a result of meteor events) as reported in news since 1900. Part III will be an analysis the Chelyabinsk Zinc Plant Warehouse collapse. I started out to just blog about the warehouse collapse but I decided to put this event in historical context with other meteor events reported in the news.

Before I start, I should explain some of the terminologies. A meteoroid is an object like an asteroid but smaller than 1 meter. A meteor is the visible streak of light seen in the sky made by a meteoroid as it impacts the atmosphere. A meteorite is a fragment of a meteoroid that has hit the surface of the Earth. This blog was inspired by the historic meteor event at Chelyabinsk Russia where a substantial industrial building collapsed under the influence of a shock wave. I began research to see if there were other similar events. I was surprised to find five events with reports of the destruction of houses that were apparently caused by meteors since 1900. In these reports, there was an understanding that a meteor was involved, but that somehow a meteorite must have hit the structure, although no meteorites were found with any of these events. In three reports the damage is related to the word meteorite, but no meteorite was found, and in two of the reports the phrase "meteoric stone" or "meteoric shower" is used, but again no meteorite was found. Two of these events were criticized as "meteorite events" and I think appropriately so, and I provide links to these criticisms. Where's the meteorite if these structures were destroyed that way? It is important to realize that when the first 4 events happened there was no scientific understanding about meteor airbursts, so a meteorite was the only possible explanation. Airburst science was not developed until the 1950s and early '60s. I think that these events I list below are all shock wave damage events and these events only make sense if they are interpreted in that way. So I want to make it clear that this is my interpretation of these events. 

Prior to the historical meteor event at Chelyabinsk Russia in February, I think some scientists discounted these accounts of meteor events where houses were destroyed. Maybe they still do. One problem is that in only a single account (where a structure was destroyed) prior to Chelyabinsk were scientists were reported to be involved in the investigation. Because of the event at Chelyabinsk in February and the event at Tunguska in 1908 we have some first-hand accounts of the effects of destructive meteor shock waves that represent real events. We also can look at well-documented accounts of meteorite impacts with houses and other structures where the meteorite is found. There are lots of these events. Most recently on April 19, a meteorite crashed through the roof of a house in Wolcott Connecticut. That is the third meteorite to penetrate a roof in Connecticut since 1971. The two previous meteorites in Connecticut damaged houses 1.64 miles and eleven years apart. One thing seems clear in these events and is that the damage was light enough so that the house could be quickly repaired and more importantly the meteorite in these cases was fairly easy to find. Halliday, et al. (1985) states that "it is clear that an impact on a building greatly increases the probability of recovery of the meteorite since only a small percentage of falls will strike buildings, whereas half of the recent recoveries have such involvement." In a couple of instances that I found, the meteorite penetrated the whole house and was found in the crawl space. 

If we compare these real meteorite events with the events I list below, we must conclude that they are shock wave events, because the damage is much greater than would be expected from a typical meteorite and yet no meteorite is found.

An important test of a shock wave event is that someone saw a bright meteor and reported a loud bang or a series of bangs that are commonly associated with cannon fire by witnesses in older accounts. Based on that test only 2 or 3 of the 6 events (I list below) where structures are destroyed would pass; an event in Mississippi, Chelyabinsk and maybe the Jakarta event. But as a structural engineer, I see a potential public safety issue that I feel we need to look at, so I want to err on the conservative side consider all reports of structures being destroyed that have not been proven to be false. There is another potential problem and that is, prior to 1900 there were unscrupulous newspaper accounts of meteors that were made up to help sell papers. I could not find any evidence of this happening after 1900 so I decided to start the inventory of meteor events in the year 1900.  Scientific papers written on structural damage from meteors have been made by Fessenkov, 1955, La Paz, 1958, Graham et al., 1985 and Yau, K., Weissman, P., & Yeomans, D. (1994) listed 10 reports of structural damage in China from 588 to 1879.

The following is an account in the New York Times of what I think is a shock wave that destroyed a large house in 1900 in Mississippi. This event and the event in Shiraz in southwestern Iran are not included in any lists of historic meteorite events that I have seen. I think they were excluded because if it is stated in the report that no meteorite is found then the event is not relevant. But I could not find where anyone has ever compiled an exclusive list of meteor events where structures were reported as being destroyed. Someday I will expand this list to before 1900 and include hut structures that were knocked down in a separate category. 

METEORITE IN MISSISSIPPI. Visitor from the Heavens Explodes and Wrecks a House. Special to The New York Times. NEW ORLEANS, La., July 12.—The little village of Bellefontaine, in Webster County, Miss., thirty miles in the interior from this place, was the scene last night of the fall of an aerolite, or meteoric stone, which completely wrecked the large storehouse of Hodge & Mabry, and destroyed the stock of goods contained in it. The fall of the aerolite occurred between 9 and 10 o clock, during a perfectly clear moonlight night. The destruction of the building was preceded by the appearance of a ball of fire passing swiftly through the air. It gave off during its passage enough light to greatly increase the light from the moon. As it came near a loud explosion was heard and a shower of fire burst forth from all sides of the blazing mass, having the appearance of hundreds of falling stars. The storehouse was wrecked simultaneously with the explosion. The explosion of the aerolite caused a report like the sound of distant thunder or the roll of far-away cannon. The debris of the house is being cleared away in search of the aerolite... Many cinders of a gray gritty metal appearance have been found in the wreckage.

ANALYSIS of the Bellefontaine Event: This event is listed as a "Meteor Wrong" on the internet, but my research shows that this really happened. This expression is a kind slang term for rocks that look like meteorites but are not. Because Webster County Mississippi has good genealogical records, I was able to find the full name of one of the store owners and then verify that the event happened from a living granddaughter. The store owner was, George Clark Mabry (1874-1952). This is one of the better meteor airburst descriptions that I have read. If anyone can contribute information about this event or other listed events (below) please contact me.

Here is an account of 5 other events (2 with very limited information) since 1900 where houses and one building have been reported to be destroyed by meteor events:

!  In the publication of the Meteoritical Society (Meteoritics 29, 864-871) 1994 there is this report: Sept. 5, 1907, Hsin-p’ai Wei in Weng-li, China meteorite caused a house to collapse, killing a family; “the whole of Wan Teng-Kuei's family was crushed to death.” ANALYSIS: An event scrutinized by the Meteoritical Society, should be a credible event. In a paper by Yau, K., Weissman, P., & Yeomans, D. (1994) it is stated: "The Wan family probably died from the collapse of the house, rather than from a direct fall of the meteorite."

!  The Straits Times (A Malaya Newspaper) on May 17, 1946, reported: Mexico City, Thurs. –The Government announced today that a meteorite destroyed the hamlet of Santa Ana in Nuevo Leon state of Mexico. Eight people were killed and 26 injured. – Reuter. Also in the New York Times 5-17-46 and listed by John Lewis in Rain of Iron and Ice.

!  August 15, 1951, Teheran, Iran near. Sixty-two houses were destroyed by a meteoric shower. Twelve people were killed and nineteen injured. In addition, 300 livestock animals were killed. The event was reported by Iranian newspapers and the United Press. This event is reported to have been published in the Lowell Sun (Massachusetts) on the next day (p.19) but I have not as of yet seen the archives.

!  An event on April 29, 2010, at 4:30 in the afternoon in Durensawit, East Jakarta according to news reports by the Jakarta Globe was investigated by The Institute of Aeronautics and Space (LAPAN) and the National Police's Ballistics and Metallurgy Center (BMC). The head of the BMC said that tests had ruled out a bomb or a gas-canister explosion and said the object was similar to the one that hit in Bone in South Sulawesi on October 8, 2009. "LAPAN ruled out the possibility that the object may have been space debris…" Sri Kaloka Prabotosari estimated the meteorite was 30 centimeters in diameter and had an impact velocity of 10 kps. According to Berita Jakarta, a government website, Thomas Djamaluddin with LAPAN said: "it was not detected by a transmitter, possibly because of its small size."  The blast has only left dust with the color of a bit grayish (as translated). The homes of Sunarti, Sudarmojo, and Marzuki were damaged in the incident with Sudarmojos house taking the brunt of the impact. It blasted a hole in the second floor of the house, sending furniture falling to the first floor, and tore big holes in walls. No one was home when this happened. ANALYSIS: Getty Images owns twelve high-resolution photos of the damage made by Romeo Gacad originally posted on May 4, 2010. Unfortunately, these photos have recently been removed from their website. One, however, was re-posted at WSJ BLOGS(scroll down 14 images). All the photos appear to be of the same Sudarmojo residence. Photos show the roofing material is completely missing above at least 2 adjoining rooms, most of the wood roof framing is still in place but a few pieces are broken and a few appear to be missing and there are piles of clay tile roofing fragments on the floors. One photo shows a large wood (glass?) frame knocked down inside the house. This area appears to be a very high-density living area and it is odd that the damage was limited to only 3 adjacent structures. I looked for photographic evidence for an explosion outside vs inside. I think an inside explosion of this magnitude would have removed significant roof framing and left few, if any, roof tiles inside the structure. Photographic evidence suggests a small air burst above the roof at a low altitude but above the roof, that shattered the roof tiles and knocked them down into the house. Witnesses saw a localized flash at about 4:30 in the afternoon but not a fireball. Investigators shied away from using the expression "shock wave" but compared this event with the larger shock wave event at Sulawesi (6 down in TOP TEN table below) the year before. This was the first event in this list where shock wave science appears to be involved, but the estimated velocity of the impact object is 1.2 km/s slower than the 11.2 km/s the theoretical lower limit of meteors. A blog questioning this event was made by Ian O'Neill.  More on this subject after the following event.

!  February 15, 2013, the Chelyabinsk meteorite was visible in the early morning as a brilliant superbolide that created a series of shock waves that damaged some 7,200 buildings and according to the official Chelyabinsk website, 1613 people were injured in the region and 69 people were hospitalized. It also apparently triggered the collapse of about 1050 square meters or 11,300 square feet of roof at the Chelyabinsk Zinc Plant (see part I of this blog series). The building described as a concentrate storage facility was constructed with concrete columns and a precast concrete roof deck. This is the most substantial building ever destroyed by a shock wave. My previous blog and my next blog after this one will provide more details about this building collapse so I have cut this short.

Principles of physics limit the range of velocities of asteroids and meteoroids when they first enter the Earth's atmosphere. The escape velocity of Earth controls the lower limit (personal communication with Gareth Collins) and the full range is between 72 and 11 km/s (161,000 to 25,000 mph). Hills and Goda (1998) state that "asteroids with diameters smaller than 50-100 m that collide with the Earth usually do not hit the ground as a single body; rather they detonate in the atmosphere." A shockwave can be generated when the combined forces on the meteoroid cause it to break apart. A potentially damaging shock wave can be produced when the object is a larger meteoroid or asteroid. John Lewis (1996) states that the density of air roughly doubles for every 5 kilometers of altitude. He also states that "our normal intuition would suggest that the faster a projectile is moving the more deeply it will penetrate its target. But the exact opposite is true for meteors: the fastest moving meteors produce the highest pressures and are crushed to dust at higher altitude." Because of the high speed of the meteoroid, a pocket of high air pressure in front of the meteoroid is created. When the pressure differential becomes higher than the strength of the meteoroid it breaks apart. When this happens it is sometimes called a bolide or an airburst. The shock wave energy released is dissipated with distance. So a shockwave that begins high in the atmosphere loses most of its energy by the time it gets to the ground. But at the same time, it distributes the energy over a larger area on the ground. These events often have sound effects that can be heard. The larger events rattle windows and the even larger events break windows. On July 23, 2001, a bolide meteor shattered windows in towns west of Williamsport Pennsylvania (see Meteorites Don't Pop Corn).  In the reports I have reviewed (above), we might consider that the smallest meteoroid capable of destroying a structure to be roughly about 30 centimeters in diameter (Jakarta event). However, there is a nifty online "Earth Impact" tool to compute the effects of different size meteoroids with different masses, traveling at different speeds and traveling at different angles. This tool created by Collins, Melosh, and Markus, makes assumptions and approximations that limit the accuracy of the result, particularly for smaller objects. To get more accurate results full computer simulations must be run. So for what it's worth, I used this program to see what the smallest meteoroid was that would produce a damaging shock wave. I went through numerous iterations  and I found the following object:

1.5 meters (4 feet 11 inches) in diameter, a density of 7907 kg/m³,  impact velocity of 11.2 km/s, and an impact angle of 85 degrees. The resulting meteor burst occurred at a height of only 170 meters (559 feet) with an energy of .21 kilotons producing a peak overpressure of 17.4 psi at ground zero. The overpressure drops to 3.7 psi at .5 kilometers from ground zero. This event if it were to happen would be very serious but over a fairly small area. All wood-framed structures would be destroyed over an area of about 2 square miles or (5.4 square km). However, there are two versions of the impact tool and the new version computes different results without an airburst for the same inputs. So one or maybe both tools have an error. So a mini-shockwave event like this may not be real when you include all the physics. But the original tool indicates that the object needs to impact at the low end of possible velocities and needs to be strong and have a density close to that of an iron meteorite. These factors allow the meteoroid to penetrate deeper in the atmosphere before it explodes. The Chelyabinsk meteor, by comparison, was an object estimated to be 17-20 meters (55.8-65.6 feet) and hit at an angle just under 20 degrees and was initially traveling at about 30 km/s (67,100 mph). It only had a 10% meteoric iron content and it slowed to 18.6 km/s (41,600 mph) when it burst at a height of 23.3 km (14.5 miles).

A list of air burst events has been listed on the internet at en.wikipedia.org. I have reproduced this table showing only the top ten events and sorted them according to the average estimated yield. By doing that the Chelyabinsk event shows to be #3 and it has widely been reported as being #2. The problem is that there is a very wide range of estimates for the Curuçá River event. A Tunguska type event is believed to occur once every 1000 years and events like the Sikhote-Alin occur every 2 years somewhere on Earth.

                                   TOP TEN SHOCKWAVE EVENTS

EVENT	DATE	YIELD (kiloton's)	AVG.YIELD
Tunguska	6/30/1908	10,000-15,000	12,500
Curuçá River	8/13/1930	100-5,000	2,550
Chelyabinsk	2/15/2013	500	500
Offshore, S Africa	8/3/1963	176-356	266
Arroyomolinos de Leon	12/8/1932	190	190
Sulawesi Indonesia	10/8/2009	31-50	40
Eastern Mediterranean	6/6/2002	12-20	16
Queen Maud Land	9/4/2004	12	12
Marshall Islands	2/1/1994	11	11
Sikhote-Alin, Russia	2/12/1947	10	10

By studying the videos and pictures in the area of Chelyabinsk we can see the effects of shockwave damage. The exterior of a structure that protects it from wind and rain from a structural perspective is called the envelope. Once the envelope is damaged (from doors blown open or by windows blown out) by a shockwave, pressure changes occur inside the building. Interior doors can then be blown open and drop ceilings can be damaged by the pressure changes. Once the envelope is compromised, flexible walls, ceilings or floors can act as a membrane and bounce under the impact of a shock wave. This matches the description of the energy of the impact at Jakarta.

SUMMARY

The following is a summary of the events listed above that I think are shockwave events that mostly occurred before the science was developed:

                                     POSSIBLE SHOCKWAVE
            EVENTS SINCE 1900 THAT DESTROYED STRUCTURES

			NUMBER OF
EVENT	DATE	FATALITIES	STRUCTURES
			DESTROYED
Bellefontaine Mississippi	7/12/1900	0	1
Hsin-p’ai Wei, China	9/5/1907	a family	1
Santa Ana, Mexico	5/17/1946	8	a hamlet
Shiraz, Iran	8/15/1951	12	62
Jakarta, Indonesia	4/29/2010	0	1
Chelyabinsk, Russia	2/15/2013	0	1

We don't know the number of structures destroyed in the 1946 event and we don’t know the number of people killed in the 1907 event. But if we add all the occurrences together, we get a total of 6 events since 1900 with an average occurrence of a meteor with a shock wave that destroys at least one structure every 19 years. In 67% of these events, only a single structure was destroyed but in the remainder, maybe 3 to 62 houses were destroyed. On average about 11 to 15 structures will be destroyed by the event, and the fatality rate per event (when structures are destroyed) is 4 to 5. We can compare these possible shock wave events with known damage from meteorites. Halliday et al. (1985) estimated that 16 buildings per year could receive some damage from meteorites that weigh at least 500 grams (1.1 pounds). Adjusted to the population of Earth in 2013 this becomes 23 buildings per year damaged by meteorite impacts. They used a 20 year period in North America as a sample interval.

The fatality rate from meteors, in general, is a little higher because only events destroying structures were included (above). But if we add in all meteor events since 1900 we get maybe 2 or 3 more fatalities. There are conflicting reports of one or two deaths related to the Tunguska event in 1908 and a male guest was killed in a bridal party in Zvezvan Yugoslavia while riding a carriage in 1929 by a 40-centimeter meteorite according to a New York Times article. If we add up all the fatalities and assume 3 were killed in 1907 we get a total of 24 or 25 deaths since 1900 with one person killed every 4 ½ years on average. Every one of these fatalities except maybe the one in Yugoslavia I think are the result of shock waves and not being struck by a meteorite. But the occurrence rate of destroyed structures and fatalities must be related to the population of Earth that is now over 4 times as high was it was in 1900. Also note that in the Mississippi, Chelyabinsk and Jakarta events no one was killed, but probably because there was no one in the structures when the shock wave hit. Yau et al., 1994 used data from China to compute that a fatal incident will occur somewhere on Earth every 3 ½ years, adjusted for Earths population change that would be an incident every 2.8 years today. For comparison, I find 5 incidents worldwide since 1900 where someone was killed that is an average of once every 22.5 years. If I normalize that for a 4x population increase, we might predict an incident with at least one fatality every 9 years.

CONCLUSIONS

I have reviewed 5 events of destruction to residence type structures reported to be caused by meteorites or "a meteoric shower". However, these are all "missing meteorite" stories where the damage is much heavier than would be expected from a meteorite. I interpret these events as shockwave or airburst events. It is important to realize that the first four events happened before the science of meteor shock waves was developed so there was no chance for these events to be correctly understood when they happened. They also predate the sensor grid. The Chelyabinsk meteor event is the first event in history where a structure was destroyed and there is an official meteorite associated with the general event.  And it appears that the damage here was not caused by a meteorite object, but by a shock wave. The Jakarta Event just 3 years ago was not picked up by the worldwide grid of sensors. From personal communication with Peter Brown, University of Western Ontario; he would not rule out this as a shock wave event but added "I think it is unlikely that an airburst capable of producing significant overpressures at ground level would go unrecorded overland (or even the ocean). There are some 20 photos and several newscasts on Utube of this event and I interpret this event as a small, low altitude, airburst mainly because roof tiles were shattered and knocked down into the house. On average 23 buildings per year could be damaged by meteorites but none are destroyed. Meteorites maybe could destroy structures, but the occurrence rate must be considerably larger than 20 year study period used by Halliday et al. (1985) and larger than the 113 year study period I used.

I think all of the meteor related fatalities since 1900 except one, were caused by shock wave events and that most of these deaths are related to the collapse of a structure. It appears that only the fatalities at Tunguska and Zvezvan, Yugoslavia were not related to a structural collapse. It is possible that some of these shockwave events were large events that created limited damage because they occurred in sparsely populated areas. 

From this study, I have determined that on average 23 buildings per year are damaged by meteorites but none are probably destroyed. However, shockwave events that destroy at least one structure appear to occur once every 19 years. The average fatality rate from all meteor events since 1900 is one every 4 1/2 years with only one attributed to a meteorite. If these rates are anywhere near correct then we are living in denial of the potential damage to humanity from meteor shockwave events.

In the future structural damage events (like the ones discussed here) where no meteorite is connected with the damage, needs to be studied by engineers that understand blast damage. A damage map should be made that shows blast displacements. This helps us to understand the forces at work and ultimately the energy of the blast. The specific science that addresses this kind of damage is called structural dynamics. The key evidence is that a meteor shock wave will create a higher overpressure outside the structure and thus weaker parts of the structure (like doors and windows) will be knocked inward. It is also essential to collect witness reports that include descriptions of the sounds, the track of the object, vibrations, damage, and location of the observer. There are online forms to report witness information at the International Meteor Organization, the Canadian Space Agency's Meteorite and Impacts Advisory Committee, and the American Meteor Society. I  also see the benefit of mapping glass breakage events because it is a direct result of the energy released. Also, did the glass just break or was it knocked out of the frame like we saw at Chelyabinsk?

These reports since 1900 indicate that a shock wave event will destroy one or more structures once every 19 years or so. If these events are real, the greatest danger to human life and to the structures we build, come from shock wave events and not meteorites. Also, these events must result from objects smaller than the one that hit Chelyabinsk because objects that size hit Earth about once every 90 years. Depending on the structure, the event must produce an overpressure at the ground level of 3-4 psi for a structure to be destroyed.

REFERENCES

1. BROWN, P., SPALDING, R.E., REVELLE, D.O., TAGLIAFERRI, E., &WORDEN, S.P. (2000). The flux of small near-Earth objects colliding with the Earth. Nature. 403:165-166. doi:10.1038/35003128

2. COLLINS, G.S., MELOSH, H.J., AND MARCUS, R.A.,(2005). Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth. Meteoritics & Planetary Science 40, Nr 6, 817–840.

3. HALLIDAY, A. T. BLACKWELL, and A. A. GRIFFIN. (1985). Meteorite impacts on humans and on buildings. Nature. 318: 317. doi:10.1038/318317a0

4. HILLS, J.G. & GODA, P. (1998). Damage from the impacts of small asteroids. Planet Space Sci. 46, 21-19-229.

5. LEWIS, JOHN (1996). Rain of Iron and Ice. Helix Books. 236. ISBN 0-201-48950-3

6. SCHIERMEIR, QUIRIN. (2013). The death of the Chebarkul Meteor. Nature. doi: 10.1038/495016a

7. SPRATT, CHRISTOPHER. (1991). Possible Hazards of Meteorite Falls. Roy. Astr. Soc. Canada, 85, 263-280.

8. Yau, K. Weissman, P. and Yeomens, D. (1994) Meteorite falls in China and some related human casualty events. Meteoritics 29, 864-871.

Google Earth Photos on the Day of the Meteor at Chelyabinsk

By Britt Maxwell

This blog is Part I of a three part series about meteor shock wave damage to man made structures. Part II is about structures that have been destroyed (as a result of meteor shock waves) as reported in news since 1900. Part III will be a detail analysis of why the Chelyabinsk Zinc Plant Warehouse collapsed.  

Google Earth Image of Zinc Plant Warehouse at Chelyabinsk

Google Earth (GE) has posted satellite photos of Chelyabinsk and Lake Chebarkul that were made on February 15, 2013, the day a meteor blasted this area with a devastating shock wave. Presently you can only see these images by selecting the image history tool. The first photo is of the Chelyabinsk Zinc Plant Warehouse where the roof collapsed. It shows us for the first time exactly how much of the roof collapsed. The fallen roof area is black in the photo because it is now just a shadow. You can tell from the photos made on the ground that a significant part of the roof fell beyond where the brick wall fell on the west side of the building. But now we can see that a small part of the roof on the south end of the building appears to still be intact. Using the Google Earth measurement tool we can see that roughly a roof area of 21 x 50 meters collapsed. This is 1050 square meters or 11,300 square feet.  

In the GE image, the street traffic seems to be normal. Earlier in the day after the meteor hit, vehicles on the west side of street were forced to cross the median to avoid brick debris in the street that came from a partial collapse of the west exterior wall of the Zinc Plant Warehouse (see photo below). This is an important photograph because it shows us exactly what the debris looked like before the clean up crews had arrived. My dimensional analysis of the site shows that the brick was blasted some 70 feet from the building by air pressure that was trapped inside the building when the roof fell. I will include more details on this in Part III.

Until now we did not know if there was snow load on the roof. The GE photo also shows us that there apparently was no snow load on the roof when it collapsed. At least we can tell that the remaining roof on each side does not have snow on it. Apparently the roof surface was dark enough to melt the snow that can be seen on other roofs in the area. This is significant because snow load can be a contributing factor to roof collapses. I had been thinking that snow load added to the meteor shock wave had caused this collapse, but now it looks like snow was not a factor. The best view on the ground of the snow conditions on other roofs can be seen in two nice 360 degree panoramic images that have been uploaded by Rustam Gadrakhmanov at 360cities.net. These photos clearly show some roofs with snow and some without.

Google Earth Image of Meteor Hole in Lake Chebarkul

The whole world now knows the exact location of the hole in the ice at Lake Chebarkul made by the meteor thanks to satellite images posted on the Internet by astrium-geo.com and by the Google Earth images of the area. The image at astrium-geo was made a week after the meteor on February 22 by a Pléiades satellite. Below is a screen shot of the meteor hole from Google Earth made the day the meteor hit on February 15. In this photo there appears to be 4 vehicles and people walking around the larger meteor hole. These maybe the first aerial images of a meteor crater on Earth created in ice. There is a serious effort underway to retrieve the object that made this hole. In this pursuit a 3-D radar image of the bottom of the lake has been made. Another crater on the bottom about 10 meters away from the surface crater was found. Scanning the lake area you can see other locations that attracted attention. A second round hole in the ice that is much larger can be seen about 2.8 kilometers to the northeast. Since nobody was paying attention to that, I assume this hole was created by some other means. It looks like there is some kind of warm water runoff going to this spot from a drain pipe.

In closing I have to say that a lot important information was released with these photos that can help scientists unravel the details of this remarkable event. The Google Earth team seems to make a concerted effort to release images whenever a disaster occurs. You typically have to wait a few weeks, but eventually the images will appear. I first utilized this remarkable tool when Hurricane Katrina ravaged Mississippi and Louisiana in 2005. Since then the Google Earth team has uploaded images of all the significant tornado events that I have researched. They also posted images of the tsunami disaster in Japan in 2011. Good work and keep it up.

Designing and Repairing Slab Foundations built on Yazoo Clay

by Britt Maxwell P.E.

The Yazoo clay is an Eocene epoch expansive clay deposit that outcrops in a tilting band through Central Mississippi. Structures built over this clay with improper foundations occasionally produce over 12 inches of differential movement. The movement rate is a function of the depth to the top of the clay and available moisture. In this blog, I will talk only about residential slab construction and I will update this blog as new information becomes available. Because this blog has been very popular I have updated it several times. Updates were made on 6-22-2018, 10-29-2018 and 5-19-2019. I have also added a companion blog for drainage issues DRAINAGE FOR YAZOO CLAY. (See Blog Archive in the panel on the right side click on the arrow to expand. It is my Dec 2019 post.)

There is an interesting cosmic connection with the Yazoo clay that I think you will enjoy, but I will leave that for the end of this blog. There are a lot of ideas about construction and Yazoo clay so I will use the most reliable information available (data subject to peer review). I start with four myths about slab foundations built on Yazoo clay.

·       If there is 7 to 8 feet of select fill over the top of the Yazoo clay, it is believed that the expansive soils have been effectively mitigated. FALSE: Not even close (I explain below).

·       It’s a good idea to water your foundation. FALSE: This may work in other areas of the country, but in general it’s a bad idea with the Yazoo clay. There are a few exceptions but, in general, drainage improvements are a better strategy.

·       Underpinning your foundation will stabilize it. FALSE: Sometimes this is true, but if a foundation is heaving (uplift) from expansive soils, underpinning will not stop the movement. The key point is that expansive soils can cause an upward movement (heaving). When these movements are serious, a different approach to foundation repair is needed.

·       Some foundations cannot be fixed. FALSE: Any foundation can be repaired if you are willing to spend the money for the necessary repair. A qualified structural engineer can if necessary, design a replacement foundation, if the original foundation was not suitable for the soil conditions or the site. See my Example Damage Blog with this photo: 

The first myth is very understandable because 8 feet of fill is commonly specified (locally) for the construction of slabs for residences and small buildings by geotechnical engineers. Locally, a layer of soil above the Yazoo that is not expansive is called a “buffer.” It is also called “the cover.” If the cover is manmade then it is select fill otherwise, it is just a natural deposit, usually silty clay. Based on foundations designed in Mississippi during the 1950's and the 1960's that I have seen, some engineers of that era also misunderstood the real effect of the buffer. But at the time there was very little data to make this determination. Foundation designs have definitely evolved in response to an improved understanding of the real behavior of Yazoo clay.

The buffer is very important but it does not remove the threat of Yazoo clay unless it extends much deeper. There is adequate evidence from detailed published studies of foundation failures that show this (see reference "Field Measurements of Yazoo Clay...).  The actual purpose of the buffer is to reduce the differential movements in a slab foundation created by the expansive soil to within acceptable limits. When I say a slab foundation, I mean a typical stiffened slab (with beams built into the slab) without deep drilled piers or piles. The use of a buffer is a legitimate strategy for the design of shallow foundations on expansive soils. Locally the depth of the buffer that engineers have specified has changed. In the 1970s and early 80's we typically saw 6 feet as the required thickness of the buffer. In the last 15 years or so we now often see 7 or 8 feet. For slabs built more than 18 inches above grade, a thicker buffer is needed. The same applies to slabs built below grade.

If you want to completely negate the primary effect of Yazoo clay you need about twice the depth of the typical buffer that we use today. The depth of buffer material needed to remove heave potential is called the active zone. At one site in Madison County, the depth of the active zone was 12.7 feet. At two other sites, it was deeper than that, because related movements were detected where the clay was at that depth. There are different ways to determine the active depth of an expansive soil, but I have relied on field measurements of movements as opposed to theoretical methods.

An important fact is that you can not always count on a buffer of 7 or even 9 feet to keep foundation movements in an acceptable range. So in addition to the buffer, we must also minimize moisture infiltration around the structure. Infiltration of moisture is minimized by using a fill above the clay that has a low soil percolation rate (low permeability) and positive drainage away from the structure.
So if you use this overall strategy the chances for undesirable foundation movements are almost eliminated. Do we accept a small percentage of buffer failures or do we analyze these failures and try to prevent them? It is clear that with all these issues that the foundation is not an area where you would want to cut corners when there is Yazoo clay on the site. Yet today many homes in Central Mississippi are built with a 4-inch post-tensioned slab when the remainder of the United States prefers a 5-inch slab when an expansive clay is present. So require the builder to use a 5-inch slab and hire an experienced engineer to design the slab and to check the reinforcement placement. That experience should include commercial foundation design and repair. We know from  past failures and it is clear that there is a higher risk for foundation movements when the buffer strategy has been used in the following circumstances:

·      The larger the floor footprint area of the structure, the higher the chance for a problem.

·      The more complex the floor plan, the higher the chance for a problem. This is now a part of the Post Tension Institute design procedure. The simplest plan is a square; the more complicated are long narrow wings. The more complicated floor plans will actually kick you out of the PTI design process.

·       Hillside sites that are terraced to create a level lot. The highest risk sites often have retaining walls or should have been built with retaining walls because of the steep slope. With these sites maintaining positive drainage away from the structure on the uphill end of the structure is critical. There are countless foundations failures where the surface drainage on the uphill end of these structures was inadequate. One issue often ignored is that the yard at this location may heave (degrading the slope). So if you construct minimum drainage in a few years it could be gone.

·      There seems to be a higher risk when the boundary between the buffer and Yazoo clay is natural (not man-made). The alternative is to over-excavate and cut into the Yazoo and then replace the natural buffer material creating a man-made inclining boundary above the Yazoo clay that will shed percolating water.

There are other aspects but all of these risks have a connection with each other. That is, with each one of these situations there is an increased risk that water at some point around the foundation will work its way down to the expansive soil. This is clearly a critical issue. Once water infiltrates down to the clay there becomes a potential for upward movement or heave at that location. In the worst case, a perched water table develops at that location. A natural surface for the top of the Yazoo makes this more likely because there can be subsurface pockets (dips) for water. A perched water table is to Yazoo clay like a match is to gasoline. A perched water table is a water table that develops in the buffer material above the Yazoo clay.  Since the permeability in the clay is very low, water can stand on the surface of the clay just like water in a pond. Unfortunately, this condition could exist on a site and not be discovered by the soil borings. If the soil above the Yazoo also has a low permeability then water from the water table might not flow into a boring hole until it has been left open and monitored. The situation is complicated because the surface of the Yazoo clay is an ancient eroded surface that is uneven and might contain old buried stream channels. If you have a serious foundation problem there is a very good chance it is related to a perched water table. As a result, we have to look closely at all the complex elements that produce these conditions.

Field measurements made so far have given us a good hint of the kinds of movements that are possible with Yazoo clay where there is an established water table. Unfortunately, there is not enough solid data for us to understand how much variability exists. We recently measured 17 ½ inches of differential heave in a slab that was constructed during the mid-1970s. In this case, the Yazoo clay was only about 4 feet from the top of the slab and there had been apparent problems with plumbing leaks for years. The historic Manship House was releveled in1980 but by 2010 it was 13 1/2 inches out of level. At another site, we were able to compare the rate of heave over 5 years over a large area where the depth of the clay varied from about 7 feet to about 14 feet. The rate of heave at 7 feet was just over ¾ inch per year using regression on the data. The rate of heave when the buffer was 9 feet was ½ inch per year. If we apply this movement at a single spot to a residence and we define a failure to occur when it is 5 inches out of level, then failure occurs at about 6 ½ years with 7-foot buffer and failure occurs in 10 years with a 9-foot buffer. In these scenarios, there is the assumption that there is only one point under the structure where water gets to the Yazoo clay. Fortunately, this is rare, but it does happen. This shows us how critical a plumbing leak or poor drainage can be.


 RECOMMENDATIONS FOR BUILDING HIGHER RISK STRUCTURES:

1. Make at least 3 soil borings roughly inline along each segment or wing of a structure to see how much variability occurs in the geology. With only 2 borings we assume a steady transition between the borings and we never see the real transition. If part of the structure is to over an area where the existing grade will be cut, then a boring is needed here at the exterior face where the cut is deepest.

2.  If the expansive clay is found above the active zone in a boring, 1 or 2 borings need to be left open at the low elevations of the site (but in the footprint of the structure) for a long enough period to determine if a natural water table exists. Some of the local geotechnical engineers already do this.

3.  Constructed surface drainage can severely degrade due to expansive soil movements, erosion, and other factors. This means that you should compensate for this with more initial runoff slope in your drainage system. We already knew this but there is added emphasis.

4. Provide positive drainage around the entire structure and eliminate flower beds and structures that trap water around the exterior. Plan a check every 3 to 5 years to be sure this drainage has been maintained after the structure is built.

Now it’s time for my cosmic connection. The picture on the left is the south polar region of Mars, taken by the Mars Global Surveyor. The picture on the right is Google Earth image of a field 6.7 miles northeast of Waxahachie, Texas. The origin of the Martian polygons are still being debated, the Texas features are gilgai landforms also called hogwallows that formed in expansive clay. There is a big difference in scale of the pictures. The Martian photo shows an area of 1.9 miles where as the Texas photo shows an area of about 300 feet. So the whole Texas photo would easily fit inside a single Martian polygon. On Mars we are looking at a very large fracture pattern, on Earth we are seeing a change in relief of the surface. The darker patterned areas are depressions. Each depression is surrounded narrow polygonal ridge area. These landforms on Earth only occur in expansive soil deposits, like the Yazoo clay. Note the linearity of the gilgai that is not seen in the Mars image. So clearly there are different processes at work. If you want to find this image on Google Earth the image date is 2/27/2001 and it's located at 32°26'34.64"N  96°45'14.11W. The primary clay mineral in Yazoo clay is Smectite and this mineral has also been discovered on Mars in areas like the Mawrth Vallis region using the CRISM instrument on the Mars Reconnaissance Orbiter. So maybe someday future residents of the Mawrth Vallis region will also need a good foundation engineer.

One last thought: From a strictly scientific point of view it would seem that polygonal landforms are more common in the universe than life. Maybe the Curiosity rover on Mars will change that in the near future. Good Luck Curiosity!

 REFERENCES

Maxwell, Britt.(1994) “Influence of Horizontal Stresses on Gilgai Landforms”  Journal of Geotechnical Engineering, Vol. 120, No. 8,  pp. 1437-1444. http://dx.doi.org/10.1061/ (ASCE)0733-9410(1994)120:8(1437).

Maxwell, Britt.(2011) “Field Measurements of Yazoo Clay Reveals Expansive Soil Design Issues” Journal of Performance of Constructed Facilities, Vol. 25, No. 1, pp.18-23. http://dx.doi.org/10.1061/(ASCE)CF.1943-5509.0000069

Taylor, Angela C.(2005) "Mineralogy and Engineering Properties of the Yazoo Clay Formation, Jackson Group, 

Central Mississippi." Mineralogy and Engineering Properties of the Yazoo Clay Formation, Jackson Group, Central Mississippi. Mississippi State: Mississippi State University.

Bishop, J. L., et al. (2012), Mineralogy and morphology of geologic units at Libya Montes, Mars: Ancient aqueously derived outcrops, mafic flows, fluvial features and impacts, J. Geophys. Res., doi:10.1029/2012JE004151.