About science; About soil engineering

This is about science produced by the California Institute of Technology in collaboration with station KPCC Pasadena California. The programs are made available to the station by national educational radio. This program is about soil engineering with host Dr. Robert McGrath Leighanne and his guest Dr. Ron Scott. Here now is Dr. McGregor. And I think the expression soil engineering is one that is perhaps not too well in the public domain. I wonder Ron if you might express what this encompasses. I have 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 and I'm not at all interested in the professional sense and soil as an agricultural material as something to grow things in. With that established the the thing that I am interested in then is soil as an engineering

material or structural material. Something you build buildings on top of or make Erith dams from or put your highways on. So I'm interested in how it behaves when you put loads on it or how it behaves both under normal conditions and under earthquakes and how it behaves in different parts of the world and how the soils differ from one another in their mechanical properties. What are some of the properties or characteristics of soils which are the useful measures or parameters by which you express these properties. The thing that. Most soil engineers minds when they start investigating the soil is its strength. First of all how much of pressure or load or stress can you put on the soil before it begins to slip or yield or rupture. So it's more than a compressive strength

that they have so sheer strength. It's a shearing strength of the material we're interested in and most of our laboratory tests and a great deal of the research work that goes on and so I can exist directed that finding out how. Sharing strength varies with with all sorts of things the amount of water in the soil the type of soil sand or clay or gravel and finding better ways of describing it sharing strength. This is the 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 we look at geological maps and ask her geologist acquaintances and friends colleagues do help us describe the area geologically and what does that encompass with home geologically.

We'd like to know something about the rocks of the area the bedrock how it might outcrop if it does crop. I was going through to the surface going through to the surface yes. Where it where it's 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 geologist is extremely helpful in telling us what the size of the soil grains in the soil might be what distribution of sizes to expect if the soil are arrived. If the soil at the site was formed on the bottom of the sea or the bottom of a lake and lifted it up or whether it was washed down by a river these these help us decide what sort of structure to. That we can put on it or

sometimes it helps us. Most of the time it helps us layout where we'll make moorings and take soil samples to the time and the properties for the structure we're interested in. And I presume 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 at soil engineering now compared to 50 or 60 years ago. It used to be that there were well-known areas along the coasts on Delta those at the miles of rivers rather where extensive deposits of soft are weak so Island people would just sedimentary sedimentary that was it. Yes the people would just avoid these areas they had found their buildings where possible

on on firm soil or on our rock and and I thought the engineer would not be used because you would select where to put your structure to avoid the problems. I think you know me probably if I were what I wear of some of the classic instances where people didn't. You should have consulted a silent Johnny or he if there was one available if there was one available. Around the 12th or 13th centuries when they got the Tower of Pisa underway it would have been helpful to have a class you can do that too boring there. And there are other circumstances that we know about where structures collapsed because of foundation problems. You indicated that the sharing 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 puts down 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 object is is the stuff that's in the hole not just accomplishing the hole which is what you do for a while our water wells. So the techniques are aimed at getting the soil samples back in as natural a state as possible. Disturbing them. Making them as. Keeping them as happy as possible contented some of samples literally as a core as you say you have to remove it bodily remove it bodily we advance the hole put the hole down in various ways and then remove samples of soil from all that say every five feet is a common measurement we

take soil samples every five feet carefully bring them back to the laboratory take little pieces of them first of all four very general tests that are called identification tests we want to find out if the soil is a clay or a silt or a sound so we do sort of analysis we sieve the soil had grain size and so we have to define that as grain size. Generally speaking one. Gets more trouble with the finer grained so aisles of the clays are much more softer and more plastic than sounds and so we like to know more about them and we do these identification tasks first which usually are a good indication of what soil layers make of trouble. As this is integrated into intricate chemical analysis on occasion yes on occasion we would that 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 X-ray diffraction work to help us to get the mineral structure is on. 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 soil deposits are mixtures of heterogeneous layers of gravels and clay. Yes they're all mixed up together and so it's very hard to describe the soils and nice neat mathematical way in order to make calculations of the size of a building or of the pressure you can put on the ground. And because of this soil 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. It's a vague phrase means that we we expect our minds to

function as magical computers using data that we would hesitate to put into any real computer or the data that you've forgotten your division is somehow coughing up the result. Sort of an integrated result right. Do you call when you've seen silent looked like that before and what happened to a building in a similar location and things like that. So I think you're right we use these preliminary identification tests they tell us what the problem materials might be at a given site and then we do. Rather more careful tests on the soil enters close to the natural condition as we can get it. That is we. There are some soils which one finds and the St. Lawrence River the 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 these soil sensitive soils and so 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 in its tubes and throw it on the back of a truck and it gets a rough ride back to the laboratory and it might even get frozen or you don't know what on earth you don't know then and then you test it in a 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 it is in the original at the original site. So there's a question and they handling of the sample that you're going to test. And after you make this preliminary sort of physical identification yes then you proceed I presume to obtaining these mechanical properties. Yeah these are so several types of shearing test that we do one and one of them is rather a simple one in which we take a little square of soil is not quite a cube but that 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 to shear the two halves of the apparatus pull the two parts of the half part lighting and the soil that has to fail along this plane between the two parts the apparatus and we measure the force of the pressure that's required to do that. And then this is our shearing strength for the soil. What range of values might this have and you might compare that to other materials that might be of common knowledge. Well probably the best known material might be steel. Let's say and conventional steels might have it. Let's rupture strength or a yield strength a shearing strength of something like 30 or 40000 pounds per square inch. In

recent years much better these Mar aged steels have been developed to get up to 100000 or so. But the conventional ones around 30 or 40000 pounds per square inch concrete is around three or 4000 pounds per square inch depending on how you make it. And then the soils are a long way down behind us. Something like 20 or 30 pounds per square and sharing strength. So we're talking really about a relatively weak subset Yes it's fairly soft they might go from one pound per square inch up to maybe a hundred for a very densely compacted place you'll fill so i'll one place you're sitting on top of it has been compressed for a long time. Well you've indicated the ways in which you determine these properties if you can get the sample out of the ground and back into the laboratory. Is it possible and. I imagine perhaps it's even necessary at times to make a measurement Insitute.

Yes yes. In recent years the problem of getting an undisturbed sample has become much more prominent or better recognized and so are a variety of devices have been evolved to try and get the soil strength in the ground. Not trying to work out what it was like in the ground after you've taken it out but I just tested it right there. I think the simplest of these devices is a thing that looks just like a paddle or it's called a vane Shiri device it has commonly it's got four little veins at right angles to one another. You put it on the end essentially of 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 under stirred soil below the bottom of the borehole and then twist it. Yes and it

shears out the soil around it. I presume you measure the force you measure the torque and the amount of torque that's 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 then push the borehole ahead a little further and put it in and test the material for other had. There are a number of devices of this type that people use now. Let me answer on that. I presume the bed rock is a sort of foundation you'd like to build on but usually in the absence of that what are attractive kinds of soils and what are unattractive kinds of soil as a foundation for a building. The best materials of course are the most compact or most dense ones. Wow. If I remember correctly contrary to what it says in the Bible the Bible I think remarks that a man who is building his house on sand is doing a poor job or it's not foresight for so contrary to that

sound is fairly good in general. If it's been fairly well densify there compacted it's a good foundation material. If it's not then we can do something about it. There are several techniques of making the sand a little bit stronger gravels densely compacted coarser soil sands and gravels are usually fairly good. Then there's the soil. If you look at soils with progressively smaller grain size then one gets into more and more trouble down as you approach a closed as you approach the place the the silt the soils that is soils whose grain sizes smaller than sadness. I think the best way to say it to describe a stilt is to say you can you can taste the salt you can't see the grain size and it feels fairly smooth but if you touch it on your tongue it's gritty whereas a clay which is finer still you can't count it doesn't feel gritty it's so pretty or smooth. So

the soils the grains are so so small that they they don't tend to pack together very readily. They they're they're a mass or weight of each grain 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 I lose condition. They're also easily to make them weak. And it tends to make them weak. And they they tend to be affected by vibration. So after you build your building vibrations might shake them down into a denser state and then you have trouble you're building there goes your building right. So these are among the kind of the things that you look for when you're exploring a particular area by taking cores for a new building here for us. What kind of difficulties do you have with soil in terms of the problems that might generate later when a building is put up.

If the well if the material is so like and it can get shaken down by an earthquake or by vibrations then then that would be something to look out for that is under static normal loadings usual loadings it would be OK if something happened to give it a jolt or change it. Then you might get trouble with something like that in the clay soils which are usually the space between the grains is all filled with water we call them saturated soils. Then the clay soils the grain size is so small in the pore spaces are so small that when you put a load on top of a clay the clay 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 and that's right under 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 too. 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 aisles are 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 fine 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 has bent beyond what it can take so pipes break walls cracked all sorts of.

I remember in the case of the anchorage earthquake. There is mention of slides slumping and so forth. Yeah this reflects a kind of problem or is that something else I think. Well there are still arguments going on as to what that they happened in Anchorage. There there are in Anchorage there are bluffs around the coast which are maybe 60 or 70 feet high. These are just like embankments the Heils the dirtiest. This is natural soil and it's. It's deposited in the level of the sea has changed. The sea is now lower and the cliffs are bluffs around but their soil not rock cliffs. When that land was shaken by the earthquake there. There are soil layers below the surface clay soils which are related to the sensitive ones I talked about before in the course of an earthquake. The shearing stresses and pressures the shearing

forces acting in these sensitive clay layers. Can are higher than they are when they're just sitting there static away and with no earthquake and so it's possible that these soils just failed by shearing. They just got more stressed during the earthquake than they could take and so the bluff which put shearing stresses on the soils slid out. And then since the front part of the bluff slid out we were left behind it. Another bluff. If the earthquake still goes on then this one slide progress and this progresses backwards. And this is the Seems to have been what happened there. If I remember correctly the much of the damage that was caused was in fact by these major displacement so the soil was yes there are damages and there were many slides. Several major slides in Anchorage. The best known the most spectacular was the one that turned again 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 take that kind of thing. The impression one gets is that the soil is generally a weak material for a building. Is there anything that one can do to enhance the properties of the soil that perhaps can build on it with greater confidence. The if you have poor enough soil as a foundation if your 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 oh and coastal areas 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 at all possible. And this is with the bigger construction equipment that we've got now this is perhaps the easiest thing. But there are occasions when one might have a very loose sand 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. It's sometimes been tried to set off explosives to shake it down. However this does make us more dense make it more dense the denser Sanders the stronger it is and the less likely anything is to happen to it in future. Already the explosions explosives are really not popular with out neighbors so they are kind of a coarse or gross way of getting effect you want. This actually was tried in Anchorage following the earthquake as an attempt to stabilize some of the soil there for future air in case of future earthquakes what

other ways are trying to inject chemicals into the soil to harden chemicals that harden or cement the grains together or plug up the voids or make a cement I mean really make a cement out of it. Fact I suppose I could say point out that that concrete is just the stabilized soil happens to be a soil that we carefully choose what constituents is going to have we to get proportion of sand and gravel and then add some cement and water and then we get a very stabilized soil at the end of it. But mostly in the field. The modification of the soil properties by stabilization is expensive. I think you can easily imagine under normal substantial structures say a six or eight story building that might be worth stabilizing the soil for the volume of soil that stressed them by this building as the building puts pressure on is enormous. And you have to substantially

increase its 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 around the nation are put deep foundations down to better soil or something like that. But. With the stabilize ation there there are these two problems. The one if you don't mix the chemicals Very well then you have to use a lot of chemical unification because it's an if if the mixing is inefficient you have to add a lot of chemical. Well in one cubit yard you can picture one through a mass of soil three feet a cube three feet on a side that weighs twenty seven hundred pounds. So if you're only going to add 1 percent of chemical by weight you've got twenty seven pounds of chemical. Now if it if you can get it for a few hundredths of a cent per pound fine.

But if it's stuff if it's one of the more modern types of chemical the proxy or something like that then you're talking about dimes up under or dollars a pound. And this even then you're only achieving Salem's astronomical. It's a tremendous amount of money to to do. Just the cost of the chemical along then on the other hand if you want to use less Cameco then you must mix it thoroughly and the problem of taking up a large quantity of something while Roblin the soil movement of the material and the energy and so on. So in general stabilize ation has been used only for emergencies. Problem the server 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 silo makes 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 here. How about some of the newer frontiers in the field of Soylent Green. Well are we going to see attention of men such as yourself go these days to the problems that intrigue you. I'm myself I'm I'm interested in how soils got the way they are that is I feel I can learn something about the soil as it exists on the Earth's surface now by learning a little bit about its history and how it was formed. So I've been in many soils of course are deposited by rivers or other fall fall and settle out in the ocean and for I'm 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 strongly are what they look like how do they behave. This 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 it yeah it takes on the coast of the holiday. Yes and it's become certainly a frontier for other purposes too. Yes there must be a certain practical questions that you're grappling with. Well Ron I'm afraid that we've pretty well run out of time I appreciate very much and Jay very much a conversation. Thanks nice to have you. This was about science with host Dr. Robert McGregor and his guest Dr. Ron Scott join us again for our next program when Dr. McGregor will lead a discussion about exobiology about science is produced by the California Institute of Technology in cooperation with station KPP

C. Pasadena California. The programs are made available to this station by national educational radio. This is the national educational radio network.