About science; About soil engineering

This is about science produced by the California Institute of Technology in cooperation with station K PPC Pasadena California. The programs are made available to this station by a national educational radio. This program is about soil engineering with a host of Dr. Robert McGrath and his guest Dr. Ron Scott. Here now is Dr. McGregor. I think the expression soil engineering is one that is perhaps not too well known in the public domain. I wonder Ron if you might express what this encompasses and try. I think probably the easiest thing to say first of all is what it isn't because I'm frequently asked. On occasion to help people with their gardens I'm not appall interested in the professional sense and soil of the Niagara cultural material as something to grow things in with that established the thing that I am interested in then the

soil as an engineering material or structural material. Something you build buildings on top of or make air from or put your highways on. So I'm interested in how it behaves when you put loads on it how it behaves both under normal conditions and under earthquakes and how it behaves in different parts of the world and so Isles differ from one another in their mechanical properties. What are some of the properties are characteristics of soils which use full measures or permit or is by which you express these properties. The thing that. Most So islands in those minds when they start investigating the soil is its strength. First of all how much pressure or load or stress can you put on the soil before it begins to slip or you will die or or rupture.

So it's more than a compressive strength that the sheer strength it's a shearing strength of the material. We're interested in most of our laboratory tests and a great deal of the research work goes on and so it can exist as direct to finding out how sharing strands varies with with all sorts of things the amount of water in the soil the type of so I'll sand or clay or gravel and finding better ways of describing it searing strength. That when we when we have a particular site or a building the first thing we always have to do is. Find out something about the geology of the site. It's past history this is usually a useful clue as to what to expect in the area where you look at geological maps and ask her geologist acquaintances and friends colleagues to help us describe the area geological age. What does it encompass. Oh logically we'd like

to know something about the rocks of the bedrock how it might crop if it does crop was through to the surface that roots of the surface yes were not overlain by soil profile whether we might expect the area to be glaciated. If it's an area where earthquakes are common or where faults might occur in the bedrock geologists extremely helpful in telling us what the size of the soil grains in the soil might be what distribution of sizes to expect the soil or arrived at the so I let the site say was formed on the bottom of the sea or the bottom of the lake and stood up or whether it was washed down by a river or the help us decide what sort of structure or two. That we can put on it or sometimes it helps us. Most of the time it

helps us layout where we'll make mornings and take soil samples and the properties for the structure we're interested and to determine areas to avoid. For structures yes this is becoming a more interesting problem as we're covering more of the land surface with structures that used to be that and I suppose this is one of the reasons why people like me are making a living so I will engineering now compared to 50 or 60 years ago it used to be that there were well-known areas along the coast. So on Delta those miles of rivers were either for extensive deposits of soft or weak so Island people would sediment through sedimentary that was it the people would just void the their founder buildings where possible on firm so I were our rock

and I thought our engineer would not be used because you would select where to put your structure to avoid the problems I think you you know we probably heard some of the classic instances where people didn't. I should have consulted the soil and he was one available there was one available around the 12th or 13th century so when they got the Tower of Pisa underway it would have been helpful to have left you boring there. And there are other circumstances that we know about were structures collapse because of foundation problems. You indicated that the shearing strength was one of the principal parameters of interest in determining the strength of the soil. How are such measurements accomplished actually.

One goes out with a drilling crew in the field put the borehole. We have rather specialized ways of putting down bore hole big bore holes because we're not interested in putting down a water well our our object is is the stuff that in the hole not just accomplishing the hole which is what you do for a while over water wells. So the techniques are aimed at getting the soil samples back in as natural a state as possible. Disturbing them making them. Keeping them as happy as possible contented sort of literally Who are you. Yeah we move it bodily remove it bodily advance the hole put the hole down in various ways and then remove samples of soil from the hole every five feet. Common

measurement pulls every 5 feet. Carefully bring them back to the laboratory. Take little pieces of them first of all for very general tell us who are called identification with us we want to find out the soil is a clay or a silt so we do the analysis we saw on the ground who have defined it as grain size. Generally speaking one. Gets more trouble with the finer grained so I was the clays are much more softer and more plastic than sound so we like to know more about them and we do these identification tasks first which is usually a good indication of what soil layers may give trouble or as this isn't a crude chemical. On occasion yes on occasion we would that so they like to find out the mineral composition of the material to actually identify the clay minerals in the soil. So sometimes

we get into. We do an X-ray diffraction were to the mineral structure. Occasionally other techniques as well. These things are all helpful in establishing a background for the material. I think you can see that you know most. So I'll deposit mix. That's really genius. Layers of gravel. They're all mixed up together and so it's very hard to describe the soils and I need a mathematical way in order to make TALK OF THE NATION the size of a building or a pressure you can put on the ground. And and because of this. So engineers have to exercise a good deal of judgment and use their intuition if they have any or we all work very hard at developing our intuition which means that we we expect our minds

to function as magical computers using data that we would have to put into any real computer or the data that you've forgotten and coughing up the result of an integrated result. Do you call when you've seen soil like that before and what happened to a building in a similar location and things like that. So they rate we need a preliminary identification they tell us what the problem materials might be and if they get inside it and then we do. Rather more careful tests on the soil in as close to the natural condition as we can get it. That is we. There are some soils which one finds on the St. Lawrence River. River valley and Canada which are so sensitive to being distorted or disturbed that in fact it's possible to make them entirely liquid. We call the

soil sensitive soils and so on. If one gets a soil like that and it's subjected to a great deal of disturbance while it's being taken and then the drillers take the soil and it tubes and throw it on the back of a truck and it gets a rough ride back to the laboratory it might even get frozen and you don't know what on earth you don't know. Then you test it in the lab. It's very soft and soupy. This may or may not be anything like it is in the field. Probably it's a lot softer than the original at the original site. So there's the question the handling the sample the test. Do you make this preliminary sort of physical identification. Yeah. Then you proceed I presume to obtain these mechanical properties. You know the there are so several types of shearing that we do one and one of them as rather a simple one and which we take a little square of soil is not quite the cube but the

rectangular shape and we put it in a piece of apparatus the bottom half of the square of soil is in the lower portion of the apparatus and the top part is in the upper part and we can apply a pressure to the soil and then see are the two halves of the apparatus pool to part. So you know part lighting and the soil then has to fail along this plane between the two parts the apparatus and we measure the force or the pressure that's required to do that. And then this is our shearing strength for the soil. What range of values might have and you might compare that to other material that might be common knowledge. Well probably best known with two might be steel and conventional steels might have. Let's rupture strength or yield strength hearing strength of something like

30 or 40000 pounds per square inch. Recent years much better. These marriage deals have been developed to get up to a hundred thousand or so but the conventional ones around 30 or 40000 pounds per square inch concrete around three or 4000 pounds per square inch depending on how you make it. And then the soils are a long way down behind something like 20 or 30 pounds per square and sharing strata. So we're talking really about a relatively weak substance. Yeah it's fairly soft they might go from one pound per square inch up to maybe a hundred for a very densely compact the place you'll kill. So I'll one that you're sitting on top of it has been compressed for a long time. Well you have indicated the ways in which you determine these properties if you can get the sample back into the laboratory. Is it possible and I

imagine perhaps even necessary at times to make a measurement. Yeah yeah. In recent years the problem of getting an undisturbed sample has become much more prominent or better recognized. And so a variety of devices have been evolved to try and get the soil strand in the ground. Not trying to work out what it was like in the ground after you've taken a clock but right there I think the simplest of these devices is a thing that looks just like a paddle. It's called Change your device it has commonly it's got four little veins at right angles to one another. You put it on the end essentially over a long rod. You lower it to the bottom of the bore hole as far as it's gone at that time push it into the more or

less undisturbed soil below the bottom of the borehole and then twist it shears up the soil around it. I presume you measured of course you measure the amount of torque required to do this and you've got a value of the strength of materials and you can do this continually as you pull it out and push the borehole ahead a little further and put it in and test the material further had There are a number of devices of this type people use. I presume the bed rock is sort of like to build them in the absence of attractive kinds of soils that were unattractive. The soil foundation core building the best materials of course are the most compact the most dense one. If I remember correctly contrary to what it says in the Bible and the Bible I think

remarked who's building his house on sand. Doing a poor job or not foresight for contrary to that found is fairly good in general. If it's been fairly well densify dirt compacted it's a good foundation. If not then we can do something about it. There are several techniques for making the sand a little bit stronger gravels then stay compact the coarser sauce and gravels are usually fairly good. Then of the soil. If you look at soils with progressively smaller grains then one gets into more and more trouble and as you approach the cliff as you approach the silt the soils that those whose grain size is smaller than sand. I think the best way to describe a still to you can you can taste the salt. You can't feel the grain size and it feels really smooth. But if you touch it on

your tongue it's good to play which is finer still you can count doesn't feel gritty so clear smooth ice so the soils and the grains are so so small that they they don't plant the pack together very readily and their mass or weight gain is so small that when it settles out in water it doesn't really fall into the densest structure that it could so quite commonly lose condition. They're also easily to make them weak. That tends to make them weak and they they tend to be affected by vibration. After you build your building vibrations might shake them down into a denser state and then you're going to build it there goes your building. So these are among the things that you look for when you're exploring particular by taking cores for sure. What kind of

difficulties do you have with soil in terms of problems that might generate later when a building is put up. Well if the material is so black and it can get shaken by an earthquake or by vibration then then that would be something to look out for. Under static normal loadings usual loadings it would be OK if something happened to to give it a jolt or change it and you might get trouble with something like that in the clay soils which are usually the spaces between the grains is all filled with water we call them saturated so it was then the clay saw as the grain size is so small the pore spaces are so small that when you put a load on top of clay the grains themselves don't take the load immediately. Some of the load is taken by the water in the pores of the soil

and then they transmit pressure. That's right in the water. Because the water pressure in that bloated part underneath the building is higher than it is somewhere else. The water tries to flow it tries to come out of the ground surface and it flows laterally through. And this means that since the water is coming out of the soil the soil is compressing and because there are so aisles these place oils or the grain size is so small and they're relatively impermeable and this process takes time and so the best instance of this is Mexico City which is built on a flying plate a layer derived from volcanic ash there. And buildings built there are quite satisfactory and stable when they're built and then in the course of time settlements grow and grow and grow and the building can crack and fail. That is the soil hasn't failed by shearing but the building is bent

beyond what it can take so pipes break walls cracked all sorts. I remember in the case of the croods there was mention of slumping and so forth. This reflect the kind of problem with that. I think well there are still arguments going on of the walk that today happened in Anchorage. There are and I think roads are blocked around the coast to observe maybe 60 or 70 feet high. These are just like embankment. This is natural soil that's deposited in the level of the sea has changed so the sea is now lower and the cliffs are bluffs around but their soil not rock cliff when that land was shaken by the earthquake there. Their soil layers

below the surface clay soils which are related to the sense that the ones I talked about before in the course of an earthquake the shearing stresses and pressures of the shearing forces acting on these sensitive players. Are higher than they are when they're just sitting there static with no earthquake. And so it's possible that the soils just failed by steering just got more stressed during the earthquake than they could take. And so the block which put shearing stresses on the so I was slid out and then since the front part of the block slid out you were left behind another block and here still goes wrong then this one slide progress and progresses backwards and this is seems to be what happened there. The firm member correctly much of the damage that was caused was in fact by these major displacement.

So yes there were damages and there were many slides several major slides in Anchorage. The best known the most spectacular was the one that turned the gun where the nice residential area of Anchorage and it was spectacular and the number of houses were destroyed. But I think the major damage was due to the large slides that weren't spectacular but in which masses of soil are millions of cubic yards moved maybe 10 or 12 feet during the earthquake. And there is no way of designing a building to make that kind of thing. When we get to the soil is generally weak material for building. Is there anything that one can do to enhance the properties of the soil. Perhaps build it with greater confidence and if you have poor enough soil that the foundation here preliminary investigation tells you the soil is very poor it is frequently done.

Construction practice is frequently followed of just removing the poor stuff like for one coastal area as one might find peat deposits very soft compressible organic deposits. We call the soil but we don't really like to work with them so we just take them out if it all possible. This is with the bigger construction equipment that is perhaps the easiest thing. But there are occasions when one might have a very loose for instance and the construction conditions require that you build there you can't choose an alternative site. In that case you might try to compact it by vibration. Something has been tried to set off explosives to shake it down make it more make it more down to the denser sand as the stronger it is and the Left likely anything has to happen to it in future explosions

explosives are not popular with the Corps or groups who are getting what you want. This actually was tried in Anchorage following the earthquake an attempt to stabilize some of the soil there for future in case of future. Other ways are trying to inject chemicals into the soil too to harden chemicals that harden or cement the grains together or plug up the voids or make a cement make the cement out of it but that's because I could say point out that the concrete is just the stabilized soil happens to be a soil. We carefully choose what constituents is going to have that proportion of gravel and then add some cement and water and then we get a very stabilized toilet at the end of it. But mostly in the field. The modification of the soil properties by stabilization is expensive.

I think you could easily imagine under normal substantial structures six or eight story building that might be worth. Stabilizing the soil for the volume of soil stress whether it's building the building puts pressure on is enormous and you have to substantially increase strength everywhere in this volume. So either you've got to try and impregnated everywhere or you try to take it out and mix it and put it with a chemical and put it back or put deep structures are put deep foundations down to better soil or something like that. With the stabilizer and there are these two problems the one if you don't mix the chemical Very well then you have to use a lot of chemical. You know it's in the mix things and you have to add a lot of chemicals.

Well then one cubit yard you can picture one morass of soil three feet cubed three feet on the side. That way twenty seven hundred pounds. So if you're only going to add one per cent of chemical by weight you've got 27 pounds of chemical. Now if it if you can get it for a few hundredths of a cent per pound flying through the stock. If it's one of the more modern types of chemical the proxy or something like that then you're talking about dimes or dollars a pound. And this even that and you're only achieving astronomical. It's an tremendous amount of money to to to. Just the cost of the chemical alone. Then on the other hand if you want to use less D'Amico then you must mix it thoroughly and the problem of taking up a large quantity of the oil plume of

soil mold movement of material and energy and so on. So in general stabilize ation has been used only for emergencies and problems over isn't. Or in places where you can actually pick it up mix it and put it down fairly readily like along a road where you can pick up the natural sorrow mix a certain amount of say Portland cement with it and lay it down again. So we do have the soil cement roads in which the mixing is done with fairly conventional mixing machinery. We've been talking about the kind of problems one encounters and normal practice. How about some of the newer contours and so Islands. Well are going to the attention of men such as yourself go the terms of the problems that treat you. Eyeing myself I'm interested in how

soils got the way they are that is and I feel I can learn something about the soil as it exists on the Earth's surface now by learning a little bit about it history and how it was formed. So I would be in many soils of course are deposited by rivers or other fall fall and settle out in the ocean and form ocean floor soils which are subsequently uplifted. So in the last year or two I've been paying more attention to the way soils are on the ocean floor how strong we are and what they look like how do they behave. This there are lots of interesting problems there. I think it has a subsidiary benefit in that it takes you to the nicer places to the sun to the coast of the holiday. That's because up front here for other purposes you have to let you know certain practical questions that you're grappling with.

We'll run them through that we've pretty well run out of research very much and conversation. This was about science with host Dr. Robert McGovern again that is guest Dr. Sheldon pretty glad are joining us again for our next program but another subject of interest to scientist and layman will be discussed. About science is produced by the California Institute of Technology and is originally broadcast by station KPCC Pasadena California. The programs are made available to this station by a national educational radio. This is the national educational radio network.