Pile capacity calculator for driven and bored piles

Describe the pile, its load and the strata it goes through. Rischio’s engine works out the shaft resistance stratum by stratum, the toe resistance and, for a group, its capacity as individual piles and as a block, as the studio does. It’s free and needs no account.

How it calculates

A pile carries its load by friction along its shaft and by bearing at its toe: $Q_u = Q_s + Q_p$.

The shaft is cut at each stratum boundary. In each stratum the pile adds $Q_{s,i} = f_s\,p\,\Delta L_i$, with $p$ its perimeter and $\Delta L_i$ the length inside the stratum.

• In clay, short term, the α method: $f_s = \alpha\,c_u$, with the adhesion factor $\alpha$ from the ratio $c_u/p_a$. It goes from 1 for a very soft clay down to 0.34 for a very stiff one.
• In sand, the β method: $f_s = K\,\sigma'_v \tan\delta$, with $\delta = 0.8\,\phi'$ and $K$ a multiple of $K_0 = 1 - \sin\phi'$: $K_0$ for a bored pile, $1.3\,K_0$ for a driven pile under 0.3 m and $1.7\,K_0$ for a larger one. $\sigma'_v$ stops growing at the critical depth, $z_c = 15D$.

The toe bears $Q_p = q_p A_p$ on the stratum it rests on, after Meyerhof. In clay, $q_p = 9\,c_u$. In sand,

with $q'$ the effective stress at the toe and $N_q^{\ast}$ Meyerhof’s factor for $\phi'$, which grows quickly with it: 115 at 34°, 346 at 40°.

A group of $n$ piles can fail pile by pile, $n\,Q_u$, or as a block: the piles and the soil between them, as one large pile with the group’s outline. The weaker governs, $Q_g = \min(\eta, 1)\,n\,Q_u$ with the efficiency $\eta = Q_{block}/(n\,Q_u)$. A single pile, or a group, needs a factor of safety of 3 or more on its load.

Each step, with its tables, is in the docs: shaft resistance, toe resistance and groups.

How to use it

1. Pick how the pile is installed, driven or bored, and its section, round or square.
2. Give its diameter or side, its length below the ground and the vertical load. For a group, give the piles along each side and their spacing, centre to centre; the load is then the group’s.
3. Turn on the water table if there’s one, with its depth.
4. Describe the strata from the ground down, up to three: clay with its undrained strength $c_u$, or sand with its friction angle $\phi'$. The last one goes on down below the toe.

The results show the pile’s or the group’s capacity and its factor of safety, the pile drawn through its strata with the shaft resistance, a table of each stratum’s share and another of the toe and the group. Restore the example brings back a 15 m driven pile through soft clay into dense sand. To keep the model, add more strata, or get the calculation report with every step, design it in Rischio’s pile foundation tool.

Frequently asked questions

How do you calculate the capacity of a pile?

Add up the friction along the shaft, stratum by stratum, and the bearing at the toe: $Q_u = Q_s + Q_p$. In clay the shaft friction comes from the undrained strength (the α method), and in sand from the effective stress and the friction angle (the β method). Divide by a factor of safety to get the load the pile can take.

What is the difference between skin friction and end bearing?

Skin friction, or shaft resistance, is the soil’s grip along the pile’s sides. End bearing, or toe resistance, is the soil’s support under its tip. A friction pile in clay carries most of its load along the shaft, and an end-bearing pile on dense sand or rock mostly at the toe.

What factor of safety is used for piles?

Usually between 2 and 3. The more the capacity is confirmed on site, with load tests or dynamic testing, the lower the factor can be. This calculator asks for 3, the usual value when the capacity comes from a static calculation alone.

What is pile group efficiency?

The ratio between what the group carries as a block and the sum of its piles one by one, $\eta = Q_{block}/(n\,Q_u)$. Below 1, the block fails first, which happens with closely spaced piles in clay. At 1 or above, the piles fail one by one and the group carries $n\,Q_u$.

Does a driven pile carry more than a bored one?

In sand, usually yes: driving displaces and densifies the soil around the pile, so the engine takes a larger $K$ for it, up to $1.7\,K_0$ against $K_0$ for a bored pile. In clay, the α method uses the same factor for both.