CRENSHAW DISTRICT — Metropolitan Transportation Authority contractors ended a second week of exploratory soil drilling along Crenshaw Boulevard on Tuesday.

Officials said testing is necessary to assess conditions along the stretch of Crenshaw that will be underground as part of the Crenshaw to LAX Transit Corridor Project. The 8.5-mile route that will connect passengers between the Green Line and the Exposition Line and traverse the cities of Los Angeles, Inglewood, Hawthorne, and El Segundo, as well as unincorporated areas of the County and LAX .

“In order to do a proper design of the trackways, the station structures, and the tunnels, we need to know what the soil conditions are so that we design effectively for the soil conditions,” said MTA Transportation Planning Manager Roderick Diaz. “That way, we know what we are constructing and how we need to construct it. That is a very key component.”

Drilling 100 feet into the ground, thus far engineers have only extracted clay, which they say is preferable to hard rock. Clay, despite its softness, is much easier to plow through. With clay, engineers use what is called a shield, an iron or steel cylinder that is pushed through the soft soil. It carves out a perfectly round hole that supports the surrounding earth while workers remove debris.

On the other hand, getting through hard rock almost always involves explosives or fire-setting. Explosives cause noxious fumes which must then be vacuumed out. This can be a time-consuming process that can pose health risks to workers. Fire-setting is a technique in which the tunnel wall is heated then quickly cooled with water. The expansion and contraction caused by temperature change causes large chunks of rock to break off.

Though the stand-up time for solid rock may last for centuries, breaks in the rock or pockets of fractured rock are a possibility and therefore must be supported by bolts, sprayed concrete or rings of steel beams, as well as the addition of permanent concrete lining.

Clay also requires extra support for the tunnel roof and walls because cave-ins are a threat. In areas where clay is the predominate material underground, workers will install permanent lining made of cast iron or pre-case concrete.

“We want to find out what the soil types are and how strong they are and how they would affect construction and tunnels,” said Mike Kapuskar, senior engineer with Earth Mechanics Inc., which specializes in geotechnical and earthquake engineering. “If the soil is too soft or unsuitable the foundation will be engineered to overcome that. For example, if the soil is very soft and liquifiable, we will punch pile through it to bridge between the foundation — what’s at the top, the structure — and the frame below it.”

Further soil drilling will take place in the coming weeks south of the Crenshaw District, in an area known as the Harbor Subdivision that includes the city of Inglewood and parts of the Los Angeles International Airport.

Drilling along the Harbor Subdivision will occur between March 23 and April 9.
Residents and businesses in the area should, however, expect traffic delays that will result from some lanes being closed or blocked. Hours of operation will be from 8 a.m. to 5 p.m. At the end of each workday, lanes that were closed will be re-opened for regular use. Noise and vibration will be minimal, with officials saying the decibel levels are no higher than that of a bus motor.

All together there will be up to 40 sites drilled for soil sampling, according to Diaz.

Below ground tunnels for the proposed light rail transit line will be from 39th Street on the north to 48th Street on the south, 59th Street to the turn on Victoria Avenue and near airport runways.

Elevated sections will include major streets, such as La Brea Avenue, La Cienega Boulevard, Manchester Boulevard and Century Avenue.

Construction on the line is expected to begin in 2012 and be concluded by 2018, transporting an estimated 20,000 passengers per day.

“We want to make it work to the fullest extent possible,” said Los Angeles County Supervisor Mark Ridley-Thomas, “doing what needs to be done on traffic congestion relief, air quality improvement and economic development.”

Workshops that will focus on various segments of the corridor and their components will be held March 23 at Ascension Lutheran Church Hall, 5820 West Blvd.; March 24 at Westchester United Methodist Church, 8065 Emerson Ave.; and March 25 at Saint Mary’s Academy School Gym, 701 Grace Ave. All meetings will begin at 6 p.m.