Walls, fill, drainage, soil, and water — everything before the first plant goes in
Site Assessment
Location & climate. Lafayette, CA. USDA Zone 9b. ~200–400 winter chill hours — low-chill cultivars only.
Lot shape. Pentagon. 30 × 60 ft with the back-left (NW) corner cut at 45°. Net area ~1,687 sq ft.
Surrounded by pavement. Driveway on the east, driveway/walkway on the west, driveway on the north, patio + house on the south. No soil overflow — all roots live within the lot's retaining walls.
House. 14 ft tall, occupying the east 2/3 of the south boundary; patio wraps the west 1/3. Winter-noon shadow reaches ~16 ft into the east portion of the lot.
Topography. Existing grade is level at the south edge (flush with the patio / house slab) and drops to ~4 ft below datum at the north edge. About 4 ft of cross-fall over 60 ft (~3.8°).
Soil. Compacted clay (East Bay Diablo/Altamont — expansive).
Orientation. Lot back edge is ~26° east of true north. "North" in this document refers to the lot's back edge (the new north wall).
Existing site condition. A small pile of leftover construction materials currently sits on the lot. It needs to be cleared and hauled off before any base course or wall block goes down.
Build-Up Lot, Trenched Wall Toes
The lot interior is built up on existing grade. The retaining walls get a shallow toe trench.
All leveling across the lot is achieved by adding material on top of the existing surface — walls go up, fill goes in, planting cap goes on. The native clay subgrade across the lot interior is left undisturbed. The one exception is a narrow ~6 in deep × ~14 in wide trench at each wall line, which embeds the first block course below the outside grade and locks the toe against sliding under fill pressure. The goal is for the finished lawn to sit flush with the house and patio at the south edge so the family can walk straight out from the slab onto the lawn without a step.
What "build-up lot" means in practice:
Pre-work cleanup. Clear the existing construction-debris pile off the lot. Surface-rake any rubble, then lightly scarify the top 1–2 in of clay where new fill will land — just enough to give the first lift something to key into.
Walls. Dry-stack lip-style segmental concrete block (e.g., Oldcastle 6 in H × 16 in L × 9 in D gray-char block, or any equivalent retaining-wall block with a cast-in setback lip). Cut a ~6 in deep × ~14 in wide toe trench along each wall line, place 4–6 in of compacted ¾ in-minus gravel in the bottom, and embed the first block course in the trench so its top is roughly at the outside finished grade. Above-grade courses dry-stack with the lip setting the batter — no mortar, no rebar. The north wall retains 3 ft of fill above the buried toe course; the east wall tapers down to grade at the south end.
French drain. The toe trench also bedrooms the drainage column and the 4 in perforated drain pipe behind each wall. See the French Drain section below.
Structural fill. On-site clay goes on the existing surface and is placed in 6–8 in compacted lifts.
Topsoil cap. Imported sandy-loam / compost blend goes on top of the structural fill, 12–24 in thick (12 in over the central lawn, 16–24 in in the planting band where trees and shrubs go). All tree and guild root zones live in this cap.
Walls & Fill
Item
Quantity
Notes
Retaining walls
~141 linear ft total
North (~15 ft) + 45° diagonal (~21 ft) + east (~60 ft) + west (~45 ft, recently built). All four wall segments use the same lip-style segmental concrete block (Oldcastle 6 × 16 × 9 in or equivalent). First course is embedded in a ~6 in deep × ~14 in wide toe trench on a 4–6 in compacted ¾ in-minus gravel base — locks the toe against sliding under fill pressure. Subsequent courses dry-stack with the lip setting batter. No mortar. The west wall is already built to spec; the north, diagonal, and east are still to build.
Wall height
3 ft retained (max)
The north wall retains the 3 ft elevation difference between existing grade (4 ft below datum at the north) and finished grade (1 ft below datum at the north — the 1° drainage slope). That's 6 above-grade block courses plus 1 buried toe course in the trench = 7 total courses per linear foot at the deepest stretch. The diagonal and east walls taper as the lot's existing grade rises toward the south (the buried toe course runs the full length of every wall regardless). 3 ft retained is at or under every lip-block manufacturer's gravity-wall rating — no geogrid required. California Building Code exempts walls up to 4 ft from bottom of footing with no surcharge.
Block count (new walls only)
~380 blocks
Slope-aware. Per-wall: north (15 ft × 6 visible + 1 buried = 7 courses ≈ 78), diagonal (21 ft, courses 1–5 full length + top course ~14 of 21 ft + buried full length ≈ 105), east (visible courses stair-step 60 + 50 + 40 + 30 + 20 + 10 lin ft + buried full 60 ft ≈ 202). Total ~385. West wall already built — no blocks to order for it. Sample block: Oldcastle 6 × 16 × 9 in gray-char (Lowe's).
Drain behind west wall
~45 linear ft
Add the gravel drainage column + 4 in perforated pipe + filter fabric behind the existing west wall — identical spec to the new walls' drains (see French Drain section).
Total fill volume
~85 cubic yards
Pentagon lot 1,687 sq ft × avg fill depth 1.4 ft = ~2,360 cu ft. South edge: zero fill (existing already at finish). North edge: ~3 ft total (18 in structural clay + 18 in topsoil cap).
Structural fill
~40 cu yd
Your on-site stockpiled clay, moisture-conditioned and compacted in 6–8 in lifts.
Topsoil cap
~45 cu yd
Imported sandy-loam / compost blend. 12–24 in thick — 12 in over the central lawn, 16–24 in in the 6 ft planting band around the perimeter where trees and shrubs go.
Finish-grade slope
~1° south → north
About 1 ft of drop across 60 ft (~1° ≈ 2%). South edge sits flush with the house/patio; water sheds toward the north wall, where the French drain handles it.
North-south cross-section through the lot. Existing clay slopes 4 ft from south (flush with patio) down to north. Finish lawn surface slopes 1° south → north for drainage. Topsoil cap varies by zone: 12 in over the lawn, deepening to 18 in across the 6 ft planting band (measured from the wall back, so the drain is the wall-side end of this band), then 16 in directly over the drain. The drain column sits above the structural-fill top at the boundary because the planting-band cap is deeper than the over-drain cap. North wall caps just above the finish soil. Vertical exaggerated ~3× for clarity.
How to layer and compact
Prep the existing surface. After construction-debris cleanup, lightly scarify (rake or rototill) the top 1–2 in of clay. This keys the first new lift into the original surface so the new fill doesn't shear along the interface during rain or seismic events.
Set the walls first. Cut the toe trench, place 4–6 in of compacted ¾ in-minus gravel in the bottom, embed the first (buried) block course, lay the perforated drain pipe at the back of that course, and start the gravel drainage column behind the wall before any clay fill is placed against it. Build the wall up in step with the fill — never top out a 3 ft wall and then dump 3 ft of clay against it (the lateral load on an un-bedded wall will push it out of plumb). At 3 ft retained, the wall is at the standard gravity-wall rating for lip-style segmental block — no geogrid reinforcement needed.
Moisture-condition the clay. East Bay clay compacts well only at optimum moisture. The field test: squeeze a handful into a ball — it should hold its shape but not smear water on your palm. Too dry, it powders and won't densify; too wet, it "pumps" under the compactor and will not consolidate at all. If a lift is too wet, spread it thin and let it dry a day. If too dry, mist with a hose and turn it with a rake.
Place in 6–8 in loose lifts. A 6–8 in loose lift compacts to roughly 5–6 in finished. Spread evenly with a rake before compacting — uneven lifts compact unevenly and leave a wavy interface that holds pockets of water.
Compact each lift before the next one goes down. For clay, rent a jumping-jack (rammer) compactor from a tool yard — not a flat plate. The rammer's high-impact, low-frequency blow is what drives air out of plastic clay; plate compactors are for granular base rock and will just polish the top of a clay lift. Plan 4–6 passes per lift, with each pass running perpendicular to the previous direction.
Field-check compaction. A boot heel pressed hard into a compacted lift should leave a print no deeper than ~¼ in. If it sinks in or feels mushy, add a pass or two. If the surface bounces or cracks, the lift is too wet — pull it back, re-spread, and let it dry.
Build the 1° finish slope into the structural fill, not just the cap. When you reach the top of the structural lifts (~18 in below finished grade), grade that surface itself at the same ~1° south-to-north slope as the final cap. That way, any water that ever percolates through the topsoil cap still has somewhere to go and won't perch on a flat clay shelf.
Top with the topsoil cap. 12–24 in of imported sandy-loam / compost (12 in lawn, 16–24 in planting band). Don't compact the cap hard — one or two light plate-compactor passes, or just foot-walked. Root zones need air. Final grade the surface with a landscape rake to the 1° drainage slope.
Using your existing materials
The clay you already have is structural fill, not topsoil. It is excellent at compaction and lousy at supporting roots — exactly the right material for the bottom 18 in.
The wood chips you already have become the mulched circles around every tree, the pathway network through the planting band, and the twice-yearly top-dress on the cap.
French Drain
Each wall gets a gravel drain column with a 4 in perforated drain pipe at the base, sloped to a single controlled daylight outlet through the north wall 6 ft west of the east wall (the same point where the surface swale terminates). Water enters the gravel from the soil column, falls by gravity to the pipe, and the pipe conveys it to the outlet. Filter fabric on the soil side and top keeps clay fines out of the gravel.
Don't skip the filter fabric. Without it, clay fines migrate into the gravel and the drain silts up within a few seasons.
Cross-section of the drain (behind every wall):
Toe trench / base. ~6 in deep × ~14 in wide trench at the wall line, with 4–6 in of compacted ¾ in-minus gravel in the bottom. This is both the wall-toe foundation and the bedding for the drain pipe and gravel column.
Perforated pipe. 4 in perforated PVC (or equivalent) at the base of the drain rock, sitting on the toe-trench gravel with perforations facing down. Sloped ~1% along the wall run, draining toward the outlet through the north wall (6 ft west of the east wall). Add vertical cleanout risers (capped at finished grade) at the NW and SE ends of the pipe run so the line can be rodded out if it ever silts.
Gravel column. Clean ¾ in drain rock (washed, angular, no fines), built up around the pipe and up the back of the wall. Width: 12 in for east, west, and 45° diagonal walls; 18 in along the north wall (the low edge of the lot — most subsurface flow concentrates here). Terminates 12–16 in below finished grade so the topsoil cap extends over the top.
Filter fabric. Wraps the soil side and the top of the gravel column. Either non-woven geotextile or a biodegradable natural-fiber felt is fine — contractor's choice. The fabric stops between the wall blocks and the gravel — water has to be able to leave through the pipe.
Cap on top of the drain. 16 in of topsoil cap sits over the gravel column, sealed off from the gravel by the filter fabric. That's enough rooting depth to plant shallow-rooted shrubs, groundcovers, and guild perennials directly above the drain — see Planting band notes below.
Outlet. One controlled outlet through the north wall, 6 ft west of where the north wall meets the east wall. The 4 in pipe daylights here onto a small rock-apron splash pad on the driveway side. The natural drainage gaps between bottom block courses along the rest of the wall remain as a secondary backup overflow path.
Surface swale. A three-segment turf swale runs 6 ft inside the wall: (1) follows the 45° diagonal wall 6 ft to its lot-side, from the inset-W / inset-diagonal junction up to the inset-diagonal / inset-N junction; (2) continues east 6 ft south of the north wall to a point 6 ft west of the east wall; (3) cuts north to the wall outlet. ~6 in deep × ~18 in wide, sloped ~1% along its full run, lined with lawn turf. Catches surface runoff from the lawn before it pools at the diagonal or north wall and routes it to the same outlet as the perforated pipe (via a small grated surface inlet next to the pipe daylight).
Cross-section through the north wall at the deepest stretch (3 ft retained). First block course embedded in a ~6 in toe trench (shown dashed below grade). Drain rock column 18 in wide on the north wall (12 in elsewhere) with a 4 in perforated PVC pipe at its base, sloped ~1% to a controlled daylight outlet through the north wall, 6 ft west of the east wall. Cap depths inside the 6 ft planting band: 16 in over the drain → 18 in across the rest → 12 in over the lawn. Mulch is mounded over the planting band — peaks ~8 in above finish grade in the middle, tapers to 0 at the sides. Surface swale at the south edge of the planting band (green U-shaped dip just inside the lawn) — 6 in × 18 in, turf-lined, drains east to the same outlet. Existing subgrade slopes upward going right. Proportions illustrative.
Top-down detail of the drain column behind any wall section. Same 12 in width applies everywhere the drain runs.
Where the drain runs. Behind every wall — existing west, new north, new 45° diagonal, new east — forming a continuous "U" around three-and-a-half sides of the lot. The south side has no wall and no drain; the lot meets the patio at grade there, and the 1° finish slope carries any surface water away from the house toward the north.
Top-down view. The orange dashed lines are the gravel drain column behind every wall — west, 45° diagonal, north, east. The blue swale (6 in × 18 in turf channel) runs 6 ft inside the diagonal wall, then continues 6 ft south of the north wall, then cuts north to the outlet at 6 ft west of the east wall. Both the 4 in perforated drain pipe (under each drain column) and the swale terminate at that single outlet through the wall. The south side has no wall and no drain — the lot meets the patio at grade there.
Soil Cap & Mulch
Don't till the existing clay subgrade — the build-up sits on top of it. Within the new topsoil cap, sheet-mulch rather than till.
Soil-test a composite sample of the new imported topsoil once it's spread — pH, N–P–K, organic matter. A $30 mail-in test (UMass, A&L Western Labs) tells you what amendments the cap actually needs.
Cap the topsoil with your stockpiled wood chips (cardboard sheet + 4–6 in chips) in the planting band; leave bare topsoil where the kid lawn will be seeded.
Inoculate at planting — every tree and guild plant gets a teaspoon of granular mycorrhizal inoculant in the planting hole.
Chop-and-drop forever — comfrey, borage, prunings stay on site.
Refresh wood-chip mulch twice yearly, 2–3 in deep, pulled 4 in back from every trunk.
The 6 ft Perimeter (Planting Band)
The planting band is a ~6 ft strip running along each of the four walls (north, 45° diagonal, east, west). The east and west strips run the full length of the lot down to the patio — T15 sits at the south end of the west band, T16 at the south end of the east band. The middle of the south side has no planting band — the lawn meets the patio directly. The diagram below shows the cap-thickness pattern from above.
Top-down view of cap thickness. The 6 ft strip along each wall (north, 45° diagonal, east, west) is planting band with a deeper cap. East and west strips run continuously down to the patio — T15 lives at the south end of the west band, T16 at the south end of the east band. The middle of the south side is lawn all the way to the patio (no planting band there).
The rules that follow keep the planting band working with the wall + drain system:
Cap thickness 16–24 in here, vs. 12 in over the central lawn. Trees need deeper rooting depth than turf, and the structural clay underneath isn't where you want feeder roots living. Order the topsoil delivery with this in mind: the cap is thicker in a ring around the perimeter and thinner in the middle.
Trunk distance from any wall: 3 ft minimum — applies to all four wall sections equally (north, 45° diagonal, east, west). Mature fruit-tree feeder roots concentrate within 2–3 ft of the trunk; 3 ft gives the tree its own root zone in cap soil and keeps pressure off the wall.
The strip directly above the drain is plantable — that's the whole point of running the gravel column down to 12–16 in below finish and capping over it. Good fits for this zone: bridge shrubs (gooseberry, pineapple guava, pomegranate shrub form), groundcovers (alpine strawberry, trailing rosemary, creeping thyme), and guild perennials (comfrey, yarrow, chives). Don't put fruit-tree trunks directly over the drain — let their canopy and outer roots reach over the drain, but plant the trunk a few feet inboard where the cap sits on the full clay backing.
Place the aggressive-root species furthest from the wall. Fig (T9 Black Mission, plus the wall-trellis Violette de Bordeaux on the east), mulberry (T4 Pakistan), and pomegranate (T10 Wonderful) all have notably exploratory roots. Sit those trees toward the lot-side edge of the planting band rather than tight against the wall, and accept that some root infiltration into the gravel column is likely over 15–20 years (the drain may need a partial rebuild at that point — design for it).
Drip emitters on the lot side of each tree, not the wall side. Water trains roots; emitters between the tree and the wall pull roots toward the drain and the wall. Place them 12–18 in inboard of the trunk instead.
Mulch right up to the wall is fine. The 3–4 ft mulched circle around each trunk can run all the way to the wall — the trailing rosemary planned for the wall-cascade tier will eventually overhang most of it.
Sourcing the Topsoil Cap
~50 cu yd of sandy-loam / compost blend is the single biggest material order in this phase. Around Lafayette, the realistic options:
Yard
Location
What to ask for
American Soil & Stone
Richmond (~25 min)
"Diestel Structured Compost" + "Sandy Loam" custom blend, roughly 60/40 loam-to-compost by volume. Delivers in 10–18 yd loads.
Acapulco Rock & Soil
Pleasant Hill (~10 min)
Closest yard. Ask for their "Garden Mix" or "Planting Mix" — sandy loam with compost. Cheaper delivery given proximity.
Lyngso Garden Materials
San Carlos (~45 min)
Top quality, screened. "Vegetable Garden Mix" or custom 50/50 sandy-loam + organic compost. Delivery surcharge for the distance.
Grab N Grow / Soil Tech
Walnut Creek / Concord
Backup options. Useful if the main yards are booked during spring rush.
What to specify on the order:
Texture: sandy loam, not "topsoil" — generic topsoil is often screened native clay.
~25–35% organic matter / aged compost by volume.
Screened to ½ in minus.
No biosolids, no construction-screened fill.
pH 6.0–7.0.
Delivery in 2–3 dumps over a day or two — staging 50 yd in one drop overwhelms the access route. Confirm the truck can reach the lot edge across the patio / driveway without driving on it.
Don't accept "free fill dirt" from a Craigslist or Nextdoor listing for the cap. That material is typically clay-heavy spoils from a construction site — exactly what you're trying to not have under your trees. Use those leads only if you ever need extra structural fill.
Other Prep Considerations
Material staging & access
Stockpile zones. The lot is surrounded by pavement, so material has to stage somewhere. Identify zones up front: imported topsoil in one corner, drain rock in another, block on pallets in a third. Lay plywood under any pile that sits on the driveway to protect the surface.
Sequence the deliveries. Drain rock + wall block arrive first (you build walls + drain column first). Imported topsoil cap arrives last (after structural fill is in place). Receiving 50 cu yd of topsoil on day one means it sits getting weedy and compacted for weeks.
Timing
Best season for the earthworks: late summer through early fall (August–October). Bay Area clay is at the right moisture for compaction then — dry but not bone-dry. Winter is the worst window: lifts will be too wet to compact and the work site turns to mud.
Plant in the following winter / early spring. Bare-root fruit trees go in January–February, after the cap has had a month or two to settle.