Waterloo Park: The Use of ESCS Lightweight Aggregates in Stabilized Horticultural Subsoils

by Eric Nelson, Arcosa Lightweight

The Amphitheater Terrace
The Amphitheater Terrace provides shaded relief and panoramic views of the park. The stabilized horticultural subsoil was placed beneath the planting soil for weight reduction and proper drainage. The media contains available pore space essential for oxygen exchange and microbial growth.

Some things in life are worth waiting for. An urban park that took over 10 years to plan and 4 years to construct was opened and celebrated on August 14, 2021. Waterloo Park is the largest urban park in Austin and it showcases nature, civil engineering precision and architectural beauty. The 11-acre park is phase one in a series of green spaces and amenities tied to a 1.5-mile stormwater improvement plan between 15th Street and Lady Bird Lake.

Located just two blocks from the State Capital, Waterloo Park seems to have everything. Park visitors are greeted with hike and bike trails, an elevated skywalk, functional lawn areas, a pavilion for concerts, secluded nooks for quiet and reflection, a children’s play area and raised decks for lounging. As you explore the site you will find more than 500 trees, 100,000 plants, 50 feet of elevation change with wetlands and lush landscaping reminiscent of Central Texas. A diverse design team of professionals created this urban masterpiece; it featured the disciplines of architecture, civil and structural engineering, landscape architecture, hydrology, soil science, ecology, and horticulture, among others.

The design elements you don’t see contribute to the park’s success. An underground stormwater channel measuring 10.5′ in diameter solved the flooding issue at the site. Rainwater is collected via catch basins and underground pipes, then stored in a cistern before it is pumped into a series of raingardens for filtering. Engineered concrete structures were buried to support program elements like walkways, concession and restroom areas. Specially designed soils and irrigation systems were specified for optimum growing conditions for the many trees, plants and turf zones.

The firm of Olsson Associates crafted the soil specifications for Waterloo Park. Differing soils were blended to meet the agronomic conditions of the performance lawn, street trees, wetland plantings, landscape beds, general turf areas and plantings on structure. A key component of the specifications was stabilized horticultural subsoils. These included a mix of 2 parts sandy loam, 3 parts lightweight aggregate (expanded shale) and 1 part compost; they were placed beneath the planting soils at varying depths, depending on their use. This method was utilized for the soils over structure, the street trees and performance lawn.

Expanded shale, clay and slate (ESCS) lightweight aggregates serve an important function in stabilized horticulture subsoils. They are durable, yet porous, so they can absorb water and nutrients, slowly releasing them into the soil as it dries out. The product is stable and angular-in-shape. It forms a stabilized matrix when used at the 50%± rate. It will also add total pore space to the soil, which increases oxygen exchange and internal drainage. In the case of Waterloo Park, this soil is part of the system of rainwater capture and filtering. It allows the performance lawn to drain within minutes of a rain event before a concert. No more muddy pits for concert goers. The stabilized horticulture subsoils set up firm as needed to support chairs during concerts or, in the case of street trees, support the pavement. They also provide great growing conditions for the tens of thousands of plants installed on the project given the added moisture, oxygen and soil microbes the media generates. These soils promote the development of larger root systems which ultimately lead to a healthier landscape.

Waterloo Park provides a natural relief from the urban core. One can find quiet spaces for relaxation, reading or strolling. The dome of the State capital can be seen from most places in the park.


Sidewalks, a raised skywalk and informal pathways meander more than 1.5 miles through plantings common to Central Texas. Of the 8 live oaks at the park, 4 were transplants from nearby construction sites.
Performance lawn
The soils of the performance lawn were designed to drain quickly and support the concert chairs. The stabilized horticultural subsoils included 50% expanded shale to achieve this. Latitude 36 Bermuda was the turf chosen for its durability in high traffic areas and quick recovery time.