What are the Common Types of Collapsible Soil in United States?

Collapsible soil, in general, can be termed as soil that suffer or may susceptible to sudden and large volume changes under change in moisture content. The volume change is always reduction and change in moisture content means get wet of this soil.

The determination of collapse potential of collapsible soil, geotechnical properties and performance under rainfall etc. were discussed in our previous posts.

As discussed in last post collapsible soil induced foundation settlement is considered secondary foundation settlement as they can suffer settlement without increasing load or overburden pressure.

The common types that are found in United States are of two:

1. Fill deposit
2. Natural soil

1. Fill deposit:

Here we are concerned with uncontrolled fill, now what is uncontrolled fill?

Uncontrolled fills are that

• Not documented under compaction test while fill materials are placed including

1. dumped fills
2. under water dumping of fill materials
3. fills placed hydraulic 

• Fills compacted but have not documented about testing or effort types or amount of required compaction. The effort type is important as same compaction mechanism have different efficiency in different soils.

Uncontrolled fill is observed now in rural areas having less care in inspection or structures which were built in many years ago when rigorous compaction requirements were not developed.

In loose deposit reduction in negative pore water pressure do the harm to foundation soil. Such loose soil deposit has structure that is collapsed due to reduction of negative pore water pressure; also called capillary tension. The capillary tension reduced under wetting of fill.

The factors that influences degree of 1-D collapse for a specimen of fill submerged in water (considered distilled water) are 

  

• Soil type


• Moisture content at compaction


• Dry density of compacted fill


• Vertical pressure

Generally 1-D collapse of a fill will increase when

• Dry-density is decreased


• Moisture content is decreased


• Vertical pressure is increased

For a moisture content and dry density, 1-D collapse is decreased with increase in clay fraction when clay content exceeds optimum value (generally small percentage).

2. Natural soil

The natural soil deposits of collapse behavior are found in arid climatic condition of south western part of United States, the general mechanism involved in collapse is a result of breaking down of bonds within soil particles.

Bonds break down due to weathering, at contact of coarse particles, of fine-particles brought by the surface tension within evaporating water.

In some cases, colluvium or alluvium having unstable structure (like pin hole porosity) may collapse while wetting cycle passes through soil.

Rebound of Fiber in Shotcrete Operation

Shotcrete is special type of concrete; the difference is conveying and placing. As in concrete; steel fibers are added to shotcrete to improve its properties.

Steel fibers improve durability of concrete; as it helps to improve resistance against cracks thus increase ductility of concrete. Same performance can be expected from shotcrete with the addition of steel fibre. It have added advantage over welded mesh reinforcement as their require

• Less labor
• Less time of construction

In special work like lining in tunnels and in industrial floors for the former one it is suggested to use small fibers and later one to use longer fibers.

In case of welded mesh/ or conventional reinforcement, we can increase tensile strength in some particular direction; whereas fiber shotcrete offer us a multidirectional reinforcing effect.

While projecting shotcrete material (at a high speed) on to a target (say backup surface), it is not usual that all portion of projected materials to adhere to expected target surface. The fraction of materials rebounds or falls down. In our previous post we learnt that coarser particles are rebounded.

Here we will learn about steel fibers in steel fiber reinforced shotcrete, also known as SFRS, their performance in shotcreting in respect of rebound.

We know coarse particles rebound more and in case of SFRS steel fibers rebounds more than any other aggregates of shotcrete materials.

So in a hardened concrete surface we will have richer concrete than that is batched. ‘Rich concrete’ term is used in respect of cement content but the dose of fiber becomes less in hardened surface than expected in batching.

That is the purposes of introduction of fibers are not served. Again we will be billed for total amount of concrete materials and fibers too. Thus our exception will be lowest possible rebound.

Rebound, as expressed as percentage, depends on many parameters. The factors those are important-

• Parameters of concrete pump
• Distance of nozzle
• Spraying angle
• And most important is skill of nozzzlemen
• Method of shotcreting

Again fiber size and shape have influence on rebound and direction of shotcreting has also influence. Say

• More rebound in shotcreting to overhead than shotcreting in horizontal direction
• Regarding shape, flat ended fibers have less rebound than other shape
• Regarding size, very light and very fine fibers have more rebound than heavier fibers.

An overall reduction in rebound can be achieved by using micro-silica in concrete mix both of fibers and aggregates. This also reduces dust during shotcreting.

Foundation Scouring Scenario of Bridges in United States

In our previous posts we have discussed about scouring produced foundation failure of different bridges of New Zealand. Our aim is, here, same and our concern is United States. Scouring during floods and sometimes are the main causes of foundation failure of bridges in United States.

Here we are providing some failure of bridges in United States due to foundation scouring:

1. Schoharie Creek Bridge
2. I-5 Bridge
3. Bridge AA-438
4. CRANDIC bridge

1. Schoharie Creek Bridge:

Location: On Schoharie creek in New York near vicinity of fort hunter.

Date of failure: 5th April, 1987

Causes of failure: 

Melting water of snow together with heavy rainfall produced cumulative foundation scouring which led to collapse of this bridge.

Casualties: 

Five vehicles were on the bridge at this time and bodies of victims were found downstream throughout three weeks. 10 people were killed during this collapse. This was one of the most catastrophic failures due to foundation scouring of bridges.

2. I-5 Bridge:

Location: 

on Los Gatos creek in the vicinity of Coalinga, California.

Date of failure: 10th march, 1995.

Causes of failures: 

Failed due to scouring produced by large flood. A minimum potential scour depth of 25.2 ft was determined by standard analysis approach provided by HEC-18 & 20. This resulted column to bend at less embedment. More over significant exposed portion have no reinforcing steel. In I-5 Bridge the casualties are as follows:

• 1 truck and 4 cars fell into Arroyo pasajero
• 7 people were killed.

Bridge AA-438:

Location: prairie country in the vicinity of Saugus, Montana.

Date of failure: 19th June, 1938.

Cause of failure:

The velocity and volume of flowing water reached beyond expectation which led two of pier at central portion to undermine, the weight of locomotive passing over the rail bridge resulted subsidence of bridge piers and finally total collapse occurred.

Casualties:

• Locomotive fell into creek
• 47 people (at least) killed.

CRANDIC Bridge (Cedar River Railroad Bridge):

Location: Spanning over Cedar River in Iowa.

Date of failure: 12th June 2008.

Causes of failure: Three steel spans out of total four spans were swept out.

Casualties:

15 rail cars of CRANDIC were swept with bridge. No injuries or life loss were recorded.

Of these bridge failures, collapse of Schoharie creek bridge drawn attention from Federal Highway Administration and they provided regulation to monitor conditions of scouring of bridge over all states of United States.

Around 19% of federal emergency funds for Highway restoration work are required to restoration for bridges in United States. Within 1980-1990, the Federal Authority spent an average 20 million dollar per year for restoration of bridges and we know restoration works involve scouring of foundation elements.  

Why is Concrete Superior to Other Construction Materials?

We know concrete is the widely used construction material all over the world. Now there have no material that can be compared to concrete. The question is why? About 11 billion tones of concrete are consumed in the world in every year.

Is concrete stronger than steel? Or is concrete tougher than steel?

The answers are no? Steel is mentioned here, as it is the most comparable material with concrete in the world depending on availability and structural properties.

The reasons behind this are of three fold; namely

a. Its excellent performance against water

b. Flow characteristics of concrete

c. It is cheapest and most available materials for the job.

Regarding water resistance, we can include, concrete can withstand without having serious deterioration against water. This property makes it superior to wood and normal steel. So water tight structure can be constructed with concrete. The application is to

• Control Water
• Store Water
• Transport Water

The Romans used concrete as waterfront retaining structure (wall) and aquaducts which were the earliest application of this material. Now this material leaves its footprint, on

• Dams
• Canal Lining
• Pavements

So structural elements that are subjected to exposure of moisture or erosion of water, reinforced concrete and sometimes prestressed concrete perform successfully with expected degree of durability. Some examples of these are:

• Piles
• Foundations
• Footings
• Floor
• Beams
• Column
• Roof
• Exterior Walls
• Pipes Etc.

Concrete member can easily be formed into any expected shape and as well as advantage of providing formability makes it universal material, as no other material have shown such characteristics.

This advantage is achieved from plastic consistency at early age. The early age means before it becomes stiffening. The terms stiffing, solidification and hardening will be discussed in upcoming posts. 

For this post, just keep in mind that stiffening means losing consistency of fresh concrete. After this stage the concrete can also be reshaped with revibration; retempering are required which is the act of professionals.

This property allows concrete to flow into formwork that are fabricated according to designed shape. The formwork, after concrete gaining sufficient strength, is removed and reused in another project or further extension of structure.

Regarding availability and cost involvement in concrete construction, it can be included that concrete is readily available and cheapest material for the job. The main components that makes concrete are aggregate, Portland cement and water. All these components are relatively inexpensive.

The components that comprise concrete are generally available all over the world. The consideration which makes concrete superior than steels as an construction materials are :

a. Maintenance

b. Fire resistance

c. Resistance against cyclic loading.

Bridge Foundation Scouring Scenario of New Zealand

In simple word, scouring involve removal of material from river or other stream bed and bank of them as well, form foundations of bridges and essentially these are happened due to water flow. We will learn elaborately about scouring, its types and measures to avoid such phenomena.


Scouring results maximum damage to bridges and consequent failure of many bridges all over the world. Now we will learn some failures in New Zealand. Here we will discuss about some failures of bridges related to scouring failure-

1. Bull’s road bridge
2. Waitangitaona river bridge
3. Blackmount bridge
4. Waipaoa river rail bridge
5. Oreti river bridge
6. Bullock creek bridge etc.

Of these we will discuss first four bridge failures.

a. Bull’s Road Bridge

Location:

• On Rangitikei River
• State highway -1

Failure date: 15th June in 1973

Causes of failure:

• Scouring produced one pier to collapse
• Structural hinging

Waitangitaona River Bridge:

Location:

• On Waitangitaona River
• State highway-6

Failure date: 12th march in 1982

Causes of failure:

• One pier washed out
• Two spans of this bridge lost due to accumulation of debris resulting severe scouring with flow concentration around these pier.

c. Blackmount Road Bridge:

Location:

• On Mararoa River
• Road from clifden-manapouri

Failure date:

Considered failure between (25-26) august in 1980 

Causes of failure:

Scouring, undermining in a pier.

d. Waipaoa River Rail Bridge:

Location: 

• On Waipaoa River
• On Palmerston north to Gisborne line

Failure date: between (7-8) march in 1988

Causes of failure:

• One abutment outflanked and undermined
• Three piers were affected under both general scour and local scour.

Regarding cost involvement, we can include an expenditure calculation, done by Department of Scientific and Industrial Research (DSIR). This report prepared by coleman and Melville in 2000. An annual expenditure of 36 million dollar (NZ) was involved to restore flood scours.

DSIR, New Zealand provided that 50% of its budget was required for restoration of bridges and maintenance of them as well. Of these 70% expenditure was concentrated to bridge scour. Besides these there have many indirect expenditure and long term impact on local economy.

Danger of Rebound in Shotcrete

In definition of shotcrete, we have provided that impact of jet exerts such amount of force that materials are compacted not to sloughing or sagging. The expectation or target must be this but actual situation is never the same as that of practical cases.

The high velocity for projection exerts impacts on surface to be shotcrete which results some materials to rebound. Now we know shotcrete may be concrete or mortar.

In case of mortar, the coarsest particle is fine aggregate but in concrete, we have to provide some coarse aggregate. Obviously the coarsest particles of the mix rebound more.

Thus concrete has more rebound than mortar as materials of shotcrete. The additional information is that due to lack of coarse material, the projected in-situ shotcrete is richer than expected mix proportion according to batch.

What do you think the richer mix produce shotcrete of greater strength?

Obviously strength is increased with cement content but with this advantage, this will lead to additional shrinkage (slight increase) and require additional care.

Rebound of coarse particles produce rich shotcrete which results little increase in shrinkage. The rebound percentage is not uniform throughout the shotcrete projection.

Many factors influence the rebound percentage, of these; most important factor is thickness of section to be produced by shotcrete. A thin layer of shotcrete suffers more loss than thicker section.

The probable cause of more initial loss is the highest percentage of rebound is occurred at the beginning of shotcreting when the material is impacting on hard backup surface.

The loss is generally diminished as the progress of shotcreting. This is due to impacting of materials on plastic cushion provided by shotcrete which is built up gradually.

The typical values of rebound of materials are:

The degree of rebound with previously discussed plastic shrinkage; another economic consideration is waste of materials. Not only loss incorporated in rebound but also materials are accumulated in critical locations where subsequent layers of shotcrete produce a greater section than expected.