Six hundred millimetres, and nowhere to batter
The drawing that arrives with the enquiry is usually harmless-looking: a rectangle on a plot, 8.5 m to formation, a note saying temporary works by contractor. What decides the price is the dimension nobody has drawn, which is the gap between the box line and the flank wall of the building next door. At two metres you have options. At six hundred millimetres you have two, and they are priced on entirely different arithmetic — a bored pile wall installed before anything is dug, or a face nailed and sprayed as the dig goes down.
Estimators tend to reach for the ground investigation first. That is the wrong document to open first on a job this tight. Open the title plan. A pile wall keeps every piece of permanent and temporary steel inside your own boundary; the bore, the cage and the capping beam all sit in a strip you own. A nailed face does the opposite by definition: the reinforcement only works if it reaches past the surface the ground would fail on, which on an 8.5 m cut puts grout and steel somewhere between five and eight metres inside land belonging to someone who has not yet been asked. That is a rights question before it is a geotechnical one, and it can kill the cheaper option outright at a stage where you still have time to price the other.
Both routes end at the same gate. Before a rig is booked, somebody has to demonstrate on paper that the chosen geometry works — that the piles at the spacing you priced still overlap at the depth you priced, or that the nails at the length you priced still develop the load with the margin the specification demands. Neither demonstration is difficult. Both are routinely skipped at tender and discovered at the toe.
The two systems, and what each one is actually buying
A bored pile wall is a row of drilled shafts installed from existing ground level, before the excavation exists. Contiguous walls leave a deliberate gap between piles and retain soil but not water. Secant walls are bored so the piles physically intersect: primaries go in first at alternate positions, secondaries are then bored so the auger cuts into the two primaries either side, and the wall becomes continuous. The trade names the three variants by the primary mix — hard/soft, where the primary is a low-strength cement-bentonite type material serving only as a water cut-off; hard/firm, where it is a weak concrete cut with standard equipment; and hard/hard, where both are structural and heavy cased or rotary equipment is needed to make the cut. BS EN 1536 covers execution of bored piles and BS EN 1538 the diaphragm wall panels that take over when a pile wall runs out of depth.
A nailed face is the opposite construction logic. Nothing is installed in advance. The face is cut in lifts, and each lift is drilled, grouted, meshed and sprayed before the next one is taken out beneath it. The reinforcement is passive: it does nothing until the ground behind the face begins to move and mobilises tension in the bar. That is why the sequence is unforgiving — the ground has to stand unsupported, on its own, for the hours between cutting a lift and closing it, and that standing time is a soil property you cannot buy.
The two do not fail in the same way, which is the part worth carrying into the tender. A pile wall fails commercially — obstruction, deviation, over-consumption — and all of it surfaces as variations. A nailed face fails physically, through a lift that will not stand, and the consequence reaches the neighbour before it reaches the account.
| The site fact | Bored pile wall | Nailed face |
|---|---|---|
| Reinforcement must cross the boundary | It does not — the shaft, the cage and the capping beam sit inside your own title strip | It must, and the bonded length is the part that crosses; a wayleave or an easement is needed before the length can be priced |
| The face will not stand unsupported for a shift | Irrelevant — the wall is complete before the first bucket comes out | Disqualifying in running sand, soft clay and made ground with no cohesion, whatever the bond table says |
| Groundwater sits above formation | A secant wall toed into a low-permeability layer excludes it; a contiguous wall does not | The face cannot be cut wet, so the water has to be taken down first and kept down for the whole open period |
| Headroom or access is restricted | A rig needs mast height, a certified platform and a route in; restricted-access rigs exist but cost and slow the programme | Drilling and spraying plant is small and works off the face itself, which is often the only reason it wins |
| Obstructions in the made ground | Cased or rotary boring chews through old bases and services at a price; CFA does not | An obstruction on a nail line moves the nail, which moves the grid, which changes the load on every nail around it |
| The shoring is wanted as permanent structure | Routine — the wall becomes part of the basement box with a liner or a facing | Possible but a separate design case; permanent nails bring corrosion protection and design life into the specification |
The overlap is a bet on verticality, not on spacing
The nominal check is trivial and everybody does it. Overlap is pile diameter minus centre-to-centre spacing. Six hundred diameter at four hundred and fifty centres gives a hundred and fifty millimetres of bite, which sits inside the hundred to a hundred and fifty millimetres commonly named as a design target — though the number that governs is the one written into your own specification, not the customary range. Put the same two numbers the other way round and you have not designed a poor secant wall, you have designed a contiguous one with a gap, and the wall will not hold water at any depth.
The check that matters is what is left of that bite at the toe. Piles are bored to a verticality tolerance expressed as a fraction — read the figure off your own piling specification rather than off a rule of thumb, because BS EN 1536 and the ICE Specification both state it and the project may tighten it. Whatever the fraction is, the arithmetic behind it is the same and it is worth doing by hand once. Two adjacent piles are each permitted to lean by that fraction, and nothing stops them leaning in opposite directions. At a tolerance of one in a hundred, over a depth z, the pair can separate by two hundredths of z. At eight metres that is a hundred and sixty millimetres. At twelve metres it is two hundred and forty.
Set that against the hundred and fifty millimetres of design bite and the conclusion is uncomfortable: a 600 diameter wall at 450 centres has, in the worst permitted case, lost its overlap somewhere around seven and a half metres down. Above that line it is a secant wall. Below it, on paper, it is a row of piles with a slot between them — and the slot is at the depth where the head of water is greatest and where nobody can see it. This is the single reason deep secant walls go to larger diameters and tighter spacings than a first pass suggests: 750 at 550 centres gives two hundred millimetres of bite, which survives ten metres at one in a hundred and only seven and a half at one in seventy-five.
There are three ways to buy the margin back and all of them are priced. Increase the design overlap, which means more piles per metre and more concrete. Tighten the tolerance in the specification, which pushes you toward cased or rotary boring rather than CFA and changes the rig. Or case the upper section, since deviation accumulates from the top and a bore held straight through the first six to ten metres arrives at depth with far less of its budget spent. A well-built guide wall does part of that job for free, which is why it belongs in the method statement rather than in the list of things the gang improvises on the first morning.
Run the nominal case first, then run it again with the diameter reduced by twice the tolerance loss at your formation depth — the second answer is the one that tells you whether the wall you priced is still secant at the bottom of the box.
The diameter of each drilled pile.
The distance between adjacent pile centers.
Pile overlap (or gap)
0.5 ft overlap
True secant configuration — adjacent piles physically overlap, forming a continuous water-resistant wall (assuming the secondary piles are cut into the primary piles as designed).
- Spacing-to-diameter ratio
- 0.75
They open the calculator with your figures already in it
Secant / Contiguous Pile Wall Overlap Calculator: 0.5 ft overlap — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- This is a geometry check on the two numbers you type, not a wall design. It says nothing about pile capacity, embedment below formation level, wall stiffness, or the earth and water pressures the wall has to hold, and it does not stand in for a temporary works design.
- Verticality is not in the arithmetic. A bored pile drifts off plumb with depth, so a nominal overlap that holds at the top can be gone at the toe; the deeper the wall, the more design overlap it needs, and this does not tell you how much to add.
- It assumes every pile is full diameter and set exactly on its layout point. Setting-out error, casing or auger wander, and over- or under-break in the bore all eat into overlap, and none of them are modelled — the number is nominal, not as-built.
- Nothing here covers the concrete or the sequence: primary pile mix strength (the soft, firm or hard secant distinction), the window in which the secondary pile must be cut into the primary, reinforcement, or guide-wall setting-out. A geometrically valid overlap can still be undrillable.
- A positive overlap is not proof the wall will hold water. Sealing depends on joint quality, construction tolerance and any grouting or membrane treatment, and a contiguous wall — spacing at or beyond the pile diameter — needs a separate water-control measure whatever this returns.
What a metre of wall costs before anybody prices labour
A pile wall is bought by the pile, not by the metre of wall, and the conversion between the two is the spacing. At 550 centres you are buying 1.82 piles for every metre of perimeter, so a 44 m box perimeter is eighty piles before anyone thinks about corners — and corners always add, because a secant wall cannot turn without either a purpose-made corner pile or an overlap that is geometrically different from the run. Count the corners off the plan at tender. They are the commonest single reason a pile schedule comes back longer than the estimate.
Each pile is then a cylinder of concrete: area times length, with the length running from cut-off level down past formation to the toe. The toe is not a detail. On a propped wall the embedment below formation is what provides passive resistance and vertical support, and it is set by the retaining wall design rather than by the depth of the basement — which is why a wall for an 8.5 m dig is routinely thirteen or fourteen metres of bore. Price the bore, not the box.
Then add what the cylinder does not include. Over-consumption of concrete is real and it is reported: BS EN 1536 and the ICE Specification both require the pour record to set actual volume against theoretical, because the difference is diagnostic — a bore taking noticeably more than its geometry says has lost ground somewhere. The allowance you carry at tender is a commercial judgement backed by your own pour records on comparable ground, and that is the only defensible source for it. Add the cut-down and trimming of every pile head to sound concrete, the disposal of the trimmings, and the capping beam that ties the row together. Reinforcement usually sits in the secondary piles only, so cage tonnage lands on roughly half the count — but cage length is set by the bending moment envelope, not by the dig, and a cage stopping short of where the moment is still significant is the kind of saving that gets found during the excavation.
Take one pile at its full bored length rather than the retained height, multiply by the count the spacing gives you, and you have the concrete line of the tender before any allowance for over-consumption is argued about.
The diameter of the straight drilled shaft.
The straight shaft length, not including any bell at the base.
Whether the pier has an enlarged base (bell) for extra bearing capacity.
Total concrete volume
3.03 yd³
Assumes an idealized frustum-shaped bell — actual drilled bell geometry can vary with the drilling equipment and soil conditions; verify final volume against the driller's as-built log.
- Straight shaft volume
- 3.03 yd³
- Bell volume
- 0 yd³
They open the calculator with your figures already in it
Drilled Pier Concrete Volume Calculator (Straight Shaft / Belled): 3.03 yd³ — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- This is the theoretical volume of one drilled hole, with no allowance for over-break. A drilled shaft almost always takes more concrete than its design diameter implies, because the auger and any sloughing leave a hole wider than the drawing, and no waste or over-break factor is applied here. Add one before ordering, and multiply by the number of piers.
- The bell is treated as a plain cone frustum running straight from the shaft to the bell diameter. A real under-ream has a short vertical toe at the base and follows the shape of the reaming tool, so entering a bell height measured off a drawing understates the concrete the toe holds.
- Nothing here is a foundation design. Shaft diameter, depth, bell size, bearing pressure and skin friction all come from a geotechnical report and the structural drawings; this takes those dimensions as given and returns a volume, and says nothing about whether they are adequate.
- Whether the ground will hold an open bell is not assessed. Under-reams need cohesive soil that stands unsupported while the bell is cut; in granular, running or water-bearing ground a bell cannot be formed safely and the pier is built straight-shafted or cased instead.
- Excludes concrete placed above the design cut-off, which drilled shafts are normally over-poured to reach sound material, and the extra taken when temporary casing is withdrawn and concrete slumps out into the annulus.
The pour is where a secant wall is won
The secant sequence contains a timing window no other piling operation has. A primary pile has to be soft enough for the secondary auger to cut into it cleanly, and hard enough that the cut face stands rather than mixing with the fresh secondary concrete. Too early and the auger smears instead of cutting. Too late — and hard/firm mixes gain strength faster in warm weather than the programme assumes — and the auger deflects off the primary rather than biting into it, taking the secondary out of line and reopening the overlap problem from the section above. The primary mix is normally retarded to widen that window, and the window belongs in the method statement with a stated maximum age, not in the ganger's head.
Placement is the other half. Where a bore holds water or support fluid the concrete goes in through a tremie, and the discharge end has to stay buried in the concrete already placed for the whole pour. Break that seal even briefly and water or slurry gets between two lifts of concrete at a depth nobody will ever inspect, in a wall whose entire purpose is to be continuous. The pipe is raised as the level rises, and the amount that can safely be raised is simply how much concrete is standing above the discharge end minus the embedment the specification requires.
- Confirm the primary mix age against the stated window before the secondary rig sets up over the position — not after it has started boring.
- Record concrete volume against theoretical volume as the pour proceeds rather than as one figure at the end, so a loss shows up at the depth it happened.
- Keep a weighted tape on the rising concrete surface and derive the embedment from it, rather than trusting the count of pipe sections removed.
- Withdraw the tremie in increments that leave the required embedment intact at every stage, and stop withdrawing rather than stopping the pour if the two disagree.
- Log the cut-off level and the concrete level separately; the difference is the over-pour that gets trimmed off and disposed of, and it is a quantity, not a rounding error.
This is a pour-day check rather than a design one: enter the concrete depth measured off the weighted tape and it tells you how much pipe is available to come out before the seal is the thing you are gambling with.
The horizontal cross-sectional area of the shaft or excavation being poured.
How deep the concrete already placed currently measures, from the base of the pour.
The full length of the tremie pipe currently in the shaft.
Safe pipe withdrawal available
5.079 ft of pipe safely withdrawable
The pipe can be raised by up to this amount and still keep at least 1.5 m (5 ft) of embedment in the placed concrete.
- Concrete volume placed so far
- 5.98 yd³
- Remaining pipe length above pour surface
- 23 ft
They open the calculator with your figures already in it
Tremie Pipe Safe Withdrawal & Embedment Calculator: 5.08 ft of pipe safely withdrawable — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Embedment is taken to be the whole depth of concrete placed, which only holds while the pipe's discharge end is still sitting at the base of the shaft. There is no input for the current tip elevation, so once the pipe has been raised the withdrawal figure is too generous by exactly the height the tip has already travelled.
- The volume line is a plain prism, cross-sectional area multiplied by depth, and it is never reconciled against the concrete actually delivered. It therefore cannot reveal overbreak, sidewall collapse or loss into a permeable stratum, each of which shows up as a shaft taking more concrete than the calculated rise accounts for.
- Embedment is only half of what keeps a tremie flowing. The concrete head inside the pipe has to stay above the water or slurry head outside it, and a mix that stiffens between loads can plug the pipe or let slurry in with embedment still well above 1.5 m (5 ft). None of that is checked here.
- The 1.5 m (5 ft) embedment is a hardcoded constant rather than an input, and it is the common minimum, not the design value for a particular shaft. Many specifications require deeper embedment on deep or slurry-supported shafts, and some also cap the rate of withdrawal; the governing figure is the one in the project specification rather than the one built into this page.
- The depth entered is counted as sound concrete throughout. The top of a tremie pour carries laitance and slurry-contaminated material that has to be overpoured and cut off, and neither that cut-off allowance nor the extra concrete it consumes appears in the volume shown.
Nailing a face on ground you do not own
Soil nailing reinforces the ground rather than replacing it, and the arithmetic follows from that. A nail only resists once the mass behind the face starts to move, and only the portion of the nail that lies beyond the potential failure surface contributes anything — the length in front of it is moving with the block it is supposed to be holding. FHWA Geotechnical Engineering Circular No. 7, the soil nail wall reference manual, is the document most specifications lean on, and its preliminary sizing puts total nail length at a substantial fraction of the wall height — commonly quoted as somewhere between six-tenths of that height and the full height. For an 8.5 m face that is nails of five to eight and a half metres, installed at a downward rake, with the bonded portion landing several metres behind the face.
Which brings the boundary back. The bonded length is the working part of the nail, and on a tight urban box it is under the neighbour's garden, the neighbour's building or the highway. Grout and steel are left in the ground permanently even when the wall is temporary; nobody comes back to extract a nail. That needs a right, granted in writing, before the length appears on a drawing. In England and Wales the excavation itself already triggers notice under the Party Wall etc. Act 1996 where you are digging within three metres of a neighbouring structure and below the level of its foundations, or within six metres where the dig would cut a line drawn downwards at forty-five degrees from the base of those foundations; the Act deals with the excavation and the support, not with your right to leave reinforcement in their soil, and the two are commonly and expensively confused.
The capacity arithmetic itself is short. Pullout resistance is the nominal bond strength between grout and soil, multiplied by the circumference of the drilled hole, multiplied by the bonded length — so it is the hole diameter that matters, not the bar. GEC-7 sets the customary minimum pullout factor of safety at 2.0. The design load on a single nail comes out of the overall stability analysis and scales with the area of face each nail is responsible for, which is why a grid of roughly a metre and a half each way recurs: tighten the grid and every nail carries less, loosen it and every nail carries more, and the bonded length has to follow.
The soft number in all of this is the bond strength. Published tables give ranges by soil type and drilling method, and those ranges are wide enough that one description of ground yields capacities differing by a factor of two or three depending on whether the hole was rotary drilled open, drilled with casing, or driven. Taking the top of a published range at tender is the commonest way a nailed face gets underpriced. Take the bottom of it, say so explicitly, and price the test programme that will move it.
What holds a nailed face up
- Sprayed concrete facing — measured as face area times nominal thickness, with rebound and overspray sitting outside that geometry and inside the sprayer's own allowance Concrete Calculator
- Head plates and nuts — the only part of the nail that can be inspected or re-tested once the face is closed, which is why proof testing is sequenced before the covering pass
- Facing reinforcement — carries the load between nail heads rather than into the ground, so its lap detail is set by the nail grid and not by the roll width delivered
- Strip drains against the cut — vertical geocomposite strips taking seepage down to a collector at the toe, because a sprayed face with no relief behind it becomes a dam
- Grouted nail bodies — only the length beyond the failure surface earns its keep, so the drilled hole diameter and the bonded length are the two figures the capacity actually turns on Soil Nail Pull-Out Safety Factor Calculator
- Battered cut in lifts — taken down in stages that have to stand unsupported until the lift above them is nailed and closed, which is a soil property rather than a programme decision
Enter the bonded length only — the part beyond the failure surface, not the whole nail — and run it twice, once at the low end of the published bond range and once at the high end, because the gap between those two answers is the size of the risk you are carrying until a nail is pulled.
The grout-to-soil bond strength, from FHWA GEC-7 tables or site-specific pullout testing.
The diameter of the drilled hole for the nail and grout.
The length of the nail bonded (grouted) beyond the potential failure surface.
The nail's design tensile load.
Pull-out safety factor
3.56 (target ≥2.0)
The design load on this nail is below the most the grout-to-ground bond would carry at a pullout safety factor of 2.0 shown with it — FHWA's minimum for soil nailing gives it. Site-specific pullout testing can establish a bond strength the assumed value does not. Being under the allowable pressure is not the whole ground question: settlement, groundwater and the footings alongside are all untouched here.
- Pullout capacity
- 63.97 kips
They open the calculator with your figures already in it
Soil Nail Pull-Out Safety Factor Calculator: 3.56 (target ≥2.0) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- This checks the grout-to-ground bond and nothing else. The nail bar's own tensile capacity, the grout-to-steel bond, and the head plate and facing connection are separate checks — a nail can pass this one and still fail by yielding the bar or punching through the shotcrete face.
- It is a check on one nail, not a wall design. Global and sliding stability, bearing at the toe, facing reinforcement and the analysis that produced the design load per nail all sit outside it, and the design load you type in is taken as given rather than tested.
- Only the bonded length beyond the potential failure surface earns capacity, and the calculation cannot locate that surface for you. Enter the whole nail length instead of the portion past the failure plane and the answer is overstated in direct proportion.
- Bond strength is treated as one uniform value along the whole bonded length. Soil layering, change with depth, drilling method, groundwater and grout pressure all move it, and a tabulated figure remains an assumption until verification and proof pull-out tests on site confirm what the ground actually delivers.
- Static, short-term conditions only, and the 2.0 target is the common static minimum. Seismic loading, construction surcharge, creep in fine-grained soils and corrosion over a permanent wall's life are not in the arithmetic, and a project specification or a load-and-resistance-factor design can demand a different margin. This does not replace a geotechnical engineer's design.
A bond value is an assumption until a nail is pulled
Testing is what converts a table into a design, and on a nailed job it is a priced item with its own programme, not an afterthought. GEC-7 separates two kinds of test. Verification tests are taken on sacrificial nails, installed specifically to be pulled, loaded well past working load until the bond gives or the rig does, and their job is to establish what the ground will actually deliver by the installation method you are using. Proof tests are taken on a proportion of the production nails, to a lower multiple, and their job is to confirm that what the verification test found is still what the gang is achieving three weeks later. The multiples and the sample proportion belong to the project specification; BS EN 14490 covers execution of soil nailing on the European side and BS 8006-2 the design, and both expect testing rather than treating it as optional.
Two practical consequences fall out of that. The first is sequencing: a proof test needs access to a head that is free to move, so it happens before the closing pass of shotcrete rather than after, which puts a hold point in the middle of a lift the crew wants to finish. The second is that a failed test is not a linear problem. Lengthening the bonded portion raises capacity in proportion, which is the easy fix. Adding nails does something else — it reduces the tributary area each nail serves and therefore the design load per nail, so the safety factor improves from both directions at once, but the grid change ripples into the facing reinforcement and the head detail. Neither fix is free, and both are far cheaper found on a sacrificial nail in week one than on a production nail in week six.
Propping decides the wall, and the sequence decides the props
An embedded wall can be a cantilever or it can be propped, and on a deep box next to a building it is propped, because a cantilever's whole design philosophy is that it deflects to mobilise resistance and deflection is the thing you have promised the neighbour you will limit. Once props enter the picture the pressure distribution changes shape. A free cantilever attracts something close to a triangular active distribution; a wall held at two or three levels does not, because it cannot rotate about its toe. Braced excavation design works instead to an apparent earth pressure envelope, empirically derived and closer to rectangular or trapezoidal, which is a different diagram and gives a different answer for a strut near the top of the wall. Do not size a prop off a triangular active pressure and then wonder why the measured load is higher.
The strut load itself is a tributary calculation: the design pressure, times the height of wall the level is responsible for, times the horizontal spacing between struts at that level. That gives an axial load, which is where the engineering starts rather than ends — buckling over the free length, the connection at the waling, and bending in the waling between piles are separate checks. The last one catches people on a pile wall, because the waling spans between discrete pile faces rather than bearing on a continuous surface.
The sequence you choose changes all of these numbers. Bottom-up with temporary steel props is fast and keeps the box clear for plant, but it accepts more wall movement and puts steel exactly where the crane wants to lift. Top-down, casting the permanent slabs as you descend and using them as the props, is the low-movement option and is often what a party wall surveyor is really asking for, at the cost of working under a lid with restricted access. Ground anchors remove the props from the box entirely and are the obvious answer until you remember where the anchor bond zone goes: under the neighbour, exactly like a nail, with the same need for a right and the same permanent steel left behind. On a constrained site the anchor solution is frequently the best engineering and the least available option.
Work each prop level separately, taking the tributary height as half the distance to the level above plus half the distance to the level below, and treat the answer as the starting load for a buckling and connection check rather than as the design.
The design lateral earth pressure for the braced excavation.
The vertical height of wall each strut level is responsible for.
The horizontal center-to-center spacing between struts at this level.
Strut axial load
109 kips
This is a simplified tributary-area estimate — real braced excavation design uses the full Peck/Terzaghi apparent pressure envelope for the specific soil profile, and should also check strut buckling capacity, connections, and wale bending, all outside this calculator's scope.
They open the calculator with your figures already in it
Excavation Bracing Strut Load Calculator: 109 kips — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 109 kips — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- It gives the axial load only, not a strut. Nothing here checks buckling capacity, unbraced length, end fixity, wale bending, or the connections at each end, and the strut's own self-weight and the bending it causes on a long span are ignored. Choosing a section from this number is a separate design.
- The apparent pressure is taken as given, never derived. No excavation depth, soil strength, unit weight or wall type is asked for, so the calculator cannot tell you whether the figure you entered matches a Peck envelope for your ground — it simply multiplies what you type.
- Water and surcharge are not added. Hydrostatic pressure from an unrelieved water table, and load from spoil heaps, cranes, traffic or adjacent foundations, must already be inside the pressure you enter. Water alone is roughly 10 kPa (1.5 psi) for every metre (3.3 ft) of head.
- The tributary rectangle assumes a straight, uniform run of wall with regular strut levels at regular spacing. Corner struts, rakers, unequal bays, struts at a change in excavation depth, and the redistribution that happens as levels are installed, preloaded and removed all fall outside it.
- This is a load estimate, not shoring design. Basal heave, wall embedment, ground movement and damage to neighbouring structures are untouched, and none of this replaces a temporary works design and check by a competent engineer.
What the neighbour is measuring is movement
The acceptance criterion on a job like this is not the wall. It is the crack width in someone else's plaster, and the assessment framework behind it is a damage classification — Burland's categories, running from negligible through very slight and slight to moderate and worse — applied to predicted settlement and distortion at the neighbouring structure. Trigger and alarm values in the monitoring regime are derived from that assessment. They are not a rule of thumb, and a contractor who invents them will find they have committed to something either unachievable or meaningless.
The baseline survey has to be read before anything arrives on site, including the piling rig, because the rig is one of the things that moves the ground. What actually causes the movement, roughly in order of how often it appears in the record: over-excavation below a prop level before the prop is installed, delay between reaching a level and stressing the frame at it, drawdown outside the box during dewatering, loss of ground into an unstable bore, and the elastic response of the wall to each stage of unloading. Only the last of those is in the design. The first two are programme decisions taken on site by people who may not know what the monitoring regime committed them to.
Getting to the point where the rig can be booked
All of this is temporary works, and temporary works carries a procedure. BS 5975 sets out the framework used on most UK jobs: a register, a designated coordinator, a design brief, and a design check at a category proportionate to the consequence of failure. A deep basement against an occupied building sits at the serious end of that scale, and the check is independent by definition. Eurocode 7 governs the geotechnical design either way, and the ground investigation has to be adequate for it — which for an 8.5 m box means boreholes taken comfortably below the toe of the wall rather than to formation, and enough of them to say something honest about the sand lens the first one found.
Against all of that, the two demonstrations this article exists for are short. For the pile wall: state the diameter, the spacing, the specified verticality tolerance and the depth, and show the residual overlap at the toe under the worst permitted deviation of adjacent piles. For the nailed face: state the drilled hole diameter, the bonded length beyond the failure surface, the bond strength you are assuming and where it came from, and show the pullout factor against the design load per nail with the grid you priced. Each is a line of arithmetic and each closes off the failure mode that actually costs money on this kind of job.
The estimator's version of the same decision is simpler still. A pile wall is expensive and predictable, and the boundary does not have a vote. A nailed face is cheaper and conditional, and the conditions are a neighbour's signature, ground that will stand for a shift, and a water table below your formation. Price the pile wall properly and offer the nailed face as an alternative with those three conditions written on the face of the tender. That is a defensible position on the day the ground turns out to be worse than the borehole suggested — which, on an urban site with a hundred and fifty years of made ground on top of it, is most days.
What has to exist before either price is real
Both options are bets on a number nobody has measured yet. These are the items that turn each bet into something an engineer will put a signature against.
- Title plan with the ownership line marked, not the fence line — Decides whether a nailed face or a ground anchor is available at all, because both leave permanent steel beyond that line.
- Boreholes taken below the wall toe, not to formation — A wall for an 8.5 m dig is commonly thirteen or fourteen metres of bore, and the passive resistance at the bottom is what the investigation has to reach.
- Verticality tolerance from the piling specification, as a fraction — Twice that fraction, times the depth, is the overlap two adjacent piles can lose between them before the wall stops being secant.
- Certified working platform for the rig — A tracked piling rig needs a designed and certified platform under it; BRE BR 470 is the reference most UK jobs work to, and it is a separate cost line.
- Pullout test programme, priced separately — Sacrificial verification nails in week one and proof tests on a proportion of production nails, with the multiples taken from the specification rather than assumed.
- Monitoring baseline read before the first plant movement — Trigger and alarm values come out of the damage assessment for the neighbouring structure and belong on the excavation drawing, not in an appendix.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.