RapidRoot™ Design Calculator — Australia

AS/NZS 1170 | AS 4100 (steel) | AS/NZS 1664.1 (aluminium) | AS 2159 (piling)

Pile selection

Pile material

CHS pipe size

Wall thickness (mm)

Piles per cap

Embedded length L_ent (m)

Stickup L_ext (m)

Number of pile caps

Pile cap reference

RR8

—

Service actions — per cap (AS/NZS 1170 nominal)

G — permanent vertical (kN)

Q — imposed vertical (kN)

Wx — lateral X (kN)

Wy — lateral Y (kN)

Mx — moment X (kNm)

My — moment Y (kNm)

Mz — torsion (kNm)

Strength combo: 1.2G + 1.5Q on verticals, 1.0W on horizontals (AS/NZS 1170.0 §4.2.2). Uplift: 0.9G − Wu (§4.2.2(f)).

Design parameters

Design working life (yrs)

Soil corrosivity (steel only)

n_h — subgrade modulus (MN/m³)

Max settlement δ_max (mm)

Groundwater

Water table

No groundwater correction applied.

AS 2870 site classification — reactive soils

Site class

Active zone depth H_a (m)

Characteristic y_s

0 mm

For Class M sites, shaft friction in the active zone is excluded from compression and tension resistance, and a swelling uplift force is added to the tension demand. Pile embedment must be at least H_a + 1 m for proper anchorage.

Soil data source

Enter SPT-N blowcounts (blows/300 mm per AS 1289.6.3.1) per layer. Leave unused rows blank.

Soil layers (up to 10)

#	Thick. (m)	Type	γ (kN/m³)	SPT-N	c_u (kPa)	φ′ (°)	DPM-30/20 raw blows	N_SPT equiv (×0.006842)

Appendix — typical soil parameter ranges

Guidance only. Always prefer measured values from a geotechnical investigation (AS 1726).

Cohesive (C) — c_u (kPa)

Very soft clay	< 12
Soft clay	12 – 25
Firm clay	25 – 50
Stiff clay	50 – 100
Very stiff clay	100 – 200
Hard clay / mudstone	> 200

Non-cohesive (NC) — φ′ (°)

Loose silty sand	25 – 30
Loose sand	28 – 32
Medium dense sand	32 – 36
Dense sand	36 – 40
Very dense sand	40 – 45
Sandy gravel	34 – 38

SPT-N (blows/300mm)

Very loose sand	N < 4
Loose	4 – 10
Medium dense	10 – 30
Dense	30 – 50
Very dense	> 50
Soft clay	2 – 4
Stiff clay	8 – 15
Hard clay	> 30

Unit weight γ (kN/m³)

Peat / organic	10 – 14
Soft clay	16 – 18
Firm / stiff clay	18 – 21
Loose sand	15 – 20
Dense sand	18 – 22
Sandy gravel	19 – 22

n_h (MN/m³)

Loose sand (dry / submerged)	2.5 / 1.4
Medium dense (dry / submerged)	7.5 / 5.0
Dense sand (dry / submerged)	20 / 12

Steel corrosion (60-yr loss)

Undisturbed natural soils	0.60 mm
Non-aggressive fill	1.20 mm
Polluted / industrial	1.50 mm
Aggressive natural soils	1.75 mm
Non-compacted aggressive fills	3.25 mm

References: Terzaghi (1955); Reese & Matlock (1956); Davisson (1970); AS 1289.6.3.1 (SPT); AS 1726 (geotechnical site investigation); AS/NZS 2312 + AS 4312 (corrosion).

Limit state checks — worst-case pile

Pile section at end of design life

Auto design — service actions (factored internally)

Soil data is read from the Soil profile tab. Results show minimum pile length for each cap/pile combination, ranked by cost efficiency.

G — permanent vertical (kN)

Q — imposed vertical (kN)

Wx — lateral X (kN)

Wy — lateral Y (kN)

Mx — moment X (kNm)

My — moment Y (kNm)

Soil parameters — from soil profile tab

SPT-N, c_u and DPM data are read automatically from the Soil profile tab.

Weighted avg SPT-N

— enter layers in Soil profile tab

Weighted avg c_u (kPa)

—

Data source

—

n_h (MN/m³)

Soil corrosivity (steel)

Design working life (yrs)

Minimum pile length

Maximum pile length

Pile options to include

Steel C350L0 — CHS 60.3 × 4.0 Steel C350L0 — CHS 48.3 × 4.0 Alu 6061-T6 — OD 60 mm Alu 6061-T6 — OD 48 mm

Calculation methodology

This page documents the engineering basis used by this calculator so a qualified design professional can audit the approach against the project context, the controlling code, and their own check calculations.

1. Code framework

Calculations follow the Australian / New Zealand structural code framework. The controlling standards are:

Reference	Title / scope
AS/NZS 1170.0	Structural design actions — general principles
AS/NZS 1170.1	Permanent, imposed and other actions
AS/NZS 1170.2	Wind actions
AS 4100	Steel structures
AS/NZS 1163	Cold-formed structural steel hollow sections (CHS)
AS/NZS 1664.1	Aluminium structures — Limit state design
AS 2159	Piling — Design and installation
AS 2870	Residential slabs and footings — reactive soil classification
AS 1289.6.3.1	Standard penetration test method
AS/NZS 2312 + AS 4312	Protective coatings for steel — corrosion classes

2. Partial / capacity reduction factors

2.1 Load combinations (AS/NZS 1170.0 §4.2.2)

Combination	Reference
1.2G + 1.5Q	Eq 4.2.2(b) — strength
1.2G + W_u + ψ_cQ	Eq 4.2.2(d) — strength with wind
0.9G − W_u	Eq 4.2.2(f) — uplift

The calc uses 1.2G + 1.5Q on verticals, 1.0W on horizontals and moments, and 0.9G for the counter-action in the uplift check.

2.2 Capacity reduction factors

Quantity	Symbol	Value	Source
Steel bending	φ_b	0.90	AS 4100 §3.4
Steel compression	φ_c	0.90	AS 4100 §3.4
Steel shear	φ_v	0.90	AS 4100 §3.4
Aluminium bending	φ_b	0.85	AS/NZS 1664.1 §1.6
Aluminium compression	φ_c	0.75	AS/NZS 1664.1 §1.6
Geotechnical compression	φ_g	0.55	AS 2159 §4.3 (SPT-based, no load test)
Geotechnical tension	φ_g,t	0.40	AS 2159 §4.3

AS 2159 §8.2 allows φ_g to be raised to 0.7–0.9 when a project-specific static load test confirms capacity. The calc does not model that; for high-utilisation designs a confirmatory load test on site is recommended.

3. Load distribution to inclined piles

The pile cap is assumed to distribute the resultant action evenly between the n piles in the group. The piles are sufficiently spaced that single-pile group efficiency factors are not applied (a deliberate conservatism). For a cap of radius r with n piles arranged on a circle:

Σ x_i² = (n/2)·r²(1)

Σ (x_i² + y_i²) = n·r²(2)

The worst-pile vertical share, combined with the cap moments M_x and M_y, uses the envelope distance x_m = r/√2 (which corresponds to the pile at 45° from the moment axis, where M_x and M_y contributions superimpose):

H_z,gov = F_d/n + (x_m·M_y,d + x_m·M_x,d) / Σx_i²(3)

Per-pile horizontal shares include the torsion contribution:

H_xi = H_x,d/n + (x_m·M_z,d) / Σ(x_i² + y_i²)(4)

The piles are installed at an angle α = 25° from vertical (specifying a fixed rake reduces parameter complexity for designers). At the pile head, the vertical and horizontal force components from the cap are resolved into the pile's axial (along-axis) and shear (perpendicular-to-axis) components:

N_axial = H_z,gov·cosα + √(H_xi² + H_yi²)·sinα(5)

V_shear = √[(H_xi·cosα)² + (H_yi·cosα)² + (H_z,gov·sinα)²](6)

All subsequent capacity checks compare these axial and shear forces against the corresponding axial and shear capacities along/across the pile axis. Action and resistance are projected to the same frame; group efficiency factors are not applied.

4. Pile section — structural capacity

4.1 Steel piles — C350L0

Property	Symbol	Value	Notes
Yield strength	f_y	350 MPa	AS/NZS 1163 C350L0
Modulus of elasticity	E	200 000 MPa	AS 4100
Available CHS sizes	OD × wall	48.3 × 4.0 mm / 60.3 × 4.0 mm	—

Section properties for the corroded annulus follow equations (12)–(14) from §4.1.

4.2 Aluminium piles — 6061-T6

The aluminium pile is a 6061-T6 extrusion to AS/NZS 1664 with 6 internal longitudinal ribs at 60° spacing. Two sizes are offered (areas verified against measured mass per metre):

Size	OD (mm)	Wall (mm)	A (mm²)	I (mm⁴)	Z (mm³)	Mass (g/m)
AluPile 48	48.0	3.6	651.3	151 674	6320	1758
AluPile 60	60.0	4.5	1017.0	370 299	12 343	2747

Material property	Symbol	6061-T6	Notes
Yield strength	f_y	240 MPa	AS/NZS 1664.1
Modulus of elasticity	E	70 000 MPa	AS/NZS 1664.1

4.3 Capacity formulas

φ·N_s = φ_c · χ · f_y · A(15)

φ·M_s = φ_b · f_y · Z_el(16)

Buckling reduction χ = 0.30 is the same calibrated factor as the EU calc.

4.4 Corrosion approach

The two pile materials corrode by fundamentally different mechanisms, and are modelled separately. Steel corrodes uniformly enough to be treated as a year-on-year wall-thickness deduction. Aluminium corrodes by localised pitting — stochastic, sparse, and a poor fit for a uniform-loss approach — and is modelled through the FDOT pit-progression service-life envelope.

4.4.1 Steel piles — uniform corrosion deduction. Per AS/NZS 2312 and AS 4312 (which align with EN 1993-5 Annex D loss categories), scaled linearly with the design working life L_D:

Soil category	60-yr loss (mm)
Undisturbed natural soils	0.60
Non-compacted non-aggressive fills	1.20
Polluted natural / industrial	1.50
Aggressive natural soils	1.75
Non-compacted aggressive fills	3.25

t_DWL = max(t_nom − Δt · L_D/60, 0.5 mm)(17a)

4.4.2 Aluminium piles — FDOT pit-progression model. Aluminium does not corrode uniformly. Once the natural oxide layer is breached, attack proceeds as a small number of localised pits that grow downward roughly logarithmically. The wall around each pit stays essentially intact. The implications:

Average mass loss is small, but maximum pit depth grows faster than the area-mean section loss — the deepest pit at any time is typically 3 to 5 times deeper than the average loss (Bushman & Lytton ratio, FHWA-RD-89-186).
The classical engineering failure point for a pipe is “first pit-through” — one pit reaches the inner wall and the pipe leaks. For a pile this is not a structural failure: a 60 mm OD pile with a 0.5 mm hole has lost < 0.001% of cross-sectional area, with no measurable effect on axial, moment, or elastic-length capacity.
A uniform Δt · L_D deduction therefore overstates the structural impact of pitting by an order of magnitude. This calculator does not apply a uniform aluminium corrosion deduction.

Instead, the calculator reports a service-life envelope per the Florida Department of Transportation method published in the FDOT Drainage Design Guide, Appendix M (January 2024) — the most thorough public-domain treatment of buried aluminium infrastructure:

SL = T_p / (R_pH + R_r) [years to first pit-through](17b)

where
T_p = wall thickness (mm)
R_pH = pit-progression rate from soil pH (mm/yr)
R_r = pit-progression rate from soil resistivity (mm/yr)

The FDOT table is published for 16 ga aluminium pipe (~1.5 mm wall). The calculator scales linearly with the actual wall thickness of the RapidRoot pile (T_p/1.5).

4.4.3 Soil corrosivity tiers. The user’s soil corrosivity selection is mapped silently into one of five aluminium-durability tiers. Resistivity values shown are typical ASTM G57 ranges:

Soil category	Tier	Typical resistivity	AluPile 48 (3.6 mm) SL	AluPile 60 (4.5 mm) SL
Undisturbed natural soils	Optimal	> 5,000 Ω·cm	~480 yrs	~600 yrs
Non-compacted non-aggressive fills	Mild	2,000–5,000 Ω·cm	~240 yrs	~300 yrs
Polluted / industrial	Moderate	700–2,000 Ω·cm	~144 yrs	~180 yrs
Aggressive natural soils	Aggressive	200–700 Ω·cm	~96 yrs	~120 yrs
Non-compacted aggressive fills	Severe	< 200 Ω·cm	Out of scope — not recommended

4.4.4 Threshold checks shown on Results. Rather than reporting a precise design-life year (which would imply more precision than the stochastic pitting model can deliver), the calculator reports two binary checks at thresholds well above typical building design life (50–60 yrs):

≥ 75-year durability envelope: passes for Tiers 1–4 on both AluPile sizes. Generally a non-issue.
≥ 100-year durability envelope: passes for Tiers 1–3 on both sizes; AluPile 60 also passes Tier 4. For AluPile 48 on a Tier 4 site, the result is flagged “Marginal” with a recommendation to upgrade to AluPile 60 or switch to steel.

The 75-yr and 100-yr thresholds are deliberately coarser than the corresponding steel deduction. The intent is to give the designer a clean go/no-go on aluminium suitability rather than to suggest the calc can predict pile life to a year. Aluminium pit depth at any point in time is best understood as a distribution — the FDOT formula gives the median/expected first-perforation time, and real-world maximum pit depth could be reached earlier or later by a factor of 2–3 (Gumbel extreme-value distribution, Aziz & Godard 1989).

5. Geotechnical capacity — Meyerhof SPT method

Meyerhof (1976) SPT correlation for unit shaft friction in cohesionless soils, with the standard upper cap at N = 50:

q_s = 2·min(N, 50) [kPa](7)

The cap at N = 50 reflects field observation that shaft friction does not continue to scale linearly above this density level (Tomlinson 1994). The factor of 2 is the original Meyerhof value, consistent with NAVFAC DM-7.2 for driven small-diameter piles.

Meyerhof (1976) SPT correlation for unit base resistance, with the standard upper bound at fully developed end bearing in dense sand (≈ 4 N MPa, i.e. 40 N at full penetration depth ratio):

q_b = min[0.4·N·(L/D), 40·N] [kPa](8)

The (L/D) ratio is the penetration ratio of the pile into the bearing stratum, capped at 100 by the second term. For small-diameter micropiles (D ≈ 0.05 m) the L/D term grows quickly and the cap is reached at modest embedment.

Characteristic shaft and base resistances are then:

R_s,k = q_s·π·D·L_eff(10)

R_b,k = q_b·π·D²/4(11)

where
D = pile outside diameter (m)
L_eff = effective shaft length (m), equal to L_ent in non-reactive ground; reduced for reactive sites — see §7

Where the embedment passes through multiple soil layers, the SPT-N value used in equations (7) and (8) is the average weighted by the thickness of each layer intersected by the pile:

N_avg = Σ_i(N_i·t_i) / Σ_i t_i(9)

Layers beyond the pile toe are not included (the weighting is clipped to the actual embedment depth). This avoids overstating capacity when a strong stratum sits below the pile toe.

6. Lateral analysis — Reese-Matlock T-method

The elastic-foundation characteristic length T is computed for both cohesive and non-cohesive sub-models, and the larger of the two is used (conservative envelope on mixed profiles):

T_clay = (EI / (67·c_u))^1/4(17)

T_sand = (EI / n_h)^1/5(18)

Depth to maximum moment, capped at 1.3 T:

Z_o = min[(0.25 + 0.8·ln(L_ent/T))·T, 1.3·T](19)

Worst-case bending moment combining cantilever stickup and in-ground peak:

M_max = V·L_ext + 0.64·V·Z_o(20)

L_ext is the pile stickup above ground; the 0.64 coefficient is the Broms / Reese-Matlock dimensionless moment maximum for a fixed-head flexible pile.

7. Reactive / expansive soils

7.1 Why this matters

Expansive (reactive) clays undergo cyclical volume change with seasonal moisture content. As the soil swells in the wet season it engages the pile shaft in uplift, generating tension in the pile and the cap connection; as it shrinks in the dry season the shaft friction in the upper soil is lost. A pile that is bonded entirely within the active zone will translate up and down with the seasons, taking the building with it.

7.2 Site classification

The user selects a site classification from the standard band system used in the controlling code (§7.6 below). Each class carries a characteristic surface ground movement y_s and an assumed active zone depth H_a. Both can be overridden by the user when a site-specific geotechnical investigation provides better values.

7.3 Triangular suction profile

The disruption to shaft friction follows the suction-change profile, which is approximately triangular — full disturbance at the surface, zero at the bottom of the active zone H_a. Integrating the triangle gives an average disruption factor of exactly 0.5 across the active zone. This calculator therefore uses an effective active-zone depth of:

H_a,eff = 0.5 · H_a(20)

This is materially less conservative than treating the active zone as fully disengaged. The 0.5 factor is consistent with AS 2870 Appendix D, EN 1997-1 Annex H and the FHWA-IF-99-025 guidance on drilled piers in expansive soils. Bostani-Taleshani et al. (2020) field-measured 70% movement attenuation on a 1.2 m pile in H2 ground — i.e. effective engagement of ~30%. The 0.5 factor sits between this measurement and the fully-conservative uniform model.

7.4 Effective shaft length

L_eff = L_ent − H_a,eff = L_ent − 0.5·H_a(21)

Shaft friction in equations (10) and (15) uses L_eff rather than L_ent when the calc determines the active zone is engaged.

7.5 Swelling uplift force on the pile

P_swell = q_s,swell · π·D·H_a,eff·cosα(22)

where q_s,swell is a class-specific unit uplift friction (8 to 50 kPa, increasing with site class). This force is added to the tension demand on the pile.

7.6 Anchor depth check

Independent of disruption magnitude, the pile must reach stable ground below the full active zone. The calc applies:

L_ent ≥ H_a + (anchor margin)(23)

with anchor margin = 1.0 m for EU/AUS and 3 ft (0.91 m) for US.

7.7 NC soil override

Non-cohesive (sand, gravel) soils do not swell. If every layer that intersects the active zone has layer type = NC, the reactive adjustments are switched off automatically — regardless of the selected class. This avoids penalising a sand site that happens to share a classification with a clay site.

7.8 Hard reject for severe sites

The two highest reactivity classes (VH and P; AS 2870 H2, E and P) trigger a hard reject. With a 3 m pile-length envelope (10 ft in US) the pile cannot be reliably anchored below an active zone that exceeds ~2.5 m. The calc displays an out-of-scope notice with the recommended response: longer pile system, or project-specific load test.

7.9 Site classification table

Class	y_s range	H_a default	q_s,swell	Status
A	0 mm	0 m	0 kPa	Pass — non-reactive
S	0–20 mm	1.2 m	8 kPa	Pass with adjustment
M	20–40 mm	1.5 m	15 kPa	Pass with adjustment
H1	40–60 mm	2.0 m	25 kPa	Pass with adjustment (marginal)
H2	60–75 mm	2.5 m	35 kPa	OUT OF SCOPE
E	> 75 mm	3.5 m	50 kPa	OUT OF SCOPE
P	site-specific	3.0 m	40 kPa	OUT OF SCOPE

8. Serviceability — settlement check

Serviceability is checked using a 1.0G + 1.0Q service load combination (no factors) per the simplified approach in EN 1997-1 §2.4.8 / AS 2159 §8.4 / IBC §1810.3.3.3.

8.1 Elastic shaft compression

Assuming linear load transfer to the shaft (the average axial load along the shaft is N_service/2):

δ_shaft = N_service·L_ent / (2·A·E)(24)

where A and E are the pile section area and Young's modulus.

8.2 Tip movement

Empirical, proportional to base mobilisation. Per Tomlinson (1994) and AS 2159 commentary, tip movement at full base mobilisation is approximately D/100 for small-diameter piles:

δ_tip = (N_service / R_n)·D/100(25)

8.3 Total and permissible settlement

δ_total = δ_shaft + δ_tip ≤ δ_max(26)

The user sets δ_max on the Inputs tab (default 25 mm / 1.0 in).

9. Limitations of this calculator

Pile diameter restricted to 48 mm and 60 mm equivalents.
Maximum pile embedment 3 m (10 ft).
Group efficiency factor not applied (single-pile resistance summed across the cap).
Project-specific static load testing is not modelled. The code permits favourable adjustment of resistance factors on the basis of load test; that adjustment is the engineer's call.
Refusal on rock, soil reinforcement, and pile-cap moment transfer are not modelled.
Output is a preliminary sizing only. A qualified design professional must verify the methodology, the input parameters, and the conclusions before any construction work proceeds.

10. References

AS/NZS 1170.0:2002: Structural design actions — general principles.
AS/NZS 1170.1:2002: Permanent, imposed and other actions.
AS/NZS 1170.2:2021: Wind actions.
AS 4100:2020: Steel structures.
AS/NZS 1163:2016: Cold-formed structural steel hollow sections.
AS/NZS 1664.1:1997: Aluminium structures — Limit state design.
AS 2159:2009: Piling — Design and installation.
AS 2870:2011: Residential slabs and footings.
AS 1289.6.3.1:2004: Standard penetration test.
Meyerhof, G. G. (1976). Bearing capacity and settlement of pile foundations.
Reese, L. C., & Matlock, H. (1956). Non-dimensional solutions for laterally loaded piles.
Davisson, M. T. (1970). Lateral load capacity of piles.
NAVFAC DM-7.2 (1986). Foundations and Earth Structures.
Bostani-Taleshani, S. A., Evans, R., Gad, E., & Disfani, M. M. (2020). Performance of driven battered mini-pile group against expansive soil induced ground movement. E3S Web Conf 195.
Mehdizadeh, A. et al. (2016). Field static load testing of concrete-free reticulated micropiles. Proc. ISC5.

Disclaimer. This app is for use by qualified design professionals only, who have reviewed the design methods and verified the output with their own calculations. It is a preliminary sizing tool, not a substitute for project-specific engineering judgement or stamped calculations.