For deep foundations (i.e., piles and caissons) – has the engineer specified a drilled pile or mini-caisson when a driven pile option would suffice?

Engineers and clients may be nervous about vibration and noise, but we have successfully driven piles all over the City. Most of the time, driven piles are much faster and much less costly (even though they typically have a greater tendency to deviate). Sometimes, piles will be specified as 100% drilled due to a sensitive adjacent condition (a TA structure or a historically significant building) when in fact only some portion of the site adjacent to that condition needs to be drilled. Usually, the savings from driven piles easily outweighs the marginal inefficiency of mobilizing a pile driving rig for only a portion of the site. When making this change, depending on the clients’ appetite for risk and the structure of our contract, it will often make sense to include an allowance for structural redesigns necessitated by deviation. (As mentioned earlier, driven piles can sometimes deviate considerably.)

Can the pile specification be modified based on the market?

In the last several years, “secondary market” pipe has had a lower price than traditional pipe piles or H-piles. Secondary market pipe is cast-off steel piping from the oil and gas industry (which has far more stringent quality control standards for their pipelines than we do for our piles). Because it is dependent on what pipe is being purchased and rejected at the time, the market is very volatile and the material specification available changes constantly. Our goal in preconstruction then is to make sure the geotechnical and structural engineer are open to alternative materials that are structurally equal to or better than the specified pile.

Can we get more capacity from the mini-caissons?

The cost of mini-caissons is predominantly generating by drilling time. Often, mini-caissons can be relied upon for additional capacity with relatively minor changes—increased internal reinforcement, casing wall specification, etc. If increasing the capacity of each individual caisson can reduce the overall quantity of caissons, the savings in drilling time, lineal feet of casing, and grout will often easily outweigh the increased cost of reinforcement.

Is a secant wall specified when a soil mix wall will do?

If soil conditions are right, we can sometimes save cost by using a soil mix wall instead of a secant wall. Engineers less familiar with this technique may be more likely to specify a traditional secant wall. Soil mix walls are similar to secant walls. But instead of 100% concrete they are made of site soil mixed (on-site, during drilling) with grout or concrete to form a slurry. In this way they save material (concrete) and time. Vertical reinforcing elements can be inserted similarly to a secant wall.

Can support of excavation (SOE) soldier piles be driven or vibrated?

SOE engineers will often, by default, show drilled soldier piles for SOE. Like the permanent deep foundation elements discussed above, we can typically achieve savings by going to a driven option. If drilling is required for one reason or another, we typically want to avoid drilled-in H-piles and prefer a drilled mini-caisson. This is based on bidder preference at the time of this writing. And their preferences can change by the week.

Are secant sections too small?

Engineers will often show a secant wall with the smallest possible section in order to save on concrete when, in fact, larger sections—even though they add concrete material—will save enough drilling time to be far less expensive. (This is another one that is highly dependent on bidder preference so proceed with caution.)

Can pipe pile fill be eliminated?

Pipe piles are often specified with grout fill that is unnecessary. Often the pipe pile is perfectly capable of supporting the loads required (based on the steel section) without the added fill and this scope can be eliminated altogether. If the pipe pile cannot carry the required loads with the steel section alone, we may consider increasing the wall thickness as necessary to avoid concrete fill. Most of the time, increasing the steel will be less expensive than providing the fill. The engineer of record may look for extra, sacrificial thickness if they are expecting ongoing corrosion.

Does the engineer really need the pile capacity they say they do?

When the geotechnical engineer writes their report, they speak to the subsurface condition and, for buildings with deep foundations, they will typically recommend a pile design and state that design’s capacity. (And they are likely to be doing this before they know anything about the building design whatsoever.) Let’s say for example that the geotechnical report speaks to a 200-Ton compressive capacity pile. The structural engineer will often then copy and paste that pile design and that compressive capacity onto her drawing set and call it a day. The problem is that the structural design of the building may not need a 200-Ton pile. The loads on each pile cap may be such that a 125-Ton pile will suffice. For example, if she has a column load of 250 tons and doesn’t question the pile spec, she is going to specify a pile cap with two 200-ton piles (and 150 tons of wasted capacity). There will always be a difference between the capacity of the pile the geotechnical engineer recommends and the capacity of the pile that the structural engineer needs. It is important to question this at the outset of each project to make sure we are not spending money on heavier piles than are truly necessary.

Are splicing requirements onerous?

Piles will often need to be driven deeper than the length a driving rig can comfortably manage and this leads to splicing. This splicing can be the most expensive part of the whole deep foundation operation. The splice will typically require the rig to hold the new section in place. In this way the entire duration of the splice represents lost productivity for almost the whole crew. If a design engineer is not conscious of this, they may specify a full-penetration weld splicing method (or some other time-consuming method) when a drive-fit splicer or another faster method would suffice. Some relevant factors for allowable splicing methods will be the predicted ultimate depth of the splice (since the coupling is done above grade and then driven below grade), and whether the deep foundation element will be subjected to uplift forces.

Are all the load tests noted on the drawings actually necessary?

Unnecessary pile load tests are often included in drawing notes and/or specifications. Under certain thresholds, uplift and lateral load testing can be avoided. Most of our buildings in fact require only compressive load testing. Unnecessary load tests are often specified because the specifications or stock drawing notes for deep foundations are written before the foundation loads are finalized (and they are written as conservatively as possible) …and nobody remembers to go back and edit them.

Can PDA be eliminated?

Pile Driving Analysis (PDA) is a sophisticated method of measuring the strain in steel piles while they are being driven to predict their overall capacity. It is often specified for projects using friction piles (instead of end-bearing). It provides the engineers with another indicator of capacity (aside from the traditional blow count measures) in advance of load testing. However, it is a non-trivial cost and more importantly it is fickle. It can often under-estimate a pile’s ultimate capacity leading to confusion and problems during site indexing and load testing. Furthermore, the blow count method has proven reliable on countless projects. For these reasons we will often try to eliminate it as a project requirement.

Rock Anchors

Rock anchors are used to prevent foundation uplift due to wind, earthquake, and/or water. Since rock anchor installation requires that the grout reach full strength as well as cyclical load testing, it can severely impact foundation schedules (depending number of anchors required). For this reason, they should be considered only as a last resort for controlling foundation uplift. If rock anchors are being specified by the engineer in order to control for wind and earthquakes under shear-walls or steel trusses, we may instead be able to control those forces by connecting the shear elements to nearby columns or walls using strap beams. The idea in that case is to use the weight of the building to counterbalance the wind. Providing a strap beam is typically more cost effective (and faster) than drilling in rock anchors. Where rock anchors are used to control for water or flood forces, it is often cheaper and easier to simply increase the foundation thickness (1 foot of concrete can pin down 2.4 feet of water). It should be noted that the additional concrete should reduce reinforcement requirements (but at the same time it may increase dewatering, excavation, and SOE). The balance between dewatering, excavation, concrete, reinforcement, SOE, and the elimination of rock anchors is worthy of exploration for most projects where rock anchors are specified.

Are deep foundations really required throughout the site?

There are a handful of conditions that may trigger a deep foundation: poor soil, high groundwater, neighboring structures, transit authority adjacency, and differential settlement are some examples. However, through close study of the geotechnical report and boring logs, it may be possible to establish a more precise contour of rock elevations and find areas where deep foundations can be avoided. It wouldn’t be sensible to penalize the entire site if only a portion of the project requires a deep foundation. Instead, a hybrid system with both shallow and deep foundations should be considered and proposed to the engineer of record. Monadnock completed similar hybrid foundations recently at Riverwalk 8, Second Farms, and Lambert 3A.