RETAINING WALLS, PILES & CAISSONS

What are the  Basics of Retaining Walls?

Retaining walls are used to make a transition in finish grade elevations; the wall keeps soil at a higher elevation behind it. In order to accomplish this task, there are some rather tricky considerations concerning the soil behavior and wall structure. The figure below shows the typical details of three common types: cantilever retaining wall, gravity retaining wall, and basement (propped cantilever) retaining wall. Concrete retaining walls will be discussed here, masonry, or even aluminum, retaining walls are sometimes used and most of the principles will be applicable for these types of walls also.

As in many areas of construction, understanding the way an item fails helps to make the design assumptions and details more clear. The retained soil experts a pressure, or load, against a retaining wall. This load can cause the wall to fail in typically one of three ways. The first method of failure is overturning. The details in the previous figure show a dashed line where the soil fails due to overturning. Basically the wall remains structurally intact, but rotates forward about its toe and lifts the soil above its heel. The second method of failure is sliding. Again the wall stays structurally intact but slides horizontally forward due to the pressure of the retained soil. The third method of failure is structural wall failure and is similar to any other reinforce concrete failure.

The method of determining exactly how much pressure a soil will exert on a retaining wall is very complicated. In fact engineers have studied the problem for centuries and revised solutions are continually being proposed and discussed. A good starting point for discussing pressure on a retaining wall is to consider the retained material to be water. The pressure against the wall, then, is determined by the material density (62.4 pounds per cubic foot) and the wall height. For example, a 10' wall retaining water would have the following pressures against the wall.

1. 1' down from top of wall=1' x 62.4  lb/cf  = 62.4 lb/sf
2. 5' down from top of wall=5' x 62.4 lb/cf  = 312 lb/sf
3. 10' down from top of wall=10' x 62.4 lb/cf  = 624 lb/sf

Retained soil does not generally act like a fluid. The horizontal pressure in soil is less then the vertical pressure and is described by "K"- the coefficient of lateral earth pressure. The 10' wall example now retains soil with a density of 105 PCF and a K factor of .3; with the following results:

   Vertical:

1. 1' down from top of wall=1' x l05 lb/cf  = 105 lb/sf
2. 5' down from top of wall=5' x 105 lb/cf  = 525 lb/sf
3. 10' down from top of wall=10' x 105 lb/cf  = 1050 lb/sf

   Horizontal: (against wall)

1. 1' down from top of wall=1'x 105 lb/cf  x 3 = 35 lb/sf
2. 5' down from top of wall=5 x 105 lb/cf  x 3 = 175 lb/sf
3. 10' down from top of wall=10 x 105 lb/cf  x3 = 350lb/sf

The real significance for the Construction Supervisor of studying lateral pressure on retaining walls is in the following step. The lateral earth pressure K is subdivided in Ka (active), Ko (at rest) and Kp (passive). The active state occurs if the wall can move slightly (this usually occurs by the wall rotating a minute amount). The at rest state occurs if the wall is absolutely rigid and can not move (as in the case of a basement wall). Finally the passive state occurs when a structure pushes against, or into, soil (as in the base of a retaining wall pushing against soil to resist sliding).

A simplified example of design of a retaining wall may be helpful here to illustrate the above principles. If the 10' retaining wall retains soils with a density of 105 lb/cf, Ka = 0.3, K.= 0.5 and Kp = 3.0 the following conclusions can be drawn:

1. If the wall can rotate slightly, the pressure 10' down from the top of the wall = 10' x 105lb/cf   x 0.3 = 350 lb/sf
2. If the wall is supported at the top (basement wall), the pressure 10' down from the top of the wall = 10' x 105lb/cf  x 0.5 = 525 lb/sf
3. Part of the resistance to sliding consists of the passive soil pressure from the toe to the finish grade (say 3')=3' x 105lb/cf  x 3.0 = 630 lb/sf

Far too many retaining walls fail because the above principles are not clearly understood. Rarely does the Design Professional design for the worst case that may be encountered in the field. Regardless of the contractual arrangements it is important that the Construction Supervisor advise the Design Professional of any safety concerns regarding the retaining wall. A Construction Supervisor that understands the concepts above should check with the Design Professional in instances like the following:

1. The retaining wall base is on solid rock. Therefore the retaining wall will not be able to rotate at all and the active lateral pressure is invalid and the higher at rest lateral pressure must be used.
2. The water table in the area is higher then anticipated.
3. During excavation in the area, a soft clay seam is discovered several feet under the retaining wall base. This soft clay seam could allow sliding to occur and should be checked.
4. The only readily available material to backfill the retaining wall is clay.
5. No drainage or weephole information is given on the design drawing.

CANTILEVER:Reinforced concrete cantilever retaining walls are commonly used because of their efficient use of materials. However, since it is an efficient design, it is critical that design details be followed explicitly. The base is used to translate the lateral soil load to a downward load on the toe and an upward load of the heel. Therefore there is a couple (or a moment) in the base that is resisted by the top and bottom rebar. The rebar in the base that runs parallel to the wall is for temperature and shrinkage resistance. The vertical rebar in the stem (nearest the retained soil) carries the tension load down into the base. This is the main structural rebar in the wall. The horizontal and front face vertical rebar is for temperature and shrinkage resistance.

Cantilever retaining walls are generally designed with gravel backfill immediately behind the wall and a drainage and weephole system. These drainage systems allow the wall to be designed for a much lower soil pressure, and if the drainage is not properly installed the wall may fail.

GRAVITY:A reinforced concrete, or plain concrete, gravity retaining wall is designed on the same soil principles as a cantilever retaining wall. However the resistance to overturning is based on the bulk weight of the concrete. The formwork, rebar and pour sequencing is significantly simpler with a gravity retaining wall. Often the only rebar required is for temperature and shrinkage at the face and top.

BASEMENT WALLS:
A reinforced concrete basement retaining wall is structurally quite different from a cantilever retaining wall. The main difference is based on the simply supported beam versus a cantilever. In a basement wall the main tension rebar is on the inside wall face while in a cantilever wall the main tension rebar is on the outside (or retained earth) face. In a basement wall the base footing does not transmit a moment to the soil, but only resists sliding. A basement wall can be detailed as shown in the previous figure or can be supported various other ways at the top of the wall. For example a cable imbedded in the top of the wall and tied back to a concrete deadman in the soil creates a basement wall condition.

It is essential for the Construction Supervisor to differential between cantilever retaining walls and basement retaining walls during construction. Basement walls must be supported at the top prior to backfilling, which can be very difficult in some instances. It is regretfully common to see Design Professionals make retaining wall details that are structurally sound upon completion but extremely difficult to build. The Construction Supervisor must recognize these situations, understand the various factors as well as possible, and proceed as a team member to help resolve the problem.

What to Know about Driving Piles?

The two basic classifications for piles are bearing piles and sheet piles. Bearing piles are used like columns to transmit foundation loads downward to rock or deeper soils. Sheet pilings are used as bulkheads to retain soil or water. Piles are driven vertically into the ground by power hammers or vibrators to a certain amount of resistance or load determined by the Geotechnical Engineer. This information is usually given in blows per inch for the hammer being used.

The allowable piling load is usually not determined by the structural capacity of the pile, but by the piles' ability to transfer its load to the soil. This load is transferred in the following three ways:

1. End-bearing-all the load is transmitted from the bottom tip of the pile to the rock below.
2. Friction-all the load is transmitted by friction between the surface of the entire length of the pile and the adjacent soil.
3. Combination of the above

There are historically many variations of types and materials used as piles. Wood was the pile material first used and is still in common use due to economy and workability. The main drawbacks to wood are in the areas of durability and heavy load capacity. Precast concrete piles can carry heavy loads but are bulky to handle with a tendency towards cracking. Precast, pre-stressed piles help eliminate many of the cracking problems. There is a wide variety of proprietary cast in place concrete piles. If these piles are used on a project, the Construction Supervisor should try to obtain and review the manufacturer's literature. Steel H piles are made of rolled steel H sections and are often used due to their combination of high strength and relative economy. Steel pipe piles are often somewhat higher in cost but have the advantage of a uniform section in any direction and the full length of the interior can be inspected after driving. Composite piles, finally, are a combination of one material for the lower part and another material for the upper part of the pile.

Sheet piles are defined as piles driven very closely together that interlock, thus forming a continuous wall or sheet. Wood, concrete, or steel are the most common materials for sheet piling. Some common uses for sheet piling include:

1. Permanent bulkhead to retain fill
2. Permanent deep enclosure at base of waterfront structure to prevent erosion or scour.
3. Permanent forms for cofferdams, retaining walls, piers, etc.
4. Temporary construction wall to retain earth at excavations

When working with sheet piling it is important to consider both the method of driving and the method of removal before the work begins. Plan the type of machine to use and its working location. A good Piling Contractor will know many "tricks" for effectively using the different types of piles. Conversations and planning with the Piling Contractor prior to work can be extremely helpful.

One of the four common uses for sheet piles listed above is the temporary construction wall to retain earth at excavations. Often the Contractor has responsibility to design, install, maintain and remove these sheet piles. There are some simple, but important considerations when deciding what method of sheet piling to use. The first decision must be whether the sheeting wall must retain water. Steel interlock sheet piling is designed to retain water as will the various wood interlock systems shown. The concrete interlock sheet piling is generally less effective at retaining water since it lacks the positive interlock of steel and the swelling characteristics of wood. Wood sheeting without interlocks or soldier piles (vertical driven steel H-piles with horizontal timber sheeting) do not retain water.

The use and location of the excavation will dictate if water must be retained by the sheet piling. The Construction Supervisor must understand if the sheet piling wall actually will retain water or let water pass through the sheeting. The load resisted by the sheet piling is 2 to 4 times larger if water is retained. Many failures result from not taking this simple fact into consideration. The next important consideration is whether to cantilever or simply support the sheet pile wall. A cantilever sheet piling has no horizontal bracing and transfers the retaining wall load to the soil at the sheet piling toe. It is important to know that the pieces of vertical sheeting have the structural capacity to carry that load and that the soil at the sheeting toe can also resist the load. The soil at the sheeting toe resists the load as a structural couple, so increasing the amount of sheeting imbedment in the soil significantly increases the load capability. A simply supported sheet piling wall has horizontal whalers supporting the sheeting. The soil at toe of the sheeting then resists horizontal load only and no moment. The whalers must be appropriately designed and supported at the ends for the retaining wall load.

What to Know about Drilling Caissons?

Caissons, likes piles, are a deep foundation type that are most often used when shallow, spread footings are not feasible. The first distinction of caisson types is pneumatic or open air. Pneumatic caissons (or compressed air caissons) have air tight sides and top and are open on the bottom only. Usually a double air lock door is used at the top of the caisson and the caisson is pressurized.

Open caissons can also be broken down into two types: sheet pile box and drilled pier. The sheet pile box caisson is formed using conventional excavation equipment and some type of sheet piling (see above discussion of sheet piling). There are several different sequences for excavating and driving sheet piles that yield an acceptable sheet pile box caisson. The specific circumstances will dictate the sequence of work for the Construction Supervisor. Some projects require the caisson sheeting to be removed while other projects require the sheeting to remain. This question must generally be approved by the Design Professional or Owner.

Drilled pier caissons diameters are generally chosen by the design professional by determining the load from the superstructure and distributing that load to the rock beneath the caisson. For example a building column may have a load of 100 tons and the allowable rock bearing of 20 tons per square foot.

100 tons / 20 tons/sf    =      5 sf of rock bearing required

Since a 3' diameter circle has an area of about 7 sf, a 36" caisson would be selected.

There is a method of increasing the rock bearing area without making a larger diameter caisson. It's called a bell caisson. A bell is formed by a special auger that increases the diameter caisson immediately above the rock bearing surface therefore the 100 ton load used in the example above could be carried by a 24 inch diameter caisson with a 36" bell. Bells can not be used if the soil immediately above rock is not suitable to be undercut.

Another method of increasing load in drilled pier caissons uses a rock socket. If a caisson is drilled several feet in solid rock, the load from the superstructure can be transferred to both the rock below the caisson and the sides of the rock (rock) socket.

Another important consideration with drilled pier caissons is verification of a solid, level bearing rock surface. Generally caissons 30 inches diameter or larger can be temporarily protected with steel casing and inspected. If the rock surface is sloping steeply, the inspector may require that the bottom be jack-hammered to a more level surface. The inspector can also check the quality of the rock at bearing. Finally, in sinkhole prone areas, a small diameter (2" - 3") hole may be drilled several feet below the bearing area to insure solid rock and no voids.

Payment for drilled pier caissons can vary from totally unclassified to unit price for rock or earth drilling. The method of payment should be clearly understood by the Construction Supervisor and a mutually acceptable record-keeping system should be instituted from the beginning of the project.

What Public Domain Documents are Available for Further Study?

The US Dept of Defense Deep Foundations Manual provides excellent detail for design and installation of many kinds of deep foundations. This 195 page handbook is officially called UFC 3-220-01A 16 January 2004.

The US Dept of Defense Pile Driving Equipment Manual provides lots of information to understand pile driving. This 151 page handbook is officially called UFC 3-220-02 16 January 2004.

Tricks of the Trade & Rules of Thumb for Sitework Structures & Deep Foundations:

1. To avoid retaining wall failures, make sure the water drainage system at the retaining wall actually works as designed.
2. When backfilling retaining walls, understand if they are designed as cantilevers or basement walls. 
3. Understand any extra payment terms for deep foundations prior to the start of work.