Walk up almost any hillside on the edge of Lima and you will see homes stacked on terraces cut into loose sand and rock. What keeps that ground in place? Often, a retaining wall.
So, how do retaining walls work? In simple terms, they hold back soil that would otherwise slide downhill, and they do it by resisting a constant sideways push from the earth behind them.
In this article, you will learn the basic engineering behind retaining walls and why they matter so much in Lima's hillside settlements. You will also see how a community in Lima is building one right now with Safe Homes Movement, working side by side with local engineers, architects, and student volunteers.
How do retaining walls work? The engineering basics
A retaining wall holds soil at two different levels: higher ground on one side, lower ground on the other. Its job is to keep the higher ground from collapsing onto the lower one.
Lateral earth pressure: the force a wall resists
Soil on a slope always wants to move down and out. The weight of the soil above presses on the soil below, which pushes sideways against anything in its way. Engineers call this lateral earth pressure. It increases with depth, so the base of a wall carries more load than the top.
Engineers check every design against four main failures:
- Sliding: the wall slides forward at its base.
- Overturning: the wall tips outward.
- Bearing failure: the ground beneath can't support the wall's weight.
- Global failure: the entire slope fails around the wall.
Why drainage matters
Wet soil is heavier than dry soil, and water trapped behind a wall adds its own pressure. That's why drainage is a core part of design. Free-draining backfill, such as gravel, and openings that let water escape help keep that pressure down. Many failures trace back to poor drainage rather than weak materials.
Common types of retaining walls
Gravity walls rely on their own mass of stone, masonry, or concrete. Cantilever walls use reinforced concrete with a base slab that the soil itself helps hold down. Anchored walls are tied into the ground behind them with cables or anchors. Engineers choose among these types based on wall height, soil conditions, space, budget, and the materials and skills available locally.
Why retaining walls matter in Lima, Peru
Homes built on loose, steep slopes
Over the past decades, many families moved to Lima from other regions of Peru and built homes wherever land was available, often on the steep hillsides surrounding the city. Close to a million people now live in these settlements on Lima's outskirts. Many of their homes are made of lightweight materials such as plywood or reed mats.
The hillsides themselves are part of the problem. They are made largely of loose sand and unstable rock, and much of the construction happened without technical assistance.
Earthquakes and El Niño rains
Lima sits in a seismically active zone, and even a minor tremor can trigger a rockslide on an unstabilized slope. The city rarely sees rain, but El Niño years can bring heavy rainfall and mudslides, which you can read more about in our post on El Niño in Lima's settlements.
When a slope fails, it can destroy homes and cut off the footpaths families use to reach school, work, and health care. This is why engineers across the region focus on building resilience against natural catastrophes before a disaster rather than after.
A real example: a community-led retaining wall in Lima
What the wall is for
In 2026, Safe Homes Movement launched a retaining wall project in one of Lima's hillside human settlements. A local resident working on the construction explained its purpose in plain terms. The wall holds back the collapse that a pile of loose rocks could cause, which endangers children and everyone else in the neighborhood.
That is lateral earth pressure in practice. The community understands exactly what the wall is resisting, because they live with the risk every day.
Community-led, professionally supervised
The project follows a 50/50 model. Community members take the lead in building the wall, designed to protect homes and stabilize the neighborhood, while local architects and engineers lead the design and supervise construction. Student volunteers on Service Learning Trips receive a briefing before work begins and join the construction under that professional guidance. Because residents build it, residents own it.
The same approach shaped the staircase Safe Homes built in the 2 de Diciembre community, where students and neighbors mixed cement, laid bricks, and installed handrails on a once-dangerous slope.
Why it goes beyond safety
In these neighborhoods, permanent, engineered infrastructure is often a key requirement when residents ask local governments for land titles and public utilities like electricity and running water. A retaining wall prevents rockslides, and it also gives families a stronger case for the legal recognition and basic services they need.
For engineering and architecture students, projects like this are a chance to gain real-world experience. Students see how load, drainage, and site conditions play out under real limits on access, budget, and materials, alongside the professionals solving those problems. You can see how this works on our Development Service Learning Trip in Lima.
Help build safer hillsides in Lima
Learning how retaining walls work in a classroom is a good start. Seeing one rise on a hillside, built by the families it will protect, teaches you what the textbook can't.
If you study engineering, architecture, or community development, the Development Service Learning Trip lets you learn from local professionals in Lima while contributing to projects the community itself has prioritized.
Fill out the form to receive dates, eligibility details, and next steps for the Development Service Learning Trip. Safe Homes Movement also runs projects in Cusco and Tena, so ask the team which destination best fits your goals.
To learn more about how you can get involved, check out our Safe Homes Movement brochure. And remember, for less than one coffee a month, you can make an impact in low-income communities by becoming a monthly donor and directly assisting communities in need.
