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Building of historical significance built upon creep prone deep alluvial soil

Building of historical significance built upon creep prone deep alluvial soil

Building of historical significance built upon creep prone deep alluvial soil

(OP)
Hi, I'm working for a consulting engineering, responsible for designing restoration and retrofitting plans for a two story adobe masonry building of historical significance. The building is built upon an alluvial fan with more than 20 meters of horizontally bedded layers of gravel and sand alluvial deposits with little or no bond between aggregates with some boulders of more than 80 centimeters of diameter. Based on our crack pattern surveys of the building, rophly more than 90 percent of the cracks are vertical cracks that are more intensive at ground floor or at middle or bottom half of the walls. Most of these cracks are still active. We believe that there may be a mass wasting phenomenon happening underneath some parts of this building causing this cracks.

The building is built near the apron of the fan with approximately 10 percent slop in neighboring area. It is built in an area of semi-arid climate with cold winters and hot summers. There is also rapid changes in soil water content due to flash flood incidents common in the region. There is a water feature built around, so that water flows from a seasonal spring and rain water flows are directed towards three connected water ponds built in front side, inside and back side of the building. These ponds are not insulated so well, and there is some evidence of leakage in them. Given this situation, We think the soil underneath building is prone to creep.

The building site is underlain by a layer of loom fill (mostly SC) of approximately two meter thickness, probably brought here for gardening in a somewhat inhabitant layers of soil. Walls of the building is placed on top of a rockery layer which is built to level surface of natural soil.

Speaking of creep evidence, other than the forms of cracks on the building's walls and surrounding walls of the site (which are mostly vertical and active), one can say, there is no particular sign of creep incident in the region. The building is surrounded with trees, pavements and fences, but there is no recognizable evidence of creep on them. There is also a road alongside this fan from uphill to downhill, also with no recognizable evidence of creep on it.

Given these hopefully detailed information, I would greatly appreciate to have your comments on the followings:

1. In my opinion, because all the features other than the building itself, i.e. trees, fences, pavements and the road, are built in and upon a fill layer of approximately two meter depth, so there is no evidence of creep recognizable on them (the fill itself is not creep prone). Also I think, it is weight of the two story building (estimated about 1 ton per square meter) that triggers the creep behavior of the underlain soils. Is that sounds a logical argument to you?

2. Going around the neighboring area, I could find some evidence of creep on top layer of soil right behind the building's backyard wall (attached file). Considering that these ripples that flow the contour of slop, are caused by creep in top soil layer, can we say that creep behavior also happens in the underlain bed of the building?

3. From a geotechnical engineer's viewpoint, what is your recommendation to prevent active differential lateral movements of building parts that are the main cause of vertical cracks?

RE: Building of historical significance built upon creep prone deep alluvial soil

Sometimes we get fooled into thinking our first conclusion really applies. Take a look at the shrinkage potential that trees can cause. More often than not it has been my experience that trees can cause a lot of problems by taking water up via the root system.

RE: Building of historical significance built upon creep prone deep alluvial soil

sounds like collapsible soils. big problem around here on deep alluvial soils and fans. the holocene layers are collapsible and the pleistocene are much more stable. inundation with water, seismic or overload can cause collapse. differential subsidence could also be caused by lowering of groundwater, not sure if that is an issue on your property.

RE: Building of historical significance built upon creep prone deep alluvial soil

Because the cracks are wider at the bottom than at the top, I would concentrate on the 2 meter thick SC layer. This crack pattern is typical in clay materials, not in granular. Buildings founded in sand are much more likely to settle more at the edges, resulting in cracks which are wider at the top.

RE: Building of historical significance built upon creep prone deep alluvial soil

lets just confirm my interpretation of creep is the same as yours! Creep to me is the gradual downslope movement of surficial soil layers. This happens over a very long and sustained period of time. Maybe 1-2mm of downslope movement per year.

You say the neighbouring area has a 10% slope across it. What is the slope of the ground profile immediately upslope and down slope of the building? Is the building platform made up of cut to fill ? Could it be the thicker downslope wedge of fill that is settling? Are the cracks you observed all around the building or just on the sides of the building that are perpendicular to the slope ( if there is one). A cross section and sure plan showing pond locations trees etc would be really helpful. We also need some soil strength information ? Has there been testing? Or will there be testing?

Sorry for all the questions but this the information I would need in order to provide an assessment of someone came into my office and told me exactly what you have!!!

RE: Building of historical significance built upon creep prone deep alluvial soil

(OP)
Thank you all for your kind reply, considering your comments on the subject matter, I must say:

oldestguy: According to observation pits that have been dug around the building, there is only minor traces of tree roots at top 2 to 3 meter of soil layers around the building, so I think the probability of trees causing problems here is very low.

cvg: Water table level is at very lower depths, and it wasn't observed in our 20 meter depth test boring.

hokie66: The building walls are placed at the top of a rockery layer built to level natural alluvial soil layer (mostly GP with boulders of more than 80 centimeter diameter), that we know. So I don't think the top SC fill layer can cause any problem here.

I also attached a photo here, showing some crippling in the trees near the corner of the building that has the most sever lateral movement problem of all parts. I would appreciate if you take a look at that too. It can be a sign of some kind of soil movement, I guess. I couldn't find any anomaly on near land surface though.

RE: Building of historical significance built upon creep prone deep alluvial soil

Ysna - any SPTs done in the 20m borehole? Strength parameters would be good.

Could you up load a plan? Even a sketch showing the house and direction of the slope and which way the most severe lateral movement is. Pictures are helpful but it's hard to
Interprete without knowing the site. The trees are leaning from left to right. Trees sitting on sloping ground susceptible to crew normally lean up hill as the bottom of the tree is slipping down slowly. Is it correct to say in the picture that the site is sloping from right to left?

RE: Building of historical significance built upon creep prone deep alluvial soil

(OP)
EireChch, our observations of crack movements, shows movements of approximately 1 to 2 millimeter per year. The cracks are in almost all around the building but they are most intensive at the corner near down slope of the building, which is more steeper than other areas nearby (see crippled trees in second attached file and also location shown on attached plan view). Also, most of the cracks are active cracks, though the rate of crack growth are not the same on all parts of the building. Unfortunately there is no routine crack movement observation record at hand to help us check if the cracks move faster in wetter, colder seasons or not.

The building platform is made up of step cut slopes of two steps of 60 and 100 centimeter height. As far as we know the building itself isn't placed on top of any fill layer, it is built on top of natural ground surface. I'm pretty much sure about that, because the soil layers seen on observation pits near the building, have no sign of fill material, i.e. they are horizontally bedded layers of alluvium of different colors and gradations in each layer.

There has been geotechnical testing. The SPT tests are done on 3 rotary bore holes of depths of about 15 to 20 meters using 60 degree cone tip not standard split spoon sampler (because of rubbles and gravels). Other than being refusal at some depths, probably because of presence of rubbles on those layers, it shows that alluvium layers are of apparent densities of loose to medium dense. Our observation pits also confirm those soil densities. Other routine laboratory testing have been done too, but unfortunately on disturbed samples only.

I also attached aerial view and plan view of the building, hope they can help you.

RE: Building of historical significance built upon creep prone deep alluvial soil

(OP)

Quote (EirChch)

The trees are leaning from left to right. Trees sitting on sloping ground susceptible to crew normally lean up hill as the bottom of the tree is slipping down slowly. Is it correct to say in the picture that the site is sloping from right to left?

No, the site is moving from left to right in those photos. At least our observation gouges on the building walls says so. I attached another plan view showing direction of building movements and camera point of view in those photos.

Consider that it seems that the top 2 to 3 meter fill layer (seen as level ground surface in those photos) have no sign of movements. Also pavements in those photos doesn't have any recognizable cracks on them. Is it possible that movements of underlain natural alluvial layer down slop (from left to right in photos) can cause those anomalies on trees that are rooted in fill layer above it?

RE: Building of historical significance built upon creep prone deep alluvial soil

The plan show a "vertical cut slope of about 3 m height". Is the retaining wall there showing any signs of movement? Does it have a good drainage system? Do you have cracks in those sidewalks? Appears that the trees are tilting because of a possible movement of the retaining wall at that vertical cut slope.

Also, looking at your photos, appears that the building is at higher level than the garden area (the area with trees and sidewalks) with a short retaining wall. I assume that the retaining wall which is retaining that corner of the building shows more movement than other wall areas. Does this wall have a good drainage system? It may be a problem with the retaining walls rather than creep movement...


RE: Building of historical significance built upon creep prone deep alluvial soil

groundwater around here is at 100 meters or more, but withdrawal has caused regional subsidence. In some areas this subsidence is up to 5 meters. Where you are near a bedrock high, you can have differential subsidence. causes fissures as well. It appears that you have bedrock at the surface in your photo

the fact that you have water impounded around the building could saturate the poorly consolidated layers of SC and cause collapse (not creep).

RE: Building of historical significance built upon creep prone deep alluvial soil

(OP)
Okiryu, The 3 meter height retaining wall you mentioned, is a gravity wall built to retain fill layer in order to provide level platform around building for gardening. It does show minor deflection signs on near sidewalk as long cracks along the wall, but it is not more than 5 millimeters width. The wall does not have any drainage system and also there is no recognizable tilting sign on it.

The retaining wall at the corner of the building you mentioned is a 60 centimeter height gravity wall built to retain fill layer. It's part of step cut slope built around building to provide level surfaces for pavements to built on slope. Foundation of the building is placed approximately 1.7 meter below that pavement surface on natural soil profile. So, I assume that the movements on these retaining walls, if any, are irrelevant to the building part movements.

RE: Building of historical significance built upon creep prone deep alluvial soil

The geologist in me wants to know the location. . .

f-d

ípapß gordo ainÆt no madre flaca!

RE: Building of historical significance built upon creep prone deep alluvial soil

(OP)
There are a few things that we are sure about it: One, our crack pattern survey says that, the building is suffering some kind of active differential lateral movements down slope. Two, and I can't stress this enough, the building is built on top of a rockery layer built to level natural alluvial layer's surface profile. This layer is right below the two meter top GC fill layer. Three, our masonry strength tests says that the building is one of the strongest buildings built in it's period. Four, we haven't have any major earthquake in the area, science it has been built some 100 years ago.

fattdad: The building is called "Baqcheh Jooq Palace" (https://en.wikipedia.org/wiki/Baqcheh_Jooq_Palace). It is located in border town of Maku in western Iran.

https://www.google.com/maps/place/39%C2%B017'22.1%22N+44%C2%B025'34.7%22E/@39.2792998,44.4271481,3014m/data=!3m1!1e3!4m5!3m4!1s0x0:0x0!8m2!3d39.289462!4d44.426309

RE: Building of historical significance built upon creep prone deep alluvial soil

(OP)
Somehow the google maps link above seems to be broken, please copy and paste the full url in your browser's address bar.

RE: Building of historical significance built upon creep prone deep alluvial soil

YSna - when assessing a slope stability problem such as this I would prepare a cross section to develop the geotechnical model. Do an inclinometer survey running down the slope to get the ground profile. Make sure it runs beside the building platform and through some testing points. Then plot the geotechnical information (i.e strength parameters on it). I hve marked up. Very crude assessment of what information is above. It is probably way off!! But a starting point that you could develop further!

Show what you think the building foundations are I.e that the rest on the rockery layer. Show ground profiles above and below the building platform. Show the 2m of fill that you encountered.

If the slopes are steep enough it could be the fill layer consolidating and slipping down slope? An I ground palisade wall could be a Potentisl solution but this is expensive and you'd want to be damn sure you know what the cause is before you go implementing a wall like this.

For me to assess this I need more information . I bet you well get some good advice (from engineers alot more experienced than me) of you can show is a good geotechnical cross se section!

My two cents anyways.

RE: Building of historical significance built upon creep prone deep alluvial soil

A sample geotechnical cross section that I have prepared before. Shows building platform, strength parameters (untrained shear strengths from hand held vane testing) and ground profiles. This is what we do for every site with sloping ground,, even if stability isn't an issues

RE: Building of historical significance built upon creep prone deep alluvial soil

I would not pretend to solve this problem from half a world away with limited information. I am just suggesting some principles learned over the years that can be considered.

As cvg said, alluvial fans can be collapsible. So can loosely-placed fill. I have no experience with fans, but a lot of experience with collapsible loess and loose fill. Collapse depends on water content and pressure and their histories. Many of the problems I have seen involved an increase in pressure (area fill) followed by water content increase, such as irrigation. In one case, the fill was placed 80 years before the sewer began to leak, and the settlement was rapid and highly differential. Water from localized sources tends to go downward without spreading much in permeable soils. Borings taken a few feet away from the problem may not show the cause.

Differential settlement can cause cracks that look like lateral movement. Vertical cracks in the lower part of the walls suggest settlement of the central part of the walls. I would start with a level survey of features that should have been constructed level, such as mortar joints and floor slabs. The results can be revealing.

It sound like a complex problem, especially if the movements have occurred over a long period of time and the present movement is too slow to monitor accurately. If that is not the case, and the present rate of movement is significant, ask what has changed. Water is the first suspect.

RE: Building of historical significance built upon creep prone deep alluvial soil

(OP)
Thank you EireChch for your kind advise and thanks for your detailed attachments. I will try to prepare a cross section of the slope like what you said.

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